Quantum Mechanics, qubits, superposition & superconductors with Prof. Prabha Mandayam | BP2B S2 E11

Prabha Mandayam 0:00 [upbeat music] It's too much information.

Speaker 0:01 I'm going, I'm going, I'm going to share what I understood. [chuckles]

Prabha Mandayam 0:04 Yeah.

Speaker 0:04 I'm sure I'm wrong.

Prabha Mandayam 0:05 But imagine the coin in flight, and let's say you capture it in a box as it is in flight. That's a quantum state. So I should tell you that I was one of those people in 2000 who actually did not write the JEE exam.

Speaker 0:19 Okay.

Prabha Mandayam 0:19 So, you know, I have never thought about this as is there a, you know, career path here? Is there-

Speaker 0:24 Mm.

Prabha Mandayam 0:24 It's more about what has interested me. Last year, with Google's, like, 100 qubit experiment, it has more or less come. That's like a first proof of principle that you can put 100 qubits on a chip. You can connect them all up in some way that makes them resilient to this error. Take your friend's notes, right, the night before the exam, and make a copy of the entire notebook. No, not possible. So you cannot copy quantum information.

Speaker 0:49 Hi,

Speaker 0:50 this is Amrit. We are at IIT Madras, my alma mater, and India's top university for people who like to build. We are here to meet some builders, ask them: What are you building? What does it take to build? And what makes IIT Madras the best place to build? [upbeat music] Hello, and welcome to the Best Place to Build Podcast. Today, we are sitting with Professor Prabha Mandayam, a renowned physicist and a faculty here

Speaker 1:30 at the Physics Department at IIT Madras. Her area of interest is quantum information and error correction. She's an author of a book, Functional Analysis of Quantum Information Theory, and her NPTEL lectures on quantum computing are very popular. You can see I'm looking into my notes, so I know nothing about this subject, so it's an opportunity for me to learn. Welcome to the podcast, Professor.

Prabha Mandayam 1:50 Thank you, Amrit. Thank you for having me here.

Speaker 1:53 Uh, Professor, I don't know anything about this, so, um, it's a challenge I'm offering to you. [chuckles] Please, can you tell me about quantum computing in the next half an hour, so that I can survive the next ten years?

Prabha Mandayam 2:03 [chuckles] Um, yes.

Prabha Mandayam 2:05 Uh, certainly, I will try. So, um, okay, so quantum computing, as the name suggests, is that you try to compute with objects, physical objects, that are truly quantum in nature, right? So a very, um, simple example that, uh, or a simple picture that I can give you is, uh, a kind of geometric picture to contrast how a quantum bit will be different from a classical bit, right? So we- in classical computers, you know that we encode in zeros and ones, right?

Speaker 2:34 Correct.

Prabha Mandayam 2:35 Classical communication is zeros and ones, right? Yeah.

Speaker 2:38 Yeah, so in, in what I know of classical computers is that, um, we convert everything to binary. There are a set of binary operations-

Prabha Mandayam 2:44 Correct

Speaker 2:45 ... a set of algorithms-

Prabha Mandayam 2:45 Yeah

Speaker 2:45 ... that work on those binary operations.

Prabha Mandayam 2:47 Yeah.

Speaker 2:48 At the same time, on the physical side, there's this thing called a transistor-

Prabha Mandayam 2:51 Right

Speaker 2:52 ... which mimics a bit, a zero, one state.

Prabha Mandayam 2:54 Right.

Speaker 2:54 So we build a physical hardware-

Prabha Mandayam 2:56 Sure

Speaker 2:56 ... based on transistors, which can do these computes.

Prabha Mandayam 3:00 Absolutely. So now, instead of this humble zero, one, you imagine now a sphere, a ball, and then you put the zero and one at the two poles of the sphere. So zero could be at the North Pole, one could, uh, be at the South Pole, right? But now, instead of just having these two points, um, in which you can transcribe information, you now have all the points on the surface of the sphere.

Speaker 3:25 Okay.

Prabha Mandayam 3:25 So that's what quantum does, right? If you, instead of encoding into the zeros and ones, if you encode into a quantum state-

Speaker 3:33 Okay

Prabha Mandayam 3:33 ... then that means that you somehow have access to the entire surface of the sphere, and not just two points on the surface of the sphere. So you can imagine that the space that you have to now read in, put in information, has exploded, because you've gone from just two points to an infinity of points on the surface of the sphere. And the way this happens is through this magical word called superposition.

Speaker 3:57 Okay, so, so far I've understood that I can imagine a sphere which tra- which rotates, and any po- any position of that would be a number between zero and one, which is a infinite number of numbers.

Prabha Mandayam 4:11 Correct. So you, rather than think of it as a number between zero and one, you should actually think of it as, uh... Don't think of the sphere as moving or anything, just think of a static sphere.

Speaker 4:20 Okay.

Prabha Mandayam 4:20 So how do I identify any point on the surface of this sphere? I need two angles, right? I need a latitude and a longitude, like the surface of the Earth. To identify any point on the surface of the Earth, I need a latitude, which is the angle with, like, one axis, and then the longitude, which is like, you kind of project onto a, the equatorial plane, and then you have the longitude, right? So now, these are two numbers, two angles, right? And these angles go between, like, zero to , zero to . Right? So with these two angles, you can identify this infinity of points.

Speaker 4:54 Okay.

Prabha Mandayam 4:54 And so a quantum state is that. Basically, it's like identifying a point on the surface of the sphere,

Prabha Mandayam 5:00 where one point corresponds to your classical bit zero, the other point corresponds to classical bit one, but now you have all these different possibilities in between, and that's what superposition does for you. It allows you the ability to be somewhere in between a zero and a one state.

Speaker 5:15 Okay.

Prabha Mandayam 5:15 I can think of zero as an off state and one as an on state, right? And quantum allows for the possibility that you could be in either, uh, the on state with some probability, or the off state with some probability. The other way to think about it is a coin toss, where your coin, um, imagine it's in flight-

Speaker 5:36 Right

Prabha Mandayam 5:36 ... as you're tossing it. But once it is tossed, it's either head or tail. There's nothing in between. But imagine the coin in flight, and let's say you capture it in a box as it is in flight. That's a quantum state in a very, very loose sense. I mean, I'm not being very, uh-

Speaker 5:52 Fair enough. So if I, if I take a high-speed photograph, pho- photo at any point, that will be at some point.

Prabha Mandayam 6:00 Yeah, yeah. So you can imagine that if I... It's, it's like I tossed, but I kind of-... you know, toss it and put it in a box.

Speaker 6:08 Okay.

Prabha Mandayam 6:08 So now I don't know whether it's gonna land head or tail.

Speaker 6:11 Okay.

Prabha Mandayam 6:12 But-- and it's not a fair coin, okay? It's a biased coin, which means there is some non-trivial probability of landing head and non-trivial probability of landing tail, not, uh, fifty percent.

Speaker 6:22 Okay.

Prabha Mandayam 6:22 Fifty percent would be one, uh, such state-

Speaker 6:25 Option

Prabha Mandayam 6:25 ... one option. But so now you have all these possible states with all these different possibilities, uh, probabilities for head and tail.

Speaker 6:32 Okay, so this is a qubit?

Prabha Mandayam 6:34 This is a qubit.

Speaker 6:34 Okay.

Prabha Mandayam 6:34 A quantum bit. And now you imagine computing with these quantum bits, right? So now, a task that I do on a classical computer, I have to encode in either zero or one, or in some string of zeros and ones.

Speaker 6:51 Right.

Prabha Mandayam 6:51 But now I have the option of encoding in these in-between states.

Speaker 6:56 Okay.

Prabha Mandayam 6:56 Okay? So imagine a simple task. I give you a binary function, okay? And I say, uh, "Is this function, uh, constant or balanced?" Okay, I'm getting a little bit technical here. Constant meaning f of, uh, the, the function value for bit zero is the same as the function value for bit one. Balanced means the function value for bit zero is different from the function value for bit one.

Speaker 7:17 Okay.

Prabha Mandayam 7:18 Now, you have to tell me, is the function constant or balanced? And I give, first give you a classical calculator. So you punch in zero, and you find out what the answer is. You punch in one, you find out what the answer is. So you have to query this... You have to kind of use this classical box-

Speaker 7:35 Yeah

Prabha Mandayam 7:35 ... twice. But in place of that, if I give you a quantum box, I give you a quantum processor.

Speaker 7:41 Mm.

Prabha Mandayam 7:41 And I give you the ability to prepare these in-between quantum states. Now, you can input a state which is a superposition of zero and one.

Speaker 7:52 Okay.

Prabha Mandayam 7:52 And in a way, your box can simultaneously compute both values at the same time.

Speaker 8:00 Hmm.

Prabha Mandayam 8:01 Okay?

Speaker 8:02 So if I imagine this function as a box in my classical computer, I would have to use the box twice?

Prabha Mandayam 8:06 Twice.

Speaker 8:07 And in quantum computer, I use it-

Prabha Mandayam 8:09 Once, and this is for a single bit, right? This, so this kind of, um, model in computer science parlance is what's called oracle-based model. The box is like an oracle.

Speaker 8:18 Hmm.

Prabha Mandayam 8:19 Like this Greek goddess who knew all the answers.

Speaker 8:22 Mm.

Prabha Mandayam 8:22 And, uh, but you have only one shot with her. Like, you can only ask her one question, or the number of questions you can ask her is very limited. Now, the quantum box gives you an advantage because you can, with a single query to this oracle, and, uh, you know, the output, you can actually know whether f of zero is the same as f of one, or f of zero is different from. But this is a toy problem. Now, you imagine scaling this up. So for example, instead of a single bit function, you imagine an n bit function.

Speaker 8:50 Yeah.

Prabha Mandayam 8:51 Turns out that this speed-up is actually exponential.

Speaker 8:54 Okay.

Prabha Mandayam 8:54 Meaning classically, I would need an exponential number of trials with this box and write down all the answers, and then I can decide whether the n- the function is constant or balanced. But with a quantum box, I can again do it in a single query.

Speaker 9:09 Mm.

Prabha Mandayam 9:10 Because it doesn't matter how many states I have-

Speaker 9:12 Hmm

Prabha Mandayam 9:12 ... I can make superpositions of all of these.

Speaker 9:16 You're using the word superposition?

Prabha Mandayam 9:17 Superposition.

Speaker 9:18 Okay.

Prabha Mandayam 9:18 As in to say, a combination of two different classical states.

Speaker 9:22 So i- in, in a very like, like, twelfth standard math level, it's like evaluating a function for different values at the same time.

Prabha Mandayam 9:32 In one way, if you wanna think about it, think about a vector. In math, you know, we do this coordinate geometry, and you have vectors. Now, you have the, a unit vector along X, you have a unit vector along Y, but then you have all these vectors in between-

Speaker 9:45 Yeah

Prabha Mandayam 9:46 ... which you can break down as some projection along X-

Speaker 9:50 Yeah

Prabha Mandayam 9:50 ... and some projection along Y. So at the exact analogy of this is a qubit, which you can, uh, prepare using, um, polarization states of light.

Speaker 10:01 Mm.

Prabha Mandayam 10:02 Right? So you think about your Polaroid glasses. What do they do? They basically filter out different polarizations of light.

Speaker 10:08 Yeah, yeah.

Prabha Mandayam 10:08 Right? Now, you can have light which is polarized along this direction, light which is polarized along this direction. But you can also have forty-five degree polarized light. You can have thirty degree polarized light. You can have twenty degree. All these are valid states of light.

Speaker 10:21 Mm.

Prabha Mandayam 10:21 Okay? Now, having only horizontal and vertical is like having only bit zero and bit one. But now, forty-five degree polarized light is a cos forty-five combination of horizontal, plus a sin forty-five, uh, combination of vertical.

Speaker 10:39 Hmm.

Prabha Mandayam 10:40 That's exactly what superposition is. It's like having a vector in a three-dimensional space, right? So in fact, we say quantum states live inside what are called vector spaces, mathematically.

Speaker 10:50 Okay.

Prabha Mandayam 10:51 So, yeah.

Speaker 10:51 But before, before you proceed on that-

Prabha Mandayam 10:53 Yeah

Speaker 10:53 ... I wanna ask you a question here, which comes to my mind. So far, I'm not clear why it's called quantum computer. Where did the word "quantum" come from?

Prabha Mandayam 11:01 Right. So now this polarization that I spoke about, right? Uh, the light that's coming out of this, for example, is not polarized. But now I can imagine putting a big filter or this, or sunlight, right? We use your Polaroid glass. But now there are tens of thousands of photons or millions of photons in this sunlight that is coming to us, right? When you have these many sort of particles or a collection of particles, right, then you're really not in a quantum state. You are in what we call a classical, uh, state of some sort. But now, if you use, um, certain, uh, filters, filters and so on, and you reduce the number of photons so that you're down to a single photon level. The same analogy that I told you, right? Now, what does your Polaroid glass do? It blocks a certain fraction of light.

Speaker 11:49 Yeah.

Prabha Mandayam 11:49 So there's a certain fraction that goes through and a certain fraction that is blocked, and that's why you feel better wearing Polaroid glasses, uh, in a, in, in hot sun. Now, if you had a single photon instead of this beam of light, what will that single photon do? Will it get blocked or will it go through? Now, a photon cannot split.... right. So you can't have this possibility that some fraction will go through and some other. So now you're really in this single quantum level.

Speaker 12:17 Mm.

Prabha Mandayam 12:17 And now how do you describe the state of that photon? That is what I call as superposition.

Speaker 12:23 Okay.

Prabha Mandayam 12:23 Because at a bulk level, I can still talk in terms of intensity of light and more classical notions like that, but when I'm down to a single photon level, that is when I'm truly probing the probabilistic nature of these single level particles, right? So the theory that governs these microscopic particles or microscopic objects, like an electron spin, a single electron, the spin of a single electron, the polarization of a single photon-

Speaker 12:53 Mm

Prabha Mandayam 12:54 ... um, a single atom, right? The laws that govern this are the laws of quantum mechanics. And since you're using these laws of quantum mechanics to now you treat these objects as information processing objects, you call this as quantum information theory or quantum computing. You call the kind of bit that you get out of this as a quantum bit.

Speaker 13:17 Mm.

Prabha Mandayam 13:18 Right? It is one that follows the laws of quantum physics as opposed to Newtonian laws, which are, you know, classic.

Speaker 13:25 Okay.

Prabha Mandayam 13:25 Yeah.

Speaker 13:27 Quantum mechanics as a field is quite new itself, right? Like, it may be 50 years old or something like that.

Prabha Mandayam 13:32 We are celebrating 100 years of quantum this year.

Speaker 13:35 Is it?

Prabha Mandayam 13:35 Yeah.

Speaker 13:35 Okay.

Prabha Mandayam 13:36 So we are celebrating 100 years of quantum science and technolo- quantum mechanics this year.

Speaker 13:40 Can I just say that that implies that there must have been a starting point. What is... What are you con- considering as a-

Prabha Mandayam 13:44 Yeah, so these are hard to say, in fact, you know, um, [chuckles] yeah. So the, what they're considering... So why 100? Because in 1925 was when this, um, Austrian physicist by name Schrodinger wrote down this famous equation, the Schrodinger equation-

Speaker 13:59 Mm

Prabha Mandayam 13:59 ... which governs how quantum particles behave, the dynamics, uh, the, both the statics and the dynamics of quantum systems, right?

Speaker 14:06 I remember the Schrodinger equation-

Prabha Mandayam 14:07 Yeah

Speaker 14:07 ... from my first year. I think it's also taught in, uh, class 12 sometimes.

Prabha Mandayam 14:10 Yes, yes.

Speaker 14:11 Stuff of nightmares for me. I'm sorry. [laughing] But you're saying that from Schrodinger's, uh, description of quantum physics, the field evolved, uh, and at some point... Like, okay, separately, you are saying that those laws can define what is superposition?

Prabha Mandayam 14:29 Right.

Speaker 14:29 That is the-

Prabha Mandayam 14:29 So the ideas of superposition, the idea of, uh, probabilis- this being a probabilistic theory, the idea of uncertainty principle, for example, all of this goes back to 100 years, right?

Speaker 14:39 Mm.

Prabha Mandayam 14:40 So, for example, Heisenberg also formulated his uncertainty principle, which also is something-

Speaker 14:44 Yeah

Prabha Mandayam 14:44 ... that we all learn in school and, uh, maybe first-year physics courses. That's also 100 years old now, right? So the development of the field happened sometime between 1920 to 1925. I would say those were the golden years.

Speaker 14:55 Mm.

Prabha Mandayam 14:56 Um, the field itself actually goes back, according to me, to 1900s at least, when

Prabha Mandayam 15:00 the photoelectric effect was first discovered.

Speaker 15:02 Mm.

Prabha Mandayam 15:02 Which is when you were literally probing this, you know, single particle kind of interaction. You had an electron-

Speaker 15:07 The slit experiments.

Prabha Mandayam 15:08 The double-slit, yes, but no, this is when you, um, shine, uh, light and you release a single electron out, right? And this is what then led Einstein to, uh, formulate the idea of photon, that light ex- can, uh, the, uh, energy, uh, levels of, uh, light exist in these integer, um, numbers of a basic quantity, and that the basic energy quantity is a, is what we call a photon, right? And then you have, like, integer number of photons, and so on, right? So this quantization of light, right, uh, happened in 1905, right?

Speaker 15:47 Mm.

Prabha Mandayam 15:47 That's when Einstein came up with this theory of photoelectric effect, uh, the idea of h nu, uh, the energy being of a single photon being h nu, and all that, right?

Speaker 15:57 Can I, can I just summarize? So far, you've, uh, explained to me what is a qubit. You've explained to me what is superposition, and, uh, then we are having this conversation of why it's called quantum, because it came from-

Prabha Mandayam 16:07 Quantum mechanics

Speaker 16:08 ... quantum mechanics.

Prabha Mandayam 16:08 Yeah.

Speaker 16:09 Uh, and, uh-

Prabha Mandayam 16:10 Yeah

Speaker 16:10 ... the, the starting point is [keyboard clicking] nominally, like you have to decide some starting point, so it's the-

Prabha Mandayam 16:15 Schrodinger equation, then it's 100 years, yeah.

Speaker 16:17 Great.

Prabha Mandayam 16:17 It's 100 years of Schrodinger equation. I just want to

Prabha Mandayam 16:20 say that the idea of quantum computing-

Speaker 16:23 Yeah

Prabha Mandayam 16:23 ... is about 40 years old now.

Speaker 16:25 Okay.

Prabha Mandayam 16:26 So it was in the 1980s, um, is a physicist in Oxford by name David Deutsch. So the problem that I explained to you of this, uh, identifying whether a function is constant or balanced, this is called a Deutsch problem. So he was the one who came up with this toy problem, where you could then show that using quantum mechanics, you actually achieve a speed-up over what you can do classically. And this, this little algorithm sort of way of doing it is what's called Deutsch algorithm. So it's the first quantum algorithm, which-

Speaker 16:58 So-

Prabha Mandayam 16:59 Yeah

Speaker 16:59 ... to go from, um, you know, these really complicated papers that Schrodinger, Einstein, Heisenberg are writing, to something that all of us should care about, the, the step there you are saying is important, is the fact that it can compute much, much faster than a classical computer?

Prabha Mandayam 17:15 Exactly. Exactly.

Speaker 17:16 Okay.

Prabha Mandayam 17:16 Exactly.

Speaker 17:16 So that is why we should care about it.

Prabha Mandayam 17:17 Exactly. Absolutely, yeah. So, um, the problem that I mentioned is a toy problem, but sometime in the '90s, early '90s, a mathematician by name Peter Shor at MIT, showed that you can factor- you can identify the prime factors of a number exponentially faster than the best classical algorithm to date, the best classical algorithm, if you had a quantum computer. So he came up with a quantum algorithm that can factor numbers exponentially faster, and this made everybody sit up and take notice. Deutsch's work was not really noticed, because that was a toy problem. Okay, that's a cute thing, but who cares about the toy problem? But factoring, because by then, everyone knew that what is at the heart of all secure communications today is a protocol by name RSA, which relies on the computational hardness of prime factorization.... okay. So the prob- like, I give you a very large number, let's say even a, um, a 20-bit number, let's say. And I say, "Can you find the prime factors of this?" I even tell you that there are exactly two prime factors for this number, okay? But it's very hard task. Classically, the best- so, you know, classically, we classify problems as being either NP or P.

Speaker 18:31 Yeah.

Prabha Mandayam 18:31 Uh, can you solve them in polynomial time, or can you only verify them in polynomial time? So you don't have an algorithm that solves in polynomial time, that's NP, right? So factoring is so far believed to be in NP. It's not that anybody has proven it, but the prime factorization problem is believed to be in NP, and in 1992 or '3, I have to check, uh ... maybe-

Speaker 18:54 Mm

Prabha Mandayam 18:54 ... you can show your viewers the-

Speaker 18:56 Yeah

Prabha Mandayam 18:56 ... correct year. Um, uh, Peter Shor came up with a quantum algorithm that can factor numbers in polynomial time. If you had access to a quantum computer, which can encode information in quantum bits, you do quantum gates, quantum logic gates, just like how you do classical logic, um, run a quantum algorithm, then you can do this exponentially faster. So then-

Speaker 19:19 Okay, so but this is still an algorithm.

Prabha Mandayam 19:21 Yes.

Speaker 19:21 This is an algorithm that says that if we have-

Prabha Mandayam 19:24 Yeah

Speaker 19:24 ... these quantum states.

Prabha Mandayam 19:25 Yeah, yeah.

Speaker 19:26 And then we could write math-

Prabha Mandayam 19:27 Yeah

Speaker 19:27 ... in such a way that factorization would go from a NP-hard problem to a very simple-

Prabha Mandayam 19:33 Right

Speaker 19:33 ... problem.

Prabha Mandayam 19:33 It's not known if it's NP-hard. In fact, it is not NP-hard, it's NP. NP-hard is technically-

Speaker 19:37 Sorry. I'm so sorry

Prabha Mandayam 19:38 ... different from [chuckles] NP. [laughing] Yeah.

Speaker 19:41 Uh, it, it goes from a NP problem to a P problem.

Prabha Mandayam 19:43 Yeah, yeah.

Speaker 19:44 Okay.

Prabha Mandayam 19:44 Yeah.

Speaker 19:45 Um, and you're saying that this is, this is something that it made everyone sit up, because factorization as a problem or the complexity of factorization as a problem is the basis of RSA algorithm.

Prabha Mandayam 19:55 Security, yeah.

Speaker 19:55 Um, maybe other algorithms-

Prabha Mandayam 19:57 Yes

Speaker 19:57 ... which are used in encryption-

Prabha Mandayam 19:59 Yes

Speaker 19:59 ... like every day.

Prabha Mandayam 20:00 Every day, yeah. Public, public key encryption, like the-

Speaker 20:03 Mm.

Prabha Mandayam 20:03 Yeah. Your ability to send emails over Gmail and be guaranteed that others are not reading it.

Speaker 20:09 So if I had a quantum computer-

Prabha Mandayam 20:11 Yeah

Speaker 20:11 ... and nobody else had a quantum computer, then I could technically-

Prabha Mandayam 20:13 Crack all this, yeah.

Speaker 20:15 I could crack the RSA algorithm-

Prabha Mandayam 20:16 Yes, yes

Speaker 20:17 ... daily encryption.

Prabha Mandayam 20:18 Yes.

Speaker 20:19 Okay, that sounds really powerful.

Prabha Mandayam 20:20 Yes.

Speaker 20:21 Okay.

Prabha Mandayam 20:22 Yeah. So all governments then were, got interested, uh, you know, uh, the government funding started for this program. Then people started looking at, "Can we build this?" Right? This is, like you said, a math result.

Speaker 20:34 Yeah.

Prabha Mandayam 20:34 Now, can we actually build these things? Is it possible to actually build a quantum computer?

Speaker 20:39 Correct. There has to be a transistor moment.

Prabha Mandayam 20:41 Right.

Speaker 20:42 Mm.

Prabha Mandayam 20:42 Now, that happened sometime in the, uh, early 2000s-

Speaker 20:47 Okay

Prabha Mandayam 20:47 ... with the advent of the, um, superconducting qubit, okay? And in fact, it was a group in Japan, uh, by Professor Nakamura, that first started, uh, in fact, demonstrated, I think, the first, uh, superconductor-based quantum bit, okay? Um, I must tell you, though, that there were folks at IBM, uh, like Charlie Bennett and so on, who had demonstrated this polarization qubit that I spoke to you about. That's the easiest qubit to realize, right? Because that's just light.

Speaker 21:16 Mm.

Prabha Mandayam 21:16 Now, you have to just bring it down, all the way down to single photon levels, and then you can already start seeing this quantum stuff in action, right? You can sta- start seeing superposition in action, and so on. But that was not good enough to be able to do computing, okay? At that stage, at least. But then, the superconducting qubit showed that you can actually precisely control these single qubits. Now, the challenge, unlike in the classical case where you're still treating these systems as very macroscopic, classical objects, here, you're down to microscopic levels, and you need precise control of these objects.

Speaker 21:51 Mm.

Prabha Mandayam 21:52 And it's very easy to kind of, uh, for them to move into the classical domain, from-

Speaker 21:59 Which is zero one.

Prabha Mandayam 21:59 Which is zero one.

Speaker 21:59 Mm.

Prabha Mandayam 22:01 Uh, this, um, mechanism is what is called decoherence.

Speaker 22:05 Okay.

Prabha Mandayam 22:06 Okay? And this is an important word, because it then ties in with my research, which essentially is about how do you combat decoherence?

Speaker 22:14 Okay.

Prabha Mandayam 22:15 Okay. Uh, this decoherence is what we call noise for qubits, and, um, the key challenge till date is to be able to realize quantum bits which, uh, are, uh, not very susceptible to this decoherence. And even if they are, we have sort of error-correcting codes that can somehow overcome this decoherence. So the challenge is to actually realize these, um, error-corrected qubits, which can exist in their quantum state for long times. That's too much information, I'll back track. [chuckles]

Speaker 22:52 I will, I will, I will share what I understood.

Prabha Mandayam 22:55 Yeah. [chuckles]

Speaker 22:55 I, I'm sure I'm wrong, uh, in some way, but let me just try. You're saying that in realizing a qubit, uh, first of all, we should be able to give instruction to the qubit?

Prabha Mandayam 23:05 Yes.

Speaker 23:05 And secondly, we should be able to measure and find out what state it is?

Prabha Mandayam 23:07 Absolutely. Absolutely, yeah.

Speaker 23:08 In both giving instruction to the qubit and measuring-

Prabha Mandayam 23:11 Yeah

Speaker 23:11 ... there's some error that comes in.

Prabha Mandayam 23:13 Yes, yes. It's, it's not- it's challenging to do it at a single qubit level-

Speaker 23:17 Mm

Prabha Mandayam 23:17 ... to precisely control them. So what happens is that the quantum state is very fragile, the superposition state, right? You're precisely putting it at some point on the surface of the sphere, right? Remember. Now, it's very easy for this thing to simply collapse down-

Speaker 23:33 Right

Prabha Mandayam 23:33 ... either to the equatorial plane or just go off to the North Pole, like-

Speaker 23:38 Right

Prabha Mandayam 23:38 ... the zero state.

Speaker 23:39 Right.

Prabha Mandayam 23:39 Right? Um, that's a very, uh, overpowering mechanism. So you're kind of trying to overcome that in precisely controlling this. One solution is, well, um, shut your qubit from all external influences, right? Make it so isolated th- that... Yeah.

Speaker 23:55 Uh, the electron or the pho- sorry-

Prabha Mandayam 23:57 Yeah

Speaker 23:57 ... the photon or-

Prabha Mandayam 23:58 Yeah

Speaker 23:58 ... or the-

Prabha Mandayam 23:58 Yeah

Speaker 23:59 ... the microscopic element-

Prabha Mandayam 24:00 Yeah

Speaker 24:00 ... is moving because it's getting energy from everywhere.

Prabha Mandayam 24:02 Ab- absolutely.

Speaker 24:03 Right?

Prabha Mandayam 24:03 Yeah, yeah.

Speaker 24:03 So you're saying that if I shut out everywhere-

Prabha Mandayam 24:05 Exactly

Speaker 24:05 ... then it'll just-

Prabha Mandayam 24:06 Yeah

Speaker 24:07 ... sit?

Prabha Mandayam 24:07 Yeah, yeah.

Speaker 24:08 And follow my instruction.

Prabha Mandayam 24:09 Absolutely. But then you have to interact with it-

Speaker 24:12 Mm

Prabha Mandayam 24:12 ... so that you can make it follow your instructions.

Speaker 24:14 Mm.

Prabha Mandayam 24:15 So now there's a trade-off.... how much do you interact with it? And in that process, you're kind of opening up the qubit a little bit.

Speaker 24:24 You're saying my interaction itself gives it-

Prabha Mandayam 24:26 Yeah

Speaker 24:27 ... uh, the license to move?

Prabha Mandayam 24:28 Exactly.

Speaker 24:28 Okay.

Prabha Mandayam 24:29 Yeah. So this problem is the problem of quantum error correction.

Speaker 24:32 Okay. Wow, Professor, just to get to what [chuckles] do you do, it takes me, like, very minutes to understand the... Okay, but just since we have come till here-

Prabha Mandayam 24:43 Yeah

Speaker 24:44 ... uh, I understand from, uh, see, we now have a national quantum mission and all of that. I understand that we started this conversation on why it is important with respect to security and, uh, encryption and decryption. There must be more to it, right?

Prabha Mandayam 24:59 Yeah. So factoring was the first task, like I said, which made people sit up and take notice. Subsequently, um, another computer scientist by name Lov Grover, in Bell Labs, uh, that's an Indian connection to quantum computing, uh, from the very beginning. So he, uh, showed that you can search over an unsorted database faster if you had a classic, uh, quantum computer. So this is called a quantum search algorithm. Now, this is an example of a useful task in a very application, uh, sense, right? So you can imagine now you have your route optimization problem, or you have a traveling salesman problem. Uh, you know, so any problem which you can kind of model as searching over an unsorted database can be done much faster if you had a quantum computer. So classically, if you need N attempts or N, um, uh, steps to do this, quantumly, you need only square root N steps. So as your N becomes larger and larger, you can imagine that you have a real advantage with a quantum computer. So these were the two main algorithms which brought in funding for quantum computing, both from government and from industry. Because the search is something that industry folks are then interested in. So, uh, both of these, uh, then, uh, groups, uh, started building quantum hardware. So

Prabha Mandayam 26:15 in, uh, the, one of the early groups in the US was at Yale, uh, then at IBM, they started building quantum computers. But it wasn't until 2016 that they actually showed that you could put a certain number of quantum bits on a chip, right? Because now this polarization qubit and all of that is like a tabletop experiment.

Speaker 26:35 Mm.

Prabha Mandayam 26:35 It doesn't become a quantum processor, it doesn't become a computer.

Speaker 26:38 Mm.

Prabha Mandayam 26:39 Now, I need to have the ability to like I-- how I etch transistors and this whole-

Speaker 26:44 Yeah

Prabha Mandayam 26:44 ... circuits on a chip, can I now make quantum circuits on a chip?

Speaker 26:48 Fair enough. The breakthrough in, uh, transistor is actually photolithography, which allows us to sort of print transistors on a chip. So you're saying that that moment is... Has it come? It's-

Prabha Mandayam 26:59 It started in 2016, and I would say last year, with Google's, like, hundred qubit experiment, it has more or less come.

Speaker 27:06 Okay.

Prabha Mandayam 27:06 Right? That's like the first proof of principle that you can put hundred qubits on a chip. You can connect them all up in some way that makes them resilient to this errors and this decoherence and all of that. So it was a very proo- but it's still a proof of principle.

Speaker 27:20 Mm.

Prabha Mandayam 27:20 Right? We still haven't demonstrated Shor's algorithm on a-

Speaker 27:25 Sure

Prabha Mandayam 27:25 ... a quantum computer at a, at a, at a meaningful scale. I mean, we have shown that we can factor 15 into three and five. We have shown that you can factor 21 into three and seven, and so on, but we haven't... So there are proof of principles like that.

Speaker 27:38 Mm.

Prabha Mandayam 27:38 But can you really take, like, a 20-bit number and get the prime factors out? We have not yet seen that kind of a demonstration.

Speaker 27:44 Walking back a bit, when you were talking about this realization in the beginning, you mentioned the word superconductivity.

Prabha Mandayam 27:49 Yeah.

Speaker 27:50 Superconductive qubit.

Prabha Mandayam 27:51 Yeah.

Speaker 27:51 And I sort of, uh, you know, broke you there-

Prabha Mandayam 27:54 Right

Speaker 27:54 ... in that flow.

Prabha Mandayam 27:54 Right, right.

Speaker 27:55 Can you complete that flow? What, what is a superconductive qubit? And before that, there's a photon qubit, is that it?

Prabha Mandayam 28:00 Right, right. So these are just two of them. So let me say that today there are at least four, five different architectures that people are simultaneously, um, uh, sort of exploring to build quantum computers. The earliest one was the photonic qubit, which is basically the polarization states of light. Um, the next one, which was reasonably successful, and today which is the kind of the leading architecture, is the superconducting qubit, right? You basically take, uh, uh, or you- so you etch a kind of, um, superconducting, um, material onto a chip, right? And, um, you create, uh, this, uh, sort of presence or absence of, um, a charge, or presence of absence of magnetic field as a qubit. That becomes your zero and one.

Speaker 28:51 Mm.

Prabha Mandayam 28:51 So what do you need for a qubit? You need to identify basically these two levels, the zero and the one, the on and the off level, like in a transistor and so on. But now you need to do it in a quantum way, so that you can also prepare all these superposition states and not just have on and off.

Speaker 29:06 Mm.

Prabha Mandayam 29:06 Right? So the q- superconducting qubit relies on the principles of superconductivity, which says that your, uh... For example, if you had a piece of superconducting, uh, metal, let's say, that means that... You know, so superconductor is what? It's basically you cool down, um, certain metals at very low temperatures.

Speaker 29:23 Yeah.

Prabha Mandayam 29:23 They offer almost no resistance to the flow of current.

Speaker 29:25 Yeah.

Prabha Mandayam 29:25 And this happens because these electrons, which are your charge carriers, they pair up into something called Cooper pairs.

Speaker 29:31 Okay.

Prabha Mandayam 29:32 Okay? And now you can create islands or, or pockets here, where you could either have a charge there or not have a charge there. That's a very, again, very crude description, but essentially, that's like your on and off.

Speaker 29:44 Understood.

Prabha Mandayam 29:45 Yes.

Speaker 29:45 I mean, I'm saying understood, but-

Prabha Mandayam 29:48 Uh, yeah

Speaker 29:48 ... I should say acknowledged. [laughing]

Prabha Mandayam 29:50 [laughing] No, I'm also being very loose, uh, you know, in this, but I'm hoping that people will pick up some of these words and go look up more.

Speaker 29:58 Yeah, go look up.

Prabha Mandayam 29:59 Yeah.

Speaker 29:59 Yeah.

Prabha Mandayam 30:00 Yeah.

Speaker 30:00 At the very least, they can look up your NPTEL course.

Prabha Mandayam 30:03 Sure. Uh, but-

Speaker 30:03 Can you tell me something now?

Prabha Mandayam 30:04 Yeah.

Speaker 30:05 Um, you said that the Google, uh, experiment last year had 100 qubits on a chip.

Prabha Mandayam 30:10 Yeah.

Speaker 30:11 Um, but you also said that, uh, the, the quan- quantum computing reduces the requirement-... uh, of po- of time, of quality-

Prabha Mandayam 30:20 Uh-huh

Speaker 30:20 of time.

Prabha Mandayam 30:21 Uh-huh.

Speaker 30:21 Uh, so, uh, when, when we look at classical computing chips, they are in millions and billions of transistors.

Prabha Mandayam 30:27 That's right.

Speaker 30:28 To have ... How many qubits do we need on a chip? Is that, uh, a small number?

Prabha Mandayam 30:35 No, it does not scale. The number of qubits, you don't really get an advantage with the number of qubits in these tasks, right? What you get an advantage typically is with time.

Speaker 30:44 Mm.

Prabha Mandayam 30:45 Right? The number of steps that you need to do, right?

Speaker 30:47 Mm.

Prabha Mandayam 30:47 Whether it's a number of, let's say, times you need to access this quantum box, or whether it's a number or the time to which, you know, how, how, how deep are the circuits-

Speaker 30:58 Mm

Prabha Mandayam 30:58 ... that you need to run in order to solve a particular problem, right?

Speaker 31:01 Okay.

Prabha Mandayam 31:02 Um, so if you had very, um, sort of ideal qubits, which are noiseless-

Speaker 31:08 Mm

Prabha Mandayam 31:08 ... then you could, for example, crack today's RSA with about tens of thousands of qubits.

Speaker 31:13 Okay.

Prabha Mandayam 31:14 Um, but the current, um, benchmarks that I know is if you run, if you need to run these error-correcting codes and do Shor's algorithm on error-corrected qubits, then you need at least a million qubits.

Speaker 31:28 Mm.

Prabha Mandayam 31:28 Okay? Of the order of a million qubits.

Speaker 31:30 Okay.

Prabha Mandayam 31:30 Yeah.

Speaker 31:30 So we are, we are at 100.

Prabha Mandayam 31:31 We are at 100.

Speaker 31:32 Uh, so there's a long distance to go.

Prabha Mandayam 31:33 Yeah. Yeah, yeah.

Speaker 31:34 Um, Professor, uh, now that I have understood this, uh, basic idea, and I, I can also appreciate why error correction becomes so critical. You're in the field of error correction, so can you give us an idea of what that looks like and, um, just an overview?

Prabha Mandayam 31:50 Right. So now, error correction is not something new to quantum. Uh, we do this in classical information theory all the time. I mean, Shannon's work is all coding theory. Uh, Shannon's kind of, in a way, gave rise to, uh, this idea of coding, uh, and so on. Uh, so but classically, what happens, right? So I have my, uh, classical, um, registers, my transistors, and so on. I, I put in a zero, but then it somehow flips and becomes a one, right? That's an error. That's what we call a bit flip, right? Now, classically, we say, "Oh, no, ma- no, don't worry. Every time I wanna put a zero, I'm gonna put three zeros, or I'm gonna put five zeros. And every time I put a one, I'm gonna put three ones or five ones." Now, the odds that all three are gonna flip or even two are gonna flip is much lower than the odds that only one of them will flip. Now, when I read out, I'll read out every set of three or every set of five, right? And depending on whichever is majority, so if in a set of three, I find two zeros and one one, I know that this was intended to be three zeros-

Speaker 32:46 Mm

Prabha Mandayam 32:46 ... but one of them flipped. So this is introducing redundancy.

Speaker 32:49 Yeah.

Prabha Mandayam 32:50 And that's how you protect information. Now, the, uh, in quantum, can you do the same thing? Yes and no. Yes, because zero and one in quantum are like the classical zero and one. But now what happens when I prepare this 45-degree state?

Speaker 33:05 Mm.

Prabha Mandayam 33:06 How is that- how does that get protected? Because now I, mm, I'm not just protecting zero and one, I'm protecting what is called an entire vector space. I'm protecting all possible points on the surface of the sphere.

Speaker 33:17 Mm.

Prabha Mandayam 33:17 So I have to protect superposition-

Speaker 33:19 Mm

Prabha Mandayam 33:19 ... because that's the key resource for me now, right? So if I don't protect superposition, I'm back to classical. So to protect superposition, you can say, well, every time you have 45, you say three 45s. But the problem is,

Prabha Mandayam 33:33 in quantum computing or quantum information, the very basic no-go result called the No-Cloning Theorem-

Speaker 33:40 Okay

Prabha Mandayam 33:40 ... which says that you cannot make copies of arbitrary superposition quantum states.

Speaker 33:47 Mm.

Prabha Mandayam 33:47 So you don't have a Gurunath, which probably was there when you were a student here-

Speaker 33:52 Mm

Prabha Mandayam 33:52 ... where you have a Xerox machine, and you put in, take your friend's notes-

Speaker 33:57 Mm

Prabha Mandayam 33:57 ... right, the night before the exam and make a copy of the entire notebook. No.

Speaker 34:01 Mm.

Prabha Mandayam 34:02 Not possible. So you cannot copy quantum information.

Speaker 34:05 But even in classical computing, there is a assembly code for take this value from here and put it there. It's a simple copy.

Prabha Mandayam 34:12 Right, but you cannot do that quantum.

Speaker 34:15 Okay.

Prabha Mandayam 34:15 So you can copy states which are like classical, which is your zero and one.

Speaker 34:19 Mm.

Prabha Mandayam 34:20 But an arbitrary superposition-

Speaker 34:22 Mm

Prabha Mandayam 34:22 ... I don't have a universal Xerox machine that can copy quantum information. So this is what primarily makes quantum error correction challenging. So now you have to come up with clever ways of introducing redundancy.

Speaker 34:36 Mm.

Prabha Mandayam 34:36 And here comes in another player in this whole game, which is entanglement.

Speaker 34:40 Okay.

Prabha Mandayam 34:41 Which is a word which I have not used until now. Uh, so what is entanglement?

Speaker 34:47 Mm.

Prabha Mandayam 34:48 Now, the idea is that you can have ... Now we've spoken about single qubits. Now I take this collection of three qubits, okay? Now I try to do an error-correcting code on this, means I try to say, "Okay, every time I have a single zero, it's three zeros. Every time I have a single one, it's three ones." But what about the 45-degree state? What I do now is I make a 45-degree state, not of three single qubits, but of this collection of three qubits, of this set of three qubits.

Speaker 35:17 Mm.

Prabha Mandayam 35:18 Okay? So it's like three qubits is, zero is my x-axis, and three qubit one is my y-axis, and I've made now like a superposition of this three qubit zero and the three qubit one. But this is a state which I cannot break down into a state of the first qubit, a state of the second qubit, and a state of the third qubit. It's a state which cannot be broken down into its individual parts. The information is in the whole.

Speaker 35:44 Mm.

Prabha Mandayam 35:45 So that's the idea of quantum entanglement. Again, very, very, um-

Speaker 35:48 Sure

Prabha Mandayam 35:48 ... naively. Um, but this is how, this is the simplest ... What I've explained to you is a simplest quantum error correcting code. It's called a three-qubit repetition code. So you see, it borrows elements from classical-

Speaker 36:00 Mm

Prabha Mandayam 36:00 ... but then there's a quantum twist to it. And this circumvents this idea of no copying, because now what I do is I make a circuit.... I make a circuit with three registers.

Speaker 36:12 Mm.

Prabha Mandayam 36:13 The first register is the state that you want to encode. It could be any state.

Speaker 36:17 Wait, wait, wait.

Prabha Mandayam 36:18 Yeah.

Speaker 36:18 You're using the word register. This is from the classical computing word.

Prabha Mandayam 36:21 Right.

Speaker 36:21 Register.

Prabha Mandayam 36:21 Okay, yeah. So qubit, qubit, so the first qubit. So when I say qubit or register, I kind of use them, uh, equivalently.

Speaker 36:27 Oh, okay.

Prabha Mandayam 36:27 A register is, I think of it as a single bit or a single qubit, huh? So the single qubit is in some arbitrary point on the sphere, at some arbitrary superposition. The other two qubits are in fixed states. And now by doing a circuit which involves some logical operations on all three qubits-

Speaker 36:45 Mm

Prabha Mandayam 36:46 ... at the output of the circuit, I have a s- three-qubit state which can no longer be broken down into single qubit states, but which is successfully now protected.

Speaker 36:55 The information.

Prabha Mandayam 36:56 Yeah. Yeah, yeah. [exhales] [chuckles]

Speaker 37:00 [chuckles] So I, I, I want to say that, uh, wh- when superconduct- uh, when, uh ... sorry, not superconductors. When, um, transistors were invented and then, uh, the, the digital algorithms, the zero-one algorithms got implemented into transistors, and that whole revolution took place between the '50, '60s, and '70s, and then suddenly it was apparent to the world that the world has changed.

Prabha Mandayam 37:25 Right.

Speaker 37:25 It's, you know-

Prabha Mandayam 37:25 Right

Speaker 37:26 ... the thing-

Prabha Mandayam 37:26 Yeah

Speaker 37:26 ... we are- you know-

Prabha Mandayam 37:28 Mm-hmm

Speaker 37:28 ... we could go to the moon, we could-

Prabha Mandayam 37:30 Yeah

Speaker 37:30 ... build mobile phones, computers-

Prabha Mandayam 37:31 Yeah

Speaker 37:31 ... all of that, right?

Prabha Mandayam 37:32 Yeah.

Speaker 37:32 And so it, as you're saying it, it feels to me that we are at that 1950, 1960 level.

Prabha Mandayam 37:39 Absolutely.

Speaker 37:40 And in the next-

Prabha Mandayam 37:40 Yeah

Speaker 37:40 ... 20 years or-

Prabha Mandayam 37:41 Yeah

Speaker 37:41 ... 30 years as-

Prabha Mandayam 37:42 Yeah, yeah

Speaker 37:42 ... quantum computers get built-

Prabha Mandayam 37:44 Yeah, yeah, yeah

Speaker 37:45 ... there'll be like a, will we all have quantum computers in our hand? Is that possible?

Prabha Mandayam 37:49 Yeah, my own,

Prabha Mandayam 37:50 uh ... I mean, this is crystal ball gazing at this point, right? I mean, who knew at 19- in 1950 that, uh, you know what-

Speaker 37:55 You have iPhones.

Prabha Mandayam 37:56 Yeah.

Speaker 37:56 Yeah.

Prabha Mandayam 37:56 So there's this famous statement of Thomas J. Watson of IBM saying that, uh, "Perhaps the world has a need for three personal computers or four personal computers," or something like that, in the 1930s, of, you know, when we still had, uh-

Speaker 38:07 That's the-

Prabha Mandayam 38:07 vacuubes and-

Speaker 38:08 IBM, it thought of the computing revolution as something that governments and organizations will have.

Prabha Mandayam 38:14 Right. So today, I typically say this, that maybe we will have specialized quantum computing facilities.

Speaker 38:22 Mm.

Prabha Mandayam 38:23 Right? But then who knows? Like I said, you know-

Speaker 38:25 Right

Prabha Mandayam 38:25 ... that's, the, the way things are shaping up today, it seems to me that what it is certainly, and this is already happening, but it is not happening at the scale where it can now start doing useful tasks. It's happening at a scale where you can do proof of principle, you can train people on it, you can teach them how to program a quantum computer, and so on and so forth, but it's not yet at a stage where hopefully we are cracking passwords and we are- or even doing useful things like running, solving traveling salesman or optimization problems-

Speaker 38:53 Okay, so-

Prabha Mandayam 38:54 ... and so on.

Speaker 38:55 maybe, uh, maybe in future, IIT Madras will have a facility, and I'm a researcher in, say, protein folding.

Prabha Mandayam 39:00 Exactly.

Speaker 39:00 I will like, "Here, take this problem-

Prabha Mandayam 39:02 Absolutely

Speaker 39:02 ... and get a solution in-

Prabha Mandayam 39:03 Yeah, yeah

Speaker 39:04 ... like 20 minutes."

Prabha Mandayam 39:04 Yeah, yeah. So you can imagine how IIT Madras had one of the earliest IBM computer-

Speaker 39:08 Yeah

Prabha Mandayam 39:08 ... stationed, uh, in India, I think-

Speaker 39:10 Yeah

Prabha Mandayam 39:10 ... for that matter. And there were researchers all across the city, and even from out station, who would come. They're punching cards, and, you know, you have, you kind of put in the cards, you get out your results, and so on. So at least in the next 20 years, I see this happening.

Speaker 39:22 Yeah.

Prabha Mandayam 39:22 Now, beyond that is anybody's guess, right? I mean, Microsoft came along, and then everybody had a computer in their house in the late '90s and early 2000s. So this, uh, we don't know where this is going, right, at beyond this point. But definitely we will see these computing, quantum computing facilities.

Speaker 39:40 I'm just going through all my notes I have taken. I'm just thinking that if I have to go deep into this, and if I have to understand this better to prepare myself for the future, because if this is coming in 20 years, I better know something about it. Um, I have to understand qubit, I have to understand entanglement.

Prabha Mandayam 39:56 Superposition.

Speaker 39:57 Superposition and entanglement. These are the three basic

Speaker 40:00 ideas I have-

Prabha Mandayam 40:01 Yes, indeed

Speaker 40:01 ... I have to get.

Prabha Mandayam 40:02 Yeah, yeah.

Speaker 40:02 Can you give me a, again, can you give me a bigger list? So q- I'm gonna make this list.

Prabha Mandayam 40:07 Yeah.

Speaker 40:07 So I must understand what is a qubit. I must understand what is superposition. Um, uh, third one was entanglement.

Prabha Mandayam 40:14 Yeah. Um, so from a slightly mathematical perspective, one needs to understand a little bit of linear algebra. So if I want to become a software developer for a quantum computer, right, if I want to program a quantum computer, what, what do I need to know? So the language or the mathematics of quantum computing or quantum mechanics is all linear algebra. So I nee- if I, the better I get at linear algebra, the better I get, not just at understanding what's happening, but maybe I can even start coming up with my own new quantum algorithms. I can-

Speaker 40:47 Is this because it's like A first angle, uh, whatever, A theta plus B alpha? I don't know.

Prabha Mandayam 40:53 Exactly. A0 plus B1, alpha 0 plus beta 1. I had, I had refrained from using things like this-

Speaker 40:58 Okay

Prabha Mandayam 40:58 ... but exactly. It's because it's not just zero and one, but it's alpha zero plus beta one. And this is the, an idea of what we call a vector space in linear algebra.

Speaker 41:06 Mm.

Prabha Mandayam 41:06 So this becomes kind of the basic, uh, alphabet in which you talk about qubits, and gates, and algorithms, and all of that. So this language of linear algebra becomes very important. Incidentally, it's also very important in another domain today, namely-

Speaker 41:19 Yeah

Prabha Mandayam 41:20 ... uh, AI/ML.

Speaker 41:20 Yeah.

Prabha Mandayam 41:21 So in a way, this is a kind of convergence of, uh-

Speaker 41:23 Yeah

Prabha Mandayam 41:23 ... new technologies having the same underlying math. I would also say some grounding in probability theory, but maybe that's not as a severe a constraint as linear algebra, which I think is absolutely something that people should be super comfortable with if you want to really get into this field.

Speaker 41:39 Makes sense.

Prabha Mandayam 41:40 On the engineering front, yes, you need to know how to do photonic hardware, okay? Um, you need to, um, uh, understand, um ... Yeah, I mean, there are certain, uh, sort of, uh-

Speaker 41:53 So I don't need to be better at Schrodinger equation.

Prabha Mandayam 41:56 [laughing]

Speaker 41:56 Do I need to?

Prabha Mandayam 41:58 This is a very tricky question. [laughing] So it depends on how deep you want to get into quantum computing.

Speaker 42:04 Okay.

Prabha Mandayam 42:04 Okay, so I want to say that one of the important use cases today, for example, protein folding.

Speaker 42:08 Mm.

Prabha Mandayam 42:08 How do you solve a protein folding problem on a quantum computer? You use something called a variational quantum algorithm.... okay. It's different from Shor's Algorithm, it's different from the Search Algorithm. It's a heuristic algorithm which relies much more on the laws of quantum mechanics-

Speaker 42:26 Okay

Prabha Mandayam 42:26 ... and in fact, requires some knowledge of Schrödinger,

Prabha Mandayam 42:30 right?

Speaker 42:31 Mm.

Prabha Mandayam 42:31 So it depends on how deep you wanna get into this, right?

Speaker 42:34 Fair enough, but I could choose to, maybe not today, but in five, 10 years, I could choose to, instead of focusing on this, I could choose to become a programmer, uh, who is, uh ... Is there a assembly language equivalent in quantum computing? Is there a-

Prabha Mandayam 42:48 It's not a single- So like I said, there are different architectures. In fact, I think we, uh, got sidetracked there. So photonic superconducting, then there are what are called trapped ions and neutral atom, uh, based quantum computers. There are, there are at least these three, four different approaches-

Speaker 43:03 So this, you were talking about the architectures of qubit.

Prabha Mandayam 43:05 Yeah.

Speaker 43:05 So photonic-

Prabha Mandayam 43:06 Superconducting

Speaker 43:07 ... superconducting.

Prabha Mandayam 43:07 Yeah.

Speaker 43:07 Trapped ions.

Prabha Mandayam 43:08 And neutral atoms.

Speaker 43:09 Okay.

Prabha Mandayam 43:11 Now, each of these is a different kind of physical object with different kind of control operations for each of them, which are a bit unique. The standard picture you see of a quantum computer is this dangling chandelier-

Speaker 43:22 Chandelier. Yeah

Prabha Mandayam 43:23 ... which is actually a dilution refrigerator.

Speaker 43:25 Okay.

Prabha Mandayam 43:25 That's actually a refrigerator.

Speaker 43:27 Mm.

Prabha Mandayam 43:27 Uh, the chip is somewhere deep inside, and you use this entire array of wires and so on to control the qubits that are lying deep inside.

Speaker 43:35 Mm.

Prabha Mandayam 43:35 And you're cooling it down to millikelvin temperatures-

Speaker 43:39 Mm

Prabha Mandayam 43:39 ... to avoid this decoherence as much as possible, and keep them in the state, the superposition state, that you want them to be. So this is one example of an architecture, right? Now, um, IBM has, for example, come up with a Python-based language called Qiskit, right? Using which you can program their quantum computers remotely.

Speaker 44:00 Okay.

Prabha Mandayam 44:00 Right, they have their quantum computer stationed in Yorktown Heights in New York, and you can sort of from here write code and program access these quantum computers. So it's not still an assembly line language. There are people who have built these full stacks and so on, being agnostic to the architecture, but I think it's still very much a work in progress. Because in a way, we still don't even know which of these architectures is going to win out. Today, it looks like superconducting qubits are at the lead because they have 100 odd qubits, but neutral atom-based systems are also catching up.

Speaker 44:29 Mm.

Prabha Mandayam 44:29 Um, trapped ions are catching up. And item- finally, actually, it would be very ideal if we could do photonics chips, right? Integrated photonics on chip-

Speaker 44:37 Mm

Prabha Mandayam 44:37 ... because those will be your room temperature qubits. You don't need these big chandeliers or, uh, quantum computer occupying, like, one entire section of this room. But with a photonics-based chip, you could probably have something

Prabha Mandayam 44:50 which is on the table, which is what would be ideal.

Speaker 44:53 So it's a maths, physics, but now as these things get realized more and more, it becomes a heavy engineering-

Prabha Mandayam 44:58 Today, it's very much an engineering problem.

Speaker 45:00 Engineering problem.

Prabha Mandayam 45:01 Yeah. And I would say what is going to drive the field, and what is already driving the field, is what one can call quantum engineers, right?

Speaker 45:08 Mm.

Prabha Mandayam 45:08 Engineers who understand the principle of quantum mechanics, um, and learn sort of how to sort of play around with, um, single quantum systems, right? And engineer, control, design single quantum systems.

Speaker 45:22 These guys are gonna be very valuable.

Prabha Mandayam 45:23 Yeah.

Speaker 45:24 If I were a JEE rank holder, I would probably choose to be a-

Prabha Mandayam 45:26 Absolutely

Speaker 45:27 ... quantum engineer.

Prabha Mandayam 45:27 Well, today we see many of the students who graduate from our programs.

Speaker 45:30 Mm.

Prabha Mandayam 45:30 Actually, there was a time when people who would do engineering physics and so on, would go almost entirely into theory.

Speaker 45:35 Yeah.

Prabha Mandayam 45:35 Now we see more and more of them going into experiments, and particularly into quantum hardware.

Speaker 45:41 Mm.

Prabha Mandayam 45:42 Because I think that's a tool and technique which is gonna be useful in the future, no matter, you know, how many quantum computers, facilities we have finally.

Speaker 45:51 Okay. [chuckles] Thank you so much. I have exhausted one set of questions, but I have another two sets of questions. So, uh, one line of inquiry that I wanted to talk to you about was, there's an India Quantum Mission.

Prabha Mandayam 46:02 Yes.

Speaker 46:03 Um, and this is a global player. Every count- every country will try to develop their own resources, and-

Prabha Mandayam 46:08 Yeah

Speaker 46:08 ... there'll be a little bit of a push and pull on which architectures win or which kind of systems, um-

Prabha Mandayam 46:15 Are going to scale up eventually

Speaker 46:15 ... are going to scale up.

Prabha Mandayam 46:16 Yeah.

Speaker 46:17 Uh, so where is India in all this? And, uh, honestly, the, the question is, do we have a chance?

Prabha Mandayam 46:22 [chuckles] So I actually think that we do have a chance.

Speaker 46:25 Mm.

Prabha Mandayam 46:26 Okay? Because the field is still, um, sufficiently nascent. I wouldn't say it's very nascent, I mean, but because, you know, we've had now companies like Google make tremendous progress in the last decade.

Speaker 46:37 Mm.

Prabha Mandayam 46:37 But it is still reasonably nascent, that I think we can still today catch up when it comes to ... Mainly when it comes to hardware. I would say when it comes to the theory of quantum and so on, I would say we are very well-equipped. We are no less than any other country in the world. We have excellent quantum information, quantum computing theory groups across the country. Experimentally, over the last decade, we have some- definitely made efforts to strengthen our experimental efforts. So we have the group at IIT Madras, led by Prasanil Prabhakar, who works on building quantum communication hardware, which is an aspect which I've completely not touched upon at all. Um, maybe just a word or two, since we are also a quantum communication hub under the National Quantum Mission. What is the idea here? The idea is, okay, now you say quantum computers are going to break classical crypto systems or your, uh, things like, uh, RSA. But can you now replace these classical security systems with quantum secure systems?

Speaker 47:40 Mm.

Prabha Mandayam 47:40 And here again, quantum mechanics comes to the rescue. So there's a way to do the same public key distribution that RSA does, using quantum states.

Speaker 47:49 Okay.

Prabha Mandayam 47:49 So now instead of sending your infor- now, like how we spoke about computing with quantum states, you now think about your information carriers themselves as quantum states. Again, not too hard because, you know, photons are traveling objects, and you can send them from place A to place B. And here, entanglement also plays a crucial role. So there is a protocol called Quantum Key Distribution, which allows you to distribute key over public channels in a way that is quantum resilient or quantum secure. So even if you had a quantum computer-... So the security of these kind of quantum schemes relies on the physics properties of these systems rather than on the computational hardness of a mathematical problem. So why RSA became vulnerable is because it was a problem which was hard classically, but not hard quantumly.

Speaker 48:36 Mm.

Prabha Mandayam 48:37 Now, you are kind of not even having computation assumptions, but you're having physical assumptions, like Quantum Mechanics is valid, the uncertainty principle is valid. So-

Speaker 48:48 So Professor Anil's group, uh, sorry, what is it called?

Prabha Mandayam 48:51 Uh, it's called Quantum Key Distribution.

Speaker 48:53 But what is An- Professor Anil's group called?

Prabha Mandayam 48:55 So we are, we, we call ourselves the Centre for Quantum Information, Communication, and Computing, CQUIC.

Speaker 49:00 Okay.

Prabha Mandayam 49:00 Okay? And we are one of the hubs under the National Quantum Mission. So they made four hubs, one for quantum communication, which is with IIT Madras; one for quantum computing, which is with the Indian Institute of Science in Bangalore; one for quantum sensing, which is again, another domain which I've completely not touched upon, which is in IIT Bombay; and a fourth one for quantum materials at IIT Delhi. So these are the four verticals which the government is focusing on, and most governments across the world

Prabha Mandayam 49:30 are kind of, uh, focusing on these.

Speaker 49:31 Mm.

Prabha Mandayam 49:32 And, uh-

Speaker 49:32 Very interesting

Prabha Mandayam 49:33 ... these form the pillars of the field, so to speak. Um, yeah, so quantum communication, uh, we have Professor Anil's group here. We have Professor Urbasi Sinha's group at Raman Research Institute, who's been doing, again, fantastic work in quantum key distribution. They are trying to do satellite-based-

Speaker 49:49 Yeah

Prabha Mandayam 49:49 ... quantum key distribution. Um, we have, uh, Bhaskar Kansara's group at IIT Delhi. Uh, so we have a few different groups on quantum communication.

Speaker 49:57 Mm.

Prabha Mandayam 49:57 We have a few different groups that are building quantum hardware, like, uh, Professor Vijay Raghavan's group at TIFR, Professor Uma Kant's group at IISER Pune. And again, the government has wisely distributed money across different architectures, so that you're not putting all your eggs in the same basket. You're funding superconducting qubits, you're funding trapped ions. You're also funding photonic-based, uh, groups in Indian Institute of Science, Professor Chandrasekhar's group. Um, you're also funding, uh, trapped ions, that's IISER Pune, Uma Kant. So the National Quantum Mission, I think has, uh, kind of done a, a fair job of assessing the key, um, sort of, uh, groups, technical areas in which, you know, we need funding. And-

Speaker 50:36 I'm, I'm a little confused on one thing. Um, w- what would the National Quantum Mission ... like, how, like, because it's such a nascent field, right? So what is the goal like-

Prabha Mandayam 50:47 Yeah

Speaker 50:47 ... what would be achieved?

Prabha Mandayam 50:48 Yeah, so the goal for the communication hub, for example, is to build a quantum secure, um, route, let's say, between Chennai to Bangalore, for starters. Can you set up a quantum secure link, communication link between Chennai and Bangalore?

Speaker 51:04 So quantum encrypted here, quantum decrypted.

Prabha Mandayam 51:06 Exactly.

Speaker 51:07 Mm.

Prabha Mandayam 51:07 Yeah.

Speaker 51:08 Okay.

Prabha Mandayam 51:09 Now, with present-day research and technology, this will not happen with a single link. It will require a few hops in between. Those hops will have to be trusted nodes, right? And, uh, this can be done. Now, this is not ... Okay, so China demonstrated this a few years ago-

Speaker 51:26 Okay

Prabha Mandayam 51:26 ... where they showed such a quantum secure link from Beijing to Shanghai.

Speaker 51:30 Okay.

Prabha Mandayam 51:31 Which is, of course, much longer. So one of the mission, uh, goals, a more ambitious goal, is can you now do this from Chennai to Delhi?

Speaker 51:38 Mm.

Prabha Mandayam 51:39 Instead of Chennai to Bangalore, but we're starting with Chennai to Bangalore. So IIT Madras, for example, we have now set up a communi- quantum secure link from IIT Madras, our lab in the electrical engineering department here, to the research park.

Speaker 51:51 Mm.

Prabha Mandayam 51:51 Right?

Speaker 51:52 Okay.

Prabha Mandayam 51:52 This is one such point to point link. But now you want to sort of stretch this and make it across cities, intercity, and so take it all the way to Delhi, for example, is one of the goals of the National Quantum Mission.

Speaker 52:03 Okay.

Prabha Mandayam 52:03 Right? For computing, the goal would be, can you build a chip with, let's say, 50 qubits? And you wouldn't be too far behind. I mean, IBM may have 100, 150, but if you have the technology to build and control 50 qubits, then you're not too far off from sort of realizing that. So that's another goal of the National Quantum Mission.

Speaker 52:21 Okay.

Prabha Mandayam 52:21 So these are the kind of goals that have been set, very targeted, very concrete goals-

Speaker 52:26 Mm

Prabha Mandayam 52:26 ... and teams have been identified to work towards these specific objectives.

Speaker 52:30 Yeah. Okay, and this is like a, uh, it's an engineering challenge, a photonics challenge, a electrical engineering challenge, and a physics-

Prabha Mandayam 52:37 Yes.

Speaker 52:38 Problem.

Prabha Mandayam 52:38 Yes, absolutely.

Speaker 52:38 These are the fields that are intersecting.

Prabha Mandayam 52:40 Right. Well, also computer science from the perspective of, can you come up with new algorithms?

Speaker 52:44 Okay.

Prabha Mandayam 52:45 Right? Uh, can you, uh, optimize even the existing algorithms, so that if you have noisy devices, then, you know, can you do some special tricks that can push you faster, uh, you know, towards a solution, and so on?

Speaker 52:58 Yeah.

Prabha Mandayam 52:58 So it's a, it's, I would say, a confluence of mainly computer science, physics, math, electrical engineering, right? But now I would say we also have chemists and biotechnology people in the mix, because the kind of problems we are going to solve finally-

Speaker 53:14 Yeah

Prabha Mandayam 53:14 ... which are gonna have lasting impact, will be the problems of drug delivery, drug design, protein folding, and so on and so forth.

Speaker 53:21 Right.

Prabha Mandayam 53:21 So which needs domain knowledge in chemistry, she's domain knowledge in biosciences.

Speaker 53:25 Yeah.

Prabha Mandayam 53:25 So it's kind of an interesting melting pot. [chuckles]

Speaker 53:29 Wow, it's, um, it's amazing, um-

Prabha Mandayam 53:31 So I should mention that our center itself has faculty from all these disciplines-

Speaker 53:34 Okay

Prabha Mandayam 53:35 ... that I just mentioned. We also now have faculty from management sciences, because operations research folks are very interested in quantum computing.

Speaker 53:42 Mm.

Prabha Mandayam 53:42 Because you can solve all these different optimization problems, um, you know, uh, bin packing, uh, you know-

Speaker 53:48 No, for sure. Operations research as a field started because they wanted to, uh, because the Americans wanted to distribute, uh, their supplies across the war front-

Prabha Mandayam 53:57 Absolutely

Speaker 53:57 ... in World War II.

Prabha Mandayam 53:58 Yeah, yeah.

Speaker 53:59 So computation challenge-

Prabha Mandayam 54:00 Yeah

Speaker 54:00 ... more than anything else.

Prabha Mandayam 54:01 Yeah. So for example, the center has, um, industry collaborators, so one of our biggest sponsors is Mphasis Foundation.

Speaker 54:07 Mm.

Prabha Mandayam 54:08 Right?

Speaker 54:08 Yeah.

Prabha Mandayam 54:09 And, uh, what are they interested in mainly is, uh, an entire suite of these kind of optimization problems, right? For example, EV charging, charging electrical, uh, electric vehicles.... right? So you have some configuration, uh, in your grid, so to speak. Now, what is the optimal sort of solution for this? This could be something that can be cast as a quantum, uh, problem and run on a quantum computer.

Speaker 54:30 Mm.

Prabha Mandayam 54:30 Right? Um, we have companies like Boeing and Airbus being interested in quantum computing. We have companies like Siemens and Bosch being interested. So it's kind of, uh, Mercedes.

Speaker 54:40 The EV challenge problem- EV charging problem is: how do I use my existing grid to charge so many electric vehicles at so many points-

Prabha Mandayam 54:47 Correct

Speaker 54:47 ... at what time of the day?

Prabha Mandayam 54:48 Yeah. Exactly.

Speaker 54:48 At what-

Prabha Mandayam 54:49 Yeah

Speaker 54:49 ... current and not-

Prabha Mandayam 54:50 Right

Speaker 54:50 ... completely destroy the-

Prabha Mandayam 54:51 Correct

Speaker 54:52 ... system.

Prabha Mandayam 54:52 So you want to optimize, yeah, you want to optimize power. So the, you, you can identify the cost function that you want to optimize, right? And you can, uh, cast it into a, a quantum, uh, problem.

Speaker 55:03 Professor, one area of, uh, inquiry that I have is... I mean, h- how did you get into this? What is your personal journey?

Prabha Mandayam 55:11 [chuckles] Yeah. So, um, I did a bachelor's in physics, uh, from Chennai, Ethiraj College in Chennai, and then I, uh, came to do a master's in, uh, physics

Prabha Mandayam 55:20 in IIT Madras. So full disclosure, my father himself is a theoretical physicist-

Speaker 55:25 Okay

Prabha Mandayam 55:25 ... now retired from University of Madras, and he worked on an area called quantum foundations.

Speaker 55:31 Okay.

Prabha Mandayam 55:31 [chuckles] Okay. Um, or the theory of quantum measurement, which was, at that point, considered very esoteric sort of thing. Um, he was trying to really understand the nitty-gritties of quantum measurement, and so on. Interestingly, his papers are now very well-cited today because, uh, you know, quantum measurement has become such an important problem because of quantum technology, right? So in a way, the seeds were probably sown by some, uh, discussions with him.

Speaker 55:57 Mm.

Prabha Mandayam 55:57 But really, after I joined my master's here at IIT Madras, and we did our first Quantum Mechanics course, and, uh, we were all asked to do... give, uh, presentations on some topics, and I think I chose, um, something called this, um, Bell experiment, Bell CHSH experiment, which actually won the Physics Nobel, uh, three years back. And I think in reading about that, in even talking about it, I was really excited. Somehow something clicked. Um, and then I went on to do my master's project under Professor Arul Lakshmi Narayan.

Speaker 56:27 Mm.

Prabha Mandayam 56:27 Um, we studied early papers on quantum entanglement, and I ended up working with him on something called quantum random walks-

Speaker 56:34 Okay

Prabha Mandayam 56:34 ... which is, again, different from a classical walk, and it's, again, some interesting stuff there. So all of this, I think, kind of, you know, propelled me in this direction. So I realized that I really like... First, I really liked the math of quantum mechanics, and then I really liked the idea of entanglement, the idea of doing, um, tasks like random walks with quantum particles and stuff like that. Um, I got introduced to a book, uh, by Nielsen and Chuang, which had just come out. Um, and then I applied for grad school and got into, uh, one of the, at that point, and even today, leading groups on quantum computing with, uh, Professor John Preskill at Caltech. So I think that kind of cemented the deal.

Speaker 57:16 I want to ask you a question here. Um, I understand that you were exposed to it early-

Prabha Mandayam 57:20 Yeah

Speaker 57:21 ... uh, which is amazing. Um, but still, it- at the time, you're talking about 2003, 2005.

Prabha Mandayam 57:28 Yeah.

Speaker 57:28 It's a very esoteric field.

Prabha Mandayam 57:29 Yes.

Speaker 57:30 You're committing your career to it.

Prabha Mandayam 57:32 Right.

Speaker 57:32 Isn't that like a, like a nerve-wracking decision, that, "Am I going to do this?" Uh, "What if I... What if the field doesn't move for 30, 40 years?"

Prabha Mandayam 57:43 So I should tell you that I was one of those people in 2000 who actually did not write the JEE exam.

Speaker 57:50 Okay.

Prabha Mandayam 57:51 So I knew that I wanted to do physics-

Speaker 57:54 Okay

Prabha Mandayam 57:54 ... for whatever reason, and at that point, uh, maybe the only option was to go for an integrated physics program in IIT Kanpur.

Speaker 58:00 Mm.

Prabha Mandayam 58:01 And then there was all this conversation at home. You know, everyone in class was going for JEE coaching-

Speaker 58:05 Mm

Prabha Mandayam 58:06 ... in Chennai. You know, you're also a Chennai person.

Speaker 58:07 Yeah.

Prabha Mandayam 58:08 So you know, there were specific classes [chuckles] that you would go to for JEE.

Speaker 58:11 Yeah.

Prabha Mandayam 58:11 I didn't go because, um, I, I talked to my dad and he said, "Do you want to do engineering?" I said, "No, I don't want to do engineering." Then he said, "Why are you writing the JEE exam?" [chuckles] So I said, "Okay, maybe I don't need to write the JEE exam." So, you know, I have never thought about this as, is there a, you know, career path here? Is there-

Speaker 58:27 Mm.

Prabha Mandayam 58:28 It's more about what has interested me. I've been lucky enough to kind of follow my interests that way-

Speaker 58:34 Yeah

Prabha Mandayam 58:34 ... right? Um, but I have always done that, and maybe that's why I did not think too much. For example, even in theoretical physics-

Speaker 58:41 Mm

Prabha Mandayam 58:42 ... most of my peers were, um, applying, when they applied for PhD, whether abroad or here, were applying in string theory. It was still the heydays of string theory, right? And everyone either wanted to do particle physics, high-energy physics, you know, CERN, um, you know, or string theory. But to me, what was fascinating [chuckles] was quantum, uh, information and entanglement and all of that. So then, uh... So in a way, I was bucking the trend, uh, even-

Speaker 59:08 Nice

Prabha Mandayam 59:08 ... even in theoretical physics. It's true that I'm one of the earliest, probably, who actually went on to do a PhD in quantum computing. Uh, so yeah. But well, it, it worked. [chuckles]

Speaker 59:19 It worked out. Um, I would, I would summarize that, uh, you had not only early exposure, you had a very supportive parent.

Prabha Mandayam 59:27 Absolutely. No, uh, absolutely.

Speaker 59:28 Great.

Prabha Mandayam 59:28 Yeah, yeah. No, my parents were very supportive. Yeah, yeah.

Speaker 59:30 Who didn't push you down JEE path.

Prabha Mandayam 59:32 Not at all. Not at all, no. The... Very, very supportive. Uh, yeah. Yeah.

Speaker 59:36 Amazing. Um, so at, at... You mentioned that you were a master's student here at, at IIT Madras. I think that's 2003 to '5.

Prabha Mandayam 59:44 Yep.

Speaker 59:45 Uh, what was the department like then, and has it changed a lot from then to now?

Prabha Mandayam 59:48 Well-

Speaker 59:48 I, I also wanna give you the, what I have heard from, uh, aspirants.

Prabha Mandayam 59:52 Mm-hmm.

Speaker 59:53 Uh, obviously, in 11th and 12th, a lot of, um, a lo- lot of students do want to do physics.

Prabha Mandayam 59:58 Yeah.

Speaker 59:58 They are unable-

Prabha Mandayam 59:58 Yeah

Speaker 59:58 ... to do because there's a lot of peer pressure.

Prabha Mandayam 1:00:00 Yeah.

Speaker 1:00:00 But when they-

Prabha Mandayam 1:00:01 Yeah

Speaker 1:00:01 ... want to do physics, they-

Prabha Mandayam 1:00:02 Yeah

Speaker 1:00:02 ... kind of look at IIT Madras as a place they would want to come to.

Prabha Mandayam 1:00:05 Mm-hmm. Mm-hmm. Yeah, no, I think today, um, I mean, this might not have been the case maybe 20 years ago.

Speaker 1:00:12 Mm.

Prabha Mandayam 1:00:12 First of all, 20 years ago, we barely had started the Engineering Physics program, right? I myself was a master's in physics. Like I said, you see, if I was a class 11, class 12 person who wanted to do physics, or even mathematics for that matter, or chemistry, where are the institutes where I could...? This was before all the ISERTS came up.

Speaker 1:00:30 Mm.

Prabha Mandayam 1:00:30 You know, uh, there were very few options to do an undergraduate physics program. I think IIT Bombay had an engineering physics program already by then.... IIT Madras still did not have. When I started my BSc, for example, IIT Madras did not have an engineering physics program, right? It started, in fact, in 2003, if I'm not mistaken, or, uh, 2002. So the first batch of engineering physics students graduated in 2006-

Speaker 1:00:54 Mm

Prabha Mandayam 1:00:54 ... from IIT Madras. So it was a much smaller department then, right? Um, mainly, uh, we had master's and PhD students, right? And even those numbers were fewer, right? And, um, the undergraduates who came in to attend these physics courses were, like, the really motivated, interested-

Speaker 1:01:13 Mm

Prabha Mandayam 1:01:13 ... uh, people who really wanted to do physics, despite having been pushed into JEE and pushed into taking one of these branches. But then the love for physics superseded all of that, and they came and did physics courses. But I think today, well, of course, the subject has grown, and but also the department has grown tremendously. Back then, the theoretical physics group was very small, maybe five, six faculty. Today we have maybe 15, 16 faculty, and we are g- still growing.

Speaker 1:01:40 Mm.

Prabha Mandayam 1:01:41 Um, the experimental group was mostly low-temperature physics. Today we have again grown much bigger than that. We have people who do optics, um, ultrafast lasers, we have photonics, we have, uh... We now have, uh, people who have tie-ups with all these big experiments that are running worldwide, like CERN, the particle physics experiments. We have a very active particle physics group. We have a very active cosmology, general relativity, um, uh, gravitational wave, uh, you know, group. So I think we've really expanded on all fronts. And of course, we have an active quantum, uh, computing, quantum information group. Even within physics, m- almost all of us are part of the center that I described.

Speaker 1:02:23 Mm.

Prabha Mandayam 1:02:23 Um, so I think the department has grown tremendously. We now have a dual-degree physics program. The engineering physics program now, I'm told, is, uh, ranked, uh, higher than-

Speaker 1:02:35 Than the other IITs.

Prabha Mandayam 1:02:36 Yes.

Speaker 1:02:37 Yeah.

Prabha Mandayam 1:02:37 Um, and in fact, I'm told that our ra- our closing rank this year in counseling was even better than... was next only to computer science and EE-

Speaker 1:02:47 Mm.

Prabha Mandayam 1:02:47 ... um [chuckles] which is, I think, a big achievement.

Speaker 1:02:49 Mm.

Prabha Mandayam 1:02:50 But, yeah, I think it goes to show, um, that, one, okay, the program itself, I would say... I mean, I've taught EP students since my- uh, so I joined the department back about 10 years ago, and I've taught a few batches of dual-degree physics and engineering physics students. Very motivated kids. And I think, uh, since, you know, we now have several young faculty in the department-

Speaker 1:03:11 Yeah

Prabha Mandayam 1:03:11 ... dynamic faculty, so there's something very, um... I think there's a lot of action happening-

Speaker 1:03:16 Yeah

Prabha Mandayam 1:03:17 ... on the physics front, and so the students are picking up on it, and we get some very good students today.

Speaker 1:03:22 Yeah, the movement in the ranks, uh, for EP has been quite, uh, incredible.

Prabha Mandayam 1:03:26 Right. And I feel that engineering physics students, at least from a quantum perspective, are the best placed, uh, to enter a field like quantum science and technology. Because you come with, uh, some amount of electrical engineering knowledge, which as an MSc physics person, frankly, I don't, right? I had to pick up several things along the way. Even my knowledge of computer science or something is very picked up later, right? I, in fact, attended an information theory course, uh, during my grad school just to pick up on classical coding, classical information, and all these ideas. Uh, but someone who's doing an engineering physics degree will naturally have those courses. They will have quantum mechanics, so it's very easy to, I think, get

Prabha Mandayam 1:04:05 into this field today if you're an engineering physics graduate. Yeah.

Speaker 1:04:08 Nice. Thank you so much for all of that. Um, I've already occupied your time a lot-

Prabha Mandayam 1:04:13 Yeah

Speaker 1:04:13 ... so I will let you go. But one last question I want to ask you. Are there a lot of women students taking these courses? And I'm asking you because I came from mechanical engineering, and my class had maybe a 10%, uh, uh... Even today, we get questions from parents saying, "Is this suitable for..." I don't know why.

Prabha Mandayam 1:04:32 [chuckles]

Speaker 1:04:33 I don't know why. What do they think that engineers do? But they still ask us questions like, "Oh, is, uh, naval suitable? Is chemical suitable?" I don't know. But I would ask you that question: Is this a field that sees a lot of women in it?

Prabha Mandayam 1:04:46 At present, at the moment, not yet, right?

Speaker 1:04:48 Yeah.

Prabha Mandayam 1:04:48 Our ratios, unfortunately, haven't changed much from the 10% that, uh, you saw, right? Um, in engineering physics, I think, uh, in a batch of about 40, 45, maybe we have 10 girls typically.

Speaker 1:05:00 Okay, so that's, that's 20, 20.

Prabha Mandayam 1:05:02 A little bit better.

Speaker 1:05:02 Yeah.

Prabha Mandayam 1:05:02 Right? It's 20%, uh, almost, I think, which is good. And I think also initiatives like the supernumerary, uh, you know, uh, sort of-

Speaker 1:05:11 Supernumerary seats.

Prabha Mandayam 1:05:12 Yeah, yeah. I think that helps, right? Certainly. Um, so I think to me, the biggest problem for women in STEM fields is that at some point... I mean, I would say by and large, Indian parents, uh, we are very supportive of women's education, I must say, in general, right, as a society. Uh, because I don't believe that, I don't buy this thing that we are very conservative. We are not at all conservative when it comes to educating our women, right?

Speaker 1:05:36 I mean, there, there's data on this, because in CBSC, women outperform men all the-

Prabha Mandayam 1:05:40 Absolutely. Absolutely, yeah. I mean, my son is in class four, and I can tell you that the number of girls in his class is, first of all, outnumbers the number of boys, and the number of top rankers who are girls is much higher.

Speaker 1:05:51 Mm.

Prabha Mandayam 1:05:51 Right? So this is very true. But I think what happens is, after a certain age, um, society starts imposing certain, um, not constraints, but more like expectations-

Speaker 1:06:04 Mm

Prabha Mandayam 1:06:04 ... that now you need to do this, this, and this, right?

Speaker 1:06:07 Mm.

Prabha Mandayam 1:06:07 And then these, this, this, and this come with all these constraints, but then you start second-guessing yourself. You start saying, "Should I really go for that master's, which is gonna take me two more years, and then what do I do? And then should I really go for this PhD, which is, like, five years long?"

Speaker 1:06:24 Interesting.

Prabha Mandayam 1:06:24 You know? "Or should I... At this point, I'm getting placed. Let me take this job." I mean, nothing h- wrong in that, but I'm just saying that the choices that girls start making at 2021 is, I think, what pushes down the number of women who are, you know, sort of... See, you need that extra qualification, you need that extra skilling-... right? In order to be able to enter a niche area like this.

Speaker 1:06:51 Yeah. Also, there are some fields, I feel, there are some fields if a woman, uh, uh, in 11, 12 or 8, 9, 11, 12, has decided that I want to have a long career, uh, there are some fields which are, uh, predominantly women. Like, I come from marketing-

Prabha Mandayam 1:07:06 Right.

Speaker 1:07:06 There's a lot of women in it.

Prabha Mandayam 1:07:07 Right.

Speaker 1:07:07 So, uh, by default, they choose fields where there are more women. So I think in these ca-

Prabha Mandayam 1:07:12 But that I think is a just, um, just a peer, peer thing, I think, you know? It's just, uh, because I don't see a marketing job as being any less stressful or any less, uh, demanding, um, than, let's say, a, uh, the job that I do or the job that, uh, so, let's say, a quantum engineer in Google will do.

Speaker 1:07:30 Yeah, I don't mean in terms of easy or hard, I mean in terms of, say, for a easier choice.

Prabha Mandayam 1:07:36 Yeah, but I'm saying that that is just, uh, pure, uh, peer driven.

Speaker 1:07:40 Okay, fair.

Prabha Mandayam 1:07:40 Right? It's like, for example, okay, so I went to do a PhD-

Speaker 1:07:43 It's not a really logic driven.

Prabha Mandayam 1:07:44 I don't think it's very logical. Because if you really think about how time co- demanding it is, and so on, it's not any less demanding, right? Because I see, uh, you know, uh, women... I have friends who, women who are in marketing, they're all the time having to-- you could have client calls in the middle of the night, you could have client calls early in the morning.

Speaker 1:08:00 Yeah.

Prabha Mandayam 1:08:00 Whereas if you think about it, I actually do not have calls in the middle of the night.

Speaker 1:08:03 Yeah.

Prabha Mandayam 1:08:03 I set my, uh, calendar myself, frankly, if you think about it. So in a way, I find I have more freedom sometimes. So I think that's not so logical in a choice, but I do think that there are more women there, so women kind of tend to... So like when I walked into my advisor's group meeting for the first time-

Speaker 1:08:20 Mm

Prabha Mandayam 1:08:20 ... there was only one other woman in that room with, uh, maybe 20 men. This was not even the case in my master's class in IIT Madras. We were 10 girls among 40, which is a reasonable number, you know. Among 40, 45, class of 40, 45, there were 10 of us girls.

Speaker 1:08:36 Mm.

Prabha Mandayam 1:08:36 So this PhD actually was the first time when I was in a group, which was-

Speaker 1:08:41 Yeah

Prabha Mandayam 1:08:41 ... entirely male dominated. There's one other female graduate student. At any given point of time, we would have a group of 15, 16, which is one or two women. Either one woman post-doc, one woman PhD student. So I think that maybe daunts us sometimes, right? And so we say, "Oh, but there are no women there, so why should I be really there?" But I think we are second-guessing ourselves in asking these questions. So what?

Speaker 1:09:04 Yeah.

Prabha Mandayam 1:09:05 What does it matter? So long as your colleagues treat you in a fair way, and there's no, you know... And I don't think I faced any, frankly, no such... You know, I never felt like, "Oh, my God, I'm squirming," because, you know.

Speaker 1:09:16 Yeah.

Prabha Mandayam 1:09:17 No, no one... So I think so long as everyone's respectful and treating you like, you know, anybody else in the room, there's no issue.

Speaker 1:09:24 Yeah, and this is a field where your gender really doesn't give you an edge for anything. You could be a man or woman.

Prabha Mandayam 1:09:30 Absolutely. Absolutely. I do think that the main issue that happens is, after a certain age, you maybe have additional constraints, additional responsibilities. But today, in today's day and age, we have developed mechanisms-

Speaker 1:09:42 To work

Prabha Mandayam 1:09:42 ... you know, to work with it. So I see no, uh, yeah, barrier as such.

Speaker 1:09:48 Thank you so much, Professor. That was a long conversation, and [chuckles] -

Prabha Mandayam 1:09:51 Thank you. I hope I did not bore everyone with all my quantum [laughing] I get into teacher mode very easily, so.

Speaker 1:09:58 We, we will, uh, in the, in the YouTube

Speaker 1:10:00 description, we have left links, um, on, on some of these things, including the links to her NPTEL courses, so you can check them out. Uh, Professor, any l- final thoughts before we close?

Prabha Mandayam 1:10:11 Well, I would just say, if you are, um, uh, a high school student or even an, um, undergraduate, uh, watching this podcast, uh, this, the next decade I think is a very exciting time for quantum science and technologies. I think it is a good time to get into the field, where you get to learn a lot of new things. You get to be a part of exciting new developments that are happening around. And no matter which course, eventually, uh, history will take, I think you end up learning some very nice skills, which will-

Speaker 1:10:45 Mm

Prabha Mandayam 1:10:45 ... last you throughout. So that's my... Yeah.

Speaker 1:10:48 Thank you, Professor. Thank you everyone for listening in. Please share, subscribe, uh, send this podcast as a link on WhatsApp to your friends. Thank you.