Best Place To BuildQuantum Mechanics, qubits, superposition & superconductors with Prof. Prabha Mandayam | BP2B S2 E11
EVERY SPOKEN WORD
70 min read · 13,559 words- 0:00 – 0:50
Introduction
- PMPrabha Mandayam
[upbeat music] It's too much information.
- SPSpeaker
I'm going, I'm going, I'm going to share what I understood. [chuckles]
- PMPrabha Mandayam
Yeah.
- SPSpeaker
I'm sure I'm wrong.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
So, you know, I have never thought about this as is there a, you know, career path here? Is there-
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Hi,
- 0:50 – 1:30
Welcome to BP2B
- SPSpeaker
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
- 1:30 – 2:05
Introducing Prof Prabha Mandayam
- SPSpeaker
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.
- PMPrabha Mandayam
Thank you, Amrit. Thank you for having me here.
- SPSpeaker
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?
- PMPrabha Mandayam
[chuckles] Um, yes.
- 2:05 – 5:00
What is Quantum Computing?
- PMPrabha Mandayam
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?
- SPSpeaker
Correct.
- PMPrabha Mandayam
Classical communication is zeros and ones, right? Yeah.
- SPSpeaker
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-
- PMPrabha Mandayam
Correct
- SPSpeaker
... a set of algorithms-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... that work on those binary operations.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
At the same time, on the physical side, there's this thing called a transistor-
- PMPrabha Mandayam
Right
- SPSpeaker
... which mimics a bit, a zero, one state.
- PMPrabha Mandayam
Right.
- SPSpeaker
So we build a physical hardware-
- PMPrabha Mandayam
Sure
- SPSpeaker
... based on transistors, which can do these computes.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
So that's what quantum does, right? If you, instead of encoding into the zeros and ones, if you encode into a quantum state-
- SPSpeaker
Okay
- PMPrabha Mandayam
... 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.
- SPSpeaker
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.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
And so a quantum state is that. Basically, it's like identifying a point on the surface of the sphere,
- 5:00 – 15:00
Quantum Mechanics visualised
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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-
- SPSpeaker
Right
- PMPrabha Mandayam
... 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-
- SPSpeaker
Fair enough. So if I, if I take a high-speed photograph, pho- photo at any point, that will be at some point.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
So now I don't know whether it's gonna land head or tail.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
Fifty percent would be one, uh, such state-
- SPSpeaker
Option
- PMPrabha Mandayam
... one option. But so now you have all these possible states with all these different possibilities, uh, probabilities for head and tail.
- SPSpeaker
Okay, so this is a qubit?
- PMPrabha Mandayam
This is a qubit.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Right.
- PMPrabha Mandayam
But now I have the option of encoding in these in-between states.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... twice. But in place of that, if I give you a quantum box, I give you a quantum processor.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- 15:00 – 16:20
The Origin of Quantum Mechanics Studies - Photonics 101
- PMPrabha Mandayam
the photoelectric effect was first discovered.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Which is when you were literally probing this, you know, single particle kind of interaction. You had an electron-
- SPSpeaker
The slit experiments.
- PMPrabha Mandayam
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?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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?
- SPSpeaker
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-
- PMPrabha Mandayam
Quantum mechanics
- SPSpeaker
... quantum mechanics.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Uh, and, uh-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... the, the starting point is [keyboard clicking] nominally, like you have to decide some starting point, so it's the-
- PMPrabha Mandayam
Schrodinger equation, then it's 100 years, yeah.
- SPSpeaker
Great.
- PMPrabha Mandayam
It's 100 years of Schrodinger equation. I just want to
- 16:20 – 22:00
The History of Quantum Mechanics | Algorithms Explained
- PMPrabha Mandayam
say that the idea of quantum computing-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... is about 40 years old now.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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-
- SPSpeaker
So-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... 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?
- PMPrabha Mandayam
Exactly. Exactly.
- SPSpeaker
Okay.
- PMPrabha Mandayam
Exactly.
- SPSpeaker
So that is why we should care about it.
- PMPrabha Mandayam
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.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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-
- SPSpeaker
Mm
- PMPrabha Mandayam
... you can show your viewers the-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... 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-
- SPSpeaker
Okay, so but this is still an algorithm.
- PMPrabha Mandayam
Yes.
- SPSpeaker
This is an algorithm that says that if we have-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... these quantum states.
- PMPrabha Mandayam
Yeah, yeah.
- SPSpeaker
And then we could write math-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... in such a way that factorization would go from a NP-hard problem to a very simple-
- PMPrabha Mandayam
Right
- SPSpeaker
... problem.
- 22:00 – 26:15
What is Quantum Decoherence?
- PMPrabha Mandayam
Uh, this, um, mechanism is what is called decoherence.
- SPSpeaker
Okay.
- PMPrabha Mandayam
Okay? And this is an important word, because it then ties in with my research, which essentially is about how do you combat decoherence?
- SPSpeaker
Okay.
- PMPrabha Mandayam
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]
- SPSpeaker
I will, I will, I will share what I understood.
- PMPrabha Mandayam
Yeah. [chuckles]
- SPSpeaker
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?
- PMPrabha Mandayam
Yes.
- SPSpeaker
And secondly, we should be able to measure and find out what state it is?
- PMPrabha Mandayam
Absolutely. Absolutely, yeah.
- SPSpeaker
In both giving instruction to the qubit and measuring-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... there's some error that comes in.
- PMPrabha Mandayam
Yes, yes. It's, it's not- it's challenging to do it at a single qubit level-
- SPSpeaker
Mm
- PMPrabha Mandayam
... 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-
- SPSpeaker
Right
- PMPrabha Mandayam
... either to the equatorial plane or just go off to the North Pole, like-
- SPSpeaker
Right
- PMPrabha Mandayam
... the zero state.
- SPSpeaker
Right.
- PMPrabha Mandayam
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.
- SPSpeaker
Uh, the electron or the pho- sorry-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... the photon or-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... or the-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... the microscopic element-
- 26:15 – 31:50
Google & IBM’s Experiments With Quantum Computation
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
It doesn't become a quantum processor, it doesn't become a computer.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Now, I need to have the ability to like I-- how I etch transistors and this whole-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... circuits on a chip, can I now make quantum circuits on a chip?
- SPSpeaker
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-
- PMPrabha Mandayam
It started in 2016, and I would say last year, with Google's, like, hundred qubit experiment, it has more or less come.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Right? We still haven't demonstrated Shor's algorithm on a-
- SPSpeaker
Sure
- PMPrabha Mandayam
... 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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Walking back a bit, when you were talking about this realization in the beginning, you mentioned the word superconductivity.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Superconductive qubit.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
And I sort of, uh, you know, broke you there-
- PMPrabha Mandayam
Right
- SPSpeaker
... in that flow.
- PMPrabha Mandayam
Right, right.
- SPSpeaker
Can you complete that flow? What, what is a superconductive qubit? And before that, there's a photon qubit, is that it?
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- 31:50 – 33:33
Classical v/s Quantum Error Corrections
- PMPrabha Mandayam
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-
- SPSpeaker
Mm
- PMPrabha Mandayam
... but one of them flipped. So this is introducing redundancy.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
So I have to protect superposition-
- SPSpeaker
Mm
- PMPrabha Mandayam
... 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,
- 33:33 – 37:50
The No-Cloning Theory
- PMPrabha Mandayam
in quantum computing or quantum information, the very basic no-go result called the No-Cloning Theorem-
- SPSpeaker
Okay
- PMPrabha Mandayam
... which says that you cannot make copies of arbitrary superposition quantum states.
- SPSpeaker
Mm.
- PMPrabha Mandayam
So you don't have a Gurunath, which probably was there when you were a student here-
- SPSpeaker
Mm
- PMPrabha Mandayam
... where you have a Xerox machine, and you put in, take your friend's notes-
- SPSpeaker
Mm
- PMPrabha Mandayam
... right, the night before the exam and make a copy of the entire notebook. No.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Not possible. So you cannot copy quantum information.
- SPSpeaker
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.
- PMPrabha Mandayam
Right, but you cannot do that quantum.
- SPSpeaker
Okay.
- PMPrabha Mandayam
So you can copy states which are like classical, which is your zero and one.
- SPSpeaker
Mm.
- PMPrabha Mandayam
But an arbitrary superposition-
- SPSpeaker
Mm
- PMPrabha Mandayam
... 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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
And here comes in another player in this whole game, which is entanglement.
- SPSpeaker
Okay.
- PMPrabha Mandayam
Which is a word which I have not used until now. Uh, so what is entanglement?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
So that's the idea of quantum entanglement. Again, very, very, um-
- SPSpeaker
Sure
- 37:50 – 40:00
Will We All Have Quantum Computers in Our Hands?
- PMPrabha Mandayam
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-
- SPSpeaker
You have iPhones.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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-
- SPSpeaker
That's the-
- PMPrabha Mandayam
vacuubes and-
- SPSpeaker
IBM, it thought of the computing revolution as something that governments and organizations will have.
- PMPrabha Mandayam
Right. So today, I typically say this, that maybe we will have specialized quantum computing facilities.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Right? But then who knows? Like I said, you know-
- SPSpeaker
Right
- PMPrabha Mandayam
... 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-
- SPSpeaker
Okay, so-
- PMPrabha Mandayam
... and so on.
- SPSpeaker
maybe, uh, maybe in future, IIT Madras will have a facility, and I'm a researcher in, say, protein folding.
- PMPrabha Mandayam
Exactly.
- SPSpeaker
I will like, "Here, take this problem-
- PMPrabha Mandayam
Absolutely
- SPSpeaker
... and get a solution in-
- PMPrabha Mandayam
Yeah, yeah
- SPSpeaker
... like 20 minutes."
- PMPrabha Mandayam
Yeah, yeah. So you can imagine how IIT Madras had one of the earliest IBM computer-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... stationed, uh, in India, I think-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... 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.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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.
- SPSpeaker
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.
- 40:00 – 42:30
The Maths Behind it All
- SPSpeaker
ideas I have-
- PMPrabha Mandayam
Yes, indeed
- SPSpeaker
... I have to get.
- PMPrabha Mandayam
Yeah, yeah.
- SPSpeaker
Can you give me a, again, can you give me a bigger list? So q- I'm gonna make this list.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
So I must understand what is a qubit. I must understand what is superposition. Um, uh, third one was entanglement.
- PMPrabha Mandayam
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-
- SPSpeaker
Is this because it's like A first angle, uh, whatever, A theta plus B alpha? I don't know.
- PMPrabha Mandayam
Exactly. A0 plus B1, alpha 0 plus beta 1. I had, I had refrained from using things like this-
- SPSpeaker
Okay
- PMPrabha Mandayam
... 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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... uh, AI/ML.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
So in a way, this is a kind of convergence of, uh-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... 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.
- SPSpeaker
Makes sense.
- PMPrabha Mandayam
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-
- SPSpeaker
So I don't need to be better at Schrodinger equation.
- PMPrabha Mandayam
[laughing]
- SPSpeaker
Do I need to?
- PMPrabha Mandayam
This is a very tricky question. [laughing] So it depends on how deep you want to get into quantum computing.
- SPSpeaker
Okay.
- PMPrabha Mandayam
Okay, so I want to say that one of the important use cases today, for example, protein folding.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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-
- 42:30 – 44:50
Variational Quantum Algorithms
- PMPrabha Mandayam
right?
- SPSpeaker
Mm.
- PMPrabha Mandayam
So it depends on how deep you wanna get into this, right?
- SPSpeaker
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-
- PMPrabha Mandayam
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-
- SPSpeaker
So this, you were talking about the architectures of qubit.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
So photonic-
- PMPrabha Mandayam
Superconducting
- SPSpeaker
... superconducting.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Trapped ions.
- PMPrabha Mandayam
And neutral atoms.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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-
- SPSpeaker
Chandelier. Yeah
- PMPrabha Mandayam
... which is actually a dilution refrigerator.
- SPSpeaker
Okay.
- PMPrabha Mandayam
That's actually a refrigerator.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
And you're cooling it down to millikelvin temperatures-
- SPSpeaker
Mm
- PMPrabha Mandayam
... 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.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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-
- SPSpeaker
Mm
- 44:50 – 49:30
The Interdisciplinary Nature of Quantum Computational Studies
- PMPrabha Mandayam
which is on the table, which is what would be ideal.
- SPSpeaker
So it's a maths, physics, but now as these things get realized more and more, it becomes a heavy engineering-
- PMPrabha Mandayam
Today, it's very much an engineering problem.
- SPSpeaker
Engineering problem.
- PMPrabha Mandayam
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?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
These guys are gonna be very valuable.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
If I were a JEE rank holder, I would probably choose to be a-
- PMPrabha Mandayam
Absolutely
- SPSpeaker
... quantum engineer.
- PMPrabha Mandayam
Well, today we see many of the students who graduate from our programs.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Actually, there was a time when people who would do engineering physics and so on, would go almost entirely into theory.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
Now we see more and more of them going into experiments, and particularly into quantum hardware.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
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.
- PMPrabha Mandayam
Yes.
- SPSpeaker
Um, and this is a global player. Every count- every country will try to develop their own resources, and-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... there'll be a little bit of a push and pull on which architectures win or which kind of systems, um-
- PMPrabha Mandayam
Are going to scale up eventually
- SPSpeaker
... are going to scale up.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Uh, so where is India in all this? And, uh, honestly, the, the question is, do we have a chance?
- PMPrabha Mandayam
[chuckles] So I actually think that we do have a chance.
- SPSpeaker
Mm.
- 49:30 – 55:20
Quantum Research in India
- PMPrabha Mandayam
are kind of, uh, focusing on these.
- SPSpeaker
Mm.
- PMPrabha Mandayam
And, uh-
- SPSpeaker
Very interesting
- PMPrabha Mandayam
... 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-
- SPSpeaker
Yeah
- PMPrabha Mandayam
... 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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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-
- SPSpeaker
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-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... what would be achieved?
- PMPrabha Mandayam
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?
- SPSpeaker
So quantum encrypted here, quantum decrypted.
- PMPrabha Mandayam
Exactly.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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-
- SPSpeaker
Okay
- PMPrabha Mandayam
... where they showed such a quantum secure link from Beijing to Shanghai.
- SPSpeaker
Okay.
- PMPrabha Mandayam
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?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Right?
- SPSpeaker
Okay.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay.
- 55:20 – 1:04:05
How Did Prof Prabha Get into Quantum Mechanics?
- PMPrabha Mandayam
in IIT Madras. So full disclosure, my father himself is a theoretical physicist-
- SPSpeaker
Okay
- PMPrabha Mandayam
... now retired from University of Madras, and he worked on an area called quantum foundations.
- SPSpeaker
Okay.
- PMPrabha Mandayam
[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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
Um, we studied early papers on quantum entanglement, and I ended up working with him on something called quantum random walks-
- SPSpeaker
Okay
- PMPrabha Mandayam
... 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.
- SPSpeaker
I want to ask you a question here. Um, I understand that you were exposed to it early-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... uh, which is amazing. Um, but still, it- at the time, you're talking about 2003, 2005.
- PMPrabha Mandayam
Yeah.
- SPSpeaker
It's a very esoteric field.
- PMPrabha Mandayam
Yes.
- SPSpeaker
You're committing your career to it.
- PMPrabha Mandayam
Right.
- SPSpeaker
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?"
- PMPrabha Mandayam
So I should tell you that I was one of those people in 2000 who actually did not write the JEE exam.
- SPSpeaker
Okay.
- PMPrabha Mandayam
So I knew that I wanted to do physics-
- SPSpeaker
Okay
- PMPrabha Mandayam
... for whatever reason, and at that point, uh, maybe the only option was to go for an integrated physics program in IIT Kanpur.
- SPSpeaker
Mm.
- PMPrabha Mandayam
And then there was all this conversation at home. You know, everyone in class was going for JEE coaching-
- SPSpeaker
Mm
- PMPrabha Mandayam
... in Chennai. You know, you're also a Chennai person.
- SPSpeaker
Yeah.
- 1:04:05 – 1:10:00
Do Women Pursue Quantum Computing Studies?
- PMPrabha Mandayam
into this field today if you're an engineering physics graduate. Yeah.
- SPSpeaker
Nice. Thank you so much for all of that. Um, I've already occupied your time a lot-
- PMPrabha Mandayam
Yeah
- SPSpeaker
... 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.
- PMPrabha Mandayam
[chuckles]
- SPSpeaker
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?
- PMPrabha Mandayam
At present, at the moment, not yet, right?
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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.
- SPSpeaker
Okay, so that's, that's 20, 20.
- PMPrabha Mandayam
A little bit better.
- SPSpeaker
Yeah.
- PMPrabha Mandayam
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-
- SPSpeaker
Supernumerary seats.
- PMPrabha Mandayam
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?
- SPSpeaker
I mean, there, there's data on this, because in CBSC, women outperform men all the-
- PMPrabha Mandayam
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.
- SPSpeaker
Mm.
- PMPrabha Mandayam
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-
- SPSpeaker
Mm
- PMPrabha Mandayam
... that now you need to do this, this, and this, right?
- SPSpeaker
Mm.
- PMPrabha Mandayam
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?"
- SPSpeaker
Interesting.
- PMPrabha Mandayam
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.
- SPSpeaker
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-
- PMPrabha Mandayam
Right.
- SPSpeaker
There's a lot of women in it.
- PMPrabha Mandayam
Right.
- SPSpeaker
So, uh, by default, they choose fields where there are more women. So I think in these ca-
- 1:10:00 – 1:11:01
Closing Thoughts
- SPSpeaker
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?
- PMPrabha Mandayam
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-
- SPSpeaker
Mm
- PMPrabha Mandayam
... last you throughout. So that's my... Yeah.
- SPSpeaker
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.
Episode duration: 1:11:06
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