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Â Hi everyone.

Â We are here at the University of Illinois and we are talking

Â with Professor Paul Kweeot, who is a professor here in the Physics department.

Â And this is us playing ten question, or twenty questions about quantum computing.

Â Thank you Paul for talking with us.

Â >> You're welcome.

Â >> Please introduce yourself for our audience.

Â >> So yes, I'm Paul Kweeot.

Â I'm in the Physics and also Zero-Time Appointment and

Â Electrical Engineering Department here.

Â I've been here since 2013.

Â And my area of specialty is quantum information,

Â experimental quantum information mostly with with optics, with photons, and

Â a lot of things with quantum communication and a little bit with quantum computation.

Â But I teach a quantum information course here.

Â So I guess I'm hopefully qualified to at least tell you what other

Â people know about this topic.

Â >> And you've been here since 2003, right?

Â >> 2001.

Â >> 2001, right.

Â So you work in quantum computing, obviously.

Â And I'm wondering if you can tell us what the differences are between quantum

Â computing and the traditional computing that have been around for many decades.

Â >> Yeah. So I guess I'd

Â say that there are two sort of main differences one could think about.

Â So first of all main, the classical computing was based on bits,

Â things that can be zeroes or ones.

Â And I happen to have brought here and example of something that could be a nice

Â bit, something that could be a zero or a one.

Â Obviously in a normal computer, it might be some voltage on a com,

Â on some capacitor plate or something like that.

Â 1:55

Okay we'll come back to that in a minute [LAUGH] because that's a problem.

Â So that's one very different thing.

Â And so there's a lot more information in a quantum bit than in a classical bit.

Â You can think of, sorry, I don't have too many of these props.

Â But, there,

Â we can write a general quantum state of as a combination of a zero and a one.

Â And it can have arbitrary complex coefficients

Â as long as the sum of the squares of these two coefficients is equal to one.

Â And so you can think of that as being represented on a because I have two,

Â two coefficients here, and because the sum of the squares is equal to one,

Â that corresponds to two random, two parameters, basically.

Â So you can think of a quantum bit as represented like a longitude and

Â a latitude somewhere on a sphere.

Â So if you're at the top of a sphere, for example, you could call that a zero,

Â and if you're at the bottom, you could call that a one.

Â And you can have, 0 plus 1 or 0 minus 1 or 0 plus i1,

Â basically anywhere on the surface of this sphere.

Â So that's a lot of information compared to classic.

Â 3:42

And also if you combine different gates together,

Â qubits together, where we have two qubit gates, or something like that.

Â Then you can generate much more interesting states,

Â so-called entangled states.

Â And so, just as an example of that,

Â if you had a classical computer that had three registers there.

Â We'll do three registers.

Â With three registers in a classical computer, you would have one of eight

Â numbers because you could have 000 or 001 or 010 or 011.

Â Okay, 2 times 2 times 2.

Â One of eight numbers.

Â But in the quantum case, you actually get.

Â I can do this.

Â You'll actually get all eight numbers simultaneously in

Â the computer at the same time.

Â Again ignore the fact that these are decohering.

Â These are dropping out of that, for the moment.

Â So, with just three registers, that's not so impressive.

Â But if instead you had 300 registers which, you know, a normal,

Â your iPad or something has millions of registers in it.

Â But with just 300 quantum registers,

Â you can encode up to simultaneously 2 to the 300 numbers.

Â And 2 to the 300 is like 10 to the 100 or something like that and

Â that's more than the number of particles in the universe.

Â So even if you were to count every electron,

Â you've run out of particles before you get to the number of states you could

Â simultaneously put into into a quantum register.

Â And, so that's going to enable a tremendous speed-up in solving

Â certain types of problems.

Â And, in particular I, I guess the funny way of putting it is

Â that there's an exponential speed-up on an exponentially

Â small number of problems because it's not, it's not good for everything.

Â But for some things, it definitely has a, has a huge, a huge benefit.

Â >> So, essentially when we do pattern computing say with four processors,

Â it might get sp, sped up four times.

Â It might be four times faster.

Â Essentially what you're saying is with a quantum computer, you could be as fast as,

Â as you possibly can because its essentially an exponential speed-up.

Â >> Yeah.

Â It would be, well, it would be like 2 to the 4 times faster.

Â >> 2 to the 4 times faster.

Â So we hear this term called qubits-.

Â >> Yes.

Â >> In quantum computing, can you say what it is?

Â >> Well a qubit is just the term of a quantum bit.

Â So any, any representation of this two-level system, so bit,

Â but that can be in superpositions of the two at once.

Â And so maybe I'll talk about how you might get that physically in a physical

Â thing, yeah?

Â So the simplest example maybe is

Â I guess there are sort of two that are pretty easy.

Â One is if you from chemistry, you know, there in atoms, there are electrons, and

Â electrons have these different orbitals.

Â And so, for example, you could call, if the electron is in it's lowest orbital,

Â you could call that a zero, and if it's in some excited orbital,

Â you could call that a one.

Â And you can put the electron so that it's in a superposition of zero and

Â one at the same time.

Â It's in a superposition at both of these locations at the same time.

Â If that seems really impossible or weird to you, I would say yes, true.

Â But already the fact that when it was in the zero state the electron was already in

Â all these different locations around the nucleus at once anyway.

Â So, in some sense, it's not really any, we've already had the weirdness put in and

Â now the question is, can we use that to do something?

Â That's one possible way of getting a, of encoding a cubit.

Â Another, other things that people have looked at is in superconducting coils,

Â you can have the current going clockwise being a zero and clock,

Â counterclockwise being a one.

Â And then you can get the current of electrons to be both going clockwise and

Â counterclockwise simultaneously.

Â I deal with optics a lot and so we can use properties of photons

Â the one that we often use is the polarization of the photon.

Â You know, whether that's horizontally or vertically polarized.

Â Just like you would get with 3D glasses in a movie theater.

Â And you can make superpositions of, say, horizontal and vertical and

Â that would give, say, something was diagonal, and so

Â it's real easy to see how you get those superpositions.

Â Or if there's superpositions where there's a 90 degree face shift,

Â then you get circularly polarized light.

Â And that, that's how the not the IMAX 3D glasses, but the other one, the Real 3D or

Â whatever, they use circular polarization.

Â So it's basically, the qubit is basically any

Â quantum system that can be mapped onto a two dimensional space.

Â 8:05

But in a way that's maybe a lot more controlled than you would normally have

Â with the actual system that you're trying to study.

Â So certainly, quantum simulation is something that people look at a lot as a,

Â as a possible application.

Â And I'll, I'll come back to that in a little bit.

Â The main one that really started the whole field, though,

Â is so-called Shor's algorithm.

Â So Peter Shor,

Â the theorist I think at IBM at the time, who figured out that you could

Â factor large numbers into their prime constituents using a quantum computer,

Â and wouldn't require the exponential speedup that we currently have.

Â So currently with classical algorithms, it's an exponentially, as you,

Â as you increase the number of digits then the time to do the factoring increases

Â exponentially.

Â And with a quantum computer, it, it doesn't.

Â It decreases.

Â It's, it's only polynomially.

Â And so there's a huge interest in that because,

Â of course, of that difficulty of that problem, or the perceived difficulty,

Â is what a lot of our current classical encryption is based on.

Â Like, RSA encryption is based on the fact that it should be a hard problem

Â to, to solve.

Â It's not mathematically proven that it's hard to,

Â that that's a so-called NP-complete problem, or

Â that it's a, it's a difficult problem, but it's, it's strongly believed to be.

Â So at the moment, what's driving this is this factoring sort of thing,

Â in terms of quantum computing, and this possibility of doing quantum simulation.

Â >> You mentioned quantum computing

Â being used to potentially break RSA cryptography, and is, is it possible to

Â use quantum computing to come up with new ways of encrypting and decrypting data?

Â 9:39

>> There are two answers to that.

Â The answer is yes, in both cases.

Â So first of all, people try to find algorithms.

Â Classic algorithms that are not susceptible to this exponential speedup.

Â And so if you use, I think it's elliptic curve cryptography?

Â I think it's that.

Â I don't know.

Â You're the computer science person.

Â And, maybe also Diffie-Hellman en, encoding.

Â Those, you only get the square root of speed, the square root of N speedup,

Â because it's like a search algorithm, basically.

Â So that's not such a big speedup.

Â So one thing is to try and redo your classical cryptography in ways that

Â are not so susceptible to quantum computers.

Â The other thing that you can do is quantum cryptography, which is actually

Â mostly what my group is involved with, not so much quantum computing.

Â And that's basically using single photons to,

Â to generate a random key that you can then use to encrypt information in.

Â Because you have a single quantum system, and

Â it's very fragile if you make measurements on it.

Â So it's very fragile in the sense that this thing doesn't,

Â you know, doesn't like to stay spinning, in that sense.

Â [SOUND] you can encode information in a way that you can detect it.

Â There's an eavesdropper who's been trying to monitor that information.

Â And then you, you simply never, you never have any transmissions

Â that are encrypted using a key that an eavesdropper could have

Â looked at without you knowing that an eavesdropper looked.

Â So either you know that no eavesdropper looked at it, in which case it's

Â secure and you can use it, or maybe you know that an eavesdropper did use it,

Â did look at it, and then you just never use that key to encrypt anything.

Â So in that sense, it gives you perfect,

Â perfectly secure encryption that is not breakable by a quantum computer.

Â Because basically, it's just using simple one time pad encryption,

Â where you just have your message and you add to each bit of the message.

Â You have your key, which is a random string of zeros and ones.

Â And to each bit of the message, you just add, you know, a bit of the key, modulo 2.

Â This is a, this is a string of zeros and ones that you want to send.

Â This is a random string of zeros and ones.

Â When you add these random string to this string,

Â you get a completely random string.

Â You just post that on the internet, or you call up the person,

Â or you, you put it on Facebook, or whatever.

Â And if the recipient knows the same key, then they can undo the process and

Â they get the original thing.

Â Because what's being transmitted over the internet

Â 11:57

is just a random string of numbers, there's no, there nothing to be broken.

Â There's no code, there's no structure in it at all.

Â It's just a random string of numbers, so that it's never going to be possible to

Â to do some sort of processing on that to figure out what the original thing was.

Â Now, if you don't do the implementation, the, so the whole, quantum cryptography is

Â all about distributing this key between the two players.

Â And if you don't do that right, if there, if there are, what will I say?

Â Loopholes in your experimental setup, then you can hack that kind of a system.

Â And that's also been done in the last couple years,

Â where people have started hacking quantum cryptography systems and finding out,

Â okay, there's a big difference between theoretically drawing the protocol and

Â then how you actually implement it.

Â And is, are, you know,

Â are you leaking extra information when you're implementing it?

Â >> So, so in all of this, it seems like there's a lot of ideas in quantum

Â computing and quantum cryptography.

Â 12:47

How far are we from seeing a truly powerful quantum computer, one that is,

Â say, similar to one of our data centers, or maybe even weaker than that?

Â >> In terms of that, how, how long is it until we have like a universal quantum

Â computer, or something like that, or one that could factor some very large number?

Â Yeah, there's a physicist, Bill Phillips,

Â who got the Nobel Prize a number of years ago.

Â He used to say that it's a 50-50 proposition with quantum computers.

Â There's a 50% chance that in 50 years we'll have one.

Â So, let me just put that in perspective.

Â We now have 14 or 15 cubits, that's the world record.

Â And then you might say a little about this other company, D-Wave,

Â that's been in the news a lot.

Â For universally accepted and recognized quantum computing efforts, this record is

Â about 15, with maybe hundreds of gate operations, something like that.

Â If you want to do quantum simulation, you need to have of order 30 or 40.

Â And so, 30 or 40, we're, we're going to have that in, in five years or ten years.

Â I don't have any doubt about that.

Â Whether or not we can get to this large number of gates that we need to do

Â a useful simulation, that's a different question.

Â And in order to do quantum factoring, then as I say, you need 50,000, or

Â something like that.

Â So, you know, there are people working on a lot of different efforts.

Â A lot of different systems using ions, or superconducting cubits, or photons, or

Â something like that.

Â The, the main difficulty, if I can just go back to my example here.

Â The main difficulty, assuming this doesn't fall off the table,

Â is that that doesn't like to stay in a quantum superposition.

Â [SOUND] It interacts with the environment.

Â In this case, it's interacting with the air in the room, and

Â also with friction on the table.

Â And that's causing it to collapse into beating into either a zero or

Â a one, not the superposition of zero and one.

Â This, this is obviously just an analogy, but

Â this happens with every quantum system, that it doesn't like to stay

Â in this quantum superposition because it's interacting with the environment.

Â And so, you might say, well, okay, that's fine.

Â I could certainly, I could put this in some kind of evacuated chamber so

Â it's not interacting with the environment, and somehow magnetically levitate it, or

Â something like that.

Â That's true, you could, but there's another thing you need for cubits to do.

Â You, they have to be able to talk to each other to do these gates.

Â So in this analogy,

Â it would be the, the spinning of this one- [SOUND] impacts how this one spins.

Â Which, which actually it does, a little bit, but

Â we need to interact on a really strong level.

Â So they have to inter, interact really strongly with each other and

Â not with the environment.

Â So then they say, okay, well that's fine.

Â I just take these, I'll put them up in space somewhere,

Â they're completely isolated, and they just interact with each other, and that's fine.

Â Except for the fact that there's two more things that you need for any,

Â any computer at all.

Â And that's, you have to have input and output.

Â So you've gotta be able to input into your computer,

Â 15:28

what's the number you're trying to factor, for example,

Â or what's the, what's the molecule you're somehow trying to simulate?

Â And then, at the end of the day,

Â you need to make this read out of a single quantum system out.

Â And here, it's really easy because I just- [SOUND] stuck my hand down, and I,

Â and I see this.

Â But if you're actually trying to read out what the state of a single electron is,

Â a single photon is, something like that, that tends to be a lot more,

Â you need to have strong interactions to do that.

Â So basically, we need to have very strong interactions.

Â With the qubits to put the information in,

Â we need to have no interactions with them while they're interacting,

Â while they're doing their dance and, and running the algorithm.

Â And then at the end, we need to have very strong interactions again.

Â And those are just hard, it's hard to do that.

Â This interaction with the environment that's known as decoherence,

Â it's the technical term for

Â that and it happens with any physical system that we know about.

Â So that fact of decoherence is a problem.

Â So, you need to do some sort of error correction

Â 16:36

if you, assuming that, if they all three agree, you say that's the right answer.

Â If two of them agree, you take the two and you say the one was an error.

Â And then by doing that sort of concatenation of that you can really get

Â the errors small.

Â The problem is then in a quantum computer,

Â you're not allowed to measure what the results are in the middle because that

Â stops the whole computation.

Â So you have to do more complicated schemes of encoding, and

Â it turns out you need to have of order, of let's say seven.

Â If you can have seven physical qubits,

Â then you can correct any single logical qubit error.

Â So whether an error, an error could be a flip error, so

Â something a zero becomes a one, or it could become the a, what we

Â would call a phase error which would be that this sort of gets rotated around.

Â Or if you think about this longitude and then latitude, a flip error would be doing

Â this and a phase error would be doing a rotation, an unwanted rotation like that.

Â So if you can encode in seven physical qubits,

Â one logical qubit, then you can, it turns out there are ways of then measuring them

Â in a way that doesn't kill the quantum com, computation, and

Â you can correct those errors and then you can continue.

Â >> That's pretty fascinating.

Â Is if, if one of our audience members is interested in knowing more about quantum

Â computing, is there a good book or a reference that you might suggest?

Â >> If you just Google quantum computing, I mean, there's lots of Wikipedia articles,

Â lots of tutorials on it.

Â I mean, a quantum computing tutorial and there's tons and

Â tons of webpages on it about it.

Â I, I keep wanting to say one more thing about an algorithm thing

Â 17:59

that is relatively new that might be of interest.

Â So there's a problem called the Boson Sampling Problem,

Â which is equivalent to finding the permanent of a matrix.

Â Computer pi, scientists supposedly know what this is.

Â So, I didn't know what it was, but the permanent is like the determinant of

Â a matrix, but you make all the minus signs plus signs.

Â And it turned out that finding the determinant of an n by n matrix is ex,

Â is something that's exponentially hard.

Â And it's now known, it's now believed anyway that if you take

Â that you can map that problem onto a very physical problem.

Â And the physical problem is,

Â you've got a bunch of photons which are particles of light and

Â they come into a network that has a bunch of beam splitters that connects things.

Â So the beam splitters are just like this piece of plastic that I had where

Â the light sometimes goes through and sometimes is reflected and

Â you make this complicated web and it's got maybe n inputs and n, n outputs.

Â And the question is for given inputs in here, and for

Â given basically depending on what the reflectivities and

Â transmissivities of each of the elements in this network are.

Â What's the distribution of where the things are going to show up?

Â So what's the probability distribution?

Â And that is mappable on to this permanent problem which is known to be

Â exponentially hard.

Â And yet the quantum case seems to just solve it.

Â You just send things in and you see where the photons show up, and it solves it.

Â And the reason that's relevant is supposedly then that would violate

Â the extended Church-Turing thesis, because you'd be able with the, with the system,

Â you would have a system that would be able to solve an exponentially hard problem.

Â And so that is something that's of great interest at the moment,

Â both from a theoretical point of view, and also that requires

Â of order maybe 30 photons or something like that, or 40 photons.

Â And so I guess in the next five or ten years that will also be done.

Â And I don't know,

Â either the quantum mechanics will somehow break down which would be impossible, or

Â the, the Church-Thuring, Church-Turing thesis would break down,

Â which I think the computer scientists would also say [LAUGH] is, is impossible.

Â So, so something's going to fall and that's, that's kind of interesting.

Â >> So I wanted to wrap up with a question that's changing gears a little bit

Â not really too technical, your technical work.

Â But you know your, your work in the university for

Â over a decade now, before that you were at a national lab.

Â >> Los Alamos National Laboratory.

Â >> Right.

Â So I wanted to ask about you know, how do you think what the,

Â what is the difference between working in these two different environments?

Â >> So one thing is there's more bureaucracy at a national lab.

Â There's no question about that.

Â And we have colleagues who come to us from NIST, National Institute of Standards and

Â Technology.

Â You know, and they have also travel restrictions and computer restrictions,

Â and they have to change their passwords every two months.

Â There was a talk once where someone couldn't remember the password to

Â his computer and that was really kind of annoying.

Â 21:27

On the other hand, things are a lot more expensive at a national lab, and a lot of

Â times you know, you have to get funding to work on whatever project you're doing.

Â Same thing here, but also as professors we're paid,

Â you know, to teach during the semester, during the year as I should say.

Â And that covers a lot of our salary,

Â whereas if you're really working at a national lab,

Â you have to find some sponsor who's, unless you're working on whatever,

Â it kind of depends on what kind of project you're working on at the national lab.

Â But certainly places, you know, like NIST are really fabulous.

Â I would, I would love to work at a place like NIST.

Â Not, I wouldn't really love it more than working here,

Â but I certainly wouldn't mind recommending you know,

Â people to go to someplace like that or Sandia National Lab.

Â It's really a, a good experience.

Â 22:59

But it's certainly an opportunity that's worth looking into.

Â >> So your advice to students would be don't stop asking questions

Â even if you think they're, they're stupid questions.

Â >> I, I, I've very rarely seen stupid questions.

Â Unless there's procedural things about, is the homework really due, blah, blah, blah,

Â or something like that.

Â But in terms of the science things, people I mean the questions might just reveal

Â that we really haven't given quite enough information in lecture.

Â And so, you know, people are trying to make some logic jump and

Â they don't quite have all the pieces, or they misunderstood something.

Â I mean all of these topics, whether it's, you know, quantum physics, or high end,

Â you know, computer science, they're not, they're not easy topics.

Â And so everyone's trying to put this scaffold of knowledge together in their

Â head, and sometimes it doesn't, you know, it doesn't quite fit.

Â And a question can really cause it to fit, or it can reveal that oh,

Â there's this whole other avenue that can, that can be looked at, so.

Â >> Okay, thank you for talking with us Paul.

Â >> You're welcome.

Â My pleasure. >> Thanks.

Â Professor Paul Kwiat.

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Â