So our plan today is to talk about what quantum

computers are. How people are building them. What they can do. What they can’t do. They’re not all powerful god like devices

so they do have limitations that we’ll get into as well. OK. Without further ado let me introduce our esteemed

panelists. Our first panelist is a mathematician, computer

scientist, physicist, expert on quantum information theory. She was a professor at Tel Aviv University

and a researcher at the CNRS in Paris. Pease welcome Julia Kempe. Our next participant is the professor of quantum

mechanical engineering, that’s a thing, at MIT. Director of the Keck center for extreme quantum

information theory and hopefully will explain why it’s extreme to us. Please welcome Seth Lloyd. Next up, our next guest comes to us from the

Air Force Research Laboratory in Rome in upstate New York. She’s a senior research scientist there and

the primary investigator for the Trapped-Ion Quantum Networking Group. Please welcome Kathy-Anne Soderbergh. And finally coming to us from IBM in Yorktown

Heights, just north of the city, is the Manager of the Experimental Quantum Computing Group,

distinguished research staff member. Please welcome Jerry Chow. So Seth, let me turn to you first. Quantum mechanics: weird or just unfamiliar? Definitely weird. It’s, I prefer the word funky actually. OK you heard its. This is the official terminology. It’s the James Brown of sciences. How so? I mean why and particularly in terms of computation

what is it? What’s the special quality of quantum physics? So in quantum mechanics, things that we think

of being like particles, like basketballs or soccer balls, have waves that are attached

to them. And so you know I have a ball over here and

it’s got a wave and then I have a ball over here and it’s got a wave. But the funky thing about that is that the

waves can add up so I can have a ball that’s both here and there at the same time. And if you map this to a bit in a quantum

computer, so this is zero, ball over here, and ball over there is one, then I can have

a quantum bit or qubit that is zero and one at the same time. So Julia, why would that be something you

would actually want? Why would this, I want to actually comment

on the weird first if I may. Funky please. Funky. Because I think it becomes a, it depends on

your point of view coming at it as a physicist with a lot of training in classical physics

it’s indeed probably very weird, but if you look at it as a computer scientist it maybe

becomes less weird because we are not spoiled. Our intuition is still, that it’s not, how

can I say, biased in any way. And I view it as something which is like probability

theory except the probabilities can be negative or they can be even complex that that is not

so essential. And so in that sense it’s not very weird. It’s just gets, requires a bit of getting

used to but it’s pretty natural. And so for the qubit, why is it useful to

have qubits in being both zero and one at the same time? Cause if we have many of those, we can think

of having various states at the same time and we can think of computing all these possibilities

at the same time. So that leads us to this massive what’s called

quantum parallelism. So Jerry, can you walk me through like just

a quick numeric example, you have a certain number of qubits, what that means in terms

of parallelism? Yeah. So I mean one of the, as has been mentioned

with regard to these qubits, you have these superpositions. And so you might have some number of bits

but instead of bits now you have qubits. Right. And in terms of this parallelism what you

can actually have is access to a much larger space of possibilities. So if you have n qubits you actually have,

using these principles of superposition and entanglement, you have access to a space of

up to two to the n possibilities. And so that type of, that type of exponential

space gets really, really large for rather modest numbers of qubits. In fact if you get to around n of three hundred

cubits you actually have some state-space that is greater than the total number of particles

in our universe. Kathy-Anne, walk me through an extremely simple

numerical example…suppose I, in fact we do have, but to take an example two of these

qubits. What does that mean in terms of the computational

space that we can work with? So if you have two qubits you have a four

qubit state space because you can have each one stored as a zero plus one and the other

stored as a zero plus one, so you get 00011011. Whereas classically you can only have zero

or one. You can’t have anything in between. So the computer’s in a sense an all possible

computational state space. Yes that’s right. Now Seth, on the face of it that doesn’t sound

like such a good idea because you want to get an answer from the computer and it’s just

basically telling you everything. Yeah and indeed it’s kind of dangerous. If I have a quantum bit that’s zero and one

at the same time and I say yo are you zero or are you one? Well you know you could, OK, so the electron

is over here and I bring up a very sensitive electrometer that says, yo, are you here or

there? Well it’s either going to show up here, say

with fifty percent probability or there were fifty percent probability. So it’s just going to behave like something

that’s generating a random number which is kind of useful but it’s not, if you actually

want certainty for your answers that’s not so great. So the kind of the way that these quantum

computations work is you set up all these waves and they’re wiggling on top of each

other and they’re, they’re performing multiple computations simultaneously. So you can think of an individual wave, a

wave of say electron here and not there, that’s kind of like a pure tone like ahh. An electron here and not there is like ahh. An electron here and there at the same time

is does somebody want to supply the other tone? It’s a chord. So it’s you, you get the computational power

from the interference from the kind of symphonic nature of this. So the idea of a quantum computation: you

set up all these waves, they make this beautiful music together in this symphonic way. But at the end of the day you actually want

to have an answer that says yes or no, or zero or one, and all the trickery and talent

goes into making that happen. So Julia, is that, any problem I care to pull

out of a hat or that my professors give me as a homework assignment. Can I turn it into a problem that’s amenable

to this kind of chord pattern that then reduces to a single pure tone? That might depend on your artistry but in

general quantum computers are good at certain things and we would leave, you know, a lot

of other things to our normal classical computer. And of course you probably all heard of the

problem that a quantum computer can solve very well. And that’s factoring numbers, large, very

large numbers into primes. And the way it’s done, just like Seth was

saying and I was saying, you can think of these amplitudes of a quantum computer as

positive and negative numbers. We try to arrange these waves in a way that

all the bad answers cancel out because you can, you know ,you can have several ways to

arrive at the wrong answer. And what you’ll try is have some with a positive

amplitude and some with a negative amplitude and they cancel out and then you arrive at

the right answer and you, voila you get a factor for a very big number. And that’s one of the first and most remarkable

things that was discovered a quantum computer can do. And I can elaborate on why it’s important. Most of you, all of you, I assume, are using

credit cards at the machine or over the Internet and you rely on the fact that they’re encryption,

that they are encrypted. And it so happens that the modern day encryption

is based on assumptions of hardness of factoring or problems of that type. And it’s these type of problems that a quantum

computer would be able to easily break. Interesting. Kathy, let me ask as you to drill down a little

bit more here. Sure So when I think of a computer I think of it

has, does arithmetic logical operations. How do you, I mean without going into details,

we’ll get to some of the details later, but does the quantum computer also have those

same elements of it can add, it can subtract, it can do logical comparisons of things? Yes, yes you need all of the same similar

components to a classical computer to do quantum computing. But the way that you make the gates looks

very different than in a conventional computer. And that somewhat depends on the underlying

qubit technology. OK I’m going to definitely come back to the

details on that later. I just want to establish that it’s a computer. Yes It has, you look at it and though it’s configured

differently, does have the recognizable qualities of a computer. Yes You can program it in C or Java, and it has

one additional instruction which says take this quantum bit and put it here and there

at the same time, put it in the state of zero and one at the same time. So you supplemented ordinary computer language

with just one additional quantum instruction and you’re good to go. If you can build the thing of course. Which we’ll see in a little bit. Jerry, what are we up to just in general state

of the art about how many qubits we have and what does that mean in terms of vis a vis

a classical machine. Yeah. So there’s many different physical implementations

of building a quantum computer. The underlying core of a quantum computer

we call a quantum processor. And what you basically need to build the quantum

processor is something that follows the laws of quantum mechanics and can have this quantum

mechanical zero and one. And in terms of where we are in experiments

we’re looking at building universal based quantum computers of order of ten to twenty

qubits at the moment. And that’s, that’s kind of where the state

of the art is in the field. And just for reference sake, twenty bits doesn’t

sound like a lot actually, but how does that what kind of power does that endow a machine

with? Well it’s actually very interesting because

although you might have twenty qubits you can actually then have the state space up

to two to the twenty possibilities, right? But how much you’re actually able to access

that then determines, is determined by your coherence time. So there’s a metric known as coherence time

which says how good of a quantum state can you actually keep in your quantum processor. And different types of technologies varying

from superconducting to trapped-ions, like the kind Kathy-Anne works on, have different

amounts of coherence time. So overall this type of time times the number

of qubits, we try to we try to at least my colleagues that we’ve only started thinking

about a metric for this called quantum volume to kind of describe what is the power of a

quantum computer. So walk me through. What’s a quantum volume? You can basically think of it as how many

steps of these logical operations, or these, these gate operations that you can do the

superposition or entanglement steps in the amount of time that before all the quantum

information is gone, becomes just classical. And so you have a certain number of steps

and then you have a certain amount of depth in terms of the total number of qubits that

are connected to one another. So Julia, let me ask this of you. If I just have a phone, a classical ordinary

computer and I store information in it, one hopes at least it will be able to retain that

for a long period of time. Is Jerry saying that actually it decays away? And why would it do that inside a quantum

computer? So the big challenge for a quantum computer

is indeed to maintain these coherences or these waves that are spread over, not just

you know one qubit but over a collection of, in this case, perhaps twenty qubits. And what we call entanglement these, these

correlations at a distance of these qubits. And it is true that this is what nature does

on a very small scale when we describe electrons and so on. But it’s also true that we don’t observe this

in our everyday life. I mean when you have a bit you have a big

it’s either zero or one and the reason we don’t observe it is that once we start interacting

with the environment, once this very fragile superposition is being subject to the surroundings,

it’s being subjected to noise. And these very fragile superpositions will

start to what we call decohere, so just disappear. And the point is of course that we need to

be able to address this quantum computer. We need to be able to talk to it. We need to be able to manipulate it. So it has to be exposed to us, to our you

know to the world in that sense. And so we’re living in this tradeoff situation

where on one hand we need to protect the state. So we would like to just put it in fridge

and never touch it. On the other hand we have to touch it in order

to manipulate it. And this is the big challenge that, you know,

these experimentalists are facing. To battle this decoherence invariably comes

along with the fact that we’re exposed to the, you know, to the environment. Kathy-Anne, walking into your laboratory,

what would we see and then walk me through what that represents. Sure. So I’ll talk about trapped-ion technology

which is what we work on and I believe Jerry will talk about superconducting qubits in

a few minutes. They’re very different technologies but they’re

both very advanced right now in the field. So for ions we track single atoms and we hold

them, they’re they’re charged so you can hold them using electric fields. So first we prepare a vacuum chamber, because

as you just heard these systems are very fragile, so you need to protect them from the environment. So these trapped-ions operate at room temperature

but we hold them in a vacuum chamber roughly ten to the minus twelve Torr – it’s the same

vacuum as outer space. So the only thing in there is the atoms that

you want to manipulate. And you have a neutral atom source which is

just a piece of metal in a stainless steel oven that you heat up and then that creates

a beam of neutral atoms which you can put in your, what’s called an ion trap, which

is just a collection of metal electrodes. Because as I said we trap these using electric

fields since they’re charged. So you put an oscillating electric field on

that trap. What that looks like to the atom is a rotating

saddle. So then that looks like a bowl. And if you drop a marble in a bowl eventually

it’ll come to rest at the bottom of the bowl. The trapped ions do the exact same thing and

these potentials. And so we shine the neutral atom beam near

the trap and then we have a laser that actually rips one of the electrons off the neutral

atom and that makes our ion. So that leaves it charged. And at the same time we have to shine a different

color of light in, because coming straight out of an oven the atoms are essentially screaming

hot and the trap potential just can’t catch an atom it’s going that fast. So you have to cool it down a little bit with

a laser, its laser cooling. And then that allows you to trap it in this

bowl like potential and then we shine yet another color of light on the atom and all

these different colors of light create different transitions within the atomic structure. So if you could look in the atom you would

see different energy levels inside and each color of light is resonant with a different

energy level. And so the detection light when we shine it

on. It hits a very strong transition in the atom,

which excites it from it its ground state, so the qubits themselves are held in the ground

states of these atoms. There’s two ground states. And it sends it to an excited state. It’s a very short lived state. And when it emits it emits a photon and it

does sense that this strong transition it does that hundreds of thousands of times and

then we collect those photons on a camera. The ion we’re using is ytterbium so it emits

a UV. If it was a visible color you could actually

see it with your eye, it would be a tiny speck. Hang on. You can see atoms. You can see atoms. See atoms. What do they look like? Just like when you shine a flashlight on a

ball in a dark room, right, it scatters light and then your eyes can see it and you say

‘oh there’s a ball sitting there’. The atoms do the same thing. They emit, they emit photons that if your

eye was visible through the UV you could see them. It would just be a tiny dot on a very dark

background. You can see a single atom fluorescing. The darkest, you know that the brightest star

on a very dark sky. Unfortunately we can’t see in the UV with

our eyes so we have to use a camera. But if you were to walk into our lab you would

see large optical tables that are about six feet long by four feet wide filled with lasers. Because to do all the different operations

you need different frequencies of light. And then you’d see another vacuum chamber

that holds or another optical table excuse me the holds our vacuum chambers. OK. So let me just see if I follow. You load your system Yes With ytterbium ions? Yes. And to perform a computation just, for example,

how do you clear the memory? What would be the first step in your computation? Sure. So usually you start with some number let’s

say between two and five ions is what you’d want. So you’d load two or five ions, let’s say

two for this example. So we can turn the oven on for a set amount

of time then we shine the laser that takes that rips the electron off to create the ions

and we wait till we get two ions. And then we can see them on the camera. And so we initialize the system to a zero

state just like in conventional computing you have to initialize your computer to zero

state, and then if we wanted to put those two ions in a superposition we could shine

either a laser or a microwave at them and that would create a superposition. So you could think of a qubit on what’s called

a block sphere, which is just a unit sphere where the up z axis could be your one qubit

state and then down z axis could be your zero qubit state. So you’re prepared it in a zero, and then

we shine these microwaves or lasers on the atoms and it causes the population to rotate,

basically. And so you just stop when you get to the upstate

and you can look at that, it’s a trace on a scope. So for example I mean a standard operation,

simplest possible operation you might have in a computer system is a ‘not’. That’s right. So how would you do a ‘not’? So you prepare, you would prepare your qubit

in the zero state then you would shine a microwave a laser beam on it for a set amount of time

and it would cause the population to evolve to the up state, and that’s a ‘not’ gate

if it go from zero to one that’s a ‘not’ gate. If you let that light or microwave interaction

on and it would go back down to the zero state and it would just keep rotating. So suppose I want to do something a bit more

sophisticated like an ‘and’ or something that actually combines two qubits. How would you how would you do that? So if you had two qubits in trapped-ions,

the nice thing is that because they’re charged they want to repel each other, but because

there’s a trapping potential on them they get pushed together so they find a happy medium

where they sit. But they have a shared motional mode due to

this interaction. So there are a lot like a Newton’s cradle. If you pull a ball in a Newton’s cradle you

see all the balls move together. That’s right. That’s right. And so the trapped ions do the same thing. If you start to shine a laser beam on one

and you excite some motion and actually excites motion in in both ions. And so then you have a databus that you can

get the ions to talk to each other. And if you had five ions you could actually

use this databus to get one in five to talk to each other directly. So you’re not limited to your nearest neighbor

interactions in a trapped ion system. And you can use that to combine them. You can use that combined motional mode to

get the qubit states to talk to each other and create things like controlled ‘not’

gates, say. Great. So the Jerry, can you kind of repeat that

kind of virtuoso performance for your own lab? Yeah, I’ll do my best there. But what’s it like in there actually? So our lab looks a lot different from what

Kathy-Anne described. And that, that the reason for that is because

the underlying qubit is very different. One difference that, the main difference is

that instead of actually having physical, naturally occurring qubits, in this case ytterbium

ions, that you can you know that all of this work is based off of having a really, really

stable atomic clocks. What we’re doing with superconducting qubits

is to actually engineer and build them on a chip. So it’s a little more integrated. You’re actually using lithographed techniques

that you know and love today with your silicon processors. And instead of the materials that are in your,

in your in your chipset or in your phone or your laptop we’re using slightly different

materials to build superconducting circuits. So superconducting refers to materials that,

that when they’re cold they have basically no resistance. And by using the right kind of superconductors

you can actually build quantum effects into circuit elements. So with Kathy-Anne, I have a good picture

for what the bit is. The ion is either pointing up, or you know

rotating that direction that corresponds up, or the other way so what’s the corresponding? Yeah, so the way to think about it here is

that you’re actually building an oscillator circuit. So if you if you go back to your electrical

engineering days think about the circuits that you might build with resistors or capacitors

or inductors, these are varied circuit elements. In the case of a superconducting circuit you

actually could use an element known as a Josephson junction. And a Josephson junction is basically a sandwich

of aluminum, aluminum oxide, aluminum. And what’s phenomenal about this this element

is that you can combine it with it with a standard capacitor and you can make it oscillate

in the microwave regimes, so around five gigahertz and choosing the right parameters of the capacitance

in the Josephson injunction you can isolate it to build a qubit state, so zero and one,

that that resonates at around five gigahertz. So in your case can you walk through an example. You load your computer… Right. So in this case, in this case where we have

a silicon fabrication facility that builds these circuits, we, they come out in large

wafer form and then we have to cut them up into smaller chips. These chips are packaged into a printed circuit

board like what you might see in inside your phone. But this printed circuit board carries microwave

signals and so those the printed circuit board then needs to be cooled down to really, really

low temperatures to basically have the qubits function properly. So I said that we use the superconducting

materials. And so the materials are niobium and aluminum. And for them to superconduct and for there

to be so little noise that we can actually see these quantum mechanical effects at five

gigahertz, we need to cool down to fifteen millikelvin. So that’s, since you already brought up the

space analogy, it’s colder than outer space as well. And in fact you know with the microwave background

space is around it’s a little under four kelvin there, but we’re getting down to fifteen millikelvin. Wow And so the refrigeration systems that we built,

that we use they’re commercially available but it is phenomenal that you can just turn

the turn hit a button turn a key and cool down to these these devices to such a low

temperature. So in the example of the trapped ions, if

you want to execute a ‘not’ operation you hit it with lasers or microwaves. What do you do in your case? Yes. In this case it’s more electrically controlled. So you you’re placing this chip inside this

printed circuit boards, it’s inside of the refrigerator, but then you have all these

wires that come down through the refrigerator and those carry electrical signals. And so to do say, a ‘not ‘operation, what

we’re doing is basically applying a shaped microwave pulse that’s generated at room temperature,

so on the set of electronics that sits outside of the refrigerator, we generate a five gigahertz

signal for a certain amount of time say maybe twenty nanoseconds or thirty nanoseconds. That pulse gets sent down into the refrigerator,

applies just enough energy to flip your qubit state from zero to one. And then you could do. How would you do an ‘and’ or ‘nand.’ And then with regard to two qubit gates, so

our particular architecture connects qubits on the chip. So there’s there’s other microwave circuitry

that is used to, to define particularly interactions between qubits on a chip. But then those, those interactions are again

activated using microwaves so just the way that we do the not we might send it we might

send pulses at a slightly different frequency down into the refrigerator to induce a two

qubit operation such as a controlled ‘not’ gate. So everything you’ve described is acting on

the system. So how does this system act on us to return

its information and the result of the computation? Yeah, so in the case of Kathy-Anne they’re

sending another laser beam to do the detection, and you can see it with the camera, but us

what we actually have to do is send a another microwave pulse which is resonant with a detection

cavity, so there’s actually a resonator on the chip that oscillates at a slightly different

frequency depending on if the qubit is zero or if the qubit is one. And so we interrogate this cavity with a microwave

pulse and at a very low energy levels, so single photon energy levels at say six gigahertz,

and so that that signal goes down into the fridge, gets amplified through various stages

and then we basically have to digitize it to determine whether the qubit was a zero

or one. Cool. Seth, just to kind of bring some perspective

on the technical discussion here. What would be some pros and cons of the different

techniques? Why would you use trapped-ions in some cases,

superconducting qubits in others? Well so pretty much anything at the microscopic

level will compute if you shine light on it in the right way, via either lasers or microwaves. But some things compute better than others. So what’s been happening over the last decade

and a half or so is that the technologies for instance superconducting quantum computing

have really advanced by a lot. I mean I was participating in the early experiments

to build I think the second superconducting qubit around 2000. It was a so-called ‘flux qubit’ these

super currents you have a little loop interrupted by Josephson junction. And so super current going around forever

that way you call it zero and super current going around forever that way, counter-clockwise,

sorry, clockwise for you then you call it one. And then you know super current going around

both ways simultaneously both clockwise and counterclockwise simultaneously, that’s 0

and 1 at the same time. So that’s how you get quantum bit in these

things. But you would let them sit for a little while

and then, you know, they’d get kind of completely randomized very, very rapidly. And so these originals superconducting qubits

were, well they sucked let’s face it. That’s the technical term like funky, right? So but then there was this great innovation

actually which Greg participated in, I think this was part of your PhD thesis, this was

developing – people thought oh the materials are bad, something’s wrong with how we’re

building these things. But it turned out that it was really much

more of a design issue and by being really sneaky about how you design these systems

you can make that much, much much, much more coherent so that they could you know you could

have, they could oscillate around or you could perform ten thousand logic operations before

these things got messed up. And so with superconducting systems I think

that what you did in your PhD thesis and afterwards was a really amazing innovation. And then which also allowed, because you building

them of these all these chips you can put many of them together, so there’s a clear

path towards scalability. Similarly with ion traps, the first ion trap

experiments were done in the in the late 90s in the mid 1990s but they were you know two

qubit experiments, sometimes two qubits you can still do interesting things with two cubits,

right, you know. You can search your data space with four possibilities

and you can find is it here or here or here or here by only looking once. Like how can that be, classically? But quantum mechanically. We’re about to find out actually in a little

while. Yeah. So, so what’s happening is that there ,is

that there is a really there’s been also with ion traps there have been all these advances

in integration and making ion traps larger and larger, integrating them with quantum

communication lines. So there’s been a steady advance in constructing

more and more elaborate and complex quantum information processors. Ion traps and superconducting systems are

the two technologies that are furthest along the way. But there are a whole bunch of other technologies

like nitrogen vacancies and diamond topological systems and all kinds of crazy things because

again pretty much anything will compute. And even though as Jerry was saying that the

twenty qubits, OK that doesn’t sound like a lot, but two to the twenty is about a million. Thirty cubits…two to the thirty is about

a billion. Forty cubits that’s a trillion. Well you know now you’re starting to try to

manipulate these, a trillion numbers, a billion or a trillion numbers and actually that becomes

very difficult classically. So the devices that are being built right

now are just at the threshold where we actually can’t understand what’s going on inside them

classically. Previously we were able to simulate what was

happening on a hugemungous classical computer and try to figure out what’s going on. Now we’re kind of on our own and sort of exploring

this quantum frontier and we, you know, we we are going to be able to try to figure out

what’s going on. And then the hope is that when we build these

devices we can use them to build ever-larger devices and build quantum computers that have

a thousand qubits or a million qubits or a billion qubits. So Julia-Ann, the machines, and it’s a great

moment to be in historically to be in, the machines are now crossing over and exceeding

the power of our most powerful classical computer. But then how do we know that they’re working

properly if we can’t even compare the result of the calculation of the quantum computer

to a classical computer any more? And working properly. I mean Because I don’t know about you but my computer

crashes sometimes. I mean how can we ensure that they’re working

properly is maybe a question I can answer because as you ask what, what architecture

will eventually you know win or be the best one and of course the question we need to

answer is which ones scales best for a large number of qubits? And in theory, I’m a theoretician so I you

know I mean I’m in a position where I write my papers saying let’s assume we have a quantum

computer of ten thousand cubits and then but there is a lot of theory developed, a theory

of say quantum error correction for instance, where we face the fact that no matter how

well Kathy-Anne and Jerry perform their jobs, the elements out of which they build their

quantum computers will be faulty at some level. There will be a probability that they’ll fail,

that they’ll lose their coherence and so on. And there actually is a very beautiful theory

of quantum error correction that once we are above a certain threshold with the noise in

their system, so once the noise is small enough, then we can actually build in redundancy into

these qubits, in a way that the computation will flow flawlessly. And that’s a very nice theory that will then

allow us to make the quantum computer work at a larger scale. So to fix the mistakes You can fix the mistakes. Yes. So I think this might be a good point to talk

about how they actually work. And Jerry I know you’ve got a demonstration

you’d like to present to us. Well first, the way that I want to actually

motivate this is based off of this search, a search algorithm and Seth already alluded

to this. But let’s say you have four cards, right and

you play the game of monte or you might go to a street corner somewhere. Not that we’re advocating you do that. Don’t do that. And so out of these four cards you’ve got

one of them which is different, one of them is the queen. And now, now we’re going to flip them over

and when you play this game you’re randomly going to try to find where that where that

queen is right. You’re going to try once and you’re going

to flip over a card and see whether or not it’s the queen. And so on, on in playing this game you really

only have a one in four chance of getting it right on your on your first try. But now what’s interesting about this type

of game is we can also ask well how would we how do we do this if we had a computer? What what what does a classic computer do

with this game and what does a quantum computer do with this game? OK. So in the case of a classical processor what

we’re doing is when we when we flip over these cards you can think of this as as storing

a database. In this case we can also call it an oracle. So you store the database with the hidden

set of cards where the queen is properly located. With a classical computer what you’re going

to do is in order to to find where it is you’re going to look at all the possible arrangements. Right. So you’re going to start with one particular

arrangement. Let’s, let’s start with placing the queen

in the first slot. And we’re going to take that entry, use it

as an input, we’re going to do some processing, in this case the green box where you’re gonna

do some comparison with what’s in the database and you’re going to make a decision at the

end of it whether or not it correct or not. In this case it was not correct and you get

a zero. OK. And so now then you can try the next one. And again you’re going to get to get a zero

and then the next one and this time you get it right. But of course classically you’re going to

go through all four of them. And so after you go through all four you see

that on average you would have gotten this correct, basically you would get a correct

after querying this database about two and a quarter times. Right so this problem of search in this case

with a classical computer you can only kind of do this sequential, sequentially or by

choosing at random. But with a quantum processor, this is where

a lot of the ideas of quantum mechanics can come through. And so in the next slide here with the quantum

processor you have access to superposition. And so just like we talked about how you can

be in zero and one at the same time, what you can do with two qubit system is to make

a superposition of all four of the possibilities: 00, 01, 10, 11, to represent all four of the

possible arrangements of this hidden queen. And so we can take that superposition state,

use it as as basically as an input, call the database just once. Perform some processing step, in this case

the processing step involves entanglement and it involves this quantum interference

of adding adding together the waves. And it’ll amplify the answer for exactly the

right answer. And so every time, no matter what you what

you place into the database, wherever you hid that card, you use only one call to the

database, you get the right answer using this algorithm. And this particular algorithm is known as

Grover’s algorithm. It is a simple case that gives you the sense

for what is done differently in terms of processing information on a classic computer versus a

quantum computer. And so on the next slide what would you actually

would do when you want to program an actual quantum, quantum processor is to use this

language of quantum gates. And so what you see here is actually a quantum

circuit. And as Seth alluded to the idea of music,

we call this actually a score because it kind of looks like a musical score. And the concept of time really in time with

gates really has a strong analogy here because it’s like you’re playing different notes on

these different qubits. What you see here is really just, just two

of the qubits being populated with these different operations, which realizes Grover’s algorithm. And to break it down a little bit further

in the next slide what you, what you see is that these various steps of superposition,

the stored database and the actual post-processing steps are all, are all encoded into these

various gate operations that you can apply. And in this case we can actually run it through

and get , get a result. And I can actually launch this live if you? Please by all means. While we are you switching over? Kathy-Anne, you were really one of the first

people to actually do this for real in your dissertation work. Can you describe what you accomplished? Yes sure. So we did it with trapped ions, we had two

trapped ions and as Jerry just showed you that’ll give you the four element database. And what we did in practice was we had the

computer mark a state and then we would run the algorithm similar to the diagram that

Jerry just showed, it looks very similar and then at the end we would see what the probability

was that we recovered that marked state. And at the time the untangling gate that we

used we were just starting to learn to use it, it had just been demonstrated. And so we found the mark with a probability

of about sixty percent. But Jerry just told you it should be one percent

and that’s because the fidelity, that’s one way you can measure how good a quantum gate

is, the fidelity of our gate wasn’t as high as we would have liked it to be, mostly due

to technical difficulties. This is a fundamental limit of trapped ions. They’ve since repeated this experiment with

three qubits recently, Chris Monroe’s group at the University of Maryland, and they did

quite a bit better because the technology has progressed. Now at this point at these gates are at very

high fidelity near the fault tolerance level that Julia was saying earlier that you need

to run these computers. You were telling me earlier that these are

so delicate that you can so much as look at the laser wrong and it would it would give

you the, it wouldn’t work. Yes so we used cadmium ions in my graduate

work and they, they need laser frequencies that are about two hundred fifty nanometers,

which is an incredibly difficult color of light to generate. You basically have to quadruple a laser to

get there. So you take a laser, you double it and then

you double it again. And doubling’s hard and the efficiency is

low. And one of the people in our lab just had

the right acoustic sound to his voice that he would unlock are doubling cavities and

so sometimes when he came in the room he would start to talk and we were trying to run our

experiment and our laser would shut off. They’re a very fragile system. So Jerry are you ready to go with thIs? Yeah. This is actually a live quantum computer. I wanted to start by just showing a little

bit about the interface of what we have. So this is the IBM ‘Q Experience’ and

what we actually have is a lot of content on there for anybody to get started with learning

how to program and actually use a quantum computer. So anybody can do this? Anybody can log in and sign up for an account. We have this library with various user guides

for beginners, if you’re more familiar with some mathematics like linear algebra and even

another other guide which actually leads you to our Github developer repository. But through this, through this portal you

have access to learning about the basics of a qubit superposition, entanglement, simple

algorithms such as this Grover’s algorithm. And we also have a community board feature

where we have the ability for anyone to ask questions and to, and our IBM researchers

are more than happy to answer. Julia I wanted to go back to what you were

saying about the factorizing problem, that’s in addition to the search algorithm the other

use that people we talk about with with quantum computers, so what’s what’s kind of state

of the art in that when you factor the number fifteen or, or get that high even? You can, I mean Jerry would probably be a

better person, but with twenty qubits you can imagine that you can factor perhaps with

some overhead I would guess you can maybe factor numbers up to a hundred? Which of course you can do in your head. So at this stage we are really at the level

where we demonstrate things when it comes to factoring. The cryptographic systems that I was talking

about that your credit cards rely on these usually have something maybe up to a thousand

bits? So I think once we get a quantum computer

to the order of perhaps one or several thousand qubits then you better stop using your credit

cards with the current encryption. So… One thing I would I would like to mention

though with regards to the Shor’s algorithm though is that because of the error rates

that we end up having with the physical qubits, sure if you have a thousand perfect qubits

you might start thinking about Shor’s algorithm for a thousand, thousand-digit numbers but

with, with needing quantum error-correction and a lot of the best known encodings you

have an overhead that significantly pushes that threshold further. So I think that in terms of Shor’s algorithm

and a realistic Grover’s search you’re thinking about probably needing millions, of tens of

millions qubits. So it’s it’s a bit further off but it’s still

there will come a day where There will come a day when you’ve got to worry

about your bank accounts but it’s on, the horizon for that is a bit further beyond where

we are currently at. So millions of physical Millions of physical qubits, yeah so that’s

only maybe around a few thousand logical qubits but the, the encoding that’s what’s going

to matter there. So Kathy-Anne you were describing also to

me earlier that on the one hand quantum computers take away our privacy by breaking these codes

but they might also restore. Can you describe some work, work you’ve done

for the restoration process? Yes so there’s people working now on, in addition

to quantum, computing quantum networking and what you can do with quantum mechanics for

networking communication, the easiest example to explain, and it’s been around for a while,

is called quantum key distribution, where you send a message between two parties say

Alice and Bob using single photons. And because the photons are encoded using

quantum mechanics you can actually make protocols that are ultra secure. And by that we mean they’re tamper-proof,

meaning that even if an, an eavesdropper can’t get between Alice and Bob to get the signal

because they don’t have the corresponding information that was encoded in the quantum

mechanics. But even if an eavesdropper were to try and

grab the signal the protocols are tamper evident so Alice and Bob would see that immediately

when they started to talk about the results that they’d gotten and would abandon the protocol. So yes it can do things like break, break

encryption but it can also provide ultra secure protocols too. So what are you doing in your lab now to kind

of bring that into fruition? So we’re working on quantum networking where

instead of sending key information, which that just sends information to generate a

key and then you would use the key for something else. We, and a lot of people in the field, are

moving towards quantum networking where you actually send quantum information directly

over some longer distance link. And so this allows you to do things like ultra

secure communication protocols or people are also looking at it for distributing computing. Where you don’t just have one computer sitting

there with millions, let’s say, of qubits but you distribute these qubits over a larger

space and you have smaller banks of qubits. Seth, you once told me, this is a couple of

years ago now, I don’t know if you remember the anecdote, the NSA funded some early quantum

computing work to show this wasn’t possible because they didn’t want to have unbreakable

codes. Can you walk through that? Oh yeah, so I was at the, so back in 1993

I wrote the first paper showing how you build a quantum computer using these methods of

zapping stuff with microwaves and lasers and things like that. And then we started to work with people to

build them. In 1994, I think the first U.S. government

meeting to fund, to discuss funding for quantum computing took place at DARPA in Arlington,

Virginia. And during this meeting there were a bunch

of people, including Peter Shor, they were there talking about stuff and a fellow named

stood up and he said I’m Keith Miller from the NSA and I am authorized to tell you that

the NSA is interested in quantum computing and then he sat down again. And everyone went, oh my God! Some people actually told us something. That’s incredible! But it caused such a stir that he stood up

again and he said well I believe I’m also authorized to tell you this, of course the

NSA is interested in quantum computing because our primary mission is to protect the secrets

of the country, up to thirty years for top secrets. We have a whole bunch of information that’s

out there that’s already encrypted which if someone could build a quantum computer could

be decrypted. And that would be bad. So really what we would really prefer is that

it not be possible to build a quantum computer. By the way this is a good person to have funding

you, it’s like they call up and they say how’s it going? And we say oh it’s terrible, the qubits aren’t

working. Great great! That’s wonderful. Here’s your money. That didn’t last very long. So then he said. But because of our secondary mission, if it

is possible, we want to have the first one, so. To bring this back down to more quotidian

kind of applications Jerry, you once described to me some of the molecular calculations you

were doing. Can you walk through what you’re doing with

these molecules? Yeah. So, I think one of the more kind of near-term

areas that we’d like to look at application wise with quantum computers, actually is in

chemical simulation. So what’s actually interesting is that it

dates back to Feynman around 1980s when he actually talked about, wouldn’t it be great

to actually simulate nature using something that follows the same quantum mechanical principles

of nature. And there’s been a lot of theoretical work

going into how would you actually map say problems in quantum chemistry, for example

electronic and molecular structure, onto physical quantum bits. And it’s a really neat idea in the sense that

you can you can actually try and get an analog for a physical, a real physical system such

as the energy levels of say a hydrogen molecule, but actually run it on a on a on a chip right? Run that simulation on qubits that’s inside

of one of our dilution refrigerators. And so we, our team has done various both

theoretical explorations and recently experimental demonstrations of how to do some rather simple

molecular calculations. So looking at the energy the ground state

energy of a simple molecule just like hydrogen so two H’s and then lithium hydride, beryllium

hydride, but very small at this at this at this stage. But it shows the type of trajectory, if you

will, of our application in the near term because at some point with these different

molecular structures you get to a point where there’s too many electrons in it that it’s

impossible to again, simulate in on any classic computer. And it can be rather modest molecular sizes

that that already maxed out all those supercomputing resource in the world. And there’s a lot of potential there for quantum

computing to really be a game changer in that in that field. Seth, I was wondering if you could fuse, merge

for me the two great computing tasks of our time; machine learning and quantum computing? Is there a relationship between the two? Yeah. For there is the only way to get information

right now is to, you know, sort of the only way get a grant right now is to apply to do

something with big data machine learning. And then in physics the only way to get a

grant is to do something with grapheme -the material is the future along with gallium

arsenide. So the real reason is to have something so

you can get a grant that’s you know graphene based quantum random access memories for the

analysis of big data. It’s a winner. I guarantee it. You heard it here. So it’s interesting now that we are we actually

are about to have a simple quantum computers that have, you know, tens of qubits and fifty

qubits coming up. And I think that there’s a reason reasonable

path to think of having up to a thousand physical qubits over the next five to ten years. I don’t think that’s unreasonable to expect. What will you do with these devices? Now because they are quantum mechanical and

they’re very hard to simulate classically, As Jerry was saying, quantum mechanics you

know it’s famously weird and funky and quantum systems exhibit funky and strange effects

like entanglement and Einstein Podolsky Rosen correlations, and Schrodinger’s cat, and statistical

patterns in data that are very hard to capture classically. They’re counterintuitive, it’s hard for classical

computers to capture them. So if they can exhibit these, if quantum systems

can generate these funky patterns that you can’t generate classically maybe they can

also recognize patterns that you can’t recognize classically. Now machine learning is about taking patterns

of data and trying to tease them out and show that they’re there, it’s recognizing patterns

in data. Machine learning of course very trendy right

now, justifiably so not not really because actually I think you know it’s about to supplant

human beings or anything like that but because actually it’s gotten good. You know there’s this there’s this thing called

deep learning, which when I learned about it a three or four years ago I said wow! this

is fantastic, you know computers will tell us about love and truth and you know happiness

of all this deep stuff, but no such luck. It turns out that these are just neural electronic

analogs of neural circuitry that have many many many many levels in it so they’re deep

in that sense. But they actually do do problems, they solve

problems that are hard to do. Now do you get inside of a machine learning

algorithm like say the Netflix algorithm where you know you say OK what should I watch today? And Netflix says, ‘’well I think that

you would like to see Dirty Harry’ but you, for some reason my students don’t watch Clint

Eastwood any longer, I don’t know what it is. You know what Netflix is doing is they’re

actually looking at the preferences of everybody out there who’s looking at Netflix, comparing

your preferences to theirs and then you know doing what’s called a matrix completion algorithm

to recommend something to you. Now if you were to program that in a co-op

into a quantum computer it turns out that their algorithm which they only run, they

run it twice a day because it’s so incredibly computationally intensive, that if you do

that on a quantum computer you could have a quantum computer that had say a hundred

quantum bits and you could do ten thousand operations and it would do the same set of

operations in a quantum mechanical fashion. So we decided hey this is great we’ll call

this quantum Netflix algorithm, but then I googled quantum Netflix algorithm and it turns

out that Netflix calls their own algorithms “the quantum algorithm” even though it has

nothing whatsoever to do with quantum mechanics. So using you know quantum computers, quantum

systems in general exhibit strange and counterintuitive patterns. This gives you reason to hope that they can

recognize strange patterns and it turns out that the actual stuff that they’re doing already

for things like factoring numbers is great for actually finding patterns and data. And actually this is a nice application that

people have been using to demonstrate, you know, simple versions of these algorithms

on small quantum computers Obviously the world as we know it would not

be the same without computers. They’re just everywhere, they’re ubiquitous. Will fifty years from now people say the same

thing about quantum computers? Will they be as transformative as classical

computers have been? That’s an excellent question. I view, I think a quantum computer will remain,

it might be ubiquitous but it will remain a special purpose device for various things. I don’t think it will replace the computers

as we know them in its entirety. So I view the future perhaps as you having

your laptop and then a little dongle with one of Jerry’s or Kathy-Anne’s contraptions. And then whenever you know whenever you need

to break into somebody else’s credit card or whatever it is that you want to do you

make you know you make calls to that, you know, to that special device. I think that, that’s more likely picture of

the future with a quantum computer in it then, yeah. So it’s more like a GPU inside of an Xbox

type of thing. Yeah. I guess. That won’t be literally a quantum iPhone. Although Apple may trademark that before Netflix

or Verizon I’m more optimistic, I think, you know,

they build it they will come. Right? So once we have quantum computers to play

around which we already do have thanks to IBM, I’m all over that and people will play

around, will come up with you know more quantum apps, quapps, quapps for all. Can you trademark that? I trademarked the cloud with a q. Questions. So the four card monte example I mean it’s

it looks to me like something that could basically like backwards solve any kind of cryptographic

hash like as a black box or whatever the hash is. And yet I hear about these, these quantum-proof

cryptographic methods and because it seems like it doesn’t matter what the actual hash

operations are, how does that, what is the general underlying principle for these quantum

proof hash hash hash functions? So a hash function in cryptography is a function

that just like scrambles everything up in a way where you can check to see if it’s been

scrambled up in a proper fashion. And inverting these, so undoing this hashing

is supposed to be hard and that’s the basis for a lot of cryptographic protocols. It’s still hard on a quantum computer that

is this quantum searching will allow you to get a speed up to that will allow you to you

know solve some problems that you would be able to solve classically. But this kind of hashing problem is still

hard on a quantum computer. So one of the things that’s going on right

now because exactly because quantum computers are getting more powerful, though let’s face

it we’re still you know we can basically compute our way out of a paper bag now where previously

we couldn’t compute our way out of a paper bag. So but you know even the NSA has issued an

advisory saying you know if you’re going to come up with an application that’s good and

will still be secure twenty years down the line it’s time for you to think of something

in addition because quantum computers might be there. So people are coming to trying to come up

with what’s called post-quantum cryptography and I think that you’re alluding to some of

these problems there. Now you make me wonder what post quantum cryptography

could possibly be? Can you just give a simple example? So based on a, so public key cryptography

is a way where you know I send, suppose so I buy green coffee beans over the Internet

and then roast them at home. Which you actually do. Yes I do actually. So yes it’s so much so much fresher that way. It really is. I highly recommend it. So what I like to say try to send buy something

from Sweet Maria’s in Berkeley, you know this 10 pounds of Costa Rican, then Sweet Maria’s

sends me a big number which is the product of two smaller numbers which are prime numbers. And this is called the public key, this big

number. I could use that number to encrypt my information

in a way such that only sweet Maria, who knows the two smaller numbers, can decrypt it and

that’s the basis for public key cryptography. There’s a public key, which is what they sent

out there. Anybody could encrypt but to decrypt you need

you know the private key, these two numbers. This is what quantum computers can do. If they can find the private key given the

public key which would be very disruptive thing because I frankly I like buying freshly

roasted green coffee and I would be pretty pissed off if I couldn’t get it. So the idea is to you see this as a rather

specific protocol, so what people are trying to come up with are other protocols where

quantum computers can’t break those protocols where there is a public key you can encrypt

using the public key but then it can only decrypt using the private key. But a quantum computer can’t find the private

key. And so far there’s been mixed success I would

say doing this. There’s not nothing’s ready for primetime. Yeah I should say. This is also important because even though

quantum computer is not there you might want to encode your information in a way that nobody

can decode it in the next one hundred years and one hundred years is a very long time,

right. And then we might assume the quantum computer

could be there, I mean a big one whatever. And the post refers to the fact that even

though you encrypted today maybe you don’t want it to be decrypted you know in ninety

five years by one of the successors of Jerry’s computers. And there are, there are methods nowadays

in fact that cryptographers have started to develop but they’re extremely impractical

at this stage, that the public keys you would have to transmit are so long that it would

take you, you know hours basically to do that. But it’s it it would be wrong to say that

there is no alternative but it’s not a practical alternative. More questions. I saw a bunch. How does the observer effect claim to like

retrieving information? Can you elaborate? Well it seems like if you try to like observe

the information it would collapse, right, like it would just collapse back to two bits. So like how do you maintain like the four

bits or whatever? Observing it is essential to how we actually

make use of a quantum computer, right, because it has to be something tangible that we can

put in, has to be something tangible that we can take out. So the input will be classical bits the output

will be classical bits. In-between is where we make use of superposition,

entanglement and this two to the n exponential space, state space. And the key thing is is how do you tailor

your algorithm to make use of that so that when you perform that measurement you’ve learned

something that you otherwise wouldn’t have wouldn’t have been able to calculate. So it’s all about how you define those interferences

of the waves through the operations you perform in between. You only observe at the end. Before that it’s considered to be rude to

look at somebody’s quantum computer while it’s in operation. So, that’s part of the difficulty in controlling

these very complex systems is because if you make a measurement or if the environment makes

a measurement without your knowledge the same thing happens. And so you have to control the system very

well so that you only look for, the environment only measures the system at the end of the

computation. So it sounds like a lot of the stuff that

you guys are working on is kind of like a straight analogy from a classical computer

to a quantum computer where like a bit is a qubit and you’re working not gates, I was

wondering if you could talk about how quantum annealers like what D-Wave works on how well

you guys work on factors into that and if there are any sort of limitations using the

annealing paradigm. Cause I know that’s usually better off for

like combinatorial optimization problems. But is there any other sort of limitation

on what an annealer can do versus what a quantum computer can do? Well I think that the first thing there an

annealer is a very it’s a more restricted type of problem, right, so you get your hardware,

the way that you lay down these that these circuits in an annealer you defined all the

couplings between these these these these devices and you’ve defined a particular energy

landscape that you want to say optimize or find the ground state for. In the case with the systems that we were

building where you have full quantum control over any of the qubits, you really can drive

the system to any kind of quantum quantum problem that you want. And so it’s it’s reprogrammable in that sense

and you can define your optimization landscape in more generality. But maybe Seth you can also comment on the

D-wave. Yeah, I mean, also let me say that you keep

on letting all of the people who possess prior knowledge into this room will make our lives

harder. So what’s that about? So quantum annealer as as Jerry was saying

it’s a it’s actually a very old idea and classically, there’s a notion called simulated annealing

classically where you want to solve a hard problem. So you want to find the the minimum value

of some function, many problems are like this, like the traveling salesman problem, I want

to find the shortest path that will get me through all the cities of the United States

and back to where I started. That’s a hard problem. And so what you do is you map this problem

into finding the lowest energy state of a physical system and then you try to find this

lowest energy state by cooling, annealing, that’s why it’s called, annealing to get down

to this lowest energy state. Now quantum annealing is a sneaky trick that

does this quantum mechanically. You construct a quantum system. I mean for instance D-Wave quantum annealer

is a tunable device with up to a couple thousand quantum bits, and they can tune all the couplings

between them And then you set it up so that the lowest energy state encodes the answer

to your problem.and then you try to find this lowest energy state by kind of oozing in a

funky quantum mechanical state of fashion from some known state to this unknown state. And either it works or it doesn’t. Now it is an interesting situation because

actually nobody knows if this works is supposed to work even in theory. And so it’s one of these things where if you

build it and then you see what happens. They find that some fraction of the time they

actually get the right answer. There’s a lot of argument about whether this

is happening in an intrinsically quantum mechanical way or not. But I mean these are very interesting systems

so I mean D-Wave deserves great credit for building a large scale quantum system. It’s got lots of entanglement in it. It’s got, you know, it’s it’s has thousands

of quantum bits and it’s actually a beautiful system just for doing experiments on. I’m going to a conference in Japan next week

or two weeks from now where basically people are going to report on all the experiments

that they’re doing all these different D-Wave devices to try to figure out what the heck

is going on. Great. I’m afraid I’m going to have to cut off the

questions there Thanks to all of you for coming.

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