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  • MARISSA GIUSTINA: Quantum computing--

  • it's been all over the news lately, often

  • accompanied by an air of mystique

  • or an assortment of fantastic promises.

  • But what does "quantum" mean in the context

  • of computer hardware?

  • What distinguishes a quantum computer from a regular one?

  • What does a quantum computer look like?

  • How do we build it?

  • My name is Marissa Giustina, and I'm

  • a research scientist in the Google AI Quantum Hardware Lab.

  • I'd like to unpack those questions.

  • And hopefully, in about five minutes,

  • the term "quantum computer" will have just a little more meaning

  • for you.

  • We're working to build devices that we can interact with.

  • That is, devices we can control and read out,

  • which behave reliably according to a simple quantum model.

  • In other words, we're building quantum computing hardware.

  • Quantum hardware can be used as a tool

  • for approaching certain kinds of computational problems.

  • So our ongoing efforts are both to develop the hardware

  • and to develop algorithms that leverage this hardware.

  • Let's start with the first question.

  • What does it mean for hardware to be quantum?

  • For that, we'll talk for a moment about quantum mechanics.

  • A model is the physicist's tool to make predictions

  • about what will happen when we put

  • the universe into a certain configuration

  • and poke it in a certain way.

  • For example, if you'd never built a skyscraper before,

  • you might make a Lego version before building it full scale.

  • That's a model.

  • Models can also be expressed in the language of mathematics.

  • The most fundamental model of nature we know

  • was developed in the early 20th century

  • and is known as quantum mechanics.

  • The word "mechanics" refers to the mechanisms

  • by which things happen.

  • The word "quantum" refers to discrete quantities

  • of energy or some other physical quantity.

  • Within quantum mechanics, energy comes in packets,

  • sometimes called photons.

  • And you cannot have fractional packets.

  • So what's a quantum object?

  • People sometimes think of a quantum object as being tiny

  • and a quantum leap as being large.

  • However the word "quantum" doesn't

  • dictate an object's size.

  • Actually, a quantum object is one

  • that relates in a well-defined way

  • to a single quantum of energy.

  • For instance, the photon I mentioned before is a quantum

  • object.

  • A photon is a single particle of energy.

  • Similarly, atoms are quantum objects.

  • An electron flying around an atomic nucleus

  • may be excited into a higher orbit

  • only by a particular quantum of energy.

  • There is no halfway point between the lower

  • orbit and the upper orbit.

  • If the wrong energy is provided, there simply

  • isn't a corresponding orbit for the electron to land in.

  • In a nutshell, a quantum object is one whose observable

  • behavior reflects that nature only offers

  • energy in discrete packets.

  • Now onto the next question.

  • What differentiates quantum computing hardware

  • from a regular computer?

  • In essence, quantum hardware lives

  • in a richer world than its conventional counterpart.

  • Let's consider a simple, abstract, quantum object,

  • which is entirely described by the fact

  • that it can be in one of two different energy levels.

  • Let's call those levels 0 and 1.

  • You can interpret those brackets around the 0

  • to mean this is a quantum energy level called "0."

  • And likewise for the "1."

  • Here, for example is a quantum energy state named "psi."

  • Recall the classical bit of information, a switch that

  • can take one of two values--

  • 0 and 1.

  • Because of the apparent similarity between our quantum

  • object and that classical bit of information,

  • we call this quantum analog a quantum bit, or qubit.

  • One peculiar feature about quantum mechanics

  • is the existence of superpositions.

  • A superposition is like a special mixture

  • of the energy levels 0 and 1, where the weight of each energy

  • level is given by complex constants C0 and C1.

  • If we measure the energy of our qubit,

  • we will sometimes observe 0, and sometimes 1,

  • where the value of sometimes is given by the constants.

  • An individual measurement will yield an outcome of 0 or 1.

  • There are no other options.

  • But before the measurement occurs,

  • we know at most the chances of getting a 0 or a 1.

  • We can't know the actual outcome for sure until we measure it.

  • Therefore, when we want to talk about the energy

  • state of the qubit before we've made the measurement,

  • we use this superposition to represent that the qubit hasn't

  • decided yet which outcome to display,

  • even though the chances of getting each outcome are fixed.

  • Now, even admitting that this superposition business

  • is a little unusual.

  • We can accept that it's easy enough to represent one qubit.

  • We just wrote it down right there.

  • Thinking about more qubits gets increasingly difficult.

  • Suppose we add a second qubit.

  • If these were conventional switches,

  • we could think about each switch independently.

  • But qubits are different.

  • Just as one qubit can be in a superposition state,

  • two qubits can share a superposition state,

  • where, for instance, the measurement outcome

  • is unknown, but will certainly be the same for both objects--

  • or opposite for both objects.

  • For example, here's a state where

  • a blue qubit and a yellow qubit are together

  • in a superposition state.

  • Here, they're correlated to each other.

  • Before the measurement, it cannot be known whether

  • the blue qubit will turn up 0 or 1.

  • But a measurement of both qubits will certainly always give

  • the same answer for each.

  • Similarly, in this case, measuring the blue and yellow

  • qubits will always give opposite outcomes.

  • This means that in order to fully describe two qubits,

  • we need to consider C's for all possible measurement outcomes

  • we could see.

  • To describe three qubits, we need eight C's.

  • Describing four qubits takes 16 C's, and so on.

  • Each time we add another qubit, it

  • takes twice as much information to describe

  • the whole pile of them.

  • That is the crux of what differentiates

  • quantum hardware.

  • The quantum system lives in a richer space,

  • so that representing n qubits with a classical computer

  • requires 2 to the n numbers.

  • But does this mean that a quantum memory with 100 qubits

  • corresponds to a conventional memory with 2

  • to the 100th bits?

  • Not so fast.

  • Quantum hardware is very effective at encoding

  • and processing certain kinds of information.

  • But it cannot efficiently mimic many useful aspects of its

  • classical counterpart.

  • When we say that a picture is worth 1,000 words,

  • we don't abolish words entirely in favor of pictures.

  • Adding quantum hardware to our modern computing capabilities

  • would be like adding pictures to a communication strategy that,

  • up to now, used only words.

  • So what does quantum hardware do well?

  • The exponentially growing complexity of quantum systems

  • also gives a clue about where quantum hardware could

  • be useful.

  • In the fields of chemistry and materials development,

  • simulation of molecules could be a powerful technique

  • to learn about the properties of a new molecule

  • before fully synthesizing it in the lab.

  • However, our ability to simulate chemistry on computers

  • is limited.

  • At its heart, chemistry is an application

  • of quantum mechanics.

  • And each electron we add to a model

  • doubles the number of parameters, crippling computers

  • with expensive calculations already

  • for very small molecules.

  • Suppose instead that we could build chemistry models out

  • of a quantum Lego set.

  • Then the model would be built with the same physics that

  • governs the system being modeled.

  • In fact, chemistry and materials simulations

  • have appeared as an appealing near-term problem

  • to approach using quantum hardware.

  • We've finally reached the last question.

  • What does a quantum computer look like,

  • and how do we build it?

  • Let's take a quick look at the actual hardware

  • we're building at Google.

  • Our qubits are resonant electrical circuits

  • made of patterned aluminum on a silicon chip

  • that slosh electrical current back and forth at two

  • different energy levels to encode the quantum 0 and 1

  • states.

  • Here's an example of one of our quantum chips.

  • Each chip features 72 qubits.

  • As you can see, it's about the size of a quarter.

  • We want each qubit to behave as one single quantum object,

  • with two levels.

  • Any other particle interacting with a qubit

  • from its environment pulls it away from this two-level ideal.

  • So creating a clean qubit environment

  • is a critical challenge.

  • At the same time, we want to be able to control

  • the qubits efficiently, adding and removing quanta of energy

  • and letting pairs of qubits interact

  • to exchange energy with each other on demand.

  • These requirements seem to oppose each other.

  • Ideal qubits should be perfectly clean to interact with nothing.

  • But then in specific cases, we want

  • them to interact very strongly.

  • This gives one insight into the tensions and challenges

  • of building good quantum hardware.

  • A first step toward building clean qubits

  • is to build the qubit circuits out

  • of superconducting materials, which

  • experience no electrical loss.

  • Superconductors perform only at very low temperatures.

  • And we operate our qubits in a cryostat

  • at less than 50 millikelvin, just

  • a fraction of a degree above absolute zero.

  • The cold temperatures and vacuum inside a cryostat

  • also contribute to keeping the qubit environment clean.

  • The cryostat consists of a series

  • of nested plates and cans.

  • The warmest stage is at the top, and it gets colder

  • as you go down.

  • All the equipment in the central core of the cryostat

  • is responsible for getting things cold.

  • Our hardware is installed around the edges

  • and on the bottom, coldest plate.

  • Each qubit chip must be mounted in a package, which

  • holds the chip at millikelvin temperatures and bridges

  • the gap between big cables and a small chip.

  • To address the packaged chip, electronics

  • outside the cryostat send signals

  • through cables in the cryostat.

  • Each cable must carry electrical signals from room temperature

  • all the way down to the coldest stage,

  • while leaking only the smallest amount of heat.

  • A large heat load would prevent the cryostat

  • from reaching its millikelvin base temperature.

  • A collection of filters and amplifiers

  • outfits each cable for its specific task.

  • The electronics outside the cryostat

  • are controlled by code running on a computer.

  • They generate precisely calibrated electrical signals,

  • shaped pulses of microwave radiation, which

  • are sent to control and read out the qubits.

  • This entire system-- from chip to cryostat, cables to code--

  • is all necessary to run our quantum hardware.

  • I hope you have enjoyed digging into some quantum computing

  • basics with me in these last few minutes.

  • We talked about the meaning of the word

  • "quantum," in particular, as it relates to computer hardware.

  • Considering the idea of a single qubit in superposition,

  • and then adding more qubits, we saw that each time we

  • add another qubit, it takes twice as much information

  • to describe the whole pile of them.

  • That's what really distinguishes a quantum computer

  • from a regular one.

  • Finally, I hope you enjoyed the quick lab tour

  • to get a basic sense of what our quantum computer looks like,

  • and what technology we're developing in order

  • to build it.

  • Hopefully, the words "quantum computer"

  • now have just a little more meaning to you

  • than they did five minutes ago.

  • For more detailed information about how we make and program

  • are quantum processors, I invite you

  • to have a look at the links in the description below.

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