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  • A new paper by IBM Quantum and UC Berkeley successfully demonstrates some of the largest quantum circuits ever run on a quantum computer.

  • I'm Andrew Eddins, an IBM Quantum Researcher and coauthor on the paper.

  • And in this video, we'll talk about what we did in this experiment, how we did it,

  • and in particular why quantum error mitigation is poised to play such an important role in near-term quantum computing.

  • So what did we run in this experiment?

  • We used a 127 qubit processor to run a simulation of 127 interacting spins with each qubit playing the role of a spin.

  • And to do this, we ran a quantum circuit with as many as 60 layers of two qubit CNOT gates.

  • And remarkably, we were able to measure reliable results at the end of the circuit,

  • which is exciting progress because it was only about a year ago that we started being able to run circuits with 100 qubits at all.

  • And the number of gates in these 60 layers-- or the 60 layer depth of the circuit

  • --is roughly double our previous record reported last year at IBM Quantum Summit in 2022.

  • So even though today's quantum computers are not perfect, they have some noise in the hardware.

  • We're still able to extract useful results-- or reliable results --using a class of techniques known as quantum error mitigation.

  • And so this is giving us space to start exploring what we can do with these devices

  • even before the era of fault tolerance and long term quantum computing.

  • And in particular, in this experiment, we used a technique known as Zero Noise Extrapolation, or ZNE.

  • So how does ZNE work?

  • First we'll run our circuit and get some estimate of our observable.

  • So we want to learn some observable property, O, and we want to look in particular at the average or expectation value of that property.

  • We run our experiment and we get some results.

  • However, this result may be made inaccurate by the presence of noise on the quantum hardware.

  • Ideally, we'd like to get an estimate of what the answer would be-- if we ran this --if we solved this problem without any noise.

  • So how do we correct for this inaccuracy brought about by the noise on the hardware?

  • Well, first we'll go and learn what the noise is actually doing-- how it's behaving on the device.

  • So we'll take the problem that we're studying, we'll break it up into layers,

  • and then for each layer we'll further decompose that into two pieces:

  • one that captures the ideal behavior of that layer and another representing the noise.

  • And so by doing a bit of additional work, we can go and measure how all of these noise pieces in the circuit are behaving.

  • And once we have that information, although it's hard to turn down the level of noise that's happening on the hardware,

  • we are able to use that knowledge to turn it up.

  • So by repeating the experiment in the condition where we increase the noise,

  • we can then get additional results which we can use to extrapolate back and estimate the true value in the case of no noise on the hardware.

  • So that's a bit about the basic theory underlying our experiment.

  • And with that out of the way, I'll pass past things over to my colleague and coauthor to further explain details of the experiment.

  • Thanks, Andrew.

  • My name is Youngseok Kim, researcher from IBM Quantum.

  • Like what Andrew said, I'm going to talk about a little bit more detail about what we did.

  • So to make a long story short, what we did is we perform an experiment on spin dynamics of transverse field Ising model.

  • So we perform experiment on our quantum processor, and we work with our collaborators at UC Berkeley

  • and they produce corresponding results in classical computer,

  • and we compare our results against each other to build a confidence in our method.

  • So we use ZNE as our error mitigation method.

  • We use our IBM Kyiv 127 qubit processor to study these spin dynamics.

  • To be more specific, we map our spin lattice to our hardware topology, which is heavy hex topology.

  • And this spin is governed by nearest neighbor interaction j and global transverse field h.

  • And as you can see here, we have large parameter space to explore.

  • Among this parameter space, we have some parameter that results in Clifford circuit,

  • meaning we can efficiently simulate this circuit, thereby we obtain ideal value.

  • So we utilize this nice property to examine our results.

  • So here's the circuit-- 127 qubit, depth of 60 two-qubit gates.

  • And since we know the exact solution, along the way, we check our results from quantum computer and that agrees well with each other.

  • So there's one check.

  • Of course they are large parameter space which results in non-Clifford circuit, which is in general hard to verify.

  • Instead, what we did is, we take the parameter that results in the non-Clifford circuit equal shallower circuit,

  • that's depth of 15, and we examine low weight observable.

  • In this scenario, we realize that there's a light cone where all the qubits within this light cone really matters for this particular observable.

  • And here's where our collaborator from UC Berkeley comes into play.

  • They realize that using the qubits within this light cone, they can use brute force numerics to produce exact solution.

  • So we compare the exact solution and our results from quantum hardware

  • and compare against each other, we realize that they have a reasonable agreement.

  • So here's one more check.

  • So we are building this confidence.

  • We go one step further.

  • So this time taking the same circuit, we examine high weight observable, which eventually accrues more qubits within its light cone.

  • This time our collaborators realize that brute force numerics are not feasible.

  • Instead, they use numerical approximation method, specifically tensor network method.

  • They realize that using this method, they still can obtain exact solution.

  • So we compare their exact solution against our results from quantum computer and they again agree with each other reasonably well.

  • So there's another check.

  • So note that all the results of over here are verifiable circuit, meaning we have exact solution using classical resources.

  • It's crucial step to do this work to build our confidence on our method.

  • So as a next step, we would like to go a little bit farther, namely,

  • we take the same circuit and we progress one more time step to make the circuit a little bit deeper, effectively.

  • And we reexamine similarly high weight observable that eventually includes more number of qubits inside light cone.

  • So in this scenario, our collaborator realized that it's no longer feasible to obtain the exact solution, even using numerical approximate method.

  • So now we are comparing to approximate solution against our results obtained from our quantum machine.

  • In that scenario, actually, what we ran is [the] following: so, again, revisiting the part of the space.

  • There are results in Clifford circuit here.

  • And we are actually tweaking our parameter that includes non-Clifford circuit as well Clifford circuit to verify our results by looking Clifford circuit.

  • So in this scenario we looked at Clifford circuit results

  • and there we see a reasonable agreement between ideal solution and results we get from quantum computer.

  • But for numerical approximation solution from classical computer, we start to see some deviation from its ideal value.

  • Of course, we don't have exact solution here, so any results from non-Clifford circuit represents [an] unverifiable circuit.

  • So we did this, we did the same practice, very similar practice,

  • but this time we go all the way to depth 60 and we look at low weight observable.

  • Which eventually covers all the qubits within this light cone,

  • and we observe very similar behavior that produces reasonable but unverifiable results.

  • Looking ahead, we believe that some researchers will find a way to verify our currently unverifiable circuit.

  • That's good because it means quantum is driving innovation to classical computing.

  • Using their technique if they prove that our results are reasonable, that's again, good,

  • because it means noisy quantum computer can produce reliable estimate on observable with interest.

  • And of course as hardware innovation progresses, and our hardware gets better and better, we'll have further access to deeper and larger circuits.

  • And we believe that this type of research eventually bring us one step closer to a day when a quantum computer can tackle a truly useful problem.

  • I hope you like this video.

  • Be sure to like and share this video.

  • Thank you for your time.

A new paper by IBM Quantum and UC Berkeley successfully demonstrates some of the largest quantum circuits ever run on a quantum computer.

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Quantum Error Mitigation and the Path to Useful Quantum Computing

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    葉羽修 に公開 2024 年 03 月 14 日
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