New Nature Communications paper and a bonus NPR interview

Woman Resting in Bath

Baths can be beneficial, even for qubits.

One of the most important things to try to understand about real quantum computers is how they behave in the presence of environments. Sometimes these environments are called ‘baths’ by physicists. I like this term because it’s really evocative of what’s physically going on. You can imagine any quantum system you’re building as always being ‘bathed’ in the glow of these environments.

It’s a very interesting fact that you can never get away from these baths, even in principle. No object in our universe — as far as we know — can be completely isolated from the rest of the universe. As Lawrence Krauss so eloquently describes, even ‘nothing’ is something.

Even if we were to build a quantum computer in the depths of interstellar space, and cool it to zero temperature, it would still be bathed in a bath formed of the virtual particles that boil and seethe in the fabric of space-time itself. There is no escape from our connections to the physical universe.

A Lovecraftian aside that has nothing to do with the paper or the NPR interview

By the way you Lovecraft fans out there — here is a famous bit from The Dream-Quest of Unknown Kadath:

[O]utside the ordered universe [is] that amorphous blight of nethermost confusion which blasphemes and bubbles at the center of all infinity—the boundless daemon sultan Azathoth, whose name no lips dare speak aloud, and who gnaws hungrily in inconceivable, unlighted chambers beyond time and space amidst the muffled, maddening beating of vile drums and the thin monotonous whine of accursed flutes.

"Azathoth has existed since the universe began. He dwells outside normal time and space. He is blind, idiotic, and indifferent." Now go watch Krauss describe "Something from Nothing."

“Azathoth has existed since the universe began. He dwells outside normal time and space. He is blind, idiotic, and indifferent.” Now go watch Krauss describe “Something from Nothing” and tell me they’re not talking about the same thing!

Lovecraft had an uncanny ability to grok modern concepts from physics and weave them into his stories. His descriptions of Azathoth, and the physics underlying Krauss’ explanations of what seems to be physically occurring deep inside the fabric of spacetime, are just too close to not point out. Of course they use different language. But think carefully about the context in which these ideas are being delivered. (Am I stretching making a connection between Krauss’ something that lives in nothing and Lovecraft’s description of Azathoth? Definitely. But I think it’s an interesting thing to think about how these two descriptions might not be incompatible.)

Back to qubits and baths

Anyway back to qubits and baths. This is not just fascinating science (although it is that). It is also a fundamentally important issue in constructing computing machines that harness quantum mechanics. Because all quantum systems MUST live in baths, it’s extremely important to understand in detail how these baths affect their behavior.

Not so long ago, it was suspected that these baths would always destroy the curious properties of quantum mechanics for large objects. But then this turned out to not be true. The first large objects where quantum behavior remained even in the presence of really big and hot baths were loops of superconducting metal — the great – great – great grandparents of our qubits.

Now the question of what effect these baths really have on large collections of large objects is being debated, and goes to the heart of many of the technical issues in building useful quantum computers.

The paper that just published

The paper that just published is called Thermally assisted quantum annealing of a 16-qubit problem.

It describes what I believe to be a key result in advancing this understanding. It looks very carefully at what happens to a quantum system in the presence of a bath, where both the quantum system and the bath have been exquisitely characterized. As was the case when macroscopic quantum coherence was first observed, the results are counter-intuitive.

Here is the abstract from the paper.

Efforts to develop useful quantum computers have been blocked primarily by environmental noise. Quantum annealing is a scheme of quantum computation that is predicted to be more robust against noise, because despite the thermal environment mixing the system’s state in the energy basis, the system partially retains coherence in the computational basis, and hence is able to establish well-defined eigenstates. Here we examine the environment’s effect on quantum annealing using 16 qubits of a superconducting quantum processor. For a problem instance with an isolated small-gap anticrossing between the lowest two energy levels, we experimentally demonstrate that, even with annealing times eight orders of magnitude longer than the predicted single-qubit decoherence time, the probabilities of performing a successful computation are similar to those expected for a fully coherent system. Moreover, for the problem studied, we show that quantum annealing can take advantage of a thermal environment to achieve a speedup factor of up to 1,000 over a closed system.

The key result is that for the specific type of bath acting on a real processor, the quantum effects required for quantum computation can successfully be tapped by protecting them in a specific way. Specifically — and this is a point that has caused much confusion — the decoherence time of the individual qubits, which is the time to decohere in the energy basis, does not set the timescale for losing quantum coherence in the measurement basis. Quantum coherence in the measurement basis (which is the resource tapped in this approach) is an equilibrium property of the system, as long as the bath is not so big and hot that well defined energy eigenstates disappear.

While the paper is primarily an experimental paper, the theory underlying all of this is very satisfactory in my view. Mohammad and his collaborators have developed a very good theoretical understanding of what really happens in real open quantum systems, and the agreement between these models and what is seen in the lab is striking.

So congratulations to all on this result.

The NPR interview and my proudness at working ‘meatiest’ into a national radio program

On a mostly unrelated note, here is a radio piece that Geoff Brumfiel of NPR did recently. It is of note because I managed to work in the word ‘meatiest’ into the discussion, of which I am understandably quite proud.

13 thoughts on “New Nature Communications paper and a bonus NPR interview

  1. This just showed up on /r/dwave: http://arxiv.org/abs/1305.4904

    “Recent evidence that the Dwave system is exhibiting quantum behavior is shown to be a purely classical consequence of the underlying adiabatic annealing algorithm”

    It looks like they’re just saying that the evidence currently available does not entirely rule out a purely classical process. Has this already been hashed out and I’m just late to the party or is it new?

    • Hi! It’s important to note that we’re not involved in this directly. The original work was done by a group of independent scientists working on the Rainier-based system at USC. The ‘criticism’ you’re referring to is from people at IBM.

      I suspect that the authors of the original work (John Martinis etc.) will prepare some sort of response and post it shortly. My take on the IBM criticism from a quick read of their paper is that there were some fundamental misunderstandings (for example, they seemed to even be unaware that Chimera graphs are non-planar (!?!) ) that Martinis et.al. will be able to explain to them and make everyone happy.

      As an aside, putting on my theoretical condensed matter physicist hat for a second, it ‘s bad science to propose a model that can explain one experiment ignoring all other experiments. You can always come up with a well-tuned classical model to explain any quantum mechanical experiment that has been done in history. In order to build a compelling model it needs to explain *all* relevant experiments. If your model only explains one and directly contradicts all the others (as is the case here) it’s just a bad explanation.

  2. According to the graph demonstrating Rose’s Law that was shown by Jurvetson, the 512 qubit D-Wave system is slower than a classical supercomputer. Something interesting emerges, however. At the end of 2014, a D-Wave system will perform well enough to be close to outperforming the universe, having already passed the merging of all possible classical computers on earth. That D-Wave system will have 2048 qubits.

    If the scaling of that graph proves to be true, at 2048 qubits, the D-Wave system will probably perform at around 10^60 flops for some of the most complex classes of problems. A frontier that only quantum mechanics, and not anything that relies on classical mechanics, could have reached.

    Furthermore, the overhead time of the D-Wave system has been decreased from around one second for the 128 qubit D-Wave system to around one millisecond for the 512 qubit D-Wave system, which is enough for the 2048 qubit D-Wave system to be interactive with the timing of the human mind.

    Given that the D-Wave system is capable of any logical operation of classical computing, the scaling of qubits implies that given the different speedups for the complete range of the classes of problems, the need for logical operations derived from classical computing becomes progressively irrelevant as the qubit count increases.

    The D-Wave system is dependent on storage size and storage i/o speed.

    At 2048 qubits the D-Wave system could be capable of Strong AI. Strong AI is the intelligence capable of solving problems to a degree not imaginable by the human mind.

    If the D-Wave system is capable of Strong AI, it will solve many problems in psychology, logic, and nature.

    • Hi Steven! Comparing this type of processor design to machines with FLOPs is probably not the right way to compare. The way we do such comparisons is to measure *wall clock time* to achieve some specified result (such as getting an optimization past some “good enough” bar). The D-Wave design doesn’t use floating point operations, it’s principle of operation is quite different. It mixes digital and analog ideas in a way no-one has ever done before.

      Already we are finding that there are some kinds of problems where this approach is superior (in some cases, vastly superior) to any known classical approach, even at 500+ qubits. The gap will widen as the technology matures. Some of the problem types — for example if you only give the system a short time, say 30ms — are hard to speed up using classical parallelism (the overheads in using multiple cores generally eat most of a short time-out).

      Re. the whole strong AI thing, note that this is mostly a *software/algorithm* issue. Hardware can help, but at this stage, it helps mostly by allowing people to try a lot more things. My view is that most of the pieces for strong AI are now known and what we need are a few million experiments / year for 5-7 years…

  3. Hi Geordie, yes the comparison is not perfect as the D-Wave system is going to be good in some things and not as good in others, but one day more proper benchmark comparisons will come out, given also that the D-Wave system is classically universal. The 2048 qubit D-Wave system is going to have an astronomical performance for certain very useful problems, and who knows for other kinds of problems. The interesting question is whether as the qubit count goes up, its reliance on offloading other types of problems to classical computers decreases.

    The Strong AI question is very interesting. In the way I define Strong AI, I mainly view it as a hardware issue, but the software side is very interesting too. I can’t wait to see the research D-Wave, D-Wave’s partners, and D-Wave’s clients come up with in the following months and years!

    Keep up with the good work!

    For some reason I can’t directly reply to you. It’s probably my VPN.

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