The Quantum Pontiff

QEC07 Conference

The grades are all done, and the students are gone, and now I’m conferencing. (That sentence should be sung to the tune of “Busted”) After a mere hour and a half delay at the airport I arrived last night at USC for QEC07.

Day one is a half day of tutorial talks and then a half day of talks. I’ve posted slides of my talk here: “Topological Codes and Subsystem Codes and Why We Should Care About Them…” Hopefully I’ll have some interesting things to post about in the next few days as I listen to the latest and greatest from the world of quantum error correction.

So far the highlight of the conference has been listening to Mohammad Amin talk about some of his recent calculations on adiabatic quantum computing. Look here for comments on this talk, or at least on the ongoing debate about fault-tolerance in adiabatic quantum computation. Having read up recently on ideas about spin glasses in random k-sat problems, I think I can even say something that might be interesting ;)


  1. #1 Joe Fitzsimons
    December 17, 2007

    I enjoyed your talk earlier, and actually all of the tutorial talks. It was a nice way to introduce the conference.

    It was unfortunate that there has been so little time for questions. I certainly had a few after Mohammad’s talk, but more than getting answers to my own questions (which presumably I can ask him at coffee), I missed a more general discussion.

    I’ve probably misunderstood his talk to some degree, but it seemed like he was saying that single qubit decoherence and multiqubit dephasing didn’t really effect the computation. Surely this is sufficient to make the system classically efficiently simulable (since for large time scales this would diagonalize the density matrix).

  2. #2 Robin Blume-Kohout
    December 18, 2007


    When Mohammad gave his talk at IQC, I got the impression that he was saying that the computer is unaffected by dephasing in the [instantaneous] energy eigenbasis. This seems to be valid. However, his presentation here dealt with single-qubit dynamics, and for a single qubit, the energy basis and the computational basis coincide. For many qubits, they won’t — in which case I’m not sure what conclusion can be drawn from Mohammad’s results.

    Does this jive with what you heard (I’m obviously not there to hear it)? If not, I’d be interested in hearing the conflicts.

  3. #3 Geordie
    December 18, 2007

    Hi guys,

    I posted Mohammad’s slides here.

  4. #4 Joe Fitzsimons
    December 19, 2007

    Robin, Mohammad refered several times to the 16 qubit system. Quoting from his conclusions slide

    1. Single qubit decoherence time does not limit computation time in AQC

    2. Multi-qubit dephasing (in energy basis) does not affect performance of AQC

    Point 2 makes sense, and seems to match what you are saying. The first point, however, is what worries me. It corresponds to a different basis, and so I don’t really see how higher energy levels can be neglected when we consider local decoherence, as opposed to the energy basis dephasing.

    The talks have all been recorded, so I’ll rewatch it when it appears online. I was disappointed to have left Waterloo before the DWave talk.

  5. #5 Daniel Lidar
    December 27, 2007

    Almost all of the QEC07 talks are now available for download in ppt or pdf format on
    We will post the videos sometime in January.
    The conference picture is available on