Quantum Bayesian Networks

December 5, 2011

Extra, Extra, Read All About It: Decoherence Times of Latest Superconducting Qubits Now Very Close to Times Needed For Quantum Error Correction

Filed under: Uncategorized — rrtucci @ 10:40 pm

Check out

Viewpoint: Superconducting Qubits Are Getting Serious
by Matthias Steffen, December 5, 2011 / Physics 4, 103 (2011)

(The html version of Steffen’s article is a bit mangled at the present time, but you can download (for free) the pdf version and that one is just peachy.)

This excellent article by Herr Doktor Matthias Steffen (at IBM) reviews the following paper by a group of people from Yale Univ.:

Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture, by Hanhee Paik, D. I. Schuster, Lev S. Bishop, G. Kirchmair, G. Catelani, A. P. Sears, B. R. Johnson, M. J. Reagor, L. Frunzio, L. I. Glazman, S. M. Girvin, M. H. Devoret, and R. J. Schoelkopf

The first superconducting qubit, built in 1999, had decoherence times of about 1 nanosecond. The Yale group has demonstrated superconducting qubits with relaxation times T1 = 60 μs and dephasing times T2 = 20 μs. That’s an improvement by a factor of 10,000 in ten years! Mamma Mia! Hot Dog! (Gate model-) quantum computers are so close, I can taste them!

Compare this to the decoherence times of 4 μs for the resonators of UCSB’s von Neumann architecture which I described in a previous blog post (in English, in Spanish ). The Yale qubits are bigger than the UCSB ones and they are enclosed in a 3 dimensional cavity. (See original Yale paper for a picture of their setup with dimensions indicated.)

Steffen estimates that the new Yale decoherence times slightly surpass what is needed for doing quantum error correction. And he believes that there is still plenty of room for improvement. It appears that no fundamental roadblocks have been encountered yet, although the sources and mechanisms of decoherence are still not well understood.

P.S. The “Viewpoint” articles of the APS (American Physical Society) are one of my favorite online overviews of leading edge physics. They are carefully written by an expert in the field. They contain references to the original papers. They are broader, briefer and less technical than the original papers. But they are much more technical, accurate and detailed than the garbage one reads in most university press releases or popular science articles. Plus they are freely available on the Internet. I’d like to congratulate APS for a brilliant idea, and I hope that for a very long time, they continue to perform this service which is so valuable to science.

12 Comments »

  1. So is it time yet to brush up on quantum computer software business plans?

    Comment by Henning Dekant — December 6, 2011 @ 2:11 am

  2. Absolutely, Henning. You could be a more likable version of Mark Zuckerberg

    Comment by rrtucci — December 6, 2011 @ 4:28 am

  3. I like this description of what I read in aps.org. Layman’s terms…

    Comment by Bryan Palencia — December 6, 2011 @ 6:23 am

  4. Social networking with quantum encrypted privacy protection? Calculating you social relationships in uncertain terms?

    Comment by Henning Dekant — December 7, 2011 @ 5:42 am

  5. Sorry guys but this approach isn’t even remotely scalable. Here’s one of many reasons: there is no way to scale the number of microwave lines into a chip into a reasonable number.

    Comment by Geordie — December 8, 2011 @ 11:36 pm

  6. Interesting comment Geordie. I don’t know anything about superconducting chip design so I can’t add to your comment. I must also confess that I haven’t read the Yale paper yet either, so I don’t know what their chip geometry looks like. I intend to read it as soon as they post it on ArXiv or as soon as I visit the Univ. library.

    Comment by rrtucci — December 9, 2011 @ 2:04 am

  7. Hi Quantum Bayesian Networks,

    Here is a link on the Arxiv for the paper quoted above:

    http://arxiv.org/pdf/1105.4652v4

    Yours Zelah

    Comment by Zelah — December 9, 2011 @ 1:16 pm

  8. Hi Zelah,
    Thanks very much for the link. I fixed the blog post above to include it. For some reason, when I used Google Scholar, the paper didn’t show up as being in ArXiv. I must have done something wrong.

    Comment by rrtucci — December 9, 2011 @ 1:57 pm

  9. I just read the original paper for the first time. I plan to read it a few more times. They mention the issue of scaling very briefly at the end. They say:

    “Scaling this architecture to multiple qubits is not harder than for conventional superconducting circuits. For example, more qubits can be added inside a 3D cavity such that they couple to each other. These results are therefore encouraging for all experiments with superconducting quantum circuits.”

    Comment by rrtucci — December 9, 2011 @ 3:03 pm

  10. Robert, what’s your take on this photonic approach that currently makes the news?

    http://www.extremetech.com/extreme/108573-worlds-first-programmable-quantum-photonic-chip
    http://arxiv.org/abs/1108.3309

    At first glance the paper doesn’t give me any clue about potential scalability.

    Comment by Henning Dekant — December 12, 2011 @ 4:56 am

  11. Hi Henning,
    I think that disingenuous press release from Bristol Univ. is just a lot of hot air about sub-par, uninspired work. Nothing new in that work, as far as optics is concerned, except miniaturization to chip level. I suspect that even the optical miniaturization techniques are pretty standard. Zero contribution towards a scalable quantum computer.

    Comment by rrtucci — December 12, 2011 @ 7:00 am

  12. Thank you for sharing your insights on the Bristol chip. Having only recently started to pay closer attention to quantum computing, I have a hard time assessing if something is genuinely new. Although I had my suspicions – just seemed like a lot of rehashing in the paper. It doesn’t help that at every turn academic institutions nowadays turn out press releases that are about as truthful as your average hair loss treatment ad.

    Comment by Henning Dekant — December 12, 2011 @ 10:01 pm


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