Category: Physics

I’m visiting the KITP in Santa Barbara because they are having a term long workshop on Topological Phases and Quantum Computation (directed by Sander Bais, Chetan Nayak, and John Preskill.) Unfortunately I won’t be able to stay for the entire workshop. But this isn’t as huge of a blow as it would have been years ago, because, the KITP records all of the talks and puts them online. They can be found here.
Yesterday, T. Sentil gave a talk in which he almost made it through his introduction! Anyway one point which he emphasized is something I have always found fascinating. And that is that topological phases (a loose term which I won’t even try to define) might actually be much more common than is widely thought, but the problem is that we don’t have the tools to see them! From the perspective of quantum error correcting codes, this seems fairly natural. In a quantum error correcting code, local probes without at least a global analysis, can not reveal the information encoded into a quantum error correcting code. Indeed if local probes without global analysis could get at our quantum data, then so too could an environment get to this data. Another way to state this is to say that the order parameter for quantum error correcting codes is a big complicated beast whose measurement cannot be done with the simple tools we currently have in our lab.
As a concrete example of this, consider single-molecule molecules. These are crazy molecules which can achieve permanent magnetization through a single molecule effect (usually at fairly low temperatures.) Basically these molecule have a high spin ground state and a high zero-field splitting such that there is a substantial barrier to leaving the high spin ground state. This effect is the result of a collection of interactions among the lower spin systems. What is interesting is the following observation. Take four spin one-half systems and couple them together anti-ferromagnetically with equal couplings between all four systems (mmm, tetrahedrons.) Such a system will have a spin zero ground state which is two fold degenerate. And this two-fold degenerate ground state is actually a single error quantum error detecting code (see quant-ph/0012018)! But why in the world, if you were a chemist, would you go looking for a spin zero molecule. This is the exact opposite of what you would like to find. Further, you won’t be able to see that there is a quantum error detecting code without a knob which allows you to split the degeneracy of this ground state. And doing this is not a trivial task. In short, exactly since we don’t have the tools to observe this effect, we won’t really be interested in it. You need a crazy theorist to tell you that maybe you should think about trying to engineer such a knob onto your system. What use is a crazy theorist except to tell you crazy stories.
Of course, thinking like this, that there might be hidden orders which we are not smart enough to discover is a good way to make yourself paranoid. What might we be hiding from because our glasses are so opaque? Certainly the role of instrumentation in science is one I find fascinating and, I must admit, a bit scary.

Alicki, Lidar, and Zanardi have put out version two of their paper critiqueing the assumptions of the threshold theorem for fault tolerant quantum computation. The new title of their paper is “Internal Consistency of Fault-Tolerant Quantum Error Correction in Light of Rigorous Derivations of the Quantum Markovian Limit” and is found at quant-ph/0506201. Discussions about the paper, can be found at the previous posts here and here. I’m a little streched write now to give it a good reading, but I do hope to do so in the next few days (after I finish the four talks I need to write, grade homeworks, and write the homework solution set.)
But I do think it is awesome that in the comment section on the abstract on arxiv.org, the following comment appears:

Comments: 19 pages. v2: Significantly expanded version. New title. Includes a debate section in response to comments on the previous version, many of which appeared here this http URL and here this http URL Contains a new derivation of the Markovian master equation with periodic driving

Which is now my favorite comment on an arxiv paper 🙂
Of course, it just isn’t fair competition for the greatest comment ever when you are battling up against the comment producing machines known as Chris Fuchs and Steven van Enk:

quant-ph/0205039 [abs, ps, pdf, other] :
Title: Quantum Mechanics as Quantum Information (and only a little more)
Authors: Christopher A. Fuchs (Bell Labs)
Comments: 59 pages, 5 figures, 140 equations, one simple idea

and

quant-ph/0204146 [abs, ps, pdf, other] :
Title: The Anti-Vaxjo Interpretation of Quantum Mechanics
Authors: Christopher A. Fuchs
Comments: 18 pages, not one equation. Requires sprocl.sty

and

quant-ph/0507189 [abs, ps, pdf, other] :
Title: |0>|1>+|1>|0>
Authors: S.J. van Enk
Comments: A more serious version, almost 2.36 pages, but still an unnormalized title
Journal-ref: Phys. Rev. A 72, 064306 (2005)

and

quant-ph/0410083 [abs, ps, pdf, other] :
Title: Quantifying the resource of sharing a reference frame
Authors: S.J. van Enk
Comments: Updated title as PRA did not accept the word “refbit” in the title: PRA accepts neither neologisms (=”a meaningless word coined by a psychotic”, according to Webster), nor novophasms
Journal-ref: Phys. Rev. A 71, 032339 (2005)

and

quant-ph/0207142 [abs, ps, pdf, other] :
Title: Phase measurements with weak reference pulses
Authors: S.J. van Enk
Comments: 5 pages, 5 figures. I apologize for this boring paper
Journal-ref: Phys. Rev. A 66, 042308 (2002)

The SQuInT conference program is now available online. OK, I am totally biased, but SQuInT is still one of my favorite conferences. Why? Green chiles! Just kiding. It is one of my favorite conferences because it is one of the few remaining conferences where you still see a good mix of experimental and theoretical quantum computing work and a good mix of computer science and physics. Of course there will always be theoreticians who are bored by experimental talks, and experimentalists who are bored by the obtuse theoreticians. Similarly there will always be computer scientists who don’t care much for the physics and physicists who don’t much care for the computer science. But I got into this field exactly because it does allow me to see both of these worlds. If you ask what I have missed the most in the last few years in going to conferences is that I haven’t seen as much experimental physics as I used to see. I therefore find it awesome that SQuInT still strives for a mixture of the often difuse worlds of quantum information science.
On a related note QIP 2006 starts next week. I am sad that I won’t be able to attend, but I’m guessing you will get good coverage from Scott Aaronson on his blog Shtetl-Optimized.

One of the big pushes occuring in ion trap quantum computing these days is the construction of different ion traps which will be useful in scaling up these quantum computer architectures. Chris Monroe’s group at Michigan (in collaboration with Keith Schwab at the PRL in Maryland) has a nice paper out a few days ago in Nature Physics describing a new ion trap they have built (for a news release, see here. ) This microtrap is built, basically, on a semiconductor chip, and is of the micrometer size as compared to the millimeter sized traps normally used for trapping ions. Because these traps are fabricated using semiconductor MEMS technology, it is not unreasonable to think of building traps which can stored hundreds to thousands of ions at a time.
One interesting property of the traps described in the paper is the shallow depth of the trapping potential as compared to the depth of the potential for larger, milimeter scale traps (about 0.08 eV in the former compared to order 1 eV in the latter.) What this means is that the ions they trap stay in the trap for minutes as opposed to days, and that it has not been possible to simultaneously trap two ions in the trap. Which is what I love about experiments: while this is an important step, we’re certain to see more steps in the future and it is not unreasonable to expect some good scaling up in of ion trap quantum computers in the next few years.
Another set of experiments involving traps designed to be scalable comes from Isaac Chuang’s group at MIT. A preprint of their work is available as quant-ph/0511018. Me, I just like to flip to the end of their paper and stare at their neat hexagonal trap and dream of the cool things I could do with such a trap.

Scott Aaronson and Lance Fortnow discuss What should physicists know about computational complexity?
I’m thinking that this deserves a rejoiner, “What should a computer scientist know about computational complexity?” I mean, how silly have these computer scientists been…they’ve been studying classical computers for all these years, when it is totally obvious (at least to a physicist like me) that they should have been studying quantum computers. 😉
This reminds me of my favorite way to distinguish between a computer scientist and a physicist at quantum computing conference. Physicists are the ones who think that NP stands for “not polynomial” and computer scientists are the ones who think that a Hamiltonian is some sort of sandwich.

This is the first year in a few that I haven’t been applying for jobs (You might suspect that this makes me less grumpy. Well, judge for yourself!) Now I could be wrong, but is this the first explicit advertisement by a U.S. university physics department for a theory position in quantum information science?

Quantum Information Theory
The Department of Physics and Astronomy at the University of Southern California invites applications for tenure or tenure track positions at all faculty levels in the area of Quantum Information Theory.

Well even if it isn’t the first, can we take this as a good sign?

Interesting papers in experimental ion trap quantum computation:
Creation of a six-atom ‘Schrödinger cat’ state from Wineland’s group at NIST Boulder, Nature 438, 639-642 (1 December 2005.) Schrodinger’s cat is now six qubits big! And growing! What a cute little kitten.
Scalable multiparticle entanglement of trapped ions from Blatt’s group in Innsbruck. Nature 438, 643-646 (1 December 2005.) In this paper the group discusses experiment they performed which created the so-called “W” entangled state for up to eight qubits. That’s a quantum byte, peoples! Amazingly the group performs full state tomography on these states. Wow that sounds like an awful lot of graduate student hours.