Back from Kyoto, Japan where I attended AQIS07. What time is it right now anyway? (And is there a selective pressure in today’s scientific fields towards people who suffer less jet lag?) AQIS 2008 will be held in Seoul, South Korea.
Here is a picture of me enjoying the awesome hospitality of our hosts at a delicious dinner. This was a dinner held on top of a creek in the mountains north of Kyoto (picture thanks to the quantum computing picture achive, a.k.a Charlie Bennett).
There we a lot of good talks at AQIS, the program can be found here. My favorite line of the entire conference was definitely when one quantum information theorist responded, when asking why a particular quantity was used in a proof, “because we are trying to keep Bob from doing something stupid.” Something about designing proofs guided by keeping the protocol participants from being stupid struck me as quite funny.
The talk which I liked the most was probably the talk by Alexandre Blais (Université de Sherbrooke) on coupling superconducting qubits to microwaves. Much fantastic work has been recently performed (most?) at Yale on coupling superconducting qubits to microwaves (see here for example.) What is cool about this setup is that one can achieve coupling between the superconducting qubits and light which is in a strong-coupling limit, much as is done in cavity QED. Strong-coupling means that the light and qubit coupling is much stronger than other couplings of these two systems to the rest of the world (i.e. such as the rate at which the qubit decoheres or the photons leak out of the cavity you are using.) In particular this allows for very robust coupling/transmission of quantum information between the superconducting qubit and light. What was exciting about Alexandre’s talk was at the end of his talk about recent experimental results from Yale to be published soon about the coupling of two superconducting qubits to each other using the microwave field as an intermediary. Very cool stuff. It seems to me that this offers many of the benefits of traditional cavity QED for building a quantum computer, but in a much more scalable manner than is achievable in cavity QED. It definitely will be interesting to watch as these systems become better characterized and as more complex devices get implemented.
Update: This work is now on the archive at 0708.2135. I
Quantum Peace
JohnQPublic points me to a new use of quantum theory. World peace:
[youtube]http://www.youtube.com/watch?v=TsvEkPNitdQ&mode=related&search=[/youtube]
At about 4:30 you’ll find my favorite line: “…the radiated influence of peace in the environment grows roughly as the square of the number of people doing it together…” Can we expect Grover speedups in achieving world peace if we use quantum theory?
Mesoscopic Quantum Coherence Length in a 1D Spin Chain
Interesting experiment reported in Science, “Mesoscopic Phase Coherence in a Quantum Spin Fluid,” Xu et al (available here). The authors discuss a one dimenionsional spin chain where each site has a spin 1 system. This system is coupled antiferromagnetically to its nearest neighbor. Now such systems have a ground state whose two spin correlation, [tex]$langle S_i S_j rangle[/tex] decays exponentially as a function of the distance between site i and site j. However, if you examine the more complicated correlation function [tex]$langle S_i exp [ i pi sum_{i<k<j} S_k]S_j rangle[/tex] this tends to a constant as the distance between the two sites increases. Thus a more complicated order exists in this system, one which is not revealed by a simple two spin correlation function (In this traitorous world, nothing is true or false, all is according to the color of the crystal through which you look.) This order is known as a string order. In particular the ground state of the system is roughly a superposition over Neel states (over [tex]$S_z=pm 1$[/tex]) with [tex]$S_z=0$[/tex] inserted into these states. The amplitude of each of these states in the superposition is exponetially decreasing in the number of inserted $S_z=0$ states.
Okay, cool, so there is this nice model which has a very cool ground state whose order isn't in a two spin correlation but some other, more interesting order. But what is cool about this experiment is that the authors are able to examine the excitations in this system. In particular they examine the creation of a triplet pair excitation at rest and show that these propogate over a fairly large distance before losing their coherence (roughly fifty lattice units.) Indeed, if I am reading the article correctly, it seems that this coherence is limited only by the length of the chains themselves (at low temperature, at higher temperature thermal excitations can shorten this coherence length.) Cool! This, I think, should give hope to those who are interested in using spin chains for quantum computation, although, of course, TIALWFAQC (this is a long way from a quantum computer.)
Kielpinski Spaces
Reading Light
So at the begining of the book a main character kills a guy and returns to a lab (of course, doesn’t everyone go to lab after they’ve murdered someone?) where they are working with “q-bits” [sic]. Then this choice line (p.6):
“We can slow down the rate at which the q-bits pick up phase. We’re actually doing better than Kielpinski there – I’ve had factors of four and up this week.”
Despite the Cornell spelling (it’s so cold in Cornell that David Mermin loses the “u”?), cool! Hopefully, many of you will recognize the reference to Dave Kielpinski who did some amazing ion trap quantum computing experiments at NIST and is now at Griffith in Australia. Okay cool, a reference to a real quantum computing researcher.
But it gets better! A few lines later:
Somewhere off in its parallel mazes, the Beowulf system begam modelling the decoherence-free subspaces – the Kielpinski space – of an ion pair…
Not only q-bits [sic] but also decoherence-free subspaces (no subsystems, alas)! And indeed this is a direct reference to papers Kielpinski was involved in: “A decoherence-free quantum memory using trapped ions,” D. Kielpinski, V. Meyer, M.A. Rowe, C.A. Sackett, W.M. Itano, C. Monroe, and D.J. Wineland, Science 291, 1013 (2001) and “Architecture for a large-scale ion-trap quantum computer,” D. Kielpinski, C.R. Monroe, and D.J. Wineland, Nature 417, 709 (2002). That former paper saved my butt during my thesis defense. An AMO physicist, about half way through my defense, said something like “Well this theory is all good, but what about in the real world.” My next slide was a plot yoinked from that first paper showing the first experiment which demonstrated slower decoherence in a decoherence-free subspace under ambient conditions.
And, dude, from now on I am totally calling the DFSs in ion traps “Kielpinski spaces.”
The Quantum Theory of Dick Cheney
Doctor Wilbur Von Philbert explains the laws of the quantum cheneyverse. Surely a theory of quantum government will appear on the arxiv soon, no?
Update: Robin points out this relevant link.
Teleportation? Beam Me Up, But Do It Coherently?
This news article, led me to this website, describing a scheme (arXiv:0706.0062, “Teleportation of massive particles without shared entanglement” A. S. Bradley, M. K. Olsen, S. A. Haine and J. J. Hope) for transporting matter waves between two remote BECs. The basic idea is a setup where mater wave gets converted into information in photons which then gets written back onto another BEC. A very cool idea (if probably experimentally challenging!) However, in all of the above the articles, the experiment is described as “teleportation.” Now don’t get me wrong, I think the experiment would be very cool if you could pull it off, but does this type of setting really deserve the moniker of “teleportation”? Now normally I would call a setup like what they authors describe a quantum state transfer protocol and not teleportation. In teleportation you use entanglment and classical communication to transmit quantum information. In the above setting you swap the information from the matter wave to the light field and then back out again, with no use of entanglment or classical communication. The authors, probably sensing the existence of 32-year-old-curmudgeons like me, write
Although our scheme is quite distinct from what is normally termed quantum teleportation,
we feel that it is closer in spirit to the original fictional concept and so will use the term to describe our system.
Okay, so we could argue about this nomenclature until we turn ourselves into chemists. But the real question, I think, is not one of naming rights (although seeing as how the preprint is PRL pages long, and that damn APS journal is the king of the pedantic, there might be some interesting editor/author wrestling matches ahead.) No, the real question is whether the experiment described above is actually close in spirit to the original fictional concept!
So which is more like Star Trek teleportation? The teleportation ideas of Bennett et al. which use entanglement and classical communication or the “teleportation” ideas described above?
Many Links Interpretation of Nature Magazine
If you haven’t seen it already, the front page of the July 5th issue of Nature has an amusing picture inspired by the “Many Worlds” interpretation of quantum theory. (I’ve said it before, and I’ll say it again. One world is enough for me, thank you very much.) Included in this issue is an essay on the many worlds interpretation:
All three approaches have their adherents, but for what seems to be a growing number of physicists, especially those working in quantum information and cosmology, it is Everett’s alternative that wins out…
Hmm, well we can run an experiment, err, I mean a social science experiment, aka a poll. Okay so these categories are subject to interpretation. For a rough guide here is wikipedia’s interpretation of quantum theories page.
[poll=2]
Also in this issue is an article Many lives in many worlds by Max Tegmark arguing for the many-worlds interpretation as well as an article Surfing the multiverse by Gary Wolfe describing many worlds and science fiction.
Finally, and perhaps more exciting than all this many worldliness, in this universe it appears that low enegy acoustic plasmons at metal surfaces have been observed. Word as to whether this observation has been made in other universes awaits the construction of a quantum computer 😉
FOCS 2007, 4 Quantum
The list of FOCS 2007 papers has been posted (via Geomblog). Quantum papers:
Andrew M. Childs, Leonard J. Schulman and Umesh V. Vazirani.
Quantum algorithms for hidden nonlinear structures [arXiv:0705.2784]
Oded Regev and Ben Toner.
Simulating Quantum Correlations with Finite Communication [not available online yet 🙁 ]
Dorit Aharonov, Daniel Gottesman, Sandy Irani and Julia Kempe.
The power of quantum systems on a line [arXiv:0705.4077]
Andris Ambainis, Andrew Childs, Ben Reichardt, Robert Spalek and Shengyu
Zhang.
Any AND-OR formula of size N can be evaluated in time N^{1/2+o(1)} on a quantum computer [arXiv:0704.3628 and arXiv:quant-ph/0703015]
SciRate.com Comments
Okay, so yeah, yeah, I’m going to blabber on about SciRate.com again. So if your one of those grumps who think that Digg is what you do in the dirt and MySpace refers to your own personsal space, you can stop reading now.
For those of you who haven’t checked out SciRate.com recently, you might not know that there is now a comment feature where you can comment on each paper posted. While there have only been a few comments so far, I think most readers of this blog (all three of you!) would be interested in each of these comments. So here is a digest of some recent comments:
User tobiasosborne writes a comment about 0705.0556, “Random Unitaries Give Quantum Expanders” by M. B. Hastings giving us a good expanded synopsis of the signfigance of this paper.
User Steven reads 0706.1966 so that we don’t have to.
User matt.hastings points us to 0706.3612, “Chiral entanglement in triangular lattice models” by D. I. Tsomokos, J. J. Garcia-Ripoll, and J. K. Pachos, which describes a model which just might be a chiral spin liquid.
User dabacon (now who could that be?) comments on the title of 0707.0021, “Fault Tolerant Adiabatic Quantum Computation” by Daniel A. Lidar, which he finds reason to object to. Update: And a conversation ensues.
Pretty cool, eh? Well I think it is cool, but then again, I think science benefits from open discussion (and I wrote the damn website myself, heh.)
Shiny Tiny Ion Traps Are Bad?
So suppose you are thinking of building an ion trap quantum computer. Since putting all your ions in one trapping region is just silly when you get to large numbers of ions, you are led to think about using multiple ion traping regions. Then you are led to two possible design choices: whether to move the ions between these regions or whether you can couple the ions to flying qubits like photons which can then couple back to ions. If you’re going to do the former, you are quickly led to the notion that it is better to have the qubits packed pretty densely, both for reasons of not wanting a gigantic quantum computer, but also for reducing the amount of moving you’ve got to do. So making small traps is of great import and a lot of groups have been doing just that, with trap sizes a few tens of microns in size.
But, uhoh, what happens if you make your traps smaller? Well one effect is that the heating rate of your ions goes up. This heating is believed to come from fluctuating patch potentials on the trap surface. This noise scales roughly like one over the size of your trap to the fourth power, and so you can imagine that going to small traps might make for some rough heating. And indeed for the small traps you’d like to use for ion trap quantum computers, this heating becomes pretty unbeareble. That is at room temperature it becomes unbareable. But there is evidence that this heating effect is thermally activated which is a fancy way of saying that if you cool your trap down this heating starts to go away. Indeed, Chris Monroes’ group has cooled a needle trap (think two ball point pens pointing at each other) to 150 Kelvin and seen a surpression in this heating rate. Indeed, one of my favorite lines at FTQC II was when Chris Monroe said of this [tex]${1 /d^4}$[/tex] heating that “we’ll get rid of it before we understand it.”
And indeed, today on the archive there is even more good news for getting rid of this noise. In 0706.3763 the ion trapping group of Isaac Chaung lab at MIT describes experiments with a surface electrode trap which has been cooled down to near 4 Kelvin:
0706.3763
Title: Suppression of Heating Rates in Cryogenic Surface-Electrode Ion Traps
Authors: Jaroslaw Labaziewicz, Yufei Ge, Paul Antohi, David Leibrandt, Kenneth R. Brown, Isaac L. Chuang
Dense arrays of trapped ions provide a promising avenue towards large scale quantum information processing. However, the required miniaturization of ion traps is currently limited by sharply increasing decoherence at sub-100 um ion-electrode distances. We characterize heating rates in cryogenically cooled surface-electrode traps, with characteristic sizes ranging from 75 um to 150 um. At 6 K, the measured rates are suppressed by 7 orders of magnitude below room temperature values, and are two orders of magnitude lower than previously published data. The observed heating depends strongly on thermal processing of the trap, which suggests further improvements are possible.
Yep they get 7 orders of magnitude slower heating in their trap by cooling from room temperature to 6 Kelvin (which, I guess, is near 4 Kelvin 🙂 ) And two orders of magnitude lower than for previously ion traps. Interestingly the effect depends on how the trap is made. Shiny traps are bad, apparently 🙂
So is the future of ion trap quantum computing to work at 4 Kelvin? Or might understanding the properties of the annealing of the trap which lead to [tex]$1/d^4$[/tex] heating suppresion give clues as to how to design room temperature traps without this heating present?