Friday, December 16, 2005

Bibliography on Quantum Programming Languages

This looks like a useful resource: Simon Gay's Bibliography on Quantum Programming Languages. It's surprisingly long. It's associated with a survey paper he wrote.

Saturday, December 10, 2005

Progress With Photons, and Jitter and Skew

[This posting serves as both the fourth installment in our series on scalability, and as the papers-of-the-week entry.]

Nature this week has three intriguing papers on progress in quantum information processing with photons. I'm at home, without access to our institutional Nature subscription, so all I can read is the first paragraph. Grumble.

Jeff Kimble's group at Caltech reports on Measurement-induced entanglement for excitation stored in remote atomic ensembles. Use 10^5 atoms at each of two sites, and quantum interference when one of them emits a photon creates a single entangled state. Hmm. "One joint excitation" is the phrase they use, but I'm a little fuzzy on why they're not creating a Bell state. I'm looking forward to reading the full paper.

The other two papers, from Kuzmich's group at Georgia Tech and Lukin's at Harvard, are on the use of atomic ensembles to store qubits that can be inserted and retrieved as single photons. These have the possibility to serve as memories, or at least as latches, for photons, providing an important tool for addressing problems I think are under-appreciated: jitter and skew. Without the ability to regenerate the timing of signals propagating through a large circuit, you can't claim to have scalability.

Clock handling is one of the most complex problems in classical chip design. Signal propagation across a chip requires significant amounts of time. It's subject to two significant error processes: jitter, in which the timing of a single signal varies from moment to moment as a result of noise, voltage fluctuations, etc., and skew, where different members of a group of signals arrive at different times because their path lengths vary. (Both of these problems are substantially worse in e.g., your SCSI cables.) We can also talk about "clock skew" as being a problem between regions of a chip.

In a quantum computer, as in a classical one, there are going to be times when we want things to be in sync. We may need pair of photons to arrive at the same place at the same time, for example. Electromagnetically induced transparency (EIT), sometimes called "stopped light", is an excellent candidate for helping here. A strong control beam (our latch clock) is focused on an atomic ensemble. Whether the beam is on or off can determine whether a separate photon (the data signal) is allowed to pass through the material, or is held in place. Thus, it can be used to align multiple photons, releasing them all to move out of the ensemble(s) at the same time. Up until now, EIT experiments have been done on classical waves; these are, as I understand it, the first reports of doing similar things for a single photon.

Prof. Harris at Stanford is one of the leading lights (sorry) in EIT. The Lukin group is another, and Prof. Kozuma of Tokyo Institute of Technology has some promising-looking work.

Visiting L.A.: WCQIS, USC/ISI, and USC

Next week I'll be in L.A. Tuesday I'm giving a talk on "The Design of a Quantum Multicomputer" at ISI, Wednesday I'll be visiting USC, then Thurday-Sunday I'm at the Caltech Workshop on Classical & Quantum Information Security. I might be giving a short talk at WCQIS, but I'm not sure yet.

If you see me, stop me and say hi. I'm especially looking to meet people who are interested in quantum computer architecture.

Thursday, December 08, 2005

Hayabusa: Good News and Bad News

Good news: JAXA is now saying that on Monday they recovered control of Hayabusa (the spacecraft that landed on an asteroid), and can probably bring it back to Earth as originally planned.

Bad news: the widget that was supposed to fire balls at the surface so they could collect the rebounding material apparently failed to fire.

? news: they think that just the landing might have kicked up some material into the collector, that they might be able to retrieve once the probe has returned to Earth.

Saturday, December 03, 2005

3D Mid-Air Plasma Display

Remember the animated, holographic chess set in the first Star Wars?

Yesterday Keio had its annual Techno-Mall, a mini-exhibition where about sixty professors' labs set up booths and demonstrations to show off their research for the public, some alumni, the Keio brass, and funding bigwigs.

I was there with my physics professor Kohei Itoh's group, discussing our research on the silicon NMR quantum computer. My CS prof, Fumio Teraoka, had a nice demo of fast handovers in mobile IPv6. There was heavy emphasis on sustainable, environmentally friendly engineering, including a lecture by Kengo Kuma, a famous architect who is a visiting professor at Keio. A lot of research is also taking place on assistive technologies for the elderly and disabled. The bicycle that balances itself while you pedal, and the wheelchair that can help you lift your arm both look promising. Material science and computer science in various forms also seem to be strengths of Keio.

But the most startling thing I saw, by far, was a videotape. Prof.
Taro Uchiyama and his group have developed a display that can create glowing, animated images in mid-air, floating above the device itself. So far, they can do things like a fluttering butterfly, or three characters of text, totaling a hundred pixels or so. In a year, they hope to be animating fully 3D characters of the quality of, say, a human being built out of Legos.

What's the trick? They focus a laser down to a point small enough to create, for a moment, a glowing plasma in mid-air! They can scan the focus point in three dimensions, not really fast enough yet, but tolerable. In fact, it's a bit reminiscent of laserium light shows in its current incarnation, though it's white light only.

The images are currently table-top size, but Prof. Uchiyama thinks they can scale this up a long ways, and one of their posters shows a graphic of it being used by a lighthouse at the beach, skywriting tsunami warnings. How much power does this take? I asked, and got a non-answer, but that's presumably the reason they were demoing a videotape instead of the device. Students are in the demo, wearing very heavy safety goggles.

This is mind-bending stuff; I wonder how far they'll really be able to take it. I hope they succeed, both technically and commercially.

Unfortunately, I can't find anything about this on the web. Prof. Uchiyama says he's looking for someone to commercialize it, either here or in the U.S.

Friday, December 02, 2005

6.4 Off the Coast of Miyagi-ken

As long as I'm keeping track of recent quakes, we had a 6.4 a couple of hundred kilometers north of here at 22:13 local time. There was only a little shaking here, but I was in the bathtub (oops, is that too much information?), which was an interesting experience. Not strong enough to feel quite like the agitate cycle...No reports of any damage; it only reach 3 on the Japanese subjective shaking scale.

Coming up after I can get it typed in, a report on Keio's 3D mid-air animated plasma display...

Thursday, December 01, 2005

Fireball Over Abiko?

At 22:18 Japan time, as I was walking home, I saw something I can't quite explain. A bright light came over a house that was blocking my view to the west, split in two, then forked a third. The first two were bluish-white, the third orange. It looked like a Roman candle, but didn't show any ballistic arc, just the path of a meteor. It petered out overhead, near Mars, having covered about 30 degrees of sky. It was brighter than Mars, but not impossibly bright, comparable to or slightly brighter than the brightest meteors I've seen.

This is in Abiko, northeast of Tokyo, at roughly 35.88N 140.03E.

Reading Out Multiple Ions Simultaneously

I'm catching up a little on my reading, so expect a fair number of paper reviews over the next week or so. Most of these will be quantum computing papers with a focus on how large systems are going to develop.

Starting from the top of the stack, Mark Acton and company from Chris Monroe's lab at Michigan just posted the paper Near-Perfect Simultaneous Measurement of a Qubit Register on the arXiv. This is one of a string of excellent papers to come out of Monroe's lab in the last half year or so.

Acton et al. are measuring ions in an ion trap using an intensified CCD. They use the hyperfine states of 111Cd+ ions in a linear trap as the qubits. A photocathode and fluorescent screen are used to intensify the photons coming from the ions themselves, then focused onto a small CCD. In their example, a 4x4 group of pixels is grouped into a single pixel, and a 7x7 block of those is used to detect each ion. Readout time is 15msec for the whole CCD right now; they claim reducing that to 2.5usec/ion is doable with currently available CCDs. One limit in their current setup is the waist of the detection beam is about 10 microns, compared to an ion spacing of 4um. They achieved readout accuracy of 98%, and a significant part of the inaccuracy is believed to be due to neighboring ions influencing each other. Done on a chain of three ions, the paper includes images of binary counting from 000 to 111.

This paper is important because concurrent readout of ions is critical for making quantum error correction work, as well as generally running algorithms faster. Rather than reading the ions in-place in a single trap, of course, the ions could be moved apart first, but that movement can create errors, and of course if they are separated then you have to have multiple laser beams for the detection. Beyond the beam waist problem, it's not clear to me what limits the number of ions this can scale to, but in general we are interested in only a few per trap at a time, so it should work.

When I visited Andrew Steane at Oxford in January, he had an old printout on his door showing pictures of binary states up to some larger number; 63 or maybe even 127? At first glance, this appears to be similar to that work, though I don't know much about the Steane experiments. I suspect those were more about the preparation of the numeric states, whereas this is about how to build a detector that can work reliably for multiple ions. At any rate, the accuracy of Acton's detection is remarkably good, and bodes well for continuing work on ion traps.

[Update, 12/2: the pontiff has a posting on a couple of other recent, good ion trap papers.]

Hayabusa in Trouble?

A few days ago, I reported happily about Hayabusa's visit to an asteroid. Today, there is less happy news; the probe is having engine trouble, apparently is unable to control its attitude, and may not be able to return to Earth as planned. Apparently, there's a good chance that it could come back anyway, four or five years late; they will have to make a decision about whether to continue funding for the project, or cut their losses. (The Hayabusa project home page seems to be several days out of date, in both Japanese and English; how can that be acceptable?)

The Daily Yomiuri article about the problems is extremely critical of JAXA, the Japan Aerospace Exploration Agency. JAXA was just created a couple of years ago by merging two separate agencies, and reportedly factionalism still exists inside the organization. The article ends with rather dour warnings that JAXA needs to do a better job of explaining itself and its research to the public, as well as fix its technical problems.

On a different topic, the article also makes mention of JAXA's successful test of a model for a supersonic jetliner, done in Australia in October. I somehow missed this, but there's a good article at the Register (including links to the flight data at JAXA) and video of the launch at the BBC (in both Windows Media and Real formats, though I swear it looks like somebody videotaped a TV, rather than used an actual feed). The goal is reportedly a 300-seat Mach 2 airliner that's as fuel- and noise-friendly as a current jumbo jet (which is to say, not very, but a heck of a lot better than the Concorde). Unfortunately, they are speculating that it won't fly until 2020 or 2025.

International Robot Exhibition

This is mostly a heads up for those of you around Tokyo -- the 2005 International Robot Exhibition is taking place yesterday through Saturday (Dec. 3). Some of the same robots that were at the Aichi Expo are there, but this is primarily an industry trade show, not a tourist attraction or educational exhibit. Nevertheless, I'm going to try to go on Saturday; if I manage to, I'll definitely report back, hopefully with photos.