Category Archives: Quantum Information

Wednesday Schrodinger’s Cat Blogging: Coupled Qubits

The slides for my March Meeting talk, “Variable Coupling of Two Flux Qubits”, are now available online. As promised, below the fold is a non-technical explanation of the results presented there. This work builds on the single-qubit work, about which I posted in August; it may be helpful to review that post before reading the following.

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Scary vs. gross [Open Thread]

It’s spring break, but I don’t have any vacation plans. I do have some travel lined up later on this spring: I bought my tickets for Coachella so I’ll be seeing some of you there next month.
The Hills Have Eyes: This movie was so bad I’m just going to leave V for Vendetta on the sidebar. Normally I like horror flicks, but this one seemed unclear on the concept. Specifically, the film confuses “scary” with “gross”, and so we get a lot of gore and ugly mutants but not a lot of suspense. Instead of being frightening the experience was merely unpleasant, and it wasn’t even the most disgusting thing I’d seen all week (David Bowie’s eyeball hanging out of its socket being the clear winner there). The protagonists are dumb even by horror movie standards—Roger Ebert writes pretty much his entire review on how dumb they are—and some of them are sufficiently annoying that I was rooting for the mutants within ten minutes or so. Some critics have suggested that the movie is an allegory for the Iraq war. Such a film would have been much more interesting; in reality the movie drags out a few political stereotypes but doesn’t sign on to an agenda or pursue anything as sophisticated as an allegory.
Charles Stross:Iron Sunrise: Here’s the problem with “hard sci-fi”: sometimes the author knows just enough physics to get it wrong. For example: this novel’s faster-than-light communication scheme involving EPR-style entangled qubits. Now, I’m one of the few readers of this book who actually has a pair of entangled1 qubits in his2 basement. But any competent physicist should know that information can’t be transferred this way—you just get correlated random numbers. (You can make a one-time pad this way for quantum cryptography, and indeed this has been done.)
All this shows is that I’m a big nerd. Once I stopping thinking very hard about the physics in the book, it turned into a fun pulp novel, with spies, assassins, conspiracies, and Nazi villains (or near enough). Once the plot really got going I was hooked, and it was an excellent way to pass the time while I was stuck in the airport last weekend. One non-science complaint I had was that the plot twists were all telegraphed in advance, so there weren’t any big surprises. However, the characters were well-written and just reading about their interactions was fun.
1It’s actually debatable whether they are entangled (I suspect they are) but they are definitely coupled. More on this in an upcoming post.
2Actually, UC Berkeley’s basement.
Arab Strap: The Last Romance: I felt like I am not nearly bitter enough to appreciate this album properly. And this is supposed to be one of Arab Strap’s more uplifting records! Well, the tone does get happier as the CD plays, culminating in the nearly-triumphant “There Is No Ending”. (The US version of the album has two bonus tracks, but that one is clearly the end of the album.) Overall this is a decent album with a few excellent tracks: the first song and the aforementioned last song; another one I like is “Don’t Ask Me To Dance”. For the most part I like the darker music, which probably means I should check out their other records which are supposed to be along the same lines. (This purchase finally prompted me to find out that the Belle & Sebastian album The Boy With The Arab Strap was named after this band, and not the other way around.)

A different take on quantum cuteness [Open Thread]

First, a follow-up link to the quantum interrogation post: Sean at Cosmic Variance explains the experiment in layman’s terms. I’m guessing he wrote this post immediately after reading Cute Overload.
Anyway, it’s now time to review the album I’ve been playing incessantly the last three weeks. No, not Loveless, the other one.
Belle & Sebastian: The Life Pursuit: I am hardly an unbiased source on this band, so when I say that the album is awesome you will probably not be surprised. At least I can say how it stands in relation to the other B&S records, which is what I spent the first ten or so plays trying to figure out. In general it has a somewhat different sound from their previous work. There’s still the sunny mood that ran through most of Dear Catastrophe Waitress (in fact the word “sun” appears in two of the song titles), but without the orchestral feel that characterized the earlier LP’s production. From a production perspective, it sounds fairly novel for this band. I’m not sure how I would descibe this new sound, but it’s quite appealing and a good match for the themes of the album.
It feels very cohesive compared to Waitress (in which they seemed to be experimenting with various styles on the different tracks)—these songs flow into each other very smoothly, and when “Act of the Apostle II” picks up the theme from its predecessor halfway through, it feels completely natural despite the fact that the first “Act of the Apostle” played ten tracks earlier. This is not to say that there’s no variety; “Dress Up In You”, which sounds like an old-school B&S song, is sandwiched between “The Blues Are Still Blue” and “Sukie In The Graveyard”, both of which are far peppier than is typical for this band.
On just about every Belle & Sebastian CD I’ve bought, there’s been one song that I’ve fallen in love with and played to excess. Joining “Your Cover’s Blown”, “If She Wants Me”, “String Bean Jean”, and “Like Dylan in the Movies” is “The Blues Are Still Blue” from this record. I’m not sure what it is about this particular song (maybe the cowbell) but I can’t get enough of it. Other highlights are “Funny Little Frog”, “Another Sunny Day”, and “Sukie in the Graveyard”.
The iTunes version of this album offers two bonus tracks, neither of which is particularly essential. “Meat and Potatoes” sounds as if it was written for the Dr. Demento show, and “I Took A Long Hard Look” is forgettable. (Apparently these are also on the “Funny Little Frog” single.) Anyway, this only applies if you bought the CD but were considering getting the extra tracks; spend your $0.99 on “Your Cover’s Blown” (from the Books EP) instead.

Counterfactual computation

I’ve been neglecting the blog the last few days, in favor of things like data analysis. Although I might have preferred to be doing things like Half-Life 2 instead, the data came out very well, and you will certainly see it if you attend my March Meeting talk.
In other quantum computing news, a group at UIUC has performed a very interesting experiment in which they combined quantum computing and quantum interrogation to get the result of a quantum algorithm without actually running it. (Via all over the place.) So at least one person will have a March Meeting talk that’s much cooler than mine—for us “counterfactual computation” is when our qubits don’t work—but in the spirit of quantum oneupsmanship I will note that my qubits are (allegedly) scalable.
UPDATE: John Holbo speculates about technological advances that may follow from this.

Experimentalist bloggers and Joule heating

Yesterday Chad Orzel speculated about the relative absence of experimental physicists in the blogging community. Unfortunately, I didn’t have time to comment until now, because I was busy working in the lab. (Actually we were gearing up for, and then undergoing, a major safety inspection. The inspectors, who were reminiscent of the consultants from Office Space, stood around trying to invent scenarios under which a graduate student could suffer oxygen deprivation folowing sudden helium vaporization in our dilution fridge.)
Anyway, Chad’s hypothesis was that theorists spend more time in front of computers on a daily basis, and thus blogging is just more convenient. This seems right to me: I’m one of the few condensed matter experimentalists who maintains a blog (and it probably helps that I’m a grad student rather than a postdoc or on the tenure track), and whether or not I have time to post mostly depends on how much time I’m spending on the computer, versus in front of an oscilloscope or soldering iron (or bolting power strips to desks two feet above the floor to satisfy safety inspectors).
For a period of about 10 months last year, we did not have an experiment running as we were fabricating a new sample. And due to the division of labor among the grad students on this project, I was not closely involved with the fabrication process, and instead spent my time reading papers, writing papers and reports to funding agencies, writing software, designing circuits, and doing simulations. These were all computer-intensive activities, and I was able to get a fair amount of blogging done. For the last two months, however, we’ve been doing measurements on the chip we made last year, and I’ve spent a lot of time taking data, looking at scope traces, and reconfiguring wiring. Hence, I think up a bunch of posts over the week and write them up on Saturday night, which is a bit lame.
Fortunately, I do frequently have the ability to post even under these conditions, due to the phenomenon of Joule heating: if a current I is applied to an electrical resistance at a voltage V heat will be dissipated at a rate equal to the product IV. Every time we make a measurement, we apply a current pulse to our device, which produces a voltage and a corresponding amount of heat. If this heat is allowed to accumulate on the chip, it will wipe out the quantum effects we’re trying to study, so between each measurement we have to wait long enough for the chip to cool off. In practice, this means instead of taking a million measurements in a second we are reduced to about 2,000. Furthermore, to get good statistics and sweep over an interesting range of parameters we have to take a large number of measurements, so it turns out that to get interesting results we need to measure continuously for at least 12 hours. I’ve written an overly baroque computer program to automate all this, so once I know what I want to measure, I can push a button to start the experiment, do something else for a while (usually analyzing data from the previous run), and then collect all the data hours later (or the next day). (This is only when everything is working properly; otherwise it’s back to the oscilloscope and wiring diagrams.) And in the gaps I can do a little blogging.
These days, the trend in the superconducting qubit community is towards nondissipative readout—i.e., measurements which leave the device in the superconducting state and thus produce no heat. This might threaten to take away my blogging windows, except that it would also enable measurements that require even better statistics and broader sweeps, and so there will still be reasons to do 12- and 24-hour runs. (Actually, our record is about 48 hours, but we don’t currently have the battery life to repeat that.)

2006 March Meeting Abstract

Less than two months remain before the APS March Meeting, which in terms of blogging means more short posts at odd hours, when I’m not in the lab trying to gather lots of last-minute data. Here’s the abstract for my talk:

Abstract: K40.00012 : Variable Coupling of Two Flux Qubits
5:06 PM–5:18 PM
T. Hime, P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, John Clarke
We report observations of variable coupling of two flux qubits. The qubits are coupled inductively to each other and to a readout Superconducting QUantum Interference Device (SQUID). By applying microwave radiation to the device, we observed resonant absorption in each of the qubits when the level splitting in the qubit matched the energy of the microwave photons. Using the two on-chip flux bias lines we adjusted the bias of each qubit so that the energy levels of the two qubits were equal; we then observed a splitting of the resulting absorption peak characteristic of coupling between the qubits. We varied the coupling between the qubits by changing the current bias in the SQUID in the zero voltage state, thereby changing its dynamic inductance and thus modifying the effective mutual inductance between the qubits. We compare the resulting changes in splitting with our predictions. This controllable coupling should be extendable to many qubits.

I’ll do a post explaining this in more detail around the time of my talk; some of this work is still, uh, “in progress”. (In fact we have performed all the experiments mentioned in the abstract, but we are working on collecting more/better data.) The talk immediately before mine covers some other results from these experiments.

Useful Physics Aggregators

Those of you who come here for the physics blogging (which has been somewhat absent of late) may be interested in a couple of links I found recently, via referrals and Technorati:
Mixed States aggregates the RSS feeds of a number of physics blogs (including this one). Since the included bloggers are listed by their real name, it’s a nice way to see who else in the community is blogging (although I didn’t recognize any names that I knew from physics rather than from reading blogs).
Coherence * is a blog reviewing work in superconducting quantum computing, something that should be useful to me professionally (perhaps more so than the cond-mat RSS feed, which is high volume and a bit tough to sort by topic). Above their blogroll they list professors working in the field, including former Clarke group member and current collaborator Britton Plourde, but strangely not John Clarke himself. (However, there are at least four of John’s former students/postdocs there, among other familiar names.)

Follow-up on decoherence

One of the comments on the flux qubit post asked an important question: where does the decoherence come from? I dealt with this a bit in the thread itself, but this post will be a less technical treatment.
In general, decoherence is a result of the fact that the qubit under study isn’t in isolation, but interacts with some larger environment. Through this interaction, information that starts out concentrated in the qubit dissipates out into the environment, and likewise information in the environment mixes into the qubit. Of course, the state of the environment isn’t known beforehand so the information that mixes in just looks random, and averages out over a large number of experiments.
In the case of our qubit, what matters is the electromagnetic environment—the electric and magnetic fields that act on the qubit. Any fluctuations in these fields can produce decoherence, and just about everything produces some level of field noise.

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Publication: Flux qubits and readout device with two independent flux lines

This paper contains the major results of my graduate research so far, compressed into four pages. Instead of the abstract I’m posting something closer to a layman’s explanation, which is below the fold since it got a bit long.
Flux qubits and readout device with two independent flux lines
B. L. T. Plourde, T. L. Robertson, P. A. Reichardt, T. Hime, S. Linzen, C.-E. Wu, and John Clarke
Phys. Rev. B 72, 060506(R) (2005)

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