Category Archives: Physics

March Meeting, Day 1

bear peering in glass
Actually I spent much of today working on my talk instead of going to sessions. The superconducting qubit sessions start tomorrow morning and basically run continuously until Thursday evening. I did go to some talks in the afternoon, though, mostly in D2: Ion Traps for Scalable Quantum Computation. (In some sense this is our competition.)
Ike Chuang, who is a big name in this field, gave the first talk, which laid out the challenges in making a practical quantum computer with ion traps. Most of this dealt with error correction; according to Shannon’s theorem (or maybe a quantum information version thereof) it should be possible to build an error-free quantum computer out of qubits that do make occasional errors, as long as the failure rate is below some threshold. Unfortunately in some cases they’ve looked at this requires a prohibitively large number of operations, as many as 1020. One can try to implement various error-correcting codes, such as Shor’s or Steane’s, but certain operations that are needed for a universal quantum computer don’t work within these codes. And in fact Chuang et al. have shown that there is no stabilizer code that allows a universal set of operations to be performed within the code—one has to decode first before performing at least one of the operations.
The other talks in the session were less abstract, and thus harder to understand (since I’m not terribly familiar with this architecture). The talk by Slusher described a proposal for a VLSI-based scalable ion-trap based quantum computer, which seemed impressive, except I’m pretty sure this is the one Chuang mentioned that would require 440 watts of laser power to operate.
I skipped out on the last talk to go to D8: Superconductivity: STM of Cuprates and see what the group I worked in as an undergrad was up to. However, I haven’t thought about STM of cuprates for a while now and only had the faintest idea what they were talking about.
A tempting alternative for the end of the day was Session D33: Focus Session: Quantum Foundations II. It starts out as a perfectly normal session, but somewhere around 4:30 becomes the dumping ground for crackpots. For example:

D33.00014 : Do Particles have Barcodes?
If an elementary particle shown in Fig 2 of gr-qc/0507130 has an UNSTABLE quantum connection to the rest of the universe calibrated by nature in terms of Planck times, as also proposed in my separate MAR07 abstract, there exists a possibility that each particle has a barcode of its own. Instability implies varying periods of connections and disconnections of particles to the universe, which would be equivalent to the varying widths of white and black strips of commercial barcodes. Considering the high order of magnitude of Planck times in a second, each particle and the universe generated by its radiations may have their unique birth times registered in their barcodes. My quest for the cause of consciousness, in MAR06 abstracts, as an additional implication of physics/0210040, leads to the inquiry if these unique parallel universes are like the ones that give rise to consciousness as proposed by some physicists. With all due respect, the attempts to explain TOE of inert matter may not be attempts to explain one step to climb up on a stairway at a time. They may be attempts to explain only half a step at a time to on a stairway made with only integer number of steps. The search for TOE assumes such a theory exists. Mathematics has no barrels to fire bullets that can shoot down a non-existent bird. A Hamiltonian knows no consciousness, a missing ingredient of biology made of particles or vice versa, and of realistic TOE.

The talk after that one describes a theory of Atonic Physics [sic], which sounds like an outtake from Monty Python’s bookstore sketch.

Your search requests answered

Today’s amusing search request: should I make an outline slide for my APS march meeting talk?
My physics category archive is the second hit for this search in Google. This is a surprising query to see from (presumably) a physicist: an overspecific question phrased in standard English is not the most well-formed Google search. (Some search engines are designed to take queries in this form, but Google is not one of them.) Nevertheless, the searcher lucked out: the fifth hit is a set of slides on giving good scientific talks.
I’ll answer the question anyway in case anyone else is wondering. If it’s an invited talk, the answer is almost certainly yes—a 30-minute talk will cover enough different points that an outline at the beginning will help the audience follow the transitions. If it’s a contributed talk, with only ten minutes of material it may not be necessary. If the talk divides nicely into multiple distinct sections, it’s a good idea, but if it’s centered on a single result you probably don’t need it.

Quantum mechanical Tomb Raider

Terence Tao explains quantum mechanics by analogizing to video games (particularly Tomb Raider):

Now, how does the situation look from Lara’s point of view? At the save point, Lara’s reality diverges into a superposition of two non-interacting paths, one in which she dies in the boulder puzzle, and one in which she lives. (Yes, just like that cat.) Her future becomes indeterministic. If she had consulted with an infinitely prescient oracle before reaching the save point as to whether she would survive the boulder puzzle, the only truthful answer this oracle could give is “50% yes, and 50% no”.
This simple example shows that the internal game universe can become indeterministic, even though the external one might be utterly deterministic. However, this example does not fully capture the weirdness of quantum mechanics…

He goes on to make some macabre modifications to the game mechanics in order to improve the analogy, bringing in interference and entanglement. It’s an entertaining post, but it gets truly ridiculous in the comments where he devises a Tomb Raider level to test Bell’s Inequality.

Cults and hierarchies in physics

There’s a great post at Cosmic Variance about the cult of genius in physics:

During high school or college, many aspiring physicists latch onto Feynman or Einstein or Hawking as representing all they hope to become. The problem is, the vast majority of us are just not that smart. Oh sure, we’re plenty clever, and are whizzes at figuring out the tip when the check comes due, but we’re not Feynman-Einstein-Hawking smart. We go through a phase where we hope that we are, and then reality sets in, and we either (1) deal, (2) spend the rest of our career trying to hide the fact that we’re not, or (3) drop out. It’s always bugged the crap out of me that physicists’ worship of genius conveys the simultaneous message that if you’re not F-E-H smart, then what good are you?

I remember clearly the moment I found that physics was much harder than I realized (although I had no delusions of being F-E-H smart by that point anyway): it was Ph 106a. I was used to being able to pick up concepts fairly quickly, but the subtleties of advanced classical mechanics (and Goldstein’s textbook) eluded me, and it was a serious blow to my confidence that I really didn’t get it. I worried that this was a sign that all the high-level physics concepts would be beyond my reach. Obviously that turned out not to be the case; I just needed to work a lot harder to understand these concepts. It’s striking to me how rapidly the difficulty seemed to ramp up, but this may have been due to the way Caltech structured the physics curriculum rather than an inherent property of the subject.
Chad Orzel has a related point:

Too many people approach physics as if there’s some sort of Great Chain of Being, with the most abstract theoretical particle physics at the very top and low-energy experimentalists down at the bottom, just above biologists and rude beasts incapable of speech.
This drives me right up the wall.
There’s no inherent moral worth to working on more “fundamental” and mathematical physics. A lack of familiarity with algebraic topology is not a defect in character, or a sign of gross stupidity. Low-energy physics is different than high-energy theory, but not inferior to it.

This is something I noticed a lot as an undergrad—in my freshman class almost everyone who wanted to do physics was interested in high-energy theory; I was rare in actually being inclined towards experiment at that point. Part of it is that there’s a certain glamour to working on the Theory of Everything, and there’s an apparent elegance to a simple but widely applicable theory that makes the experimental world look messy and ugly by comparison. (Although in fact the Standard Model isn’t really what I’d call simple or elegant.) Furthermore, at roughly the freshman undergrad level the major contact with experimental physics is through high school or freshman physics labs, which tend to be pretty lame.
(So how did I end up wanting to do experiment at that stage? At the end of my senior year in high school I had the opportunity to do some labs on more advanced topics, and they were less structured than what I was used to—instead of the procedure being laid out explicitly, I was given a set of equipment and had to figure out how to use it to measure a certain parameter or figure out how something worked. Although it was still pretty far removed from the actual practice of experimental physics, it gave me a better sense of the kind of problem-solving involved, which I found I really enjoyed. Plus I noticed I was better at it than I was at theory.)

Dusty physics history in the LeConte Hall attic



leconte attic, originally uploaded by arcanegazebo.

This was my Project 365 photo for Tuesday, but I wanted to do a blog post on it as well.

The attic of Berkeley’s main physics building resembles nothing so much as an inert and dusty version of the Jawa caravan in Star Wars. Filled with vintage ’70s/’80s (and older) electronics and cryogenic equipment, it contains the history of decades of cutting-edge research, now consigned to storage. Also, annoyingly elusive items that have to be accounted for in the annual lab inventory.

I was up here Tuesday afternoon looking for a particular frequency synthesizer that LBL’s records say we own. It turns out there is a frequency sythesizer up here, in among our group’s poorly-delineated junk pile, but it is a slightly different model (presumably with a bad motivator). I didn’t find the instrument I was looking for, but did take a few pictures, which all turned out blurry since there was hardly any light and the camera couldn’t acquire focus.

Perhaps the most unusual instrument is the one that’s musical rather than scientific: an old organ sitting in the corner, presumably for aspiring Phantoms of the Opera.

Contact the patent office!

Over coffee I and another grad student had a brilliant innovation: an electric guitar with SQUID pickups! Due to the high sensitivity and low noise of the SQUID, we expect the sound quality to be extremely good. Of course, the guitar will have to be filled with liquid nitrogen (we’re assuming high-Tc SQUIDs here) or equipped with a cryocooler. The LN2-filled guitar would have the advantage of producing plumes of fog on demand, and would be especially spectacular when smashed against the stage at the end of the show.

2007 March Meeting Abstract

The program for the 2007 APS March Meeting is now up. I have an invited talk this year; unfortunately it’s in an early morning session. Here’s the abstract:

Session N2: Progress in Superconducting Quantum Computing
8:00 AM–11:00 AM, Wednesday, March 7, 2007
Colorado Convention Center – Four Seasons 4
Sponsoring Units: GQI DCMP
Chair: Robert Schoelkopf, Yale University
Abstract: N2.00002 : Solid State Qubits with Current-Controlled Coupling
8:36 AM–9:12 AM
Author: Travis Hime (University of California, Berkeley)
The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum computing algorithms. We have demonstrated such control with two, three-junction flux qubits coupled together via their mutual inductances and via the dc SQUID (Superconducting Quantum Interference Device) that reads out their magnetic flux states. The flux in each qubit was controlled by an on-chip loop, and the chip was surrounded by a superconducting cavity that eliminates fluctuations in the ambient magnetic field. 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. With the qubits biased at the same frequency, the interaction produced an avoided crossing in their energy spectrum. At the avoided crossing transitions to the first excited state were suppressed and transitions to the second excited state enhanced, indicating formation of singlet and triplet states in the coupled-qubit system. The observed peak amplitudes were consistent with calculated matrix elements. When both qubits were biased at their degeneracy points, a level repulsion was observed in the energy spectrum. A bias current applied to the SQUID in the zero-voltage state prior to measurement induced a change in its dynamic inductance, reducing the coupling energy controllably to zero and even reversing its sign. The dependence of the splitting on the bias current was in good agreement with predictions. This work was performed in collaboration with P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, and John Clarke, and supported by NSF, AFOSR, ARO and ARDA.

On a related subject, I still intend to write a post about the results in our Science paper, but I haven’t got around to it yet.

Publication: Solid-State Qubits with Current-Controlled Coupling

As some of you know, we recently had a paper accepted to Science. The paper appears in the latest issue, and is now available online.
I will try to post something in the next few days that explains these results for the non-physicists in the audience. In the meantime, there’s this post from March about these experiments (from before we had the major findings), and here’s the abstract:

Solid-State Qubits with Current-Controlled Coupling
T. Hime, P. A. Reichardt, B. L. T. Plourde, T. L. Robertson, C.-E. Wu, A. V. Ustinov, John Clarke
The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum-computing algorithms. We demonstrated such control with two flux qubits coupled together through their mutual inductances and through the dc superconducting quantum interference device (SQUID) that reads out their magnetic flux states. A bias current applied to the SQUID in the zero-voltage state induced a change in the dynamic inductance, reducing the coupling energy controllably to zero and reversing its sign.