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Chad Orzel
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- September 2, 2008
- 10:06 AM
- 2,443 views
Spin polarization and quantum statistical effects in ultracold ionizing collisions
by Chad Orzel in Uncertain Principles
This is the last of the five papers that were part of my Ph.D. thesis, and at ten journal pages in length, it's the longest thing I wrote. It was also the longest-running experiment of any of the things I did, with the data being taken over a period of about three years, between and around other experiments. As usual for this series of posts, I can sum up the key result in one graph:
(No spiffy color figure this time, as the experiment never made it onto the old web page, and my original figures are three or four computers ago.)
What we found was that when we prepared samples of metastable xenon at very low temperatures, some isotopes would collide as usual, making lots of ions, while other isotopes at the same temperature and density would not collide at all. This is a pure quantum effect-- it's not chemistry, as the only difference between the samples is a few neutrons in the nucleus-- and comes about because of the Pauli Exclusion Principle.
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C. Orzel, M. Walhout, U. Sterr, P. S. Julienne, & S. L. Rolston. (1999) Spin polarization and quantum-statistical effects in ultracold ionizing collisions. Physical Review A, 59(3), 1926-1935. DOI: 10.1103/PhysRevA.59.1926
- February 13, 2008
- 12:00 AM
- 1,921 views
Trapping of Neutral Mercury Atoms and Prospects for Optical Lattice Clocks
by Chad Orzel in Uncertain Principles
I'm not hugely enthusiastic about the ResearchBlogging.org project, but it's a little ridiculous that they've been active for weeks now, and there still isn't a single post in the "Physics" category. If they're going to offer the category link, something ought to come up when you click it, so let's give them some blogging on peer reviewed physics research.
The recent paper that most seems to lend itself to a quick explanation is this Phys. Rev. Letter from the Katori group at the University of Tokyo on the trapping of neutral mercury atoms. Full disclosure: Dr. Katori was a student ... Read more »
H Hachisu, K Miyagishi, S Porsev, A Derevianko, V Ovsiannikov, V Pal’chikov, M Takamoto, & H Katori. (2008) Trapping of Neutral Mercury Atoms and Prospects for Optical Lattice Clocks. Physical Review Letters, 100(5). DOI: 10.1103/PhysRevLett.100.053001
- December 19, 2008
- 08:55 AM
- 1,897 views
Freezing Coherent Field Growth in a Cavity by the Quantum Zeno Effect
by Chad Orzel in Uncertain Principles
When I saw ZapperZ's post about this paper (arxiv version, expensive journal version) from the group of Serge Haroche in Paris, I thought it might be something I would need to incorporate into Chapter 5 of the book-in-progress. Happily, it's much too technical to require extensive re-writing. Having taken the time to read it, though, I might as well make a ResearchBlogging post of it... (My comments will be based on the arxiv version, because it's freely downloadable.)
So, "Freezing Coherent Field Growth in a Cavity by the Quantum Zeno Effect." That's quite a mouthful. What does it really mean? It means that they've used quantum measurement to prevent photons from collecting in the space between two highly reflective mirrors, even as they pump more photons in.
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J. Bernu, S. Deléglise, C. Sayrin, S. Kuhr, I. Dotsenko, M. Brune, J. M. Raimond, & S. Haroche. (2008) Freezing Coherent Field Growth in a Cavity by the Quantum Zeno Effect. Physical Review Letters, 101(18). DOI: 10.1103/PhysRevLett.101.180402
- October 2, 2008
- 11:51 AM
- 1,850 views
Creation of an Ultracold Neutral Plasma
by Chad Orzel in Uncertain Principles
This is the last of the papers I was an author on while I was in grad school, and in some ways, it's the coolest. It's rare that you get to be one of the first people to do an entirely new class of experiment, but that's what this was. It kicked off a new sub-field (or sub-sub-field...), the history and status of which was written up in Physics a little while back.
The ultracold plasma experiment may be the ultimate version of what we jokingly called the "NIST Paradigm" of cold-atoms physics research, which could be summarized as "I wonder what will happen if we stick this other laser in?" It's based on the realization that in the metastable xenon system we were working with, it's possible to ionize the atoms by hitting them with a pulse of green light. A photon at about 514 nm, combined with an 882 nm photon from the trapping laser, gives enough energy to strip one electron off a xenon atom.
At that point, the trapped sample of atoms becomes a plasma-- definitions of "plasma" vary, but it's essentially just a vapor composed of charged particles. The plasma we made was neutral, as there were equal numbers of free electrons and positive ions created (more or less by definition), and it was exceptionally cold, because the atoms we started with were at a temperature of something like 10 microkelvin (10 one-millionths of a degree above absolute zero), and the laser pulse does not significantly heat the atoms.
Of course, we didn't know quite what to expect when we started the experiment. We just kind of blasted the laser in, and what we saw was this:
So, what's going on, here?
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T. C. Killian, S. Kulin, S. D. Bergeson, L. A. Orozco, C. Orzel, & S. L. Rolston. (1999) Creation of an Ultracold Neutral Plasma. Physical Review Letters, 83(23), 4776-4779. DOI: 10.1103/PhysRevLett.83.4776
- March 13, 2008
- 12:00 AM
- 1,770 views
Lab Visit Report: Francium
by Chad Orzel in Uncertain Principles
As I mentioned a few days ago, I visited Luis Orozco's lab during our trip to DC last week. I already talked about his cavity QED stuff, but that's only one of the projects under development. He's also working on a next-generation apparatus for the laser cooling and trapping of francium, to be done at the TRIUMF accelerator in Vancouver-- francium is an element with no stable isotopes, and at most a few grams of it exist on the earth at any given moment. Luis and his students demonstrated the laser cooling of francium a few years back, using atoms made in an accelerator at Stony Brook out o... Read more »
E Gomez, S Aubin, G Sprouse, L Orozco, & D DeMille. (2007) Measurement method for the nuclear anapole moment of laser-trapped alkali-metal atoms. Physical Review A, 75(3). DOI: 10.1103/PhysRevA.75.033418
- April 22, 2008
- 12:00 AM
- 1,738 views
Quantum Computing in Diamond
by Chad Orzel in Uncertain Principles
Two weeks ago, now, I promised some peer=reviewed physics blogging, to compensate for the "screechy monkey" nonsense. Of course, I got distracted by other things, but I've been sitting on this paper for a while now, and I really need to get it off my desk.
The paper in question is "Quantum Register Based on Individual Electronic and Nuclear Spin Qubits in Diamond," by the group of Misha Lukin at Harvard, published in Science last June. It's a clever idea for a way to do quantum computing using individual nuclear spins in a diamond matrix. I really like this idea, not least because i... Read more »
M Dutt, L Childress, L Jiang, E Togan, J Maze, F Jelezko, A S Zibrov, P R Hemmer, & M D Lukin. (2007) Quantum Register Based on Individual Electronic and Nuclear Spin Qubits in Diamond. Science, 316(5829), 1312-1316. DOI: 10.1126/science.1139831
- August 5, 2008
- 12:00 AM
- 1,714 views
Optical Control of Ultracold Collisions in Metastable Xenon
by Chad Orzel in Uncertain Principles
(This is the first in a planned series of posts writing up each of the scientific papers on which I am an author. A short description and a link to a PDF of the paper can be found at the archived Optical Control page.)
The essence of the optical control paper is contained in this one figure:
"Very pretty," you're thinking, "But what does it mean?"
The graph shows the increase or decrease in the ionizing collision rate for a sample of xenon atoms (well, two different samples, of different isotopes, but they behave exactly the same) at a temperature of 100 mic... Read more »
M Walhout, U Sterr, C Orzel, M Hoogerland, & S Rolston. (1995) Optical Control of Ultracold Collisions in Metastable Xenon. Physical Review Letters, 74(4), 506-509. DOI: 10.1103/PhysRevLett.74.506
- July 30, 2008
- 12:00 AM
- 1,713 views
Nothing But Uncertainty
by Chad Orzel in Uncertain Principles
Over at Backreaction, Bee has a nice post about uncertainty, in the technical sense, not the quantum sense. The context is news stories about science, which typically do a terrible job of handling the uncertainties and caveats that are an essential part of science.
Properly dealing with uncertainty is one of the hardest parts of science. Which is why I'm particularly impressed by people who spend their whole careers measuring nothing but uncertainties-- looking for an electric dipole moment for the electron, or parity non-conservation, or Lorentz violation, or any of a bunch of othe... Read more »
Andrew Geraci, Sylvia J Smullin, David M Weld, John Chiaverini, & Aharon Kapitulnik. (2008) Improved constraints on non-Newtonian forces at 10 microns. Physical Review D, 78(2). DOI: 10.1103/PhysRevD.78.022002
- March 14, 2008
- 12:00 AM
- 1,633 views
Lab Visit Report: Four-Wave Mixing
by Chad Orzel in Uncertain Principles
The next lab visit experiments I want to talk about are really the epitome of what I called the "NIST Paradigm" in an earlier post. These are experiments on "four-wave mixing" done by Colin McCormick (who I TA'd in freshman physics, back in the day), a post-doc in Paul Lett's lab at NIST. As Paul said when I visited, if they had had a better idea of the field they were dabbling in, they would've thought that what they were trying was impossible; thanks to their relative ignorance, though, they just plowed ahead, and accomplished something pretty impressive.
The basic scheme is laid ... Read more »
C McCormick, V Boyer, E Arimondo, & P D Lett. (2007) Strong relative intensity squeezing by four-wave mixing in rubidium vapor. Optics Letters, 32(2), 178-180. http://ol.osa.org/abstract.cfm?id
- July 7, 2009
- 11:45 AM
- 1,535 views
Entanglement by Accident
by Chad Orzel in Uncertain Principles
It's been a while since we've had any good, solid physics content here, and I feel a little guilty about that. So here's some high-quality (I hope) physics blogging, dealing with two recent(ish) papers from Chris Monroe's group at the University of Maryland. The first is titled "Bell Inequality Violation with Two Remote Atomic Qubits" (and a free version can be found on the Arxiv); the second is "Quantum Teleportation Between Distant Matter Qubits" (and isn't available on the arxiv because it's in Science, but you can get it from their web site). Both of these deal with the physics of entanglement, the "spooky action at a distance" of the famous Einstein, Podolsky, and Rosen paper.
The idea of entanglement is that two quantum systems can have their states correlated in ways that no classical system can match. If you prepare two atoms in an entangled quantum state, measuring the state of one of them instantaneously and absolutely determines the state of the other. The state of either atom prior to the measurement is indeterminate-- it can be in one of two states, but is not definitely in either-- but the states of the two together are correlated-- if one atoms is found to be in State 1, the other will always be found in State 2, and vice versa.
Once you have that, you can use the entangled state to show conclusively that quantum mechanics is a non-local theory (that's the "Bell Inequality" paper), or you can use the fact that the two states are entangled to transfer quantum information from one to the other, through "quantum teleportation" (the second paper). These two papers showcase the aspects of entanglement that make it just about the coolest thing in modern physics: first, the utter weirdness of non-local quantum states, and second, the fact that these non-local states can be used to do useful tricks.
Of course, before you can do either a Bell inequality experiment or a quantum teleportation experiment, you need to somehow entangle the states of two atoms. That turns out to be a tricky business, and is the main reason why we don't see these sorts of effects all the time. The Monroe group has used a neat trick from quantum optics to entangle the states of two different ytterbium ions held in two different vacuum chambers essentially by accident. They just coax the ions into emitting photons, direct those photons onto a beamsplitter (as shown below), and 25% of the time, the ions end up with their states entangled.
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Matsukevich, D., Maunz, P., Moehring, D., Olmschenk, S., & Monroe, C. (2008) Bell Inequality Violation with Two Remote Atomic Qubits. Physical Review Letters, 100(15). DOI: 10.1103/PhysRevLett.100.150404
Kim, M., & Cho, J. (2009) PHYSICS: Teleporting a Quantum State to Distant Matter. Science, 323(5913), 469-470. DOI: 10.1126/science.1169279
- January 25, 2010
- 11:14 AM
- 1,522 views
Single-Photon Cooling: Making Maxwell's Demon
by Chad Orzel in Uncertain Principles
As mentioned previously, I've been reading Sean Carroll's Wheel arrow of time book, which necessarily includes a good bit of discussion of "Maxwell's Demon," a thought experiment famously proposed by James Clerk Maxwell as something that would allow you to cool a gas without obviously increasing entropy. The "demon" mans a trapdoor between a sample of gas and an initially empty space, and allows only slow-moving gas atoms to pass through. After some time, the empty volume is filled with a gas at lower temperature than the initial sample, while the gas in the original volume is hotter than when it started.
Purely by coincidence, I also recently read a paper by Mark Raizen's group at Texas with the exciting title "Single-photon cooling at the limit of trap dynamics: Maxwell's demon near maximum efficiency." It also figures this great figure showing both the original Maxwell demon set-up and a schematic view of their experimental arrangement. You have to love any figure involving a stick-figure demon:
(It's also discussed in this review article in Science, but without the charming little demon.)
In this paper, they demonstrate a technique for cooling a gas of atoms that is directly analogous to the Maxwell demon idea. It takes a gas of atoms held in a trap, and selectively transfers slow-moving atoms into a different trap, through scattering a single photon (hence, "single-photon cooling"). As a result, the second trap ends up filled with atoms at a significantly lower temperature than the original sample.
So, how does this work?
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Travis Bannerman, S., Price, G., Viering, K., & Raizen, M. (2009) Single-photon cooling at the limit of trap dynamics: Maxwell's demon near maximum efficiency. New Journal of Physics, 11(6), 63044. DOI: 10.1088/1367-2630/11/6/063044
Raizen, M. (2009) Comprehensive Control of Atomic Motion. Science, 324(5933), 1403-1406. DOI: 10.1126/science.1171506
- July 8, 2010
- 10:56 AM
- 1,473 views
Kids These Days: Is Our Learning Measure Valid?
by Chad Orzel in Uncertain Principles
Kevin Drum has done a couple of education-related posts recently, first noting a story claiming that college kids study less than they used to, and following that up with an anecdotal report on kids these days, from an email correspondent who teaches physics. Kevin's emailer writes of his recent experiences with two different groups of students:
Since the early 1990's, I have pre and post tested all of my introductory mechanics classes using a research based diagnostic instrument, the Force and Motion Conceptual Evaluation. This instrument is based on research by Ron Thornton at Tufts that identified a reproducible sequence of intermediate states that all people seem to pass through in the process of gaining a Newtonian understanding. So it can give me not only a do they get it/do they not measure, but also, along several conceptual dimensions, a measure of how close they are to getting it.
My first job out of graduate school was at an unranked tier 4 institution in Myrtle Beach, South Carolina. Coastal Carolina "University" to be specific. It was the 13th grade. [...] I pretty reliably got 50-60% normalized gains on the FMCE.
Normalized gain is the ratio of how much their scores increased compared to how much they could have increased -- (post-pre)/(100-pre). 50-60% is actually pretty stupendous on this particular measure. It means they were typically getting 80-90% of the questions right.
[His current employer] Spelman [College, in Georgia] is a top 75 liberal arts college, according to US News, and top 10 according to the Washington Monthly. My personal impression of the students is that the average is generally much higher than it was at Coastal. These are students who can think around a few corners.[...]
I think I'm at least as good an instructor as I used to be, and probably a lot better. I know quite a bit more about developmental psychology and cognitive science as a result of my job at Georgia Tech and I think that improves my instruction considerably.
And yet, in a good year I get about 20-30% normalized gains.
I don't really know what is different but something clearly is.
I have seen a few comments about this questioning the validity of "normalized gain." The argument is, basically, that if you start with students who know nothing, it's easy to teach them quite a bit, but if you start with students who already know quite a bit, it's difficult to raise their scores significantly.
This is true if you're talking about absolute gain, but normalized gain is supposed to take that into account. That's why it's a fairly standard measure used by the physics education research community to compare instructional methods across courses and institutions.
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Hake, R. (1998) Interactive-engagement versus traditional methods: A six-thousand-student survey of mechanics test data for introductory physics courses. American Journal of Physics, 66(1), 64. DOI: 10.1119/1.18809
Coletta, V., & Phillips, J. (2005) Interpreting FCI scores: Normalized gain, preinstruction scores, and scientific reasoning ability. American Journal of Physics, 73(12), 1172. DOI: 10.1119/1.2117109
- June 5, 2009
- 10:20 AM
- 1,445 views
Quantum Switching of Light
by Chad Orzel in Uncertain Principles
Physics World posted a somewhat puzzling story a few days back, headlined Ultra cold atoms help share quantum information:
Scientists in the US have demonstrated a novel "light-switch" in an optical fibre that could become a new tool in the communications industry. The device created by Michal Bajcsy at Harvard University and colleagues could be developed to share both classical and quantum information.
Quantum information systems could bring a revolution to global data-sharing, by encrypting, processing, and transmitting information using the properties of quantum mechanics. However, as strings of "1s" and "0s" are represented by the quantum states of individual subatomic particles, such as the polarization of photons, they are very delicate and information can be easily lost. Prototype quantum devices have been developed but the move towards commercial applications requires more robust systems to compete with established "classical" technologies.
This is puzzling not because of the quantum stuff, but because it seems to have very little to do with the actual research being described, which was written up in Physics a few weeks ago (where, incidentally, you can get a free copy of the original paper). The work in question uses quantum mechanics, to be sure, but the business about quantum information isn't in the paper at all, and appears to be a garbled reference to something about three steps removed from what's actually being done.
The work described in the original paper is plenty cool. The paper comes from a collaboration between the research groups of Vladan Vuletic at MIT and Misha Lukin at Harvard, and it would be hard to find a more intimidatingly smart pair of PI's. They have worked out a way to trap a few thousand laser-cooled rubidium atoms inside a 3-cm piece of hollow-core optical fiber, which is impressive in its own right, but they then went on to use those cold atoms to demonstrate all-optical switching of the light passing through the fiber: they could determine whether a beam of light sent into the fiber was absorbed or transmitted by sending in a second beam of light, at a different frequency.
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Dawes, A. (2009) Optical switching with cold atoms. Physics. DOI: 10.1103/Physics.2.41
Bajcsy, M., Hofferberth, S., Balic, V., Peyronel, T., Hafezi, M., Zibrov, A., Vuletic, V., & Lukin, M. (2009) Efficient All-Optical Switching Using Slow Light within a Hollow Fiber. Physical Review Letters, 102(20). DOI: 10.1103/PhysRevLett.102.203902
- August 21, 2008
- 12:00 AM
- 1,404 views
Suppression and Enhancement of Collisions in Optical Lattices
by Chad Orzel in Uncertain Principles
A paper showing that putting atoms in an optical lattice can both raise and lower the collision rate, depending on the conditions... Read more »
J. Lawall, C. Orzel, & S. L. Rolston. (1998) Suppression and Enhancement of Collisions in Optical Lattices. Physical Review Letters, 80(3), 480-483. DOI: 10.1103/PhysRevLett.80.480
- May 12, 2010
- 03:09 PM
- 1,402 views
Inconstant Constants: "Probing fundamental constant evolution with redshifted conjugate-satellite OH lines"
by Chad Orzel in Uncertain Principles
Via Jennifer Ouellette on Twitter, I ran across a Discovery News story touting a recent arxiv preprint claiming to see variation in the fine-structure constant. It's a basically OK story, but garbles a few details, so I thought it would be worth giving it the ResearchBlogging treatment, in the now-traditional Q&A format.
What did they do? The paper looks at some spectral lines in radio emission from a moderately distant galaxy with the poetic name "PKS1413+135." These lines are produced by OH molecules in interstellar gas clouds, and the frequencies they see suggest that there may have been changes in some dimensionless constants during the not quite three billion years since the light was emitted.
Dimensionless constants? What are those? Most of the things we tend to think of as fundamental constants-- particle masses, Planck's constant, and that sort of thing-- are numbers with units. The only way to really measure one of these things, though, is by measuring it relative to one or more of the others. As a result, people who think about hard-core particle physics and cosmology and that sort of thing prefer to talk about "dimensionless constants," which are ratios of these things arranged so that all the units cancel. The most famous is the "fine structure constant" α which is the ratio of the electron charge squared to Planck's Constant times the speed of light, helpfully shown in an image lifted from Wikipedia. Other dimensionless constants of interest are the ratio of the proton and electron masses, and the "gyromagnetic ratio" which relates the spin of a fundamental particle to its magnetic moment. These ratios are the things that really matter if you want to look for changes in the fundamental constants.
Changes in the fundamental constants? Aren't they, you know, constant? You'd like to think that, but they don't necessarily have to be. And a lot of the tricks particle theorists pull when they're trying to explain fundamental forces end up giving you fundamental "constants" that change in time. This is something that you can look for experimentally, and that's what the current paper is about.
How do you do that? It's not like you can get a three billion year old proton and weigh it, can you? No, but you can look at the light emitted by really old atoms and molecules. The frequencies at which atoms and molecules emit light depend on the exact values of those dimensionless constants, so if the constants change, then the frequencies change.
How can you tell, though? Doesn't the Doppler shift from the expanding universe shift all the frequencies we see, anyway? The trick is to compare the light emitted by different transitions in the same atoms or molecules. Some states will shift up in frequency with a change in the fine structure constant (for example), while other states will shift down. Doppler shifts due to the motion of the universe or objects in it will always go in the same direction, depending on the velocity of the source. If you look at the relative frequencies of light from these different states, then, you can take out the Doppler shift, and still see if there's been a change in the relative frequencies.
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Nissim Kanekar, Jayaram N. Chengalur, & Tapasi Ghosh. (2010) Probing fundamental constant evolution with redshifted conjugate-satellite OH lines. Astrophysical Journal Letters. arXiv: 1004.5383v1
- August 24, 2010
- 10:36 AM
- 1,377 views
Bunches and Antibunches of Atoms: Hanbury Brown and Twiss Effects in Ultracold Atoms
by Chad Orzel in Uncertain Principles
Two papers in one post this time out. One of these was brought to my attention by Joerg Heber, the other I was reminded of when checking some information for last week's mathematical post on photons. They fit extremely well together though, and both relate to the photon correlation stuff I was talking about last week.
OK, what's the deal with these? These are two papers, one recent Optics Express paper from a week or so ago, the other a Nature article from a few years back. The Nature paper includes the graph you see at right, which is a really nice dataset demonstrating the Hanbury Brown and Twiss bunching effect in bosonic helium-4 (top), and the analogous anti-bunching effect in fermionic helium-3 (bottom).
Very pretty. Who are Hanbury, Brown, and Twiss, and why should I care? There are only two of them-- Hanbury Brown has a double unhyphenated last name, so its really the (Hanbury Brown) and Twiss effect. Hanbury Brown and Twiss were a couple of British astronomers working on a way to make an interferometer to measure the size of nearby stars. They were looking at intensity correlations between the signals from two telescopes looking at the same star, and using that information to measure its angular size.
As a sort of demonstration, they looked at the signal from a bench-top light source, and showed that the light signal showed bunching-- that is, they were more likely to detect a second bit of light very shortly after the first bit than they were to get more light at longer times. While this is easily explained and in fact inevitable in a wave model of light (as I explained last week), it created a bit of a stir among people in the then-new field of quantum optics (Hanbury Brown and Twiss did their experiment in the 1950's), as this didn't seem like something you should get from a photon model of light. It took a little while to sort out, but the ultimate explanation is really very simple.
And that is? Photons are bosons.
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Jeltes, T., McNamara, J., Hogervorst, W., Vassen, W., Krachmalnicoff, V., Schellekens, M., Perrin, A., Chang, H., Boiron, D., Aspect, A.... (2007) Comparison of the Hanbury Brown–Twiss effect for bosons and fermions. Nature, 445(7126), 402-405. DOI: 10.1038/nature05513
Manning, A., Hodgman, S., Dall, R., Johnsson, M., & Truscott, A. (2010) The Hanbury Brown-Twiss effect in a pulsed atom laser. Optics Express, 18(18), 18712. DOI: 10.1364/OE.18.018712
- January 12, 2009
- 01:04 PM
- 1,349 views
Subtracting Photons from Arbitrary Light Fields
by Chad Orzel in Uncertain Principles
There's been a fair bit of press for the article Subtracting photons from arbitrary light fields: experimental test of coherent state invariance by single-photon annihilation, published last month in the New Journal of Physics, much of it in roughly the same form as the news story in Physics World (which is published by the same organization that runs the journal), which leads with:
A property of laser light first predicted in 1963 by the future Nobel laureate Roy Glauber has been verified by physicists in Italy.
These stories can be a little puzzling, though. After all, Glauber got his Nobel a few years ago-- why is one of his predictions just being verified now? Shouldn't it be experimental verification first, dynamite money second? And what, exactly, did he predict, anyway?
The subject of this experiment is necessarily a little esoteric, but let's take a whack at explaining it all the same. The news stories talk about proving that "the addition and subtraction of single photons from coherent light does not affect its coherence," which might seem obvious, but is actually kind of tricky, for reasons having to do with the "quantum" in Quantum Physics. Read the rest of this post... | Read the comments on this post...... Read more »
A Zavatta, V Parigi, M S Kim, & M Bellini. (2008) Subtracting photons from arbitrary light fields: experimental test of coherent state invariance by single-photon annihilation. New Journal of Physics, 10(12), 123006. DOI: 10.1088/1367-2630/10/12/123006
- January 20, 2009
- 10:08 AM
- 1,320 views
Students Know What Physicists Believe, But They Don't Agree
by Chad Orzel in Uncertain Principles
This is flagged as a ResearchBlogging post, but it's a different sort of research than I usually write up here, as this is a paper from Physical Review Special Topics-- Physics Education Research. This is, however, a legitimate and growing area of research in physics departments, and some of the findings from the PER field are really interesting.
This particular paper, though, is mostly kind of depressing. The authors, including Nobel laureate Carl Wieman, gave students in three introductory physics classes a survey about their attitudes toward physics. They asked the students to indicate both their own opinion of the question, and also what they thought a physicist would say. They compared the student responses to "expert" responses from a survey of 66 college and university physics professors.
Their main finding was reported in the title: for the most part, students did a very good job of predicting the expert response, agreeing with the real experts roughly 80% of the time. Their personal opinions, though, were considerably less expert-like-- 15-20% less. Read the rest of this post... | Read the comments on this post...... Read more »
Kara E. Gray, Wendy K. Adams, Carl E. Wieman, & Katherine K. Perkins. (2008) Students know what physicists believe, but they don’t agree: A study using the CLASS survey. Physical Review Special Topics - Physics Education Research, 4(2). DOI: 10.1103/PhysRevSTPER.4.020106
- April 26, 2011
- 11:36 AM
- 1,292 views
Treating Photons Like Atoms: "Bose-Einstein condensation of photons in an optical microcavity"
by Chad Orzel in Uncertain Principles
This paper made a big splash back in November, with lots of news stories talking about it; it even made the #6 spot on Physics World's list of breakthroughs of the year. I didn't write it up then because I was hellishly busy, and couldn't take time away from working on the book-in-progress to figure out exactly what they did and why it mattered. I've got a little space now between handing the manuscript in last week and starting to revise it (probably next week), so while it's a bit late, here's an attempt at an explanation of what all the excitement was about.
So, what's this about, anyway? The authors created a Bose-Einstein condensate (BEC) out of a "gas" of photons inside a very small optical cavity filled with laser dye. They found that, when the number of photons inside the cavity got strong enough, they would "condense" into a single mode of the cavity with a narrow spread in frequency and a narrow spatial profile. They could get a substatial fraction of the photons inside the cavity to occupy that single mode, in much the same way that a BEC of atoms or molecules consists of a substantial fraction of the atoms in the sample occupying a single quantum state.
Wait, isn't that just a laser? You might think that-- it certainly has all the normal elements we associate with lasers: a cavity with photons inside, a narrow output beam, an organic dye (Rhodamine 6G), even a pump laser providing photons to the system. The difference between this system and a laser is very subtle, and making the distinction is complicated by the fact that people studying BEC will often refer to it as an "atom laser," or use lasers as an example of a system in which large numbers of bosons occupy a single quantum state.
So, what is the difference between this system and a laser? The difference has to do with the way the photons behave. In a traditional laser, the photons are created in a single mode through a process of stimulated emission. Photons in the laser mode pass through the gain medium, and interact with atoms in an excited state, causing them to drop down to a lower-energy state by emitting a new photon with exactly the same energy as the first one. The number of photons in the mode is not conserved, but increases dramatically as energy is pumped into the system.
The key distinction between the recent experiment and a traditional laser seems to be that the number of photons in this system is constant. The BEC forms not because photons are created in that mode, but because photons that already exist in some other mode condense into the BEC mode.
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Klaers, J., Schmitt, J., Vewinger, F., & Weitz, M. (2010) Bose–Einstein condensation of photons in an optical microcavity. Nature, 468(7323), 545-548. DOI: 10.1038/nature09567
- April 13, 2010
- 11:17 AM
- 1,289 views
Measuring the Angular Momentum of Light
by Chad Orzel in Uncertain Principles
I'm teaching a junior/senior level elective this term on quantum mechanics. We're using Townsend's A Modern Approach to Quantum Mechanics, which starts with spin-1/2 and develops the whole theory in terms of state vectors and matrices. This is kind of an uneasy fit for me, as I'm very much a swashbuckling experimentalist, and not as comfortable with formal mathematics.
This occasionally leads to good things, though, such as Monday's class, on photon polarizations. the book uses some vector arithmetic to show that circularly polarized photons have spin angular momentum of one unit of h-bar. Being a formal and mathematical book, it pretty much leaves the subject there, but my immediate reaction is to look for an experiment that proves the angular momentum is real. So I did a little Googling, and turned up a paper from 1936(!) that does just that. And I talked about it in class, because I think experiments are way cool, and like to bring them in whenever possible. Having looked this up and read it carefully, I figure I might as well write it up for ResearchBlogging while I'm at it. The Q&A format worked pretty well last time, so we'll stick with that.
What's the paper? The paper is "Mechanical Detection and Measurement of the Angular Momentum of Light" by Richard A. Beth, who is the sole author listed, though he does single out a Mr. Wilbur Harris who "deserves much credit for the thoroughness and enthusiasm with which he carried out the tedious observations." Much credit, but not co-authorship, evidently...
What does the paper describe? The paper reports on a series of experiments looking for angular momentum in light. Angular momentum, as the name suggests, is related to the rotational motion of objects, and circularly polarized light is predicted to have angular momentum. The experiments in the paper looked for, and found, evidence of this angular momentum by measuring the twisting of a quartz plate when circularly polarized light was sent through it. The apparatus is shown at right.
Back up a minute-- circular polarization? Yeah, circular polarization. Normally, when people talk about the polarization, they refer to the direction of the electric field associated with the classical light wave. The electric field oscillates up and down along some direction, changing its magnitude all the time.
There's another way to make polarized light, though, which is to keep the magnitude of the electric field constant, and make the direction change all the time. In this case, the electric field starts out pointing up (say), then some time later points to the left, then down, then to the right, then up again. It completes one full revolution in the same time that it takes the light wave to complete an oscillation. There are two different circular polarization states, corresponding to the two different directions of rotation.
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Beth, R. (1936) Mechanical Detection and Measurement of the Angular Momentum of Light. Physical Review, 50(2), 115-125. DOI: 10.1103/PhysRev.50.115
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