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Chad Orzel
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September 24, 2011
06:43 PM
407 views

Faster Than a Speeding Photon: "Measurement of the neutrino velocity with the OPERA detector in the CNGS beam"

by Chad Orzel in Uncertain Principles

There have been a lot of pixels spilled over this faster-than-light neutrino business, so it might not seem like something I should take time away from pressing work to write up. It is the story of the moment, though, and too much of the commentary I've seen has been of the form "I am a {theorist, journalist} so hearing about experimental details gives me the vapors" (a snarky paraphrase, obviously). This suggests that there's still room for a canine-level write-up going into a bit more depth about what they did and where it might be wrong.

So, what did those jokers at CERN pull this time? Isn't it bad enough that they want to feed us all into a black hole, now they're messing with the speed of light? First of all, this wasn't a CERN experiment in the same way that the LHC is. The experiment reporting on this uses a particle accelerator at CERN, but it's actually an Italian collaboration who did the experiment and data analysis, with the principal detector based at the Gan Sasso underground laboratory in the Alps. The name of the collaboration is OPERA, which is one of those ghastly pseudo-acronyms where they use the second letter of one of the words in order to force it to spell something.

OK, fine, what have the Italians done? Well, the goal of their experiment is to look for an "oscillation" of neutrinos. The neutrinos are created at CERN in one of their three varieties, and on the way to Gran Sasso, they can change character and end up being detected as a different type. (They start as muon neutrinos and end up as tau neutrinos, or at least that's the plan. It's not terribly important for this experiment.)

As part of the preliminary analysis for their main experiment, they looked at about three years worth of data, and noticed something odd: the neutrinos in their experiment seem to be moving slightly faster than the speed of light. The difference is pretty big in absolute terms-- about 7500 m/s, or nearly 17,000mph-- but it's only about 1/40,000th of the speed of light. Still, the difference they see is many times larger than their uncertainty, and they can't figure out why, so they're making their results public.

Wow. How do they measure that, anyway? Conceptually, what they did is the most basic kind of velocity measurement, the sort of thing we talk about in the first few weeks of introductory physics. They measured the distance between CERN and Gran Sasso, and divide that by the time between when the neutrinos are created and when they're detected to get the speed at which the neutrinos covered that distance.

The implementation, of course, is a little more complicated than that...
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The OPERA Collaboraton: T. Adam, N. Agafonova, A. Aleksandrov, O. Altinok, P. Alvarez Sanchez, S. Aoki, A. Ariga, T. Ariga, D. Autiero, A. Badertscher.... (2011) Measurement of the neutrino velocity with the OPERA detector in the CNGS beam. CERN. arXiv: 1109.4897v1

September 6, 2011
10:48 AM
781 views

Quantum Computing with Microwaves

by Chad Orzel in Uncertain Principles

It's been a while since I did any ResearchBlogging, first because I was trying to get some papers of my own written, and then because I was frantically preparing for my classes this term (which start Wednesday). I've piled up a number of articles worth writing up in that time, including two papers from an early-August issue of Nature, on advances in experimental quantum computation (the first is available as a free pdf because it was done at NIST, and thus is not copyrightable). These were also written up in Physics World, but they're worth digging into in more detail, in the usual Q&A format.

So, have they built a quantum computer to factor big numbers and hack credit card encryption yet? No, your credit cards are still safe. These papers are reporting on some technical advances in ion trap quantum computing. Specifically, they're using techniques that allow you to control the state of trapped ions with microwaves, rather than lasers or magnetic fields.

Whoa. No wonder it got written up in Nature. Talk about repurposing everyday technology... "Let's see, do I want to pop some popcorn, or entangle the states of two trapped ions?" We're not talking about a microwave oven, we're talking about light in the microwave region of the electronic spectrum. The two groups-- one at NIST, the other in Germany-- have demonstrated the ability to manipulate the states of trapped ions using only microwave radiation.

Yeah, but haven't they already done lots of experiments manipulating trapped ions? Why is this interesting? What's interesting about this is that it greatly simplifies some of the important processes. Previous experiments have used optical frequencies to manipulate the states of the ions, using light from very complicated laser systems. They've been able to do some pretty amazing things this way-- to lift a phrase from Winter's Tale, "Light under flawless tutelage knows no limits," but the level of flawlessness required takes an awful lot of work.

Microwaves, on the other hand, are an extremely well-understood technology, and there's a vast industry devoted to integrating them with computer chips and the like, in the form of cell phones. If quantum computing operations can be done with microwaves alone , that makes life a lot easier for the people who would need to build and operate quantum computers in the future. You can even build the whole thing into a chip, which is what the NIST group has done:

The ion trap control electrodes are labeled C1-C6, and the RF frequencies needed to make the trap are brought in along the orange wires. There are also three yellow microwave transmission lines, that provide the fields used to do the state manipulation.
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Ospelkaus, C., Warring, U., Colombe, Y., Brown, K., Amini, J., Leibfried, D., & Wineland, D. (2011) Microwave quantum logic gates for trapped ions. Nature, 476(7359), 181-184. DOI: 10.1038/nature10290

Timoney, N., Baumgart, I., Johanning, M., Varón, A., Plenio, M., Retzker, A., & Wunderlich, C. (2011) Quantum gates and memory using microwave-dressed states. Nature, 476(7359), 185-188. DOI: 10.1038/nature10319

August 23, 2011
11:54 AM
870 views

The Dubious Science of Teacher Coaching: "An Interaction-Based Approach to Enhancing Secondary School Instruction and Student Achievement"

by Chad Orzel in Uncertain Principles

A while back, I Links Dumped Josh Rosenau's Post Firing Bad Teachers Doesn't Create good Teachers, arguing that rather than just firing teachers who need some improvement, schools should look at, well, helping them improve. This produced a bunch of scoffing in a place I can't link to, basically taking the view that people are either good at what they do, or they're not, and if they're not, you just fire them and hire somebody else. I was too busy to respond at the time, but marked that doen as something to come back to. So I was psyched when I saw this paper in Science about a scientific trial of a teacher coaching service, which claims that:

The intervention produced substantial gains in measured student achievement in the year following its completion, equivalent to moving the average student from the 50th to the 59th percentile in achievement test scores.

"Ah-hah!" I said, "Scientific proof that teachers can, in fact, be improved with some extra instruction." So I sat down to go through the paper for ResearchBlogging purposes. Which is when I hit a problem, because the paper is kind of awful.

The awfulness isn't primarily on the scientific side, which is reasonably sound. They ran a controlled trial in Virigina with 78 teachers and more than 2000 students, randomly assigning teachers to the control and intervention groups. Teachers in the intervention group received coaching in making their classes more interactive, and regularly recorded themselves teaching then sent the recordings off for review. Experts at the coaching service being tested reviewed the recordings, then sent pointers to the teachers on what they could do better. They also followed up with a phone conversation.

The result wasn't all that dramatic, but in the year after the coaching, the teachers from the intervention group did substantially better than those from the control group. They measured performance by comparing student scores on the state-mandated end-of-year test the previous year to their performance on the state-mandated end-of-year test for the class being studied. The year after the trial, the intervention group's students improved from a raw score of 479 the previous year to a raw score of 488 for the year being studied, while the control group went from a raw score of 495 the previous year to 482 for the year being studied. This difference is statistically significant, and that's the origin of the 50th to 59th percentile claim.

So what's awful?

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Allen, J., Pianta, R., Gregory, A., Mikami, A., & Lun, J. (2011) An Interaction-Based Approach to Enhancing Secondary School Instruction and Student Achievement. Science, 333(6045), 1034-1037. DOI: 10.1126/science.1207998

July 26, 2011
11:34 AM
821 views

The Physics of Frustration: "Quantum Simulation of Frustrated Classical Magnetism in Triangular Optical Lattices"

by Chad Orzel in Uncertain Principles

One of the benefits of having joined AAAS in order to get a reduced registration fee at their meeting is that I now have online access to Science at home. Including the Science Express advance online papers, which I don't usually get on campus. Which means that I get the chance to talk about the few cool physics things they post when they first become available, without having to beg for a PDF on Twitter. This week's advance online publication list includes a good example of the sort of coolultra-cold atom physics that I talked about at and after DAMOP, so let's take a look at this paper in the usual Q&A format:

So, the title talks about frustrated magnetism. Have they started publishing papers about experiments that just won't work no matter what you try? No, the use of "frustrated" there is a term of art. "Frustrated magnetism" is the term for a situation where there's no way for a system of spins to get into the state that they really want to be in.

OK, what? Well, every system in physics always "wants" to have the lowest possible energy, like a dog who always wants to be napping in a comfortable spot. If you're clever, though, you can set up a system where there's no easy way for all of the particles in a large collection of things to arrange themselves so as to satisfy their individual desires.

The classic example of this is a bunch of spins on a triangular lattice, like the arrangement in this picture:

The spins at the corners of the triangle act like little magnets, and they're happiest when each spin is pointing in the opposite direction from its nearest neighbors. On a triangular lattice, though, there's no way to do that with simple up-and-down spins-- if spin 1 is up, and spin 2 is down, then there's no good state for spin 3. It wants to be down, because it's next to spin 1, but it also wants to be down, because it's next to spin 2.

Because this system can't find a simple arrangement that makes all the spins happy, it's called a "frustrated" system. This turns out to be an important problem in statistical and condensed matter physics, because it's really simple to make systems like this, but figuring out exactly what they're going to do is a tricky problem with a lot of rich physics in it.

So, what do they do? Well, there are a lot of different ways for this to work itself out, depending on the details of the lattice. They show the major options in this figure:
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Struck, J., Olschlager, C., Le Targat, R., Soltan-Panahi, P., Eckardt, A., Lewenstein, M., Windpassinger, P., & Sengstock, K. (2011) Quantum Simulation of Frustrated Classical Magnetism in Triangular Optical Lattices. Science. DOI: 10.1126/science.1207239

July 12, 2011
12:34 PM
844 views

It's Magnetic Moment Season: Measuring Various g-Factors

by Chad Orzel in Uncertain Principles

Among the articles highlighted in this week's Physics is one about a new test of QED through a measurement of the g-factor of the electron in silicon ions. This comes on the heels of a measurement of proton spin flips (this includes a free PDF) a couple of weeks ago, and those, in turn, build on measurements of electrons from a few years back, which Jerry Gabrielse talked about at DAMOP. Evidently, it's magnetic moment season in the world of physics.

The media reports on the proton experiment tend to be a little garbled in a way that reveals the writers don't quite understand what's going on. So let's take a crack at explaining this in ResearchBlogging form, in the usual Q&A format.

So, what's the deal with all this stuff? These look like very different experiments, and I don't see how they're related. Well, they're all using different systems, but in the end, they're after the same thing: measurements of what's known as the "g-factor" for some particle or another. This is a dimensionless number that relates the spin of a particle to its magnetic moment.

OK, what? Well, fundamental particles like electrons have a property known as "spin," which is an intrinsic angular momentum, as if the particles were spinning. They're not literally spinning balls of charge, and the behavior is somewhat strange compared to ordinary spinning objects (as explained with SteelyKid's help last summer), but in many ways they behave as if they were little spinning charges.

One of the things that spinning charges do is they produce a small magnetic field. For a literal spinning ball of charge, you can understand this by thinking of the charge on the surface as a little loop of current, which produces a magnetic field characterized by the "magnetic moment." If electrons are going to have the properties of a spinning thing, then, they need to produce a small magnetic field of their own.

If the electron was really a spinning charged ball, then the conversion from the rate of spin to the size of its magnetic moment would be really simple. Since the electron isn't really a spinning ball, though, there's a numerical factor that enters into the calculation. This is generally given the symbol "g," and thus is known as the "g-factor" for the electron.

And this thing has been measured? And calculated theoretically. The best current measurement of the electron g-factor was made by the Gabrielse group at Harvard, and gives a value of:

g/2 = 1.001 159 652 180 73 (28)

This agrees perfectly with a theoretical calculation of the expected value using quantum electro-dynamics (QED), which is why I tend to say that QED is the most precisely tested theory in the history of science.

How do they measure all those digits? It's a really impressive trick, involving holding a single electron in a "Penning trap," which confines it with the help of a strong magnetic field.
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Sturm, S., Wagner, A., Schabinger, B., Zatorski, J., Harman, Z., Quint, W., Werth, G., Keitel, C., & Blaum, K. (2011) g Factor of Hydrogenlike ^{28}Si^{13 }. Physical Review Letters, 107(2). DOI: 10.1103/PhysRevLett.107.023002

Ulmer, S., Rodegheri, C., Blaum, K., Kracke, H., Mooser, A., Quint, W., & Walz, J. (2011) Observation of Spin Flips with a Single Trapped Proton. Physical Review Letters, 106(25). DOI: 10.1103/PhysRevLett.106.253001

Hanneke, D., Fogwell, S., & Gabrielse, G. (2008) New Measurement of the Electron Magnetic Moment and the Fine Structure Constant. Physical Review Letters, 100(12). DOI: 10.1103/PhysRevLett.100.120801

Odom, B., Hanneke, D., D’Urso, B., & Gabrielse, G. (2006) New Measurement of the Electron Magnetic Moment Using a One-Electron Quantum Cyclotron. Physical Review Letters, 97(3). DOI: 10.1103/PhysRevLett.97.030801

June 7, 2011
10:58 AM
955 views

Commanding the Power of Thor...ium: "Wigner Crystals of 229Th for Optical Excitation of the Nuclear Isomer"

by Chad Orzel in Uncertain Principles

I have to admit, I'm writing this one up partly because it lets me use the title reference. It's a cool little paper, though, demonstrating the lengths that physicists will go to in pursuit of precision measurements.

I'm just going to pretend I didn't see that dorky post title, and ask what this is about. Well, it's about the trapping and laser cooling of thorium ions. They managed to load thorium ions into an ion trap, and use lasers to lower their temperature into the millikelvin range. At such low temperatures, the ions in the trap "crystallize."

So, they've demonstrated that if you get something cold, it forms a solid? Dude, that's not shocking new physics. There are scare quotes around "crystallize" for a reason. They're not forming a real crystal, in large part because we're talking about triply ionized thorium here, so each has a charge of +3 electron charges. They repel each other pretty strongly, and if they weren't held in a trap, they'd fly apart at high speed rather than forming a solid.

The "Wigner crystal" that forms is a regularly spaced arrangements of these ions, each being more or less stationary, separated from all its neighbors because of the electric force between them.They make these really nifty pictures using light scattered by the ions during the cooling process, so that each ion shows up as a dot of light:

Pretty blue dots! That's false color-- the actual light being used is in the infrared, at 984 nm. But yes, it's a pleasing color choice.

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C. J. Campbell, A. G. Radnaev, & A. Kuzmich. (2011) Wigner Crystals of 229Th for Optical Excitation of the Nuclear Isomer. Physical Review Letters, 223001. DOI: 10.1103/PhysRevLett.106.223001

June 3, 2011
10:37 AM
1,064 views

Watching Photons Interfere: "Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer"

by Chad Orzel in Uncertain Principles

An explanation of a Science paper on using weak measurements to trace the average trajectories of photons in a double-slit interference experiment.... Read more »

Sacha Kocsis, Boris Braverman, Sylvain Ravets, Martin J. Stevens, Richard P. Mirin, L. Krister Shalm, & Aephraim M. Steinberg. (2011) Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer. Science, 332(6034), 1179-1173. info:/10.1126/science.1202218

May 27, 2011
11:02 AM
969 views

What Goes Around Is Really Round: "Improved measurement of the shape of the electron"

by Chad Orzel in Uncertain Principles

The big physics story of the week is undoubtedly the new limit on the electric dipole moment (EDM) of the electron from Ed Hinds's group at Imperial College in the UK. As this is something I wrote a long article on for Physics World, I'm pretty psyched to see this getting lots of media attention, and not just from physics outlets.

My extremely hectic end-of-term schedule and general laziness almost make me want to just point to my earlier article and have done with it. But really, it's a big story, and one I've been following for a while, so how can I pass up the chance for a ResearchBlogging post on this?

OK, you said this is about a dipole moment, but the headlines all talk about measuring the shape of an electron. What do these have to do with one another? A "dipole moment" is just a bit of mathematical apparatus used to describe a non-spherical distribution of charge. It turns out to be mathematically convenient to talk about "polar moments" of various fields in electricity and magnetism. The simplest sort of field is a "monopole," made by a point charge, which pushes other like charges directly outward from itself. Slightly more complicated than that is a "dipole" pattern, which is like what you get when you sprinkle iron filings over a magnet-- the field pushes out at one end, and pulls in at the other, and has some sideways component in between. You can make an electric dipole by putting a negative point charge close to but not exactly on top of a positive point charge.

So, an electron is made up of a little positive thing stuck to a bigger negative thing? There doesn't need to be actual positive charge present-- you can just take some of the negative charge from one pole of a spherical ball of charge and move it to the other pole. That creates a little bit of a dipole moment, too, without needing any of the opposite charge.

OK, so an electron is supposed to be like a ball of charge with a bump on one side and a divot on the other? Well, it could effectively look like that, but this measurement shows that it doesn't. Which in some ways isn't surprising, because it shouldn't be anything but round, according to the simplest models of physics.

Wait, what? These guys set out to measure something that shouldn't exist, and they're getting in all the papers for not finding it? Isn't that kind of a racket? Yes and no. The simplest models of physics tell us that the electron shouldn't have an EDM, because it would violate time-reversal symmetry.

What does making an electron lumpy have to do with time? The thing is, the electron isn't just a point charge. It also has a magnetic dipole moment, which is associated with a property called "spin," because it looks like what you would get if the electron were a spinning ball of charge (it's not literally a spinning ball, but it behaves as if it were). The magnetic dipole moment points along the axis of the spin, and the electric dipole moment, if it exists, must also point along the spin axis, in either the same direction or exactly the opposite direction.

Now, the laws of physics should be symmetric in time-- that is, if you made a movie of a simple particle's behavior, and ran it backwards, there shouldn't be any way to tell which direction the video was playing. An electron EDM violates this, though, as seen in this picture lifted from my Physics World article:

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Hudson, J., Kara, D., Smallman, I., Sauer, B., Tarbutt, M., & Hinds, E. (2011) Improved measurement of the shape of the electron. Nature, 473(7348), 493-496. DOI: 10.1038/nature10104

May 16, 2011
11:31 AM
934 views

Active Engagement Works: "Improved Learning in a Large-Enrollment Physics Class"

by Chad Orzel in Uncertain Principles

Physics is a notoriously difficult and unpopular subject, which is probably why there is a large and active Physics Education Research community within physics departments in the US. This normally generates a lot of material in the Physical Review Special Topics journal, but last week, a PER paper appeared in Science, which is unusual enough to deserve the ResearchBlogging treatment.

OK, what's this paper about? Well, with the exceptional originality that physicists bring to all things, the title pretty much says it all. They demonstrated that a different style of teaching applied to a large lecture class produced better attendance, more student engagement, and better learning as compared to a control section of the same course taught at the same time.

So, they showed that there are better methods than the traditional lecture. Haven't we known that for decades? How does that get into Science? Well, this is an exceptionally clean test, with all the sorts of controls you would want for good science. They took two sections of a huge introductory class, about 270 students each, and for a one-week period, they had one section taught by the regular professor (a highly regarded lecturer) and one section taught by a post-doc trained in a new teaching method. They covered the same material, using many of the same in-class examples and "clicker" questions, and at the end of the week gave both sections a short exam on the material just covered.

And the results were impressive? Very. The students from the experimental section got an average score of 74% on the test, compared to 41% for the control section. The two distributions were really dramatically different:

Yeah, that's a pretty dramatic difference. Are you sure they got the same test? It says that they did-- I'm not in British Columbia, so I can't confirm it. Interestingly, it notes that the experimental section only covered 11 of the 12 topics on the test due to time constraints, so they were starting with a slight handicap.

So what's the brilliant new method? Basically, making the class more participatory. Read the rest of this post... | Read the comments on this post...... Read more »

Deslauriers, L., Schelew, E., & Wieman, C. (2011) Improved Learning in a Large-Enrollment Physics Class. Science, 332(6031), 862-864. DOI: 10.1126/science.1201783

May 9, 2011
10:49 AM
903 views

On the "Hot Hand" in Basketball

by Chad Orzel in Uncertain Principles

A little while back, Jonah Lehrer did a nice blog post about reasoning that used the famous study by Gilovich, Vallone and Tversky, The Hot Hand in Basketball (PDF link) as an example of a case where people don't want to believe scientific results. The researchers found absolutely no statistical evidence of "hot" shooting-- a player who had made his previous couple of shots was, if anything, slightly less likely to make the next one. Lehrer writes:

Why, then, do we believe in the hot hand? Confirmation bias is to blame. Once a player makes two shots in a row - an utterly unremarkable event - we start thinking about the possibility of a streak. Maybe he's hot? Why isn't he getting the ball? It's at this point that our faulty reasoning mechanisms kick in, as we start ignoring the misses and focusing on the makes. In other words, we seek out evidence that confirms our suspicions of streakiness. The end result is that a mental fiction dominates our perception of the game.

Here's where things get meta: Even though I know all about Tversky and Gilovich's research - and fully believe the data - I still perceive the hot hand. I can't help but watch the NBA playoffs and marvel at the streakiness of shooters, from Kobe to Rose. (Personally, I'd love to see an analysis of Ray Allen. If that man doesn't show the hot hand, then it really doesn't exist.) And I'm not alone in my stubborn skepticism. Red Auerbach, the legendary coach of the Celtics, reportedly responded to Tversky's statistical analysis with a blunt dismissal. "So he makes a study," Auerbach said. "I couldn't care less."

Elsewhere in blogdom, Paul Waldman looks for ways the study could be wrong, and Kevin Drum pokes fun at him for doing so, noting that "it's interesting how unwilling most athletes are to accept the results of this study."

It's not hard to see why people who play basketball find this result surprising-- shooting a basketball in a game situation is a complicated process involving lots of factors-- balance, timing, sight lines, defense (though, to be fair, the study uses data from the NBA in the early 1980's, so it's probably safe to exclude the effects of defense...)-- and it seems difficult to believe that all of those combine to give a single unchanging chance of success. And if you've played a lot of basketball, you know that there are days when one or more of those things just doesn't feel right-- where you're rushing your shots for some reason, or the ball keeps slipping, or something like that.

The study itself looks pretty solid, though. There's only one real weak point to it, that I see, which is that not all shots are equal.
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GILOVICH, T. (1985) The hot hand in basketball: On the misperception of random sequences*1. Cognitive Psychology, 17(3), 295-314. DOI: 10.1016/0010-0285(85)90010-6

May 2, 2011
10:55 AM
968 views

Proving Einstein Wrong...ish: Measurement of the Instantaneous Velocity of a Brownian Particle

by Chad Orzel in Uncertain Principles

Last summer, there was a fair bit of hype about a paper from Mark Raizen's group at Texas which was mostly reported with an "Einstein proven wrong" slant, probably due to this press release. While it is technically true that they measured something Einstein said would be impossible to measure, that framing is a little unfair to Einstein. It does draw media attention, though...

The experiment in question involves Brownian motion, and since I had to read up on that anyway for something else, I thought I might as well look up this paper, and write it up for the blog.

OK, so what did they do that Einstein said they couldn't? The title pretty much give it away: they measured the instantaneous velocity of a particle undergoing Brownian motion. They made very careful measurements of the position and velocity of a tiny glass bead suspended in air, and showed that they don't fit the prediction of Einstein's model of Brownian motion.

Let's pretend that I'm too lazy to click that link above and read about Brownian motion, and have you explain it quickly here. Brownian motion is a sort of jittering motion of small particles suspended in a fluid. The motion was observed by lots of people, but takes its name from the British botanist Robert Brown, who was the first to rule out the presumed explanation that had been believed to be the cause, namely that the jittering was the motion of tiny living creatures.

What's this got to do with Einstein? Are these things jittering at the speed of light, or something? Einstein's most famous for his work on relativity, but his background was in what we'd now call statistical mechanics. His Ph.D. thesis, one of his five great 1905 publications, was about the diffusion of molecules in a solution, and provided a clever way to estimate Avogadro's number. Following closely on that work was a paper explaining Brownian motion-- Abraham Pais says in Subtle Is the Lord... that it was finished just 11 days after the thesis. Einstein's model for Brownian motion made a quantitative prediction that could be directly measured, and put together with the work of a few other people at around the same time, this helped conclusively settle the question of the existence of atoms.

Wait, what? I thought people knew about atoms in, like, ancient Greece, and stuff. The name comes from the atomist philosophies of the ancient world, but up until the early 20th century, there was active debate about whether the notional atoms used in physics and chemistry were real microscopic particles or just a convenient mathematical fiction. Einstein's work on Brownian motion helped conclusively prove that atoms are real physical entities.

OK, how? Einstein showed that the characteristic jittering of Brownian motion could be explained as collisions between the atoms making up the fluid colliding with the larger particle and causing it to move. Any object in a fluid is constantly bombarded from all sides by the background atoms, and each collision causes a corresponding change in the motion of the observed particle. When you carefully work through the implications of this, you find that on average, the displacement of the object from it starting position should increase in a very particular way-- as the square root of the time since the measurement started.

And Einstein said this could never be measured? No, what Einstein said was that this average displacement was the only thing that could be measured. That is, he said that the only thing you could hope to measure was the aggregate effect of vast numbers of atomic collisions, through the displacement, and not the instantaneous velocity changes caused by the collisions.

And that's wrong? Well, it was perfectly true in 1905. The technology for doing this sort of thing has advanced quite a bit over the intervening 105 years. Hence this paper.

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Li, T., Kheifets, S., Medellin, D., & Raizen, M. (2010) Measurement of the Instantaneous Velocity of a Brownian Particle. Science, 328(5986), 1673-1675. DOI: 10.1126/science.1189403

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 21, 2011
11:17 AM
990 views

Bouncing Neutrons for Fun and Science: "Realization of a gravity-resonance-spectroscopy technique"

by Chad Orzel in Uncertain Principles

Several people blogged about a new measurement of gravitational states of neutrons done by physicists using ultracold neutrons from the Institut Laue-Langevin in France. I had to resort to Twitter to get access to the paper (we don't get Nature Physics here, and it's way faster than Inter-Library Loan), but this is a nice topic for a ResearchBlogging post, in the now-standard Q&A form:

OK, why was this worth begging people on Twitter to send you a copy? The paper is a demonstration of a sort of spectroscopy of neutrons bouncing in a gravitational field. They showed they could drive neutrons bouncing on a "mirror" between two of the discrete quantum states of the system, and measure the energy difference between those states very accurately.

Wait, neutrons bouncing on a mirror have discrete states? Why doesn't anybody tell me these things? Well, you didn't ask. Anyway, yes, neutrons bouncing on a mirror have discrete states, just like any other quantum system. Quantum mechanics tells us that confined systems will always exist only in special discrete states-- that's what puts the "quantum" in "quantum mechanics," after all.

But how are these confined? They're confined by gravity. To do the experiments, they send a beam of extremely slow-moving neutrons above a polished glass surface. When the neutrons fall under the influence of gravity, they hit the surface and bounce back upward. On the high side, the neutrons have only a limited amount of energy, and once all the kinetic energy of their vertical motion has been turned into gravitational potential energy, they turn around and fall back down, just like a tennis ball thrown up into the air for a dog to chase after.

Yeah, but tennis balls don't have discrete states. They do, you just can't tell the difference between them very easily, because they're so close together in energy, and the wavelength is so small. A sample of slow-moving neutrons, though, can clearly show these different states, which are described by wavefunctions that look like this:

The solid lines show the probability of finding the neutron at a given height (probability increasing to the left) for the first four states of a neutron bouncing on their mirror. It's taken from an older paper (from 2002) where they demonstrated the existence of these quantized states.

How did they do that? The basic technique is the same one they used for the detection in this experiment: they put an absorber above their mirror at a set height, to block any neutrons in states that extended up too high.
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Jenke, T., Geltenbort, P., Lemmel, H., & Abele, H. (2011) Realization of a gravity-resonance-spectroscopy technique. Nature Physics. DOI: 10.1038/nphys1970

Nesvizhevsky, V., Börner, H., Petukhov, A., Abele, H., Baeßler, S., Rueß, F., Stöferle, T., Westphal, A., Gagarski, A., Petrov, G.... (2002) Quantum states of neutrons in the Earth's gravitational field. Nature, 415(6869), 297-299. DOI: 10.1038/415297a

April 18, 2011
10:35 AM
1,286 views

Wave Nature Gets Bigger: "Quantum interference of large organic molecules"

by Chad Orzel in Uncertain Principles

It's been a while since I wrote up a ResearchBlogging post, but since a recent paper forced me to update my "What Every Dog Should Know About Quantum Physics" slides with new pictures, I thought I should highlight the work on the blog as well. Not that you could've missed it, if you follow physics-y news-- it's been all over, getting almost as much press as rumors that some people whose funding will run out soon saw something intriguing in their data. So, in the usual Q&A format:

OK, what's this about? Well, the paper title, "Quantum interference of large organic molecules" pretty well says it all. A group in Austria, headed by Markus Arndt, with Stefan Gerlich as the lead author, has demonstrated interference effects using large organic molecules. When they pass a beam of these molecules through an interferometer, they behave exactly like waves, producing an interference pattern analogous to the bright and dark spots you get by sending light into a diffraction grating.

That sounds cool, but didn't they do this before? They've shown interference of organic molecules before-- in fact, Arndt did the first demonstration of this using fullerene molecules, way back in 1999. I even wrote up their results in How to Teach Physics to Your Dog, and you can see one of their figures in the preview chapter at dogphysics.com.

So why are you fired up about this new paper? Because they've scaled things up to a much bigger system, and still demonstrated clear interference.

What do you mean, "scaled up"? Well, the clearest answer I can give is to copy a figure from their paper, showing the different molecules they have used:

See that little ball labeled "a"? That's the fullerene, C60 that they used before. The big huge things on the right side of the figure, labeled "b," "c," "e," and "f" are the molecules they used this time around. They contain up to 430 atoms, and are several nanometers across, making them by far the largest molecules objects anybody has ever seen displaying wave behavior.

OK, I'll admit, that's pretty cool. So, what, they just made a teeny-tiny little diffraction grating, and shot these molecules at it? Not exactly...

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Gerlich, S., Eibenberger, S., Tomandl, M., Nimmrichter, S., Hornberger, K., Fagan, P., Tüxen, J., Mayor, M., & Arndt, M. (2011) Quantum interference of large organic molecules. Nature Communications, 263. DOI: 10.1038/ncomms1263

Hornberger, K., Gerlich, S., Ulbricht, H., Hackermüller, L., Nimmrichter, S., V Goldt, I., Boltalina, O., & Arndt, M. (2009) Theory and experimental verification of Kapitza–Dirac–Talbot–Lau interferometry. New Journal of Physics, 11(4), 43032. DOI: 10.1088/1367-2630/11/4/043032

November 19, 2010
10:38 AM
1,108 views

Interference of Independent Photon Beams: The Pfleegor-Mandel Experiment

by Chad Orzel in Uncertain Principles

Earlier this week, I talked about the technical requirements for taking a picture of an interference pattern from two independent lasers, and mentioned in passing that a 1967 experiment by Pfleegor and Mandel had already shown the interference effect. Their experiment was clever enough to deserve the ResearchBlogging Q&A treatment, though, so here we go:

OK, so why is this really old experiment worth talking about? What did they do? They demonstrated interference between two completely independent lasers, showing that when they overlapped the beams, the overlap region contained a pattern of bright and dark spots characteristic of interference.

How did they do that in 1967? What did they use, photographic plates? No, they used photomultiplier tubes, that produce an electrical pulse when a single photon falls on them.

But a PMT only detects photons in a single position. How did they make a picture out of that? They didn't, because they found a clever way to arrange it so they didn't need to. Here's a schematic of their apparatus:

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Pfleegor, R., & Mandel, L. (1967) Interference of Independent Photon Beams. Physical Review, 159(5), 1084-1088. DOI: 10.1103/PhysRev.159.1084

November 17, 2010
03:48 PM
1,094 views

Trapped Antihydrogen

by Chad Orzel in Uncertain Principles

The big physics-y news story of the moment is the trapping of antihydrogen by the ALPHA collaboration at CERN. The article itself is paywalled, because this is Nature, but one of the press offices at one of the institutions involved was kind enough to send me an advance version of the article. This seems like something that deserves the ResearchBlogging Q&A treatment, so here we go:

OK, what's the deal with this paper? Well, the ALPHA collaboration is announcing that they have created antihydrogen atoms-- that is, a single antiproton orbited by a single positron-- at low temperatures, and confined them in a magnetic trap for something like 172 ms.

Awesome! When can we blow up the Vatican? Settle down. We're not talking huge quantities of antimatter, here. In 335 runs with their apparatus, they detected all of 38 atoms of antihydrogen. You're not going to be blowing anything up soon.

What's the point of making antimatter if you can't use it to blow stuff up? The point is to understand the laws of physics better. If you can do spectroscopy of anti-atoms, it will tell us a lot about whether antimatter obeys the same laws as ordinary matter, which might provide a clue as to why everything we see seems to be made of ordinary matter. You could also use it to test how antimatter interacts with gravity, which is something we don't currently have any way to test.
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Andresen, G., Ashkezari, M., Baquero-Ruiz, M., Bertsche, W., Bowe, P., Butler, E., Cesar, C., Chapman, S., Charlton, M., Deller, A.... (2010) Trapped antihydrogen. Nature. DOI: 10.1038/nature09610

November 12, 2010
12:51 PM
1,221 views

Relativity on a Human Scale: "Optical Clocks and Relativity"

by Chad Orzel in Uncertain Principles

As mentioned in yesterday's post on ion trapping, a month or so back Dave Wineland's group at NIST published a paper in Science on using ultra-precise atomic clocks to measure relativistic effects. If you don't have a subscription to Science, you can get the paper for free from the Time and Frequency Division database, because you can't copyright work done for the US government.

This paper generated quite a bit of interest when it came out, because it demonstrates the time-slowing effects of relativity without any need for exotic objects like black holes or particle accelerators-- they deal with objects moving small distances at low speeds, and the results agree very nicely with the predictions of relativity.

I'm a little late for the buzz, but it's a cool enough experiment that it's worth unpacking a little in the usual Q&A format for ResearchBlogging:

OK, what's the deal with this? Well, they used a pair of identical atomic clocks of exceptional precision to measure relativistic effects at everyday scales. Their clocks are good enough to be able to detect shifts on the order of a few parts in 1016, which means they can see the slowing of time due to motion at a walking pace, and due to elevation changes of less than a meter.

Back up a bit-- what's this "atomic clock" business? Well, as I explained a few years ago, an atomic clock measures time by making use of quantum physics. Atoms will only absorb light of certain very specific frequencies, so you can use an atom as a perfect frequency reference to determine the frequency of a light source-- if it absorbs the light, you're at the right frequency, and if it doesn't, you can correct the frequency until it does. If you keep comparing your light to the atoms, and correcting the frequency, you can make a light source whose frequency can be used as a reference to mark the passage of time.

And this lets you test relativity? If your clock is good enough, yes.
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Chou, C., Hume, D., Rosenband, T., & Wineland, D. (2010) Optical Clocks and Relativity. Science, 329(5999), 1630-1633. DOI: 10.1126/science.1192720

Schmidt, P. (2005) Spectroscopy Using Quantum Logic. Science, 309(5735), 749-752. DOI: 10.1126/science.1114375

August 30, 2010
12:49 PM
1,152 views

Indirect Excitation Control: Ultrafast Quantum Gates for Single Atomic Qubits

by Chad Orzel in Uncertain Principles

Last week, John Baez posted a report on a seminar by Dzimitry Matsukevich on ion trap quantum information issues. In the middle of this, he writes:

Once our molecular ions are cold, how can we get them into specific desired states? Use a mode locked pulsed laser to drive stimulated Raman transitions.

Huh? As far as I can tell, this means "blast our molecular ion with an extremely brief pulse of light: it can then absorb a photon and emit a photon of a different energy, while itself jumping to a state of higher or lower energy."

I saw this, and said "Hey, that's a good topic for a blog post." And on Friday, the new issue of Physical Review Letters included a new paper on just this topic (arxiv version for those without subscription access), making it a good topic for a ResearchBlogging post. So,

So, what's this all about? The paper reports on a new way of moving atoms from one state to another much faster than is possible with more typical methods. This is potentially useful for speeding up the operation of a quantum computer.

Transition speeds are critically important for quantum computing, because all quantum information processing systems are subject to some interactions with the environment that will eventually destroy the quantum character of the information through the process known as "decoherence." If you do a really good job, you can get decoherence times that are measured in seconds, which sets an upper limit on the number of operations you can do with a simple system before decoherence kills you (you can do quantum error correction to extend that, but then things start to get complicated). If you can do your state-change operations in 50 picoseconds rather than tens or hundred of microseconds, you can pack a lot more computing into that same amount of time.
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Campbell, W., Mizrahi, J., Quraishi, Q., Senko, C., Hayes, D., Hucul, D., Matsukevich, D., Maunz, P., & Monroe, C. (2010) Ultrafast Gates for Single Atomic Qubits. Physical Review Letters, 105(9). DOI: 10.1103/PhysRevLett.105.090502

August 26, 2010
12:42 PM
1,271 views

Measuring Gravity: Ain't Nothin' but a G Thing

by Chad Orzel in Uncertain Principles

There's a minor scandal in fundamental physics that doesn't get talked about much, and it has to do with the very first fundamental force discovered, gravity. The scandal is the value of Newton's gravitational constant G, which is the least well known of the fundamental constants, with a value of 6.674 28(67) x 10-11 m3 kg-1 s-2. That may seem pretty precise, but the uncertainty (the two digits in parentheses) is scandalously large when compared to something like Planck's constant at 6.626 068 96(33) x 10-34 J s. (You can look up the official values of your favorite fundamental constants at this handy page from NIST, by the way...)

To make matters worse, recent measurements of G don't necessarily agree with each other. In fact, as reported in Nature, the most recent measurement, available in this arxiv preprint, disagrees with the best previous measurement by a whopping ten standard deviations, which is the sort of result you should never, ever see.

This obviously demands some explanation, so:

What's the deal with this? I mean, how hard can it be to measure gravity? You drop something, it falls, there's gravity. It's easy to detect the effect of the Earth's gravitational pull, but that's just because the Earth has a gigantic mass, making the force pretty substantial. If you want to know the precise strength of gravity, though, which is what G characterizes, you need to look at the force between two smaller masses, and that's really difficult to measure.

Why? I mean, why can't you just use the Earth, and measure a big force? If you want to know the force of gravity to a few parts per million, you would need to know the mass of the Earth to better than a few parts per million, and we don't know that. A good measurement of G requires you to use test masses whose values you know extremely well, and that means working with smaller masses. Which means really tiny forces-- the force between two 1 kg masses separated by 10 cm is 6.6 x 10-9 N, or about the weight of a single cell.

OK, I admit, that's a bit tricky. So how do they do it? There are four papers cited in the Nature news article. I'll say a little bit about each of them, and how they figure into this story.
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Gundlach, J., & Merkowitz, S. (2000) Measurement of Newton's Constant Using a Torsion Balance with Angular Acceleration Feedback. Physical Review Letters, 85(14), 2869-2872. DOI: 10.1103/PhysRevLett.85.2869

Schlamminger, S., Holzschuh, E., Kündig, W., Nolting, F., Pixley, R., Schurr, J., & Straumann, U. (2006) Measurement of Newton’s gravitational constant. Physical Review D, 74(8). DOI: 10.1103/PhysRevD.74.082001

Luo, J., Liu, Q., Tu, L., Shao, C., Liu, L., Yang, S., Li, Q., & Zhang, Y. (2009) Determination of the Newtonian Gravitational Constant G with Time-of-Swing Method. Physical Review Letters, 102(24). DOI: 10.1103/PhysRevLett.102.240801

Harold V. Parks, & James E. Faller. (2010) A Simple Pendulum Determination of the Gravitational Constant. Physical Review Letters (accepted). arXiv: 1008.3203v2

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