Quantum Optics
drorzel | August 30, 2010Last 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…
drorzel | August 25, 2010The Joerg Heber post that provided one of the two papers for yesterday's Hanbury Brown Twiss-travaganza also included a write-up of a new paper in Nature on Mott insulators, which was also written up in Physics World.
Most of the experimental details are quite similar to a paper by Markus Greiner's group I wrote up in June: They make a Bose-Einstein Condensate, load it into an optical lattice, and use a fancy lens system to detect individual atoms at sites of the lattice. This lattice can be prepared in a "Mott insulator" state, where each site is occupied by a definite number of atoms. As…
drorzel | August 24, 2010Two 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…
drorzel | August 23, 2010I'm a big fan of review articles. For those not in academic science, "review article" means a long (tens of pages) paper collecting together the important results of some field of science, and presenting an overview of the whole thing. These vary somewhat in just how specific they are-- some deal with both experiment and theory, others just theoretical approaches-- and some are more readable than others, but typically, they're written in a way that somebody from outside the field can understand.
These are a great boon to lazy authors, or authors facing tight page limits ("Ref. [1] and…
drorzel | August 20, 2010In my post about how we know photons exist, I make reference to the famous Kimble, Dagenais, and Mandel experiment showing "anti-bunching" of photons emitted from an excited atom. They observed that the probability of recording a second detector "click" a very short time after the first was small. This is conclusive evidence that photons are real, and that light has discrete particle-like character. Or, as I said in that post:
This anti-bunching effect is something that cannot be explained using a classical picture of light as a wave. Using a wave model, in which light is emitted as a…
drorzel | August 18, 2010Most of the time, when we talk about seeing quantum effects from light, we talk about extremely weak beams-- looking at intensities where one photon more or less represents a significant change in the intensity of the light. Last week, though, Physics Buzz wrote up a paper that goes in the other direction: they suggest a limit on the maximum strength of a laser pulse due to quantum effects, specifically the creation of particle-antiparticle pairs.
This is a little unusual, in that most of the time when people talk about really intense lasers, they end up discussing them as an oscillating…
drorzel | August 13, 2010Some folks I used to work with at NIST have looked at cheap green laser pointers, and found a potential danger. Some of the dimmer-looking green lasers are not so dim in the infrared, and in one case emitted 10X the rated power in invisible light. This could be a potential eye hazard.
You can read their full report on the arxiv. It's got a nice description of how green laser pointers turn infrared light into visible light, which is really pretty awesome-- a guy I met at a conference once declared them the coolest invention ever, because it's "quantum optics in the palm of your hand." Better…
drorzel | August 5, 2010A reader emailed me with a few questions regarding How to Teach Physics to Your Dog, one of which is too good not to turn into a blog post:
What is a photon from an experimental perspective?... Could you perhaps provide me with a reference that discusses some experiments and these definitional issues?
The short form of the experimental answer is "A photon is the smallest amount of light that will cause a detector to 'click.'" (For some reason, hypothetical light detector technology has never really advanced past the Geiger counter stage-- even though it's all electrical pulses these days, we…
drorzel | August 2, 2010Thoreau at Unqualified Offerings gets credit for inspiring two posts today with his proposed Murphy's Law experiment and this one, about an unrelated issue in quantum measurement. This is an analogy suggested by a colleague a couple of years ago, comparing the projection of a quantum wavefunction in the measurement process to the lottery.
The classic example of this problem is something like the double slit experiment with single particles. You have some position-sensitive detector that we can imagine as being made up of a large number of pixels, each having some probability of detecting a…
drorzel | August 2, 2010Over at Unqualified Offerings, Thoreau proposes an an experimental test of Murphy's Law using the lottery. While amusing, it's ultimately flawed-- Murphy's Law is something of the form:
Anything that can go wrong, will.
Accordingly, it can only properly be applied to situations in which there is a reasonable expectation of success, unless something goes wrong. The odds of winning the lottery are sufficiently low that Murphy's Law doesn't come into play-- you have no reasonable expectation of picking the winning lottery numbers, so there's no need for anything to "go wrong" in order for you…
drorzel | July 30, 2010Commenter miller asks:
It's often said that virtual particles can "borrow" energy, as long as it's for a short enough time to be compatible with the uncertainty principle. This never made sense to me, because the uncertainty principle says that product of uncertainty in energy and uncertainty in time is greater than h-bar over 2, not less than. Please explain.
The relevant equation is in the graphic at the top of this blog, just to the right of the title-- the one with ΔEΔt. It's easy to get turned around with this, due to the slightly unfamiliar business of working with inequalities.
The…
drorzel | July 30, 2010There's some good stuff in yesterday's post asking what physics you'd like to read more about. I'm nursing a sore neck and shoulder, so I'll only do one or two quick ones today, starting with James D. Miller in the first comment:
1) Is it true that our understanding of quantum physics comes from studying systems with only a small number of particles and there is a good chance our theories won't hold in more complex systems.
It all depends on how you define your terms-- what counts as a "small number" of particles, and what counts as not holding?
It's certainly true that most of the…
drorzel | July 27, 2010When one of the most recent issues of Physical Review Letters hit my inbox, I immediately flagged these two papers as something to write up for ResearchBlogging. This I looked at the accompanying viewpoint in Physics, and discovered that Chris Westbrook already did most of the work for me. And, as a bonus, you can get free PDF's of the two articles from the Physics link, in case you want to follow along at home.
Since I spent a little time thinking about these already, though, and because it connects to the question of electron spin that I talked about yesterday, I think it's still worth…
drorzel | July 26, 2010The subject of the "spin" of the electron comes up again and again, so as pointed out in a comment, I really ought to do a post explaining what it is and how it works. As a bonus, this gives me the opportunity to do the dorkiest thing anyone has ever done with a cute-toddler video, namely this one:
(That's an early version of SteelyKid's new favorite game. I'll put a clip of the final version of the game at the end of this post.)
So, electron spin. Electrons, and all other fundamental particles, have a property known as "spin." This is an intrinsic angular momentum associated with the…
drorzel | July 23, 2010This week's big story in physics is this Science paper by a group out of Austria Canada (edited to fix my misreading of the author affiliations), on a triple-slit interference effect. This has drawn both the usual news stories and also some complaining about badly-worded news stories. So, what's the deal?
What did they do in this paper? The paper reports on an experiment in which they looked at the interference of light sent through a set of three small slits, and verified that the resulting pattern agrees with the predictions of the Born rule for quantum probabilities.
What does Matt Damon…
drorzel | July 19, 2010Over at Jeff Vandemeer's blog, Rachel Swirsky has a series ofm guest posts (start here if you prefer direct post links) about the recently completed Launch Pad workshop. this is a NASA funded workshop bringing a group of writers together for six days of lectures on modern astronomy from working astronomers. From the workshop web site:
Launch Pad is a NASA-funded education/public outreach effort supplementing Mike Brotherton's space-based astronomical research. Our budget allows us to provide a workshop that is essentially free to participants. Our primary goal is to teach writers of all…
drorzel | July 9, 2010The big physics story at the moment is probably the new measurement of the size of the proton, which is reported in this Nature paper (which does not seem to be on the arxiv, alas). This is kind of a hybrid of nuclear and atomic physics, as it's a spectroscopic measurement of a quasi-atom involving an exotic particle produced in an accelerator. In a technical sense, it's a really impressive piece of work, and as a bonus, the result is surprising.
This is worth a little explanation, in the usual Q&A format.
So, what did they do to measure the size of a proton? Can you get rulers that small…
drorzel | June 22, 2010A press release from Harvard caught my eye last week, announcing results from Markus Greiner's group that were, according to the release, published in Science. The press release seems to have gotten the date wrong, though-- the article didn't appear in Science last week. It is, however, available on the arxiv, so you get the ResearchBlogging for the free version a few days before you can pay an exorbitant amount to read it in the journal.
The title of the paper is "Probing the Superfluid to Mott Insulator Transition at the Single Atom Level," which is kind of a lot of jargon. The key image is…
drorzel | June 21, 2010An experiment in Germany has generated a good deal of publicity by dropping their Bose-Einstein Cendensate (BEC) apparatus from a 146 meter tower. This wasn't an act of frustration by an enraged graduate student (anybody who has worked with BEC has probably fantasized about throwing at least part of their apparatus down a deep hole), but a deliberate act of science: They built a BEC apparatus that is entirely contained within a two-meter long capsule inside the evacuated drop tower at the Center of Applied Space Technology and Microgravity (which in German leads to the acronym ZARM, which…
drorzel | June 1, 2010While I mostly restricted myself to watching invited talks at DAMOP last week, I did check out a few ten-minute talks, one of which ended up being just about the coolest thing I saw at the meeting. Specifically, the Friday afternoon talk on observing relativity with atomic clocks by Chin-Wen Chou of the Time and Frequency Division at NIST in Boulder.
The real technical advance is in a recent paper in Physical Review Letters (available for free via the Time and Frequency Publications Database, because government research isn't subject to copyright): they have made improvements to their atomic…