After a short post-March Meeting lag, Physics World is back to announcing really cool physics results, this time highlighting a paper in Nature (subscription required) by a French group who have observed the birth and death of photons in a cavity. I’m not sure how it is that the French came to dominate quantum optics, but between Serge Haroche and Alain Aspect, most of the coolest experiments in the field seem to have been done in France.
In this particular case, they set up a superconducting resonant cavity for microwaves. Basically, this is like two mirrors facing one another, and a photon placed between the two will bounce back and forth many times before escaping. The lifetime is a bit over a tenth of a second, which doesn’t sound like much, but as they note in the paper, the light travels 39,000 km in that time, bouncing back and forth between two mirrors that are 2.7 cm apart. That’s an impressive number of bounces.
Part of the process of preparing the cavity is to cool it down to extremely low temperature– 0.8 Kelvin. One of the consequences of this is that there is a very low probability of finding a photon in the cavity. The temperature isn’t zero, though, so there is some thermal energy in the system, and this will occasionally manifest as a photon in the cavity. The photon will appear, stick around for a tenth of a second or so, and then disappear again. Averaged over a long time, there’s roughly a 5% chance of finding a photon in the cavity.
This thermal excitation is a prediction of quantum theory going all the way back to Max Planck, but it’s really hard to observe. What the ENS group has done is to employ a very clever trick to detect the presence or absence of a photon in the cavity without absorbing it, or introducing more photons into the cavity.
They do this by sending a beam of rubidium atoms through the space between the mirrors. The atoms are prepared in such a way that they won’t absorb photons at the frequency trapped by the cavity, but they do interact with the light. If there’s a photon in the cavity, it will cause a minute shift in the energy levels of the atom while it’s in the cavity.
This shift is very small, and only lasts a short time, but its effect can be measured using a sensitive enough technique. The technique they use is the same interferometric method used to make atomic clocks: they pass the atoms through a microwave cavity to prepare them in a particular state, then through the test cavity (which may or may not contain a photon), and then a second microwave cavity to analyze the state. The shift in the energy levels caused by the presence of a photon in the test cavity changes the rate at which the atoms “tick” in the clock, and changes the outcome. If the test cavity is empty, the atoms will end up in one energy state (they label it “g,” but for technical reasons, it’s not actually the ground state of the system), and if the test cavity contains a photon, the atoms end up in a different state (labelled “e”).
What they do, then, is to set up the system with an empty cavity, and then send a beam of atoms through the test cavity, one after another. They see a stream of atoms coming out in “g,” indicating that the cavity is empty, but every now and then, they get a burst of “e” atoms, indicating that a photon has appeared in the cavity. They get “e” atoms for a short time, and when the photon leaves the cavity, they go back to “g” atoms all the time.
It’s a beautiful experiment, and the results are very clean and clear. By running for a long time, they can watch many photons appearing and disappearing, and see how long each of them lasts. Putting together a large number of these measurements, they construct a graph showing the evolution of the photon state over time, and there’s a very nice figure in the paper (which I can’t reproduce here at the moment, because I can’t get the paper electronically at home) showing how single photon measurements build up a smooth ensemble average.
It’s really outstanding work. Of course, you knew that, because it was in Nature… Between this and the Aspect group’s recent delayed-choice experiment, there really can’t be any doubt that we’re in the golden age of experimental quantum optics.