Cold Atoms for Gravity Probes

This is nearly a month old, now, because I keep saying “Oh, Idon’t have time to do this justice– I’ll write about it tomorrow.” I really need to stop doing that.

Anyway, Physics News Update has a story about a scheme to measure gravity using Bloch oscillations, based on a paper in Physical Review Letters. This is especially interesting to me, because the most important paper of my career made use of Bloch oscillations to get our experimental signal.

A quick explanation below the fold:

Bloch oscillations are a weird phenomenon you encounter in condensed matter physics. The easiest way to understand it is in terms of Bragg scattering, which is the reflection of light off periodic structures. Bragg scattering works because light that is reflected off the front of a crystal can interfere with light reflected off the second row of atoms in, and the third, and the fourth, and so on. If the wavelength of the light is comparable to the spacing between planes of atoms, there will be some angle at which these beams will all interfere constructively. This is the basis of X-ray crystallography– by looking at the patterns that result when x-rays of known wavelength scatter off a crystal, you can say something about how the atoms in that crystal are arranged.

Bloch oscillations happen when you put material particles into a periodic structure, and apply a potential shift to cause those particles to move through the structure. The classic example is electrons in a periodic crystal being accelerated by an applied voltage. The applied voltage causes the electrons to accelerate in some direction, and because electrons behave like waves according to the de Broglie relation, as they accelerate, their wavelength gets shorter and shorter. Until it’s exactly equal to the spacing between atoms in the crystal, at which point the electrons Bragg diffract off the crystal, and instantaneously reverse their direction. They travel in the other direction for a while, and then reverse direction again, and the process repeats. Rather than moving smoothly through the crystal, they oscillate back and forth in a very regular way.

This works for neutral atoms held in a periodic optical potential made by a standing wave of light. The effect was first demonstrated by my postdoctoral research group (before I got there), and we used to use it to make a cleaner-looking signal in our BEC experiments. The current paper has refined this technique (and made a big gain by switching atoms, one of the rare cases of improving a cold-atoms experiment by not using rubidium) so that they can follow these oscillations over a period of several seconds, which is just amazing, as the period of oscillation is a couple of milliseconds.

In the neutral-atom system, the potential shift that causes the atoms to start accelerating and eventually Bragg diffract is gravity. The atoms are held in a vertical standing wave, and atoms that are higher up want to fall under the influence of gravity. The frequency of the oscillation is given by a very simple formula, and only depends on the mass of the atoms, the wavelength of the laser, and the acceleration due to gravity. The mass is known and the laser wavelength can be controlled, so measuring the oscillation frequency amounts to measuring the acceleration due to gravity, and if you can measure several thousand oscillations, you can really nail the frequency.

Their measurements are at least potentially sensitive enough to detect changes in the strength of gravity due to massive objects brought close to the atoms. This might allow measurements of the force of gravity on the micron scale, which is one of the few experiments with a realistic chance of testing the existence of extra dimensions. If it works, it would have the added bonus of completely different systematics than the torsion pendulum experiments at UW, which would be very cool.

Anyway, it’s a nifty experiment, and shows yet again that cold atoms are where it’s at…