As mentioned in a previous post, for the past few months the cloud of atoms in the center of the strontium optical lattice clock vacuum chamber in the Kolkowitz lab has held the (somewhat dubious) honor of being the second coldest place in Wisconsin at a temperature of ~1-2 milliKelvin, with the Saffman lab down the hall beating us out by a factor of ~1000x or so with their cesium quantum computing experiment, which reaches temperatures of ~2-5 microKelvin. We’re pleased to report that with the implementation of a second stage of narrow-line laser cooling we are now measuring strontium atom temperatures of ~4 microKelvin, making one small 0.5 x 0.5 x 0.5 mm3 volume in our lab the coldest place in Wisconsin (in a two-way tie with the Saffman lab), and one of the coldest places in the known universe.
The camera images above are “time of flight” images of a cloud of ~1 million strontium atoms after we have released them from our narrow-line magneto-optical trap. (The images are actually the shadows cast by the atoms on a laser beam, which is a common atom imaging technique called “absorption imaging.”) After they are released the atoms fall due to gravity and spread out due to their finite temperature, with the rate of expansion of the cloud providing an easy measurement of their temperature. Here, the average speed of the atoms is ~ 0.02 meters per second, or ~ 0.04 MPH, corresponding to a temperature of ~4 microKelvin. This is roughly the speed of a garden snail. For comparison, nitrogen molecules in air at room temperature have an average speed of about ~500 meters per second, or 1000 MPH.