Quantum thermometers

IN THE quantum world, simply measuring temperature can cool things down.

Quantum systems normally exist in a superposition of several states at once, but collapse into a single state when observed. Two years ago physicists came up with the prediction that gauging the temperature of quantum objects could cause this collapse to occur in a way that cooled the object. Now Gonzalo Álvarez of the National University of Córdoba in Argentina and his colleagues have put this prediction to the test.

The team looked at the alignment of the spins of carbon and hydrogen nuclei in molecules of methyl iodide. Temperature is a measure of disorder: the higher the temperature, the more disordered a system is. So a fall in the temperature of the spins equates to them becoming more ordered, increasingly aligning with each other, instead of pointing any which way.

The nuclei normally play "catch" with each other, tossing energy back and forth, so that the spins of the carbon nuclei get alternately cooler and warmer. Álvarez's team couldn't make spin measurements directly, but used brief magnetic pulses to create an effect similar to measuring the orderliness of the spins. They found that when these pulses were repeated about once per millisecond, the temperature fell.

The pulses stop the game of catch just after the carbon spins toss away their energy, so every time the game is interrupted, the spins cool down further. The work will be published in Physical Review Letters.