HOW do you take the temperature of the coldest thing in the universe? With a quantum thermometer, of course. And this new ability could provide fresh insights into black holes.
The coldest matter known is a Bose-Einstein condensate (BEC) 鈥� millions of atoms that act as a single quantum object when cooled inside a magnetic trap. Lab experiments have formed BECs at half a billionth of a degree above absolute zero. 鈥淚t鈥檚 the coldest thing we can produce in the lab; the closest thing to absolute zero,鈥� says at the University of Nottingham, UK. Even deep space is warmer.
At the scale of just a few atoms, a mercury-style thermometer is not an option. Instead, physicists can count the number of higher-energy particles relative to lower-energy ones. The more that are in a high-energy state, the higher the temperature. But this means releasing a BEC from its trap, making it impossible to take its temperature mid-experiment.
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Now Fuentes and her colleagues have come up with a way to measure the temperature of a BEC without releasing it. They suggest trapping a few 鈥減robe鈥� atoms 鈥� using lasers rather than a magnet 鈥� and moving these through the BEC. These probes must be in a quantum superposition of two distinct energy states first, each of which gets modified by a different amount that depends on the BEC鈥檚 temperature. The new difference between the states can be used to deduce the temperature of the BEC, which remains in its trap ().
This quantum thermometer could be used to test whether black holes emit small amounts of radiation, as predicted by quantum theory. Approaching a real black hole is impossible, but the idea could be tested by measuring temperature changes in black hole analogues, which can be made in the lab using BECs.
鈥淭he quantum thermometer could be used to test whether black holes emit small amounts of radiation鈥�
This article appeared in print under the headline 鈥淭hermometer to measure coldest thing ever made鈥�