In the strange world of quantum mechanics, nothing isn鈥檛 ever actually nothing 鈥� and now we have found that nothing, or the absence of a photon, can be used to cool things down.
One of the most common ways scientists cool things down is by using lasers. When particles of light, or photons, with a specific frequency hit an atom or molecule, it absorbs the photon and then fires out another photon with slightly higher energy, which cools the system overall.
But strangely, placing a photon detector next to an atom to count the photons given off results in the laser instead raising the system鈥檚 temperature.
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This is because in quantum mechanics, the act of measurement can alter the state of a system. Measuring the position of an electron, for example, changes it from a cloud of possible positions to a specific place.
Similarly, detecting a photon confirms that the system had enough energy to produce that photon in the first place, and measuring this raises its energy, or temperature. 鈥淚f you鈥檙e sitting under an apple tree and you don鈥檛 know how many apples are on the tree above you, but one falls and hits you on the head, then you might think, 鈥極h, there鈥檚 probably more apples up there than I thought initially鈥�,鈥� says at Imperial College London.
Now, Vanner and his colleagues have found that setting up a detector to look for the absence of photons has the opposite effect, and can make objects colder when there was no photon detected. 鈥淚f, instead, you sit under the apple tree for a while and none hit you on the head, then you may think there were less apples than you thought initially,鈥� says Vanner. 鈥淪o it cools the acoustic vibrations even further than laser cooling alone.鈥�
Vanner and his team fired an infrared laser at a glass bead that was 200 micrometres wide, or twice the width of a human hair. This produced sound waves that travelled through the bead and generated their own radiation, which slowed down and cooled the waves in a conventional laser cooling scenario.
Next to the bead, the researchers placed a system that looks for the absence of photons. This has two detectors: one designed to only measure single or no photons, and another that records all the photons given off by the bead. By comparing the detectors, Vanner and his team could record the times when zero photons were given off. This meant there were no sound waves energetic enough to produce light, which equates to a cooler overall system.
The cooling effect is small in this experiment, which was intended as a proof of concept, says Vanner, but it could be useful for storing information in a future quantum internet.
鈥淚f you want to synchronise quantum signals of light, or you want to controllably store and release them between operations in a quantum computer, you need a quantum memory,鈥� says Vanner. These sound waves can last for long enough to act as a quantum memory, on the order of milliseconds, and can easily be converted to the frequency of light used in quantum networks, he says.
鈥淲hat makes the quantum world interesting is that even the absence of something is still a thing,鈥� says at the National Institute of Standards and Technology in Maryland. It is much harder to exclude false positives when looking for the absence of a photon than when counting a single photon, says Nunn, and requires much more sensitive detectors. The more you can rule out false positives, the greater the cooling, so using a more sensitive detector could enhance the cooling even more, he says.
arXiv
