Clouds of atoms can hide their light. When an atom moves to a lower-energy state, it emits a particle of light called a photon. But this process can be delayed and photons kept trapped inside a dense cloud of atoms, which may eventually prove useful for quantum devices that communicate using light.

When an atom absorbs a photon, exciting it to a higher-energy state, it will always release that photon and return to its initial state in approximately the same amount of time. When this process is delayed, it is called subradiance.

Igor Ferrier-Barbut at the University of Paris-Saclay in France and his colleagues induced subradiance in clouds of 300 to 5000 rubidium atoms by compressing them into a space less than 3 micrometres across. This compression forced the atoms to act as a group, rather than individual particles. The team then shot pulses of light at the cloud to excite the atoms.

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“We tried to make the sample as small as possible, because then one photon does not interact with a single atom but with the whole cloud,” says Ferrier-Barbut. Because the atoms are all compressed so close together, they can essentially pass the photon back and forth within the cloud. “If it was perfect subradiance, they could exchange this photon back and forth forever and store it,” says Ferrier-Barbut.

The team repeated this tens of thousands of times, measuring how long it took for the photons to be released from the cloud. It took up to about 150 nanoseconds – about six times longer than it takes a single rubidium atom to release a photon.

It was also possible to control when the photons were released by firing an additional laser at the cloud to scramble the atoms so that they behaved individually instead of as a group.

The next step is to figure out how to control this subradiance more precisely so that it can be used in photonic devices, perhaps by placing the atoms in an ordered array instead of a cloud. “If you can control well enough how these atoms collectively interact with light, you might have interesting applications,” says Ferrier-Barbut. “You could make interesting light-matter interactions for quantum communications or to interface with the quantum computers of the future.”

Journal reference: Physical Review X, DOI: 10.1103/PhysRevX.11.021031