Physicists have stored pulses of light in a cloud of ultracold atoms for up to 240 ms — about 40 times longer than the previous record. They did this by arranging the atoms into a lattice-like structure in which the atoms are prevented from moving around. This state, known as a Mott insulator, can then re-emit the pulses on command.

Although light has previously been stored in solids for several seconds, the advantage of atomic gases is that light can be stored just one photon at a time, which could come in handy for building quantum information systems. Another important bonus of atomic gases is that physicists have lots of quantum-optical techniques at their disposal to control the storage process.

Indeed in this latest work, Immanuel Bloch of the Johannes Gutenberg University in Germany and colleagues were also able to control the direction that the pulse was re-emitted from the ultracold gas.

No hopping allowed

The team — which also included physicists from Israel’s Weizmann Institute of Science and Harvard University — began with an ultracold gas of about 90,000 rubidium–87 atoms in an optical lattice formed by crisscrossing laser beams. The wavelengths of the laser beam are set so they don’t correspond to the wavelengths of light that are absorbed and emitted by the atoms.

Each lattice site is occupied by one atom and the lasers are adjusted such that an atom would have to overcome a significant energy barrier to hop into a neighbouring lattice site. This configuration is dubbed Mott insulator because it is analogous to those solids in which the conduction electrons are localized due to the strong interactions between the atoms.

The researchers then applied a magnetic field to the atoms, which let them isolate a particular three-state system comprising two different sublevels of the ground state and an excited state. Transitions between one sub-level and the excited state could be triggered using a relatively weak "probe" laser, while transitions from the other other sub-level were driven using a more intense "coupling" laser.

Opaque becomes transparent

The researchers found that when the probe laser is fired on its own, the light is absorbed by the gas and therefore blocked. But if the coupling laser is fired at the same time, the probe laser passes through unhindered thanks to an effect called electromagnetic transparency (EIT), which was first seen about 20 years ago.

The trick to storing a light pulse is to turn off the coupling laser as the probe pulse is in the gas. The probe pulse is absorbed by gas, creating a spatially-varying pattern of atomic spins — a "spin wave" that is imprinted on the atomic gas. If the coupling laser is switched back on again, the probe pulse is “recreated” from the spin wave. The team used this technique to store pulses for up to about 240–ms

In other atomic gas experiments, the motion of the atoms degrades the spin wave pattern rapidly – the upshot being that the light pulse can only be stored for a few milliseconds. But because the atoms in the Mott–insulator state are not moving around, the stored pulse lasts much longer.

Several seconds possible

Indeed, Bloch told that the technique could be refined to get storage times as long as several seconds by adjusting the wavelengths of the lasers that form the optical lattices so this light is further away from the wavelength of the atomic resonance.

The team also showed that the pulse can be re-emitted in a direction different to its original trajectory. This was done by first storing the pulse in the medium and then firing a third laser beam, which changed the atomic spin wave in a controlled way. By doing this they managed to deflect the original pulse by more than 20 millradians — about one degree.

Single photon source

Bloch also believes that atomic Mott insulators could also be used as a source of single photons by exciting tiny spin waves in the gas via Rydberg atoms. These are atoms with electrons that have been excited into very high energy states — and lead to a Rydberg blockade mechanism, whereby the presence of a Rydberg atom prevents the excitation of nearby atoms.

Atomic gases also allow for strong interactions between the atoms, which could be used to create efficient quantum gates for the atomic spin states — and thus the creation of photonic quantum logic gates for quantum computing.

A preprint describing the work is on arXiv.