In recent years, physicists have successfully slowed and stopped pulses of light in gases of ultracold atoms. A ‘write’ laser drives transitions between two energy levels in the atoms, and then a signal pulse – which is resonant with one of these levels and a third level – is sent into the gas. This pulse is stopped by quantum interference effects, and can be trapped by switching the write laser off. When it is switched back on, the signal pulse is released and continues in the direction it was travelling before it was trapped – with its original frequency and phase properties.

Scully’s group has now used a similar technique to transport these pulses when they are trapped in a gas. In common with previous experiments, they shone the write laser into a gas of rubidium atoms and then sent the signal pulse in. Once the atoms had trapped the light pulse, they switched the write laser off. Fractions of a millisecond later, the team switched on a ‘read’ laser, which was separated from the write laser by six millimetres. The signal pulse was retrieved because the atoms that had trapped the pulse had diffused away from the write laser beam. The amplitude of the signal had dropped, however, because not all of the ‘trapping’ atoms had reached the read laser.

The team also demonstrated that pulses can be retrieved with a frequency that is different to that of the signal pulses. This process exploits the fact that the ‘read’ step of the process does not depend on the frequency of the ‘write’ laser. This means that when the ‘read’ laser retrieves pulses, it can endow them with a frequency corresponding to either of the two energy transitions in the atoms. Scully and colleagues believe this phenomenon could be used to make optical switches or an image storage system.

The team also showed that a retrieved pulse could be made to leave the gas in the opposite direction to which it entered. This means the ‘tail’ of the pulse leaves the gas before its ‘head’, and demonstrates a simple method for generating ‘time-reversed’ quantum pulses.