Optical information processing requires the storage of light on timescales much longer than can be maintained by a material’s quantum states before they degenerate. Byoung Ham from Inha University in Korea1 has now developed a protocol for optical storage that is independent of the short time that the quantum coherence of an energetic state can be maintained.

The storage of information from a laser pulse in a quantum system relies on the interaction between three quantum states in a crystal. Two of these electronic states are energetically close, whereas the third has a higher energy and acts as a link between the two lower states. To store information, the system is excited by an initial laser pulse that lifts electrons from both of the lower energy states into the higher state.

Fig. 1: Time reversal plot showing the perfect reversal of temporal evolution in a quantum system. The horizontal axis shows the number of initial states the system can assume, and the vertical axis charts how these states evolve with time. At 30 µs, a read pulse reverses the evolution of the system so that after 60 µs, the initial state is reached once again.

The simultaneous excitation of both ground states into the same excited state leads to a synchronized periodic oscillation of electrons similar to the back-and-forth movements of two coupled pendulums. The effect is largest if the excitation laser pulse lasts for precisely half of the intrinsic oscillation period between the states. However, if the first laser pulse is followed by a second pulse of twice that duration, physics dictates that the oscillations reverse their movement in time and trace back their steps. Eventually, the system reaches the initial state and the imprinted information can be read out (Fig. 1).

The problem with such schemes is that the information can only be stored as long as these oscillations can be maintained, which is on the order of microseconds. To extend this time, Ham proposes to use another light pulse that stops the oscillations and transfers the electrons temporarily into a separate, stable fourth state. To read out the information, the electrons are transferred back to the original quantum system in exactly the same state as before they were moved. This restarts the oscillations, and information can be read out in the conventional manner.

This scheme enhances storage times by several orders of magnitude and therefore could have important consequences for practical applications. According to Ham, experimental verification is the next step, and preliminary experimental work has already begun to demonstrate the proof of principle.