Quantum information processing can potentially perform computational operations much faster than conventional computers. However, devising effective schemes for storing information in quantum states is still a major challenge. Researchers from the Australian National University1 now demonstrate a technique that allows the storage and arbitrary retrieval of multiple quantum states by laser irradiation of atoms in a gas cloud.

Fig. 1: Gas-based quantum memory. (a) The energy levels of rubidium atoms (green) are used to store quantum information. (b) To store the various energies contained in an optical pulse (blue), a magnetic field is gradually varied and the information is stored in the energy levels of the rubidium atoms. (c) Reversal of the magnetic field enables the optical pulse to be retrieved.

A key goal of quantum memory research is to find ways of storing multiple bits of information. An effective means of achieving this is to use an atomic gas with a wide distribution of energies to create ‘broadband’ memory. The researchers, led by Ben Buchler, showed that the secret to making such memory efficient is to ensure that the atomic transition energies are distributed along the length of the storage medium (Fig. 1).

In their system, a gradually strengthening magnetic field increases the atomic transition energies along the gas such that the energy distribution of the optical pulse can be accommodated. “In principle, there is no limit to the number of optical states that can be stored, provided the energy distribution across the atoms is large enough and the atomic lifetimes long enough,” says Buchler.

To read out the stored signal, the magnetic field is reversed in sign, which causes a backwards evolution of the information stored. Eventually, the original signal can be recalled. More importantly, changing the sign of the magnetic field as well as its strength provides control over the way the stored information is extracted. Through a clever sequence of field changes, any desired part of the original initial multi-pulse signal can be retrieved at will.

The possibility of controlling multiple bits stored within a gas may have intriguing consequences for quantum information systems. “Possibilities include use as a random access memory in a quantum computer,” says Buchler. He adds that if the spatial distribution of the magnetic field along the gas is altered while the pulses are in storage, the properties of the quantum states are modified. “This type of manipulation is not possible in other systems.”