The most promising approach for achieving long-distance quantum communication is to employ quantum repeaters, but they require high-fidelity quantum memories with much longer storage times than the current state-of-the-art memories. The long-term goal of the quantum-communication community is to develop a reliable solid-state optical quantum memory that has a high efficiency, a high storage capacity and long storage times for single or few photons. To realize this goal it is critical to determine the most appropriate medium, the best coherent quantum storage protocol and the most effective control techniques.

Now, Georg Heinze, Christian Hubrich and Thomas Halfmann report light-storage experiments based on electromagnetically induced transparency (EIT) in crystals doped with rare-earth ions (Phys. Rev. Lett. 111, 033601; 2013) and realize storage times close to one minute.

In principle, ion-doped crystals are ideal for use in photon memories because they combine the advantages of solids and isolated atoms with very long hyperfine lifetimes. However, stochastic magnetic interactions with the host material substantially reduce the lifetime of the coherence between the two relevant atomic spin states, which, in turn, reduces photon storage times.

Credit: © 2013 APS

Heinze et al. achieved high storage capacities by imprinting two-dimensional image data onto the optical data pulse. In addition, they applied a combination of static and high-frequency magnetic fields to make the medium, a Pr3+:Y2SiO5 crystal, less sensitive to external fluctuations, leading to longer storage durations. The applied magnetic fields made the energy level spectrum of the medium very complicated. Consequently, they used feedback-controlled pulse shaping in combination with a self-learning evolutionary algorithm to determine an optical preparation sequence for their medium. According to them, this is the first time that this combined approach has been applied to EIT, quantum memory and the complex level schemes of doped solids in strong magnetic fields. It can also be used to support other storage protocols.

“The most important achievement of our work is the prolongation of the storage time of an EIT-driven memory up to the regime of 1 min. This is very close to the fundamental limit of the population lifetime in our medium, which is 100 s,” said Heinze. They also demonstrated the ability to store images in the solid medium for up to 1 min, which is six orders of magnitude longer than the image storage times obtained using hot atomic gases.

“If our approach could be transferred to the single-photon level, it would lead to important applications in the fields of spatially multiplexed optical quantum memory, quantum communications, quantum repeaters and deterministic single-photon sources,” Heinze envisaged.

However, their approach has a storage efficiency of only about 1%. The team is planning to overcome this limitation by either optimizing their technique or applying completely different storage protocols. They are also looking at using different media such as Eu3+:Y2SiO5, which would naturally provide longer storage durations because of its smaller decoherence effects. They also intend to extend the scheme to the single-photon level.