Reflecting on quantum memory

Tiny "mirrors" that can trap light around impurities within a diamond can boost the efficiency with which photons transmit information about the electronic spin states of those impurity atoms. The production of these spin-photon interfaces could be essential to the development of interconnected quantum memory devices that might be used in quantum computation and long-distance cryptographic systems.

Dirk Englund's team at the Massachusetts Institute of Technology in Cambridge USA, working with colleagues at Brookhaven National Laboratory in Long Island, New York, have demonstrated that the memory encoded in the electron spin state, the spin-coherence time, can persist for 200 microseconds or more; this is a record for quantum memories in such photonic traps. [Englund et al., Nature Commun. (2015), DOI: 10.1038/ncomms7173]

"Our research demonstrates a technique to extend the storage time of quantum memories in solids that are efficiently coupled to photons, which is essential to scaling up such quantum memories for functional quantum computing systems and networks," explains Englund.

The impurity atoms present in the diamond crystals studied by Englund and colleagues are nitrogen-vacancy (NV) centers. These consist of a nitrogen atom in the place of a carbon atom, adjacent to a crystal vacancy within the diamond lattice. The spin state of the center can be either up or down thus providing the "0" or "1" of binary code. Microwaves radiation can be used to manipulate the spin state and because the "0" state has a greater fluorescence than the "1" state, the researchers can use an optical microscope to read the quantum memory.

Reflecting on quantum memory

However, in order to be useful for carrying out logical operations of the kind that underpin computation, the spin states must be stable for a sufficient length of time. "It is already possible to transfer information about the electron spin state via photons, but we have to make the interface between the photons and electrons more efficient," Englund explains. Unfortunately, photons and electrons interact only very weakly. To boost the interaction, the team built an optical cavity around the NV to trap the photons using a transferred hard mask lithography technique. The cavity, nanofabricated at BNL by MIT graduate student Luozhou Li, working with BNL staff scientist Ming Lu, is made from layers of diamond and air tightly spaced around the impurity atom of the NV center. Reflection occurs at each interface between the layers so that photons entering bounce back and forth up to 10000 times, which boosts the interaction with the electrons in the NV center.

"These methods have given us a great starting point for translating information between the spin states of the electrons among multiple NV quantum memories," explains Englund. "These results are an important part of validating the scientific promise of NV-cavity systems for quantum networking."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".