Quantum memories are essential for quantum information processing and long-distance quantum communication. The field has recently seen a lot of progress, and the present focus issue offers a glimpse of these developments, showing both experimental an
d theoretical results from many of the leading groups around the world. On the experimental side, it shows work on cold gases, warm vapors, rare-earth ion doped crystals and single atoms. On the theoretical side there are in-depth studies of existing memory protocols, proposals for new protocols including approaches based on quantum error correction, and proposals for new applications of quantum storage. Looking forward, we anticipate many more exciting results in this area.
Private queries allow a user Alice to learn an element of a database held by a provider Bob without revealing which element she was interested in, while limiting her information about the other elements. We propose to implement private queries based
on a quantum key distribution protocol, with changes only in the classical post-processing of the key. This approach makes our scheme both easy to implement and loss-tolerant. While unconditionally secure private queries are known to be impossible, we argue that an interesting degree of security can be achieved, relying on fundamental physical principles instead of unverifiable security assumptions in order to protect both user and database. We think that there is scope for such practical private queries to become another remarkable application of quantum information in the footsteps of quantum key distribution.
