Quantum memories: emerging applications and recent advances

Abstract

Quantum light–matter interfaces are at the heart of photonic quantum technologies. Quantum memories for photons, where non-classical states of photons are mapped onto stationary matter states and preserved for subsequent retrieval, are technical realizations enabled by exquisite control over interactions between light and matter. The ability of quantum memories to synchronize probabilistic events makes them a key component in quantum repeaters and quantum computation based on linear optics. This critical feature has motivated many groups to dedicate theoretical and experimental research to develop quantum memory devices. In recent years, exciting new applications, and more advanced developments of quantum memories, have proliferated. In this review, we outline some of the emerging applications of quantum memories in optical signal processing, quantum computation and non-linear optics. We review recent experimental and theoretical developments, and their impacts on more advanced photonic quantum technologies based on quantum memories.

Memory-assisted quantum key distribution resilient against multiple-excitation effects

“…We consider ensemble-based memories due to their strong light-matter coupling and, in several cases, the possibility of long coherence times (up to seconds [38]) and high bandwidths (up to several GHz [35]). Furthermore, they offer the possibility of multi-mode storage [7,11]. By multi-mode we are referring to memories that can simultaneously store more than one qubit during a single storage event by encoding many qubits each into a different mode.…”

“…There are a variety of systems that have been utilized for optical quantum memories; see [11] for a recent overview. We consider ensemble-based memories due to their strong light-matter coupling and, in several cases, the possibility of long coherence times (up to seconds [38]) and high bandwidths (up to several GHz [35]).…”

“…The former involves many excitations, in which each individual excitation occupies a single distinguishable mode (or a pair as required for a qubit), while the latter concerns many excitations that occupy a single mode and thus each excitation may not be distinguished. Motivated by their impressive, and continually-improving, experimental record, we specifically consider warm vapor (Cs and Rb atomic gas) and cold atom (Rb atoms in a magneto-optical trap or atomic lattice) systems [11] that rely on the so-called Raman quantum memory protocol [60] as well as cryogenicallycooled rare-earth-ion-doped crystals that utilize atomic frequency combs [61].…”

“…Probabilistic quantum repeaters offer a solution to extend the communication distance to over thousands of kilometers [4][5][6][7][8][9][10]. However, such quantum repeaters rely on quantum memory modules [11] with characteristics that are hard to achieve with the current technology. This does not necessarily mean that the existing quantum memories cannot offer any advantages.…”

“…Early investigations suggest that quantum memories with large storage-bandwidth products as well as short access and entangling times are necessary for MA-MDI-QKD [13]. These requirements may be achieved by quantum memories based on atomic ensembles [11], with the added benefit of strong light-matter coupling offering the possibility for efficient implementations. Ensemble-based quantum memories may, however, allow for storage of multiple excitations [14], which have been shown to be deleterious to their performance [15].…”