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.

Keywords: Quantum memories; light-matter interfaces; optical quantum information processing.

Figure 1.

Prevalent noise processes in quantum memories. (a) Resonant fluorescence: In near-resonant schemes, the read pulse can excite population in the intermediate state $|e⟩$. Doppler broadening of this line can result in fluorescence at the signal frequency. (b) Spontaneous Raman scattering from thermal population: Thermal excitation can populate the storage state, anti-Stokes Raman scattering from this thermal population is a source of noise. (c) SFWM: The write pulse undergoes spontaneous Stokes scattering to generate a Stokes photon and a material excitation. The read pulse scatters from the excitation generating a photon at the anti-Stokes frequency and returning the population to the ground state. The Stokes photon can be eliminated by spectral filtering, but the anti-Stokes photon is a source of noise. (The colour version of this figure is included in the online version of the journal.)

Figure 2.

(a) Two single photon qubits are encoded in successive time bins within a single spatial mode. (b) LOQC can be carried out by controllably reordering and coupling different time bin modes. Without only controllable birefringent elements, this requires time-bins to be rotated into the orthogonal polarization to be displaced, and only one displacement can be implemented at each step. (c) A pulse sequencer, as can be achieved using a GEM, allows for arbitrary reordering of time bins. (The colour version of this figure is included in the online version of the journal.)

Figure 3.

An example off-resonant Raman memory scheme based on a three-level system. An electromagnetic field mode $\widehat{a}$ is coupled to the memory transition $\widehat{b}$ between levels $|1⟩$ and $|2⟩$ through a Raman transition to a virtual energy level detuned by $\mathrm{\Delta }$ from $|3⟩$. By adjusting the frequency of the control field-mode $\mathit{\gamma }$, both (a) beam splitter interactions and (b) two-mode squeezing interactions can be generated between the modes. The BS operation can realize read in/out from the memory. (c) Using these interactions, entangled 2 mode cluster state primitives can be generated. (d) These clusters can be stitched together using further quantum memories to create a time–frequency continuous-variable cluster state encoded in modes tiled across the available time and frequency degrees of freedom. (The colour version of this figure is included in the online version of the journal.)