While challenging, there has been considerable progress toward interfacing QD photons with atomic systems. Last, it is key that the memory does not introduce noise so that the quantum character of the input state of light in unchanged upon retrieval. The Raman ( 29) and off-resonant cascaded absorption (ORCA) memories have achieved bandwidths of 1 GHz ( 6, 14). Electromagnetically induced transparency ( 16, 27) and the fast ladder memory ( 28) have pushed the operational bandwidth to the near-gigahertz regime. A promising approach is to use two-photon absorption mechanisms in hot alkali vapors. Many of the leading memories are incompatible, having storage bandwidths in the near megahertz regime ( 24– 26). QDs emit photons with bandwidth on the order of a few gigahertz, but environmental effects such as strain, charge noise, and spin noise can cause emission wavelength inhomogeneities of order terahertz ( 23) and temporal instability such as blinking. The next challenge arises from matching the quantum memory bandwidth to that of the QD emission. Substantial advancements have been made to design and fabricate QDs that emit light with wavelengths close to atomic transitions with a typical spread of several-nanometer fine-tuning of the emitted wavelength that can then be achieved through the application of external electric ( 17) or magnetic fields ( 18), strain via piezoelectric actuators ( 19– 21), or by temperature ( 22). In contrast, it is much more challenging to match both the emission wavelength and bandwidth of QD light to atomic quantum memories.

Parametric down-conversion sources can generate heralded single photons with wavelengths compatible with quantum memories by means of choosing the appropriate choice of pump and quasi-phase matching, providing stable and terahertz-wide photons that can be filtered ( 13, 14) or cavity-enhanced ( 15, 16) to achieve highly effective matching to memories with megahertz-gigahertz bandwidth. Achieving an efficient light-matter interface between single photons and a quantum memory requires matching both the central wavelength and the bandwidth between the systems.

Quantum atomic model