High-Speed Quantum Memory Retrieves Photons on Demand

A quantum memory that offers relatively fast and simple retrieval could someday be used for building a “quantum internet.” The technology stores photons in a gas of rubidium (RB) atoms and uses a laser to control the storage and retrieval processes. It does not require cooling devices or complicated vacuum equipment and can be implemented in a highly compact setup. The stored photons are suitable for high-speed transfer and can be read out again without significant alteration to their quantum mechanical properties.

The quantum memory demonstrated an acceptance bandwidth that is suitable for single photons emitted by semiconductor quantum dots. According to the University of Basel research team, in this regime, vapor cell memories offer storage efficiency and a low noise level, and atomic collisions have negligible influence on optical coherences.

Researchers demonstrated the operation of the memory using attenuated laser pulses on the single-photon level, measuring end-to-end efficiency of the fiber-coupled memory, with a total intrinsic efficiency η_int = 17(3) percent. Researchers believe that technological improvements could further improve end-to-end efficiency, and that increasing the optical depth and exploiting the Zeeman substructure of the atoms could allow such a memory to approach near unity efficiency.

In the present memory, the unconditional read-out noise level of 9 × 10⁻³ photons is dominated by atomic fluorescence. According to researchers, for input pulses containing on average μ₁ = 0.27(4) photons, the signal-to-noise level would be unity.

Single photons transmit quantum information between the network nodes, where they are stored in an atomic gas. Courtesy of University of Basel, Department of Physics.

The ability to store and retrieve single photons from a quantum memory is a key element for quantum information processing. The research team was able to verify that their high-speed quantum memory has a very low noise level and is suitable for encoding information on single photons.

“The combination of a simple setup, high bandwidth and low noise level is very promising for future application in quantum networks,” said researcher Janik Wolters.

In the future, quantum networks could lead to unconditionally secure communication, the networking of different quantum computers and the simulation of complex physical, chemical and biological systems.

The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.119.060502).