Single-photon source, driven by composite pulse sequences for applications in a doped-solid quantum memory

• Applicant: Professor Dr. Thomas Halfmann
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410249930

Quantum memories are a key component of any quantum information architecture. As photons have negligible interaction with each other, they are ideal carriers for quantum information. Thus, optical memories for single photons are required. The storage time and efficiency are the most relevant benchmarks for quantum memory protocols. In previous years, the applicant conducted thorough investigations on light storage by electromagnetically-induced transparency (EIT) in Pr:YSO crystals. We demonstrated EIT storage times up to the regime of one minute, and storage efficiencies of up to 76 %. However, these experiments still operated at the level of classical light. To push towards an EIT quantum memory, we require now a source for narrow-band single photons.The proposal deals with the experimental setup of a narrow-band single-photon source, based on the Duan-Lukin-Cirac-Zoller (DLCZ) quantum memory approach, implemented in an ensemble of cold Rubidium atoms from a specifically designed magneto-optical trap (MOT) to provide large optical depth.As novel and powerful variants of the DLCZ approach, we propose schemes based on composites pulses (CP) and stimulated Raman adiabatic passage (STIRAP) to drive collective atomic coherences corresponding to a robust and very precisely defined excitation by a single photon. Important advantages are (i) higher duty cycle and essentially deterministic operation of the source by adiabatically-stimulated rather than spontaneous emission, (ii) larger brightness by confining the single-photon emission volume towards the forward direction, (iii) larger robustness with regard to fluctuations of experimental parameters. As an alternative to DLCZ, we propose the implementation of an EIT-based quantum filter, which transmits only single-photon states. The approaches are of significant interest for quantum technologies also well beyond light storage. As a new technical development, we will implement the concepts also in cold Rubidium atoms loaded into a hollow-core fiber. Recently we demonstrated huge optical depths OD > 1000 in such a setup, i.e. orders of magnitude beyond typical values in previous DLCZ experiments. This enables further major improvements towards lower noise and higher brightness. Finally, we will efficiently convert the single photons at 795 nm emitted from Rubidium by sum-frequency mixing with intense mid-infrared radiation at 2.55 µm in a periodically-poled lithium-niobate (PPLN) waveguide, to obtain single photons at 606 nm, i.e. matched to a Pr:YSO memory.

• DFG Programme: Research Grants