Single-Photon Generation via Four-Wave Mixing in a Thermal Rubidium Vapour at a High Magnetic Field

Abstract

We present investigations of the nonlinear optical phenomenon of four-wave mixing (4WM) in a thermal vapour at a high magnetic field. The 5S, 5P, 5P, and 5D terms in rubidium-87 are used. In a 0.62 T field, the optical transitions become separated by more than their Doppler width, effectively isolating a four-level system. Spontaneous 4WM in this system is used to generate herald-signal photon pairs and to realise a heralded single-photon source, as demonstrated by a Hanbury Brown—Twiss (HBT) experiment giving a value of 0.35~~0.02. We show that an increased correlation between herald and signal leads to a value closer to zero, and we investigate the parameters that increase this correlation.
We additionally characterise the parameters in the seeded 4WM system and show that working in the hyperfine Paschen—Back (HPB) regime leads to a good agreement between theory and experiment. The splitting of the seeded 4WM lineshape is used to extract an excited state dipole matrix element. We also study the background photons in our system, showing that they are partly a result of a collisional transfer process. We build a bespoke etalon lens filter to aid in filtering out these photons and show that the use of this filter improves the correlation between herald-signal photon pairs. In order to perform HBT experiments that take over twenty-four hours to run, we devise a novel laser locking scheme, named STROLL, that simultaneously stabilises the frequencies of two lasers to a two-photon transition over that time period.
Difficulty with optimising the alignment of photons into a fibre leads us to also develop an innovative method of using machine learning for automatically aligning laser beams; we implement this method on a physical device, which we have named the Pi Auto-aligner.