NEAR-CONCENTRIC OPTICAL RESONATOR FOR COHERENT ATOM-PHOTON INTERACTION

Abstract

Optical resonators are indispensable in engineering coherent interaction between atoms and photons, as fundamental building blocks in hybrid quantum information processing. With their interaction governed by cavity quantum electrodynamics, atom-cavity systems require photons to be contained in a small mode volume, and this is conventionally implemented with small high-finesse optical resonators. However, although largely unexplored, small mode volumes can also be achieved with relatively large optical resonators operated in a near-concentric regime, with a strongly focused cavity mode. Near-concentric optical cavities require a relatively low finesse to operate, and provide a large physical space between the mirrors, which can be useful for atomic ensembles, ions or Rydberg atoms. Furthermore, the frequencies of the cavity transverse modes are near-degenerate, and on the order of the hyperfine or Zeeman level splitting of the atoms, which can be utilised for atom-light interactions employing multiple photonic modes.

This thesis explores different facets of a 11-mm length near-concentric cavity operated extremely close (less than 5 µm) to the critical point. First, we observe a coupling of a single 87Rb atom to the fundamental mode of the near-concentric cavity, with the coupling strength g = 2π × 5.0(2) MHz exceeding the atomic dipole decay rate by a factor of 1.7(1) – on par with small cavity systems exhibiting strong atom-photon coupling. Due to a low cavity finesse of 138(2), the atom-cavity cooperativity is 0.084(4), which can be easily improved easily by using mirrors with higher reflectivities. Second, we selectively excite one or a superposition of the higher-order transverse modes of the near-concentric cavity, with mode-matching efficiencies close to the theoretical prediction. Third, we implement passive and active noise reduction strategies to stabilise the cavity length, and bring the cavity mechanical noise down to an acceptable level. We also examine a different mounting technique which provides a higher stability for future designs. Last, we explore how the near-concentric cavity becomes unstable around the critical point, and assess the limit to the near-concentric cavity performance.

These studies provide valuable insights into the workings of a near-concentric cavity system. A better understanding of such system, in terms of its mechanical stability and performance limits, enables a coherent interaction between atoms and photons, even with a moderate-finesse near-concentric cavity. Furthermore, it would be fascinating to apply near-concentric cavities in various systems, which exploit the tight-focusing geometry, near-degenerate transverse modes, or highly divergent critical behaviour, as resources for quantum technologies.