Fabrication and Automated Characterisation of Optical Cavity Mirrors for Quantum Networking

To achieve high rates of entanglement generation in trapped ion quantum networks [1], the Purcell enhancement offered by optical cavities can be used to reach near-unit photon collection efficiency [2]. In this work, we address the challenges of fabricating and characterizing microcavity mirrors with ultralow, Å-level roughness, necessary to achieve high cooperativity in ion-cavity systems. We use focused ion beam (FIB) milling to produce features with state-of-the-art surface roughness for this size and scale [3,4]. To rapidly characterize these mirrors and to drive iterative improvements of the fabrication methods used, we design and build an automated cavity mode-matching experiment. An algorithm controls the degrees of freedom of a multi-axis stage (hexapod), on which the cavity is mounted. Using this method, we achieve dominant coupling to the fundamental mode within the span of minutes, for a wide range of initial misalignments, and obtain the cavity finesse through automated measurements. These fabrication and characterisation developments offer a repeatable and scalable route to realising high finesse microcavities for integration into ion trap network nodes.

 

1. L.J. Stephenson, et al., Phys. Rev. Lett. 124, 110501 (2020).

2. J. Schupp, PRX Quantum 2, 020331 (2021).

3. P. Maier, Opt. Express 33, 19205-19219 (2025).

4. A.A.P. Trichet et al., Opt. Express 13 17205-17216 (2015).