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Quantum 2.0 Conference and Exhibition

Ultra-stable open micro-cavity platform for closed cycle cryostats

Michael Förg, Jonathan Noé, Ashly Jose, Sambit Mitra, Manuel Nutz, Kerim Köster, Andras Lauko, David Hunger, Thomas Hümmer

Published Jan 1, 2025• 0 citations
Publisher

Abstract

Open-access micro-cavities are a powerful tool for enhancing light-matter interactions for solid-state quantum and nano systems and are key to advance applications in quantum technologies. In order to achieve good coherence properties of the material, most of them have to be cooled down to cryogenic temperatures. In addition, the use of external magnetic fields is required for many emitter classes to fine-tune optical transitions. Here, we present the integration of a fully-tunable, open-access, fiber-based Fabry-Pérot micro-cavity platform into the vibrational environments of different closed-cycle cryostats reaching sub-pm mechanical stability. In addition, we show the development of superconducting coils to generate fields up to 150 mT for those cryogenic cavity scenarios.

DOI
10.1364/quantum.2025.qm4b.3

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Open-access micro-cavities are a powerful tool for enhancing light-matter interactions for solid-state quantum and nano systems and are key to advance applications in quantum technologies. In order to achieve good coherence properties of the material, most of them have to be cooled down to cryogenic temperatures. In addition, the use of external magnetic fields is required for many emitter classes to fine-tune optical transitions. Here, we present the integration of a fully-tunable, open-access, fiber-based Fabry-Pérot micro-cavity platform into the vibrational environments of different closed-cycle cryostats reaching sub-pm mechanical stability. In addition, we show the development of superconducting coils to generate fields up to 150 mT for those cryogenic cavity scenarios.
Open-access microcavities are a powerful tool to enhance light-matter interactions for solid-state quantum and nano systems and are key to advance applications in quantum technologies. For this purpose, the cavities should simultaneously meet two conflicting requirements - full tunability to cope with spatial and spectral inhomogeneities of a material, and highest stability under operation in a cryogenic environment to maintain resonance conditions. To tackle this challenge, we have developed a fully-tunable, open-access, fiber-based Fabry-Pérot microcavity platform which can be operated also under increased noise levels in a closed-cycle cryostat. It comprises custom-designed monolithic micro- and nanopositioning elements with up to mm-scale travel range that achieve a passive cavity length stability at low temperature of only 15 pm rms in a closed-cycle cryostat, and 5 pm in a more quiet flow cryostat. This can be further improved by active stabilization, and even higher stability is obtained under direct mechanical contact between the cavity mirrors, yielding 0:8 pm rms during the quiet phase of the closed-cycle cryo cooler. The platform provides operation of cryogenic cavities with high finesse and small mode volume for strong enhancement of light-matter interactions, opening up novel possibilities for experiments with a great variety of quantum and nano materials.
Open-access microcavities are a powerful tool to enhance light–matter interactions for solid-state quantum and nanosystems and are key to advance applications in quantum technologies. For this purpose, the cavities should simultaneously meet two conflicting requirements—full tunability to cope with spatial and spectral inhomogeneities of a material and highest stability under operation in a cryogenic environment to maintain resonance conditions. To tackle this challenge, we have developed a fully tunable, open-access, fiber-based Fabry–Pérot microcavity platform that can be operated under increased noise levels in a closed-cycle cryostat. It comprises custom-designed monolithic micro- and nanopositioning elements with up to mm-scale travel range that achieve a passive cavity length stability at low temperature of only 15 pm rms in a closed-cycle cryostat and 5 pm in a more quiet flow cryostat. This can be further improved by active stabilization, and even higher stability is obtained under direct mechanical contact between the cavity mirrors, yielding 0.8 pm rms during the quiet phase of the closed-cycle cryocooler. The platform provides the operation of cryogenic cavities with high finesse and small mode volume for strong enhancement of light–matter interactions, opening up novel possibilities for experiments with a great variety of quantum and nanomaterials.
Open microcavities provide a powerful platform for studying cavity quantum electrodynamics in solid-state systems. However, operating open microcavities at cryogenic temperatures, as required for many solid-state quantum emitters, typically demands bulky and cryostat-specific vibration-mitigation setups. Here we report a compact, robust, and tunable mechanical host for an open microcavity. The complete mechanical assembly fits within a footprint of 1′′×1′′×0.5′′1'' \times 1'' \times 0.5''1′′×1′′×0.5′′. Using this mechanical host, we observe no vibration-induced cavity broadening for an open microcavity with finesse exceeding 1,000 without cryostat customization or active locking. The assembly also enables in situ tuning of the cavity resonance over 3 nm, and the resonance of the same cavity remains within this range across multiple cooldowns. To further showcase the capability of this assembly, we demonstrate coupling between an InGaAs quantum dot and an open microcavity with a cooperativity exceeding unity. This platform provides a versatile testbed for fundamental cavity quantum electrodynamics and a scalable route to portable quantum light sources and spin-photon interfaces for quantum repeaters, quantum networks, and photonic quantum computing.
Open microcavities offer great potential for the exploration and utilization of efficient spin-photon interfaces with Purcell-enhanced quantum emitters thanks to their large spectral and spatial tunability combined with high versatility of sample integration. However, a major challenge for this platform is the sensitivity to cavity length fluctuations in the cryogenic environment, which leads to cavity resonance frequency variations and thereby a lowered averaged Purcell enhancement. This work presents a closed-cycle cryogenic fiber-based microcavity setup, which is in particular designed for a low passive vibration level, while still providing large tunability and flexibility in fiber and sample integration, and high photon collection efficiency from the cavity mode. At temperatures below 10 K, a stability level of around 25 pm is reproducibly achieved in different setup configurations, including the extension with microwave control for manipulating the spin of cavity-coupled quantum emitters, enabling a bright photonic interface with optically active qubits.
While nonclassical light sources are fundamental to quantum communication and computing, solid-state platforms like color centers and quantum dots require cryogenic temperatures to reach the performance levels necessary for practical applications. Over the past decade, low-temperature engineering has transitioned from manual handling of liquid cryogens to automated closed-cycle cryostats. This review details the principles behind modern cooling hardware ranging from flow cryostats to mechanical cryocoolers and dilution refrigerators, with a specific focus on the requirements of optical quantum devices. Aimed at the practicing scientist, this overview provides the technical insights and historical context needed to navigate the current cryogenic landscape and evaluate its role in the future of quantum technology deployment.