We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.
We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.
Cryogenic confocal microscopy is a powerful method for studying solid state quantum devices such as single photon sources and optically controlled qubits. While the vast majority of such studies have been conducted at temperatures of a few Kelvin, experiments involving fragile quantum effects often require lower operating temperatures. To also allow for electrical dynamic control, microwave connectivity is required. For polarization-sensitive studies, free space optical access is advantageous compared to fiber coupling. Here we present a confocal microscope in a dilution refrigerator providing all the above features at temperatures below 100 mK. The installed high frequency cabling meets the requirements for state-of-the-art spin qubit experiments. As another unique advantage of our system, the sample fitting inside a large puck can be exchanged while keeping the cryostat cold with minimal realignment. Assessing the performance of the instrument, we demonstrate confocal imaging, sub-nanosecond modulation of the emission wavelength of a suitable sample, and an electron temperature of 76 mK. While the instrument was constructed primarily with the development of optical interfaces to electrically controlled qubits in mind, it can be used for many experiments involving quantum transport, solid state quantum optics, and microwave-optical transducers.
Cryogenic confocal microscopy is a powerful method for studying solid state quantum devices such as single photon sources and optically controlled qubits. While the vast majority of such studies have been conducted at temperatures of a few Kelvin, experiments involving fragile quantum effects often require lower operating temperatures. To also allow for electrical dynamic control, microwave connectivity is required. For polarization-sensitive studies, free space optical access is advantageous compared to fiber coupling. Here we present a confocal microscope in a dilution refrigerator providing all the above features at temperatures below 100 mK. The installed high frequency cabling meets the requirements for state of the art spin qubit experiments. As another unique advantage of our system, the sample fitting inside a large puck can be exchanged while keeping the cryostat cold with minimal realignment. Assessing the performance of the instrument, we demonstrate confocal imaging, sub-nanosecond modulation of the emission wavelength of a suitable sample and an electron temperature of 76 mK. While the instrument was constructed primarily with the development of optical interfaces to electrically controlled qubits in mind, it can be used for many experiments involving quantum transport, solid state quantum optics and microwave-optical transducers.
A hybrid quantum computing architecture combining quantum processors and quantum memory units allows for exploiting each component's unique properties to enhance the overall performance of the total system. However, superconducting qubits are highly sensitive to magnetic fields, while spin ensembles require finite fields for control, creating a major integration challenge. In this work, we demonstrate the first experimental setup that satisfies these constraints and provides verified qubit stability. Our cryogenic setup comprises two spatially and magnetically decoupled sample volumes inside a single dilution refrigerator: one hosting flux-tunable superconducting qubits and the other a spin ensemble equipped with a superconducting solenoid generating fields up to 50 mT. We show that several layers of Cryophy shielding and an additional superconducting aluminum shield suppress magnetic crosstalk by more than eight orders of magnitude, ensuring stability of the qubit's performance. Moreover, the operation of the solenoid adds minimal thermal load on the relevant stages of the dilution refrigerator. Our results enable scalable hybrid quantum architectures with low-loss integration, marking a key step toward scalable hybrid quantum computing platforms.
Nanophotonic light–matter interfaces hold great promise for quantum technologies. Enhancing local electromagnetic fields, they enable highly efficient detectors, can help realize optically connected processors, or serve as quantum repeaters. In-situ fiber-coupling at sub-Kelvin temperatures, as required for testing and development of new devices, proves challenging as suitable cryogenic microscopes are not readily available. Here, we report on a robust and versatile confocal imaging system integrated in a dilution refrigerator, enabling high-resolution visualization of nanophotonic structures on a transparent diamond substrate. Our imaging system achieves a resolution of 1.1 μm and a field-of-view of 2.5 mm. The system requires no movable parts at cryogenic temperatures and features a large working distance, thereby allowing optical and microwave probe access, as well as direct anchoring of temperature sensitive samples to a cold finger setup, needed for applications with high thermal load. This system facilitates the development of scalable, integrated quantum optics technology, an imperative for research on large scale quantum networks.
Birefringence in optical fibers poses a challenge to controllably delivering polarized light. Strain-induced birefringence caused by bends in the fiber, vibrations, or a large temperature gradient can significantly alter the polarization, making it particularly difficult to deliver polarization states to low-temperature environments by fiber. In this paper, we investigate the transmission of polarized light through a fiber and discuss a method we have developed for delivering arbitrarily polarized light to the base stage of a dilution refrigerator using a standard optical fiber. We have created a compact, cryogenic optical system to identify the polarization of the delivered light, while room-temperature waveplates and a mathematical fiber model are used to fully characterize and compensate for the fiber's birefringent effects. We show here that we are able to deliver horizontal, vertical, diagonal, anti-diagonal, right circular, and left circular polarization states to milli-Kelvin temperatures, with state fidelities of greater than 0.96 being achieved in all cases. Additionally, we demonstrate that we can deliver randomly selected elliptical states through a standard fiber to the refrigerator. This opens up new opportunities for fiber-based optical experiments using polarized light, such as quantum information experiments using quantum states encoded in the polarization of single photons.
The electron spin of nitrogen--vacancy (NV) centers in diamond offers a solid-state quantum bit and enables high-precision magnetic-field sensing on the nanoscale. Implementation of these approaches in a fiber format would offer unique opportunities for a broad range of technologies ranging from quantum information to neuroscience and bioimaging. Here, we demonstrate an ultracompact fiber-optic probe where a diamond microcrystal with a well-defined orientation of spin quantization NV axes is attached to the fiber tip, allowing the electron spins of NV centers to be manipulated, polarized, and read out through a fiber-optic waveguide integrated with a two-wire microwave transmission line. The microwave field transmitted through this line is used to manipulate the orientation of electron spins in NV centers through the electron-spin resonance tuned by an external magnetic field. The electron spin is then optically initialized and read out, with the initializing laser radiation and the photoluminescence spin-readout return from NV centers delivered by the same optical fiber.