High-Performance Imaging in a Dilution Refrigerator

TL;DR

The work addresses the need for high-resolution, low-heat cryogenic imaging of nanophotonic structures essential for quantum technologies. It introduces a robust 8f confocal microscope integrated into a dilution refrigerator, with fixed optics and large working distance, achieving about 1.1 μm resolution over a 2.5 mm field of view while preserving sample thermalization. Key contributions include detailed design, focal-shift compensation during cool-down, and experimental validation through cryogenic imaging of diamond nanophotonic structures and fiber coupling. This approach enables in-situ optical probing and integration of quantum interfaces at millikelvin temperatures, supporting scalable quantum networks and device development.

Abstract

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 test 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, needed for applications with high thermal load. This system will facilitate the development of scalable, integrated quantum optics technology, as required for research on large scale quantum networks.

Figures (8)

• Figure 1: Schematic of the dilution refrigerator with all imaging components. The incoming beam is represented in yellow and the reflected light in red. For details see text \ref{['sec:setup']}.
• Figure 2: Change of focal plane due to (a) pressure and (b) temperature changes in the dilution refrigerator. Left axis ($\Delta s_\text{tel}$): Measured change of image plane (blue), error bars indicate depth of focus $\sim 6mm$. Right axis ($\Delta s_\text{obj}$): Extrapolated change of distance between sample and objective-side principle plane (red, green). Circles indicate applicable vertical axis.
• Figure 3: Exemplary images of group 7 of the USAF-1951 test target taken at room temperature (a) and cryogenic temperatures (b) with the high-magnification tube lens.
• Figure 4: MTF analysis for the Y-axes of the images in Fig. \ref{['fig:cold_and_warm_Usaf']}, at ambient conditions (blue, Fig. \ref{['fig:cold_and_warm_Usaf']}a) and $T\approx70mK$ (red, Fig. \ref{['fig:cold_and_warm_Usaf']}b). The resolution is stated as the value where the background subtracted MTF drops to 10% (dashed lines, b), yielding $831\frac{lp}{mm}$ at room temperature and $385\frac{lp}{mm}$ at cryogenic temperature in this example. The purple line indicates the estimated technical limit of the optical system ($945\frac{lp}{mm}$).
• Figure 5: Change of resolution during evacuation (left) and cool-down (right). Resolution corresponds to average MTF10% value of repeated optimization (blue: X-axis, green: Y-axis, see Fig. \ref{['fig:cold_and_warm_Usaf']}). Error bars indicate statistical standard deviation from repeated optimization. The red line indicates $945 \frac{lp}{mm}$, corresponding to 70% of the nominal resolution of the objective. The average resolution at temperatures below 100K is $443\pm78 \frac{lp}{mm}$. In the beginning of the cool-down process, a higher data collection rate was chosen.