With thermal fluctuation strongly suppressed, low temperature environment is essential for studies of condensed matter physics and developments of quantum technologies. Ultra-low temperature below 20 mK has demonstrated its importance and significance in physical sciences and information techniques. Dilution refrigeration is by far the best feasible and reliable method to generate and keep lattice temperature in this range. With a potential shortage of helium supply, cryogen-free dilution refrigerator (CFDR), eliminating the necessity of regular helium refill, becomes the main facility for the purpose of creating ultralow temperature environments. Here we describe our successful construction of a CFDR which reached a base temperature of around 10.9 mK for continuous circulation and 8.6 mK for single-shot operation. We describe its operating mechanism and the designs of key components, especially some unique designs including heat switch and alumina thermal link. Possible improvements in the future are also discussed.

A merged-element transmon (MET) device based on silicon (Si) fins is proposed, and the first steps to form such a “FinMET” are demonstrated. This new application of fin technology capitalizes on the anisotropic etch of Si(111) relative to Si(110) to define atomically flat, high aspect ratio Si tunnel barriers with epitaxial superconductor contacts on parallel sidewall surfaces. This process circumvents the challenges associated with the growth of low-loss insulating barriers on lattice matched superconductors. By implementing low-loss, intrinsic float-zone Si as the barrier material rather than commonly used, potentially lossy AlOx, the FinMET is expected to overcome problems with standard transmons by (1) reducing dielectric losses; (2) minimizing the formation of two-level system spectral features; (3) exhibiting greater control over barrier thickness and qubit frequency spread, especially when combined with commercial fin fabrication and atomic-layer or digital etching; (4) potentially reducing the footprint by several orders of magnitude; and (5) allowing scalable fabrication. Here, as a first step to making such a device, the fabrication of Si fin capacitors on Si(110) substrates with shadow-deposited Al electrodes is demonstrated. These fin capacitors are then fabricated into lumped element resonator circuits and probed using low-temperature microwave measurements. Further thinning of silicon junctions toward the tunneling regime will enable the scalable fabrication of FinMET devices based on existing silicon technology while simultaneously avoiding lossy amorphous dielectrics for the tunnel barriers.