Superconducting phonon blocked thermionic coolers

Cooling of quantum devices to their operating temperatures in mK range is traditionally achieved by bulky and expensive He-3/He-3 dilution refrigerators. Unlike our target device, these need to cool down an impractical amount of thermal mass and take a few days to reach their base temperature. We propose the concept of cascaded phonon-engineered thermionic coolers to cool from He-4 pulse tube compatible temperature level of 1.5 K to sub-100 mK range. This reduces efficiently the mass and cost requirements of operating quantum devices in e.g. spaceborne applications.

The full temperature range is realized by cascading multiple cooler stages together by 3D integration. Each stage consists of highly transparent and low leakage semiconductor-superconductor (Sm-S) tunnel-junctions, which also provides phonon isolation and mechanical support. Optimal operation point is provided for each stage by utilizing metals of different superconducting gaps (e.g. Nb, V, Al, Ti). We have demonstrated high quality silicon-vanadium tunnel-junctions [1] and cooled a suspended silicon chip down to 40% below the bath temperature [2,3]. Recently, we have also demonstrated a high thermal resistance in 3D integrated flip-chip device which paves the way for the envisioned device [4].