Note: Thermal properties of magnesium in the 60-150 mK range

Refrigerators for space and other applications working around 100 mK require lightweight components with good thermal properties. We have measured the thermal properties of high-purity (99.95%) magnesium, which is five times lighter than copper, over the 60-150 mK range and found that it is well-behaved down to these temperatures. Both conductivity and heat capacity are in good agreement with extrapolations from measurements at higher temperatures. The heat capacity per unit volume is about the same as copper and the thermal conductivity about 2.7 times lower than copper of similar residual resistivity ratio, as expected from magnesium's higher room-temperature resistivity

Copper waveguide cavities with reduced surface loss for coupling to superconducting qubits. Applied Superconductivity

Abstract-Significant improvements in superconducting qubit coherence times have been achieved recently with threedimensional microwave waveguide cavities coupled to transmon qubits. While many of the measurements in this direction have utilized superconducting aluminum cavities, other recent work has involved qubits coupled to copper cavities with coherence times approaching 0.1 ms. The copper provides a good path for thermalizing the cavity walls and qubit chip, although the substantial cavity loss makes conventional dispersive qubit measurements challenging. We are exploring various approaches for improving the quality factor of three-dimensional copper cavities, including electropolishing and coating with superconducting layers of tin. We have characterized these cavities on multiple cooldowns and found the tin-plating to be robust. In addition, we have performed coherence measurements on transmon qubits in these cavities and observed promising performance

Transformers to Readout Arrays of Microcalorimeters

We investigated the possibility of using transformers to replace SQUIDs for the readout of microcalorimeters. This simple scheme has been used in the past for bolometers, however it was discarded for the use with TES microcalorimeters because of the inadequate performance. Our work shows that, with a few simple changes, the performance of transformers as current transducers, while still not comparable to that of SQUIDs, is sufficient to read out the signal from TES microcalorimeters without any degradation in speed or energy resolution. In contrast to SQUIDs, transformers do not dissipate any power and their working principle makes them natural candidates for frequency multiplexing. Their extension to several channels is therefore straightforward