What are the requirements for microwave cables and connectors operating at millikelvin temperatures?

Microwave cables and connectors operating at millikelvin temperatures must satisfy stringent requirements for thermal conductivity, RF performance at cryogenic temperatures, mechanical reliability under thermal cycling, and materials compatibility with ultra-high vacuum. Key requirements: (1) Thermal conductivity: cables must minimize heat conduction from higher-temperature stages to the millikelvin mixing chamber. Stainless steel (SS) coax: thermal conductivity k = 15 W/m·K at 300K, dropping to 0.3 W/m·K at 4K. Heat load for a 30 cm SS coax from 4K to 20 mK: ~0.1 μW. Copper coax: k = 400 W/m·K, heat load ~50 μW over the same distance, unacceptable. Superconducting NbTi coax: k = 0.4 W/m·K at 4K (phonon thermal conductivity only, since electron thermal conductivity is zero below Tc = 10K). Heat load comparable to SS but with zero RF loss, making NbTi ideal for readout return lines. (2) RF insertion loss: SS coax at 5 GHz: 0.3-0.5 dB/m at room temperature, dropping to <0.1 dB/m at 4K (reduced skin-depth resistivity). Below Tc for superconducting coax: essentially zero loss. CuNi coax: moderate loss (0.5-1.0 dB/m), used where some loss is acceptable (control lines with 40+ dB of cold attenuation). (3) Connectors: SMA connectors are standard for room temperature to MXC connections. At millikelvin: the connector must not generate heat from poor contact resistance. Gold-plated SMA connectors with beryllium copper center contacts maintain reliable contact across thermal cycling. Connector torque: hand-tight (5 in-lb) is sufficient; over-torquing risks damaging superconducting center conductors. (4) Thermal cycling: cables and connectors must survive >1000 thermal cycles from 300K to 10 mK without connector degradation or cable damage. Standard stainless steel semi-rigid coax (UT-085-SS, UT-047-SS) is rated for this and has extensive heritage in dilution refrigerators.