Characterizing Loss of a Superconducting Transmission Line Resonator in a Strong Magnetic Field for the HAYSTAC Axion Dark Matter Search

In many quantum measurement systems, low-loss superconducting coaxial transmission lines are used to transport signals without significantly degrading signal fidelity. Although the microwave losses of many types of these cables have been studied previously [1], cavity-based axion dark matter detectors such as the HAYSTAC experiment immerse these cables in a strong dc magnetic field. This applied field increases transmission line loss due to magnetic flux vortex penetration into the bulk of the type-II superconductor which makes up the cable. In a planned HAYSTAC upgrade, the axion scan rate is enhanced by coupling the axion cavity to an auxiliary readout cavity via a simultaneous state swapping and two-mode squeezing interaction [2]. However, the coupling between the readout mode and the two transmission line modes closest in frequency to the cavity mode can significantly degrade the scan rate, an effect which is exacerbated as the loss in the transmission line increases. Here we use the resonant cavity technique to measure the attenuation constant and quality factor of a superconducting niobium-titanium (NbTi) cable immersed in a strong axial magnetic field, with the goal of determining whether a superconducting coaxial cable can be used to transport the signal in the future HAYSTAC upgrade. We find that the magnetic field increases loss in the cable by three orders of magnitude at GHz frequencies, resulting in unacceptable scan rate degradation and requiring development of a new waveguide-based technique to transport the mode. Our results are relevant for any system which requires low loss transportation of microwave photons in a strong magnetic field.

[1] P. Kurpiers et al. Characterizing the attenuation of coaxial and rectangular microwave-frequency waveguides at cryogenic temperatures, EPJ Quantum Technology (2017) 4:8

[2] K. Wurtz et al. Cavity Entanglement and State Swapping to Accelerate the Search for Axion Dark Matter, PRX Quantum 2, 040350 (2021)