High-Frequency Limits of Quantum Noise Detectors Based on Superconducting Tunnel Junction Mixers
Quasiparticle mixers (based on superconductor-insulator-superconductor tunnel junctions) have successfully been used in high-frequency detectors up to a few hundred GHz. Their sensitivities approach the quantum noise limit. This work concerns the high-frequency limit of such devices. The intrinsic limits were probed in a scale-type experiment where aluminium tunnel junctions were investigated at about 0.85 of the superconducting gap frequency fg. Calculations were based on Tucker’s theory of quantum mixing and used parameters extracted from the experiment. The conversion efficiency is affected by the decreased effective non-linearity of the mixer element at high frequency, as fewer photon points fall in the gap voltage region. Conversion gain may still be possible up to 2 fg. Josephson interference and instabilities may cause extra noise and a deteriorated conversion, but the Josephson currents could be suppressed by a magnetic field. Non-equilibrium effects are large in high quasiparticle injection rates in Al junctions but they had no detrimental effect on the conversion. They should pose a negligible problem in a high Tc superconductor. The pair breaking by high frequency photons did not cause any substantial differences between experimental conversions and values calculated from the theory that does not take the effect into account. High Tc mixers should be able to operate well into the THz range.