An End-Fire SIS Mixer with Near Quantum-Limited Performance

In this letter, we report the near quantum-limited performance of an end-fire millimetre-wavelength superconductor-insulator-superconductor (SIS) mixer. An important feature of this mixer is its use of a unilateral finline as the input antenna, which allows for a wide RF bandwidth, a simple waveguide structure with easy alignment and for the mixer chip to be aligned along the optical axis. Each of these factors is beneficial in the construction of large-format focal plane arrays. We tested the new finline mixer from 210 to 260 GHz and the best recorded noise temperature was approximately twice the quantum limit, which is comparable to conventional radial probe mixers. This suggests that end-fire SIS mixers can be used in large-format arrays, comprising of 100s or even 1000s of SIS mixing elements, while retaining state-of-the-art quantum mixing performance.