Superconducting quantum detectors

Abstract:

We have developed anSIS receiver with a wide intermediate-frequency (IF) bandwidth.This is important for reducing image integration time and simultaneously measuring multiple spectral lines. The receiver is a finline mixer-based design, which allows for ultra-wide radio-frequency (RF) bandwidth and has lower mechanical requirements compared to radial stub designs. Simulations of this receiver showed quantum limited noise in the RF frequency range of 140 to 260 GHzand from DC to 10GHz in the IF spectrum.We measured the noise temperature by comparing the receiver's response to hot and cold loads. The best noise temperature was 37.9 K at 231.0 GHz, and all of the results were below 100 K from 213 to 257 GHz (the bandwidth of our local-oscillator). We measured the IF bandwidth using a spectrum analyser, and found good results from around 3-10 GHz. The lower frequency was restricted by our IF amplifier's bandwidth but the higher frequency limit was lower than we expected from simulations. We believe that this discrepancywas due to the inductance of the bondwires that we used to connect the mixer chip to the IF board. We are currently investigating techniques to reduce and compensate for this inductance.

Abstract:

We have designed and fabricated a feed horn array to operate between 1300 - 1500 GHz. It consists of four smooth walled, two section feeds with a single flare angle discontinuity near the throat of each horn. The design was carried out using a modelling package that employs modal matching together with genetic algorithm and downhill simplex optimisation techniques. This determines the optimum horn dimensions given a set of stringent requirements on beam circularity and cross polarisation levels. The simulated far-field radiation patterns exhibit very good beam circularity, low sidelobes and low cross polarisation levels across the frequency range of operation. The array has been fabricated out of brass by the workshop of Oxford University using a direct machining technique that employs drill tools shaped to the profile of the designed horn. Far-field beam pattern measurements presented in this paper were carried out at the Harvard-Smithsonian Center for Astrophysics and suggest good agreement with the theoretical predictions.

Abstract:

We present the design and some preliminary measured results of a planar superconducting on/off switch comprising two niobium nitride (NbN) bridges deposited across the slotline section of a unilateral finline. The two bridges are separated by a distance of λ/4, such that the superconducting impedance of the bridges could be cancelled out at the resonance frequency. Both the NbN bridges were switched from the superconducting state to the normal state via a bias current exceeding the critical current of the NbN film. A millimetre wave source calibrated with a terahertz power meter is used to illuminate the switch, and the response of the switch in each state was measured using a superconductor-insulator-superconductor (SIS) chip as a direct detector. Preliminary measured results agreed generally well with our simulations, especially when the multiple wave reflection effect is included in our model.

Abstract:

We present the design and some preliminary measured results of a planar superconducting on/off switch comprising two niobium nitride (NbN) bridges deposited across the slotline section of a unilateral finline. The two bridges are separated by a distance of γ/4, such that the superconducting impedance of the bridges could be cancelled out at the resonance frequency. Both the NbN bridges were switched from the superconducting state to the normal state via a bias current exceeding the critical current of the NbN film. A millimetre wave source calibrated with a terahertz power meter is used to illuminate the switch, and the response of the switch in each state was measured using a superconductor-insulator-superconductor (SIS) chip as a direct detector. Preliminary measured results agreed generally well with our simulations, especially when the multiple wave reflection effect is included in our model.

Abstract:

In this Letter, we present the design of a planar microstrip coupler where power coupling between two parallel microstrip lines can easily be controlled. Enhancement of power coupling is done via two small slots in the ground plane underneath the microstrip pair. The ground plane slots force the field lines near the edge of each microstrip to cross to the adjacent microstrip. The magnitude of the power coupling can easily be controlled by adjusting the length of the slots. Here, we describe the operation of a microstrip coupler, and present the design of a1 X 4 power distributor array that can be used to distribute the input microwave power uniformly to four branch lines arranged in a row. We also present the measured responses of the array, and show that the performance agree very well with simulated results.