Future cryogenic far-infrared (IR) missions will require moderate-resolution far-IR spectrometers operating at the photon background limit. Full utilization of these facilities requires compact, multiplexable dispersive spectrometers with integrated detector arrays with sensitivities less than 3×10–20 W/(Hz)1/2. The detectors described here will be capable of those sensitivities.
Regular quantum capacitance detectors already have demonstrated such sensitivities with respect to the power absorbed in the device. However, they suffered from poor efficiency with respect to the incoming photon stream, resulting in sensitivity 20 times worse than required. The problem has been traced to the absorber, where Cooper- Pairs (paired electrons that are current carriers in superconducting materials) are broken by radiation coupled by the antenna, generating unpaired electrons that are then sensed by the device. Since the absorber is connected to the antenna (which is made of normal metal), the unpaired electron can diffuse to the antenna and be lost for signal generation.
A quantum capacitance detector was developed in which the device absorber is capacitively coupled to the antenna with no galvanic connection, hereby trapping the unpaired electrons so they will be sensed by the device. The capacitively coupled quantum capacitance detector (CCQCD) is based on the Single Cooper Pair Box (SCB), a superconducting mesoscopic circuit consisting of a superconducting island connected to a lead electrode (or reservoir) via a small (100×100 nm, typically) tunnel junction. The island can be biased via a gate capacitor.
In order to read out the capacitance, the island is coupled to one open end of a half-wave resonator. A capacitance change on the island will cause a resonance frequency shift in the resonator. The resonator is coupled on the opposite end to a microwave feedline, and the transmitted power through the feedline is measured using a conventional inphase-quadrature (IQ) mixer after amplification by a cold (4.2 K) low-noise amplifier followed by room-temperature amplifiers.
The SCB on which the quantum capacitance detector is based is an artificial quantum system and is extremely susceptible to noise. QCDs use this susceptibility by injecting the signal to be measured on the device and using the perturbation of the quantum system as the response. The major innovation of the capacitively coupled QCD is the separation of the absorber from the antenna, which improves the performance of the QCD by avoiding the loss of unpaired electrons that generate the device response.