Superconducting kinetic inductance detectors for astrophysics

The kinetic inductance detector (KID) is an exciting new device that promises high-sensitivity, large-format, submillimetre to x-ray imaging arrays for astrophysics. KIDs comprise a superconducting thin-film microwave resonator capacitively coupled to a probe transmission line. By exciting the electrical resonance with a microwave probe signal, the transmission phase of the resonator can be monitored, allowing the deposition of energy or power to be detected. We describe the fabrication and low-temperature testing, down to 26 mK, of a number of devices, and confirm the basic principles of operation. The KIDs were fabricated on r-plane sapphire using superconducting niobium and aluminium as the resonator material, and tantalum as the x-ray absorber. KID quality factors of up to Q = (741 +/- 15) x 10(3) were measured for niobium at 1 K, and quasiparticle effective recombination times of tau(R)* = 30 mu s after x-ray absorption. Al/Ta quasiparticle traps were combined with resonators to make complete detectors. These devices were operated at 26 mK with quality factors of up Q = (187.7 +/- 3.5) x 10(3) and a phase-shift responsivity of partial derivative theta/partial derivative N-qp = (5.06 +/- 0.23) x 10(-6) degrees per quasiparticle. Devices were characterized both at thermal equilibrium and as x-ray detectors. A range of different x-ray pulse types was observed. Low phase-noise readout measurements on Al/Ta KIDs gave a minimum NEP = 1.27 x 10(-16) W Hz(-1/2) at a readout frequency of 550 Hz and NEP = 4.60 x 10(-17) W Hz-1/2 at 95 Hz, for effective recombination times tau(R)* = 100 mu s and tau(R)* = 350 mu s respectively. This work demonstrates that high-sensitivity detectors are possible, encouraging further development and research into KIDS.