IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas. Sci. Technol. 19 (2008) 015509 (10pp) doi:10.1088/0957-0233/19/1/015509 Superconducting kinetic inductance detectors for astrophysics G Vardulakis, S Withington, D J Goldie and D M Glowacka Detector and Optical Physics Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK E-mail: g.vardulakis@cam.ac.uk Received 18 July 2007, in final form 2 November 2007 Published 17 December 2007 Online at stacks.iop.org/MST/19/015509 Abstract 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) × 10 3 were measured for niobium at 1 K, and quasiparticle effective recombination times of τ ∗ R = 30 µs 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) × 10 3 and a phase-shift responsivity of ∂θ/∂N qp = (5.06 ± 0.23) × 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 × 10 −16 W Hz −1/2 at a readout frequency of 550 Hz and NEP = 4.60 × 10 −17 W Hz −1/2 at 95 Hz, for effective recombination times τ ∗ R = 100 µs and τ ∗ R = 350 µs respectively. This work demonstrates that high-sensitivity detectors are possible, encouraging further development and research into KIDs. Keywords: kinetic inductance detector, superconducting imaging array, microwave resonator, KID (Some figures in this article are in colour only in the electronic version) 1. Introduction Within the astrophysics and particle physics communities, superconducting detectors have been used widely for over two decades for applications requiring exceptionally high levels of performance. In astronomy, transition edge sensors (TESs) and superconducting tunnel junctions (STJs) are used for x-ray and optical time-resolved photon counting spectroscopy [1, 2], and at submillimetre wavelengths TESs are used for high performance photometric observations [3–6]. Long-wavelength TESs have revolutionized experimental cosmology, and these devices are now being engineered into sophisticated imaging arrays and polarimeters [7, 8]. The current challenge is to fabricate extremely large-format imaging arrays to achieve wide fields-of-view on survey instruments, and to place imaging arrays in space. However, it is challenging to engineer TESs into fast responding SQUID-multiplexed photon-counting arrays and STJs require sophisticated fabrication techniques and have no obvious multiplexing scheme. The kinetic inductance detector (KID), on the other hand, promises high-sensitivity and large-format imaging from submillimetre to x-ray wavelengths [9, 10]. Crucially, KIDs solve the multiplexing problem by allowing up to 10 000 devices to be addressed through two coaxial cables and one cooled high electron mobility transistor (HEMT) amplifier 0957-0233/08/015509+10$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK