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Applied Physics Letters

Optical characterization of the quantum capacitance detector at 200 μm

Juan Bueno, Nuria Llombart, Peter K. Day, P. M. Echternach

Published Oct 24, 2011• 6 citations
Publisher

Abstract

We present the optical characterization at 200 μm wavelength of an antenna coupled quantum capacitance detector (QCD), a cryogenic detector based on a single Cooper pair box (SCB). The response of the device to a cryogenic blackbody source and the device noise have been measured giving an optical noise-equivalent power (NEP) at a readout frequency of 10 kHz of 2 × 10−17 W/Hz1/2 at an optical loading of 17 fW. For optical loadings of a few pW, the optical NEP was on the order of 10−16 W/Hz1/2, demonstrating that QCDs could already work as detectors for far-infrared and submillimeter wave radiation in a ground based telescope.

DOI
10.1063/1.3651277

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Applied Physics Letters•Mar 8, 2010•18 citations
We present the optical characterization at 200 μm wavelength of an antenna coupled quantum capacitance detector (QCD), a cryogenic detector based on a single Cooper pair box (SCB). The response of the device to a cryogenic blackbody source and the device noise have been measured giving an optical noise-equivalent power (NEP) at a readout frequency of 10 kHz of 2 × 10−17 W/Hz1/2 at an optical loading of 17 fW. For optical loadings of a few pW, the optical NEP was on the order of 10−16 W/Hz1/2, demonstrating that QCDs could already work as detectors for far-infrared and submillimeter wave radiation in a ground based telescope.
We propose a sensitive new detector based on Cooper pair breaking in a superconductor. The quantum capacitor detector (QCD) exploits the extraordinary sensitivity of superconducting single-electron devices to the presence of quasiparticles generated by pair-breaking photons. This concept would enable single-photon detection at far-IR and sub-millimeter frequencies with detector sensitivities that exceed that of transition-edge-sensor bolometers (TES), kinetic inductance detectors (KID), and superconducting tunnel junction detectors (STJ). The detectors we propose are based on the single Cooper pair box (SCB), a mesoscopic superconducting device that has been successfully developed at JPL for applications in quantum computing. This concept allows for frequency multiplexing of a large number of pixels using a single RF line, and does not require individual bias of each pixel. The QCD is ideal for the sensitive spectrographs considered for upcoming cold space telescopes, such as BLISS for SPICA in the coming decade, and for the more ambitious instruments for the SAFIR / CALISTO and SPIRIT / SPECS missions envisioned for the 2020 decade. These missions require large detector arrays (> 10,000 elements) which are limited by astrophysical background noise, corresponding to a noise-equivalent power (NEP) as low as 2x10-20 W / Hz1/2. Given its intrinsic response time, the QCD could also be used for energy-resolved visible photon detection, with estimated energy resolution > 100, enabling imaging low-resolution spectroscopy with an array of detectors.
Microwave kinetic inductance detectors (MKIDs) promise high sensitivity, combined with device simplicity and intrinsic multiplexibility. We have measured the response and noise equivalent power (NEP) of an all-aluminium, antenna coupled MKID at 1.54 THz, both as a function of radiation power and as a function of readout power. Eight optical filters and a box-in-box configuration at the bath temperature, allow a large range of optical powers by varying the temperature of a black body source. In the range of 80 aW-700 fW the detector is photon-noise limited, with an optical efficiency of 48±8% for one polarisation, which is limited by the antenna design. At lower optical powers we reveal a saturation level in the NEP of 3.8×10-19W/Hz1/2, competitive with other detectors for this purpose and the lowest optical NEP so far for MKIDs. The noise over the entire power range is limited by photon- and generation-recombination noise, the fundamental noise sources for these detectors.
Quasiparticle tunneling events are measured in real time using a quantum capacitance detector (QCD), allowing for the extraction of tunneling rates as a function of temperature and optical loading of radiation coming from a black body source filtered to 200 m. The measurements are used to corroborate the basic operating principles of the QCD. An estimate of the residual quasiparticle density is made, and the noise equivalent power (NEP) is assessed to be 7.2×10−20W/Hz1/2 at the lowest signal power of 9.2×10−20W. This NEP was higher than the photon noise by only a factor of 7 over a wide signal power range.
We present a proposed design for a pair-breaking photodetector for far-infrared and submillimeter radiation. Antenna-coupled radiation generates quasiparticles in a superconducting absorber, the density of which are measured using a single Cooper-pair box. Readout is performed using an electromagnetic oscillator or a microwave resonator, which is well suited for frequency multiplexing in large arrays. Theoretical limits to detector sensitivity are discussed and modeled, with predicted sensitivities on the order of ${10}^{\ensuremath{-}21}\text{ }\text{W}/\sqrt{\text{Hz}}$. We anticipate that this detector can be used to address key scientific goals in far-infrared and submillimeter astronomy.
We fabricated and tested a proof-of-concept quantum capacitance detector, a superconducting radiation detector described in a recent publication [Shaw et al., Phys. Rev. B 79, 144511 (2009)]. In this concept, quasiparticle tunneling in a single Cooper-pair box is used to measure the density of quasiparticles in a superconducting absorber. We measured and characterized the response of the device to electrical quasiparticle injection obtaining its sensitivity. Moreover, we have converted the sensitivity to electrical noise-equivalent power, which is in the 10−18 W/Hz1/2 range at loading powers between 10−15–10−16 W.