  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.
