Demonstration of an NEP of 3.8×10−19 W/Hz1/2 at 1.54 THz in multiplexible superconducting microresonator 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^-19 W/Hz^1/2, 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.