Photon-noise limited performance in aluminum LEKIDs

We have measured noise in aluminum lumped element kinetic inductance detectors (LEKIDs) in dark conditions at different base temperatures and with optical illumination from a variable temperature blackbody source. LEKIDs are photon-sensitive superconducting resonators coupled to planar transmission lines. We convert variations in the raw in-phase (e_I) and quadrature (e_Q) signals from a fixed frequency source transmitted through a transmission line coupled to the LEKID into a measure of the fluctuation in the resonant frequency of the LEKID (e_f) using the measured electrical response of the resonator to a swept frequency source. We find that the noise of the LEKID in the dark has a constant frequency fluctuation level, e _f⁰ which is rolled off at a base temperature-dependent frequency corresponding to the quasiparticle lifetime in the device. Above this frequency, the noise is dominated by amplifier noise at a level a factor of 2-10 times lower than the low frequency white noise level depending on the quality factor of the resonator. The amplitude of this noise and the frequency cutoff agree well with the expected frequency flucution level from generation and recombination of thermal quasiparticles from a simple Mattis-Bardeen model. When we illuminate the device with a variable temperature blackbody source through a bandpass filter centered at a frequency of 150 GHz, we observe a reduction in the quasiparticle lifetime and an increase in the level of frequency fluctuation noise as the blackbody temperature is increased. This indicates that the quasiparticle number is dominated by optically generated quasiparticles and that the noise in the device is dominated by photon noise.