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Fig. 7

image

a) MKID local frequency sweep results in a circle in the complex plane, shown by the blue circle. The line inside the circle is the trace of the MKID response from an increase in absorbed power from <10 aW to 200 fW with a readout tone placed 10 kHz below the MKID resonance frequency. The data are over-plotted with the sections used to calculate the NEP. b) NEP as a function of loading power with a constant LO frequency at a post-detection frequency of 60–80 Hz. We see that the measured NEP approaches the NEPBlip for powers exceeding 100 aW up to 40 fW. c) Mean value in a 60–80 Hz post detection frequency band of the phase noise power spectral density, amplitude noise power spectral density and the ratio of the two. We see that at low power the phase noise exceeds the amplitude noise. At the highest power we see that the phase and amplitude noise become similar due to a sharp drop in the phase noise level. This drop is caused by the MKID responsivity being reduced by the resonator moving too far off the readout tone. The power level where the phase noise approaches the amplitude noise, which is the same power level at which the NEP starts to deviate from the theoretical prediction in panel b), defines the maximum instantaneous source power that the device can measure.

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