Superconducting Microresonator Detectors

Figure 1: Left: Principle of operation of superconducting microresonator detectors, following[1]. The absorbed photon energy breaks Cooper electron pairs in a thin superconducting film (a) which is part of a lithographed microresonator circuit (b). The breaking of Cooper pairs causes a change in the surface reactance and resistance of the film, leading to a shift in the resonance frequency fr and a reduction of the resonator’s quality factor Qr (c), which can be sensed by measuring the phase (d) and amplitude (c) of the microwave output signal. Right: This figure illustrates the trajectory of the resonator’s transmission S21(ω) as a function of angular frequency ω in the complex plane. The transmission is unity (S21 = 1) at frequencies away from resonance. Starting at a frequency below resonance, S21 traces out the circle in a clockwise direction as the frequency is increased, crossing the real axis at the resonance frequency ωr indicated by the star, and corresponding to a transmission S21(ωr) = 1 −Qr/Qc as indicated, where Qc is the coupling-limited quality factor. The adiabatic response coefficients for changes in the resonator frequency A(ωg) and dissipation B(ωg) are shown for the case that the generator frequency ωg is tuned above the resonance. These coefficients are tangent to and perpendicular to the resonance circle, respectively.