Microwave-induced excess quasiparticles in superconducting resonators measured through correlated conductivity fluctuations

Abstract

We have measured the number of quasiparticles and their lifetime in aluminium superconducting microwave resonators. The number of excess quasiparticles below 160 mK decreases from 72 to 17 lm À3 with a 6 dB decrease of the microwave power. The quasiparticle lifetime increases accordingly from 1.4 to 3.5 ms. These properties of the superconductor were measured through the spectrum of correlated fluctuations in the quasiparticle system and condensate of the superconductor, which show up in the resonator amplitude and phase, respectively. Because uncorrelated noise sources vanish, fluctuations in the superconductor can be studied with a sensitivity close to the vacuum noise. The promise of a long quasiparticle lifetime and a long coherence time makes superconducting circuits popular for use in radiation detection and quantum computation. At low temperature the number of quasiparticles in a superconductor should decrease exponentially. Excess quasiparticles were recently suggested to limit the coherence time of superconducting qubits 1 and the tunnelling rate in single-electron transistors.2 Recently, the quasiparticle lifetime in a highquality aluminium superconducting resonator 3 was shown to be consistent with an excess quasiparticle population inferred from noise measurements. 4 There is a vivid debate on the question of the origin of those excess quasiparticles, 1,2,4 which mainly focuses on reducing the influence of the environment on the devices under study. Here we show for superconducting aluminium resonators that the environment is well enough under control in our experimental setup to reveal a new source of quasiparticles, namely, the microwave readout power of these devices. We show that the saturation in the number of quasiparticles at low temperature (100-150 mK), as inferred from noise measurements, decreases from 72 to 17lm À3 with a 6 dB decrease of the microwave power. The quasiparticle lifetime increases accordingly from 1.4 to 3.5 ms.Microwave resonators are popular devices in radiation detection 5 and circuit quantum electrodynamics. 6 The two quadratures of the microwave field in such a resonator are proportional to the real and imaginary part of the conductivity of the superconductor r 1 À ir 2 . The real part corresponds to dissipation in the quasiparticle system and the imaginary part to the kinetic inductance of the condensate. 7 We have shown recently that the real part of the conductivity shows quasiparticle number fluctuations. 4 When two quasiparticles recombine, a Cooper pair is formed and when a Cooper pair is broken it leaves two quasiparticles. Therefore, the superconducting condensate fluctuates as well, and one would expect to see these fluctuations in the reactive response of the microwave resonator. However, this reactive response is obscured by the response of two-level fluctuators in the dielectrics surrounding the resonator.8 Therefore, we study here the correlation between the dissipative and reactive part of the conductivity in an aluminium resonator. We observe correlated ...

Experimental observation of the breaking and recombination of single Cooper pairs

“…Quasiparticle tunneling into a Cooper pair box has been used to detect far-infrared radiation from a blackbody source with a noise-equivalent power of less than 10 −19 W/Hz 1/2 , potentially providing a successor technology to kinetic inductance detectors [18]. In parallel, studies on superconducting resonators have shown a saturation of the quasiparticle population at a relatively high temperature of 140 mK [19]. It remains an experimental challenge to reduce the quasiparticle temperature towards the base lattice temperature in a dilution refrigerator.…”

Evidence of a Nonequilibrium Distribution of Quasiparticles in the Microwave Response of a Superconducting Aluminum Resonator

“…The consequences of the redistribution of quasiparticles for the electrodynamic response were only studied theoretically for T > 0.5T c [10]. Redistribution of quasiparticles also explains [11] the microwave power dependent number of quasiparticles in microwave resonators at low temperatures, which we have recently measured [12]. These quasiparticles impose a limit for detectors for astrophysics based on microwave resonators [13,14].…”

“…[12,29]. Figure 4(a) shows τ qp as determined from the cross-power spectral density of quasiparticle fluctuations in the amplitude and the phase of the resonator [12]. Panels (b) and (c) show the measured Q i and f res .…”

“…These results were measured on sample B [18], on which we performed accurate measurements of τ qp as reported on in Refs. [12,29]. Figure 4(a) shows τ qp as determined from the cross-power spectral density of quasiparticle fluctuations in the amplitude and the phase of the resonator [12].…”

Equivalence of optical and electrical noise equivalent power of hybrid NbTiN-Al microwave kinetic inductance detectors

“…The lens-antenna coupled hybrid MKIDs have shown the expected photon noise limited performance in both phase and amplitude readout down to 100 fW of optical loading as well as a high optical efficiency 7,8 . A Noise Equivalent Power (NEP) in the 10 −19 W/ √ Hz range has been measured electrically for MKIDs 3,9 . The electrical NEP is determined from the MKIDs temperature responsivity, quasiparticle recombination time, superconducting energy gap and noise spectrum, all of which can be measured in a dark environment.…”