Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

Abstract

We have illuminated a thin-film superconducting Al lumped-element microwave resonator with 780 nm light and observed the resonator quality factor and resonance frequency as a function of illumination and microwave power in the 20 to 300 mK temperature range. The optically-induced microwave loss increases with increasing illumination but decreases with increasing microwave power. Although this behavior may suggest the presence of optically activated two-level systems, we find that the loss is better explained by the presence of nonequilibrium quasiparticles generated by the illumination and excited by the microwave drive. We model the system by assuming that the illumination creates an effective source of phonons with energy higher than double the superconducting gap and solve the coupled quasiparticle-phonon rate equations. We fit the simulation results to our measurements and find good agreement with the observed dependence of the resonator quality factor and frequency shift on temperature, microwave power, and optical illumination. Examination of the model reveals approaches to reducing optically-induced loss and improving the relaxation time of superconducting quantum devices.

High quality factor manganese-doped aluminum lumped-element kinetic inductance detectors sensitive to frequencies below 100 GHz

“…However, we note that gap broadening may also explain some of the structure in the residuals shown in Figure 3. The scale factors α x = 0.168 and α q = 0.106 are reasonable when interpreted as kinetic inductance fractions, and similar differences in the loss and fractional frequency responses have been observed in other aluminum resonators 23 . Non-dissipative quasiparticle states that affect x but not Q i have also been invoked to explain other MKID data 24 .…”

Anomalous Loss Reduction Below Two‐Level System Saturation in Aluminum Superconducting Resonators

“…k B T ) (valid for T ≪ T c ) seems to match the measurement sufficiently well, where N 0 = 10 47 J −1 m −3 ≈ 1.74 × 10 4 μeV −1 μm −3 is the single spin electronic density of states at the Fermi level. [7,13] The fit to the frequency shift data is shown in Figure 3, and the extracted fitting parameters indicate that the aluminum superconducting gap at zero temperature is Δ S0 ≈ 170 μeV, a value close to the BCS gap approximation which is 1.76k B T c with transition temperature T c = 1.12 K. The values of the other fitting parameters are 𝛼 ≈ 0.014, and 𝛿 0 = 9.6 × 10 −6 . The values of 𝛼 and 𝛿 0 are consistent with other results on a variety of similar superconducting resonators.…”

“…And eventually at high powers (𝜅 ≫ 1) such that w > 𝜈 for all temperatures, the equilibrium temperature dependence m = (1∕2) tanh (ℏ𝜔∕(2k B T)) in STM is recovered in the entire temperature range. The same discrete summation can be applied to Equation (13) for intermediate 𝛾 fluctuators. On the other hand, Equation (12) for high 𝛾 fluctuators is only modified with the substitution Γ 2 → √ Γ 2 2 + 𝜈 2 due to the sparse TLS assumption (See Section S3, Supporting Information).…”

“…However, the model with only thermal quasiparticle $${n}_{\text{th}}=2{N}_{0}\sqrt{2\pi {k}_{\mathrm{B}}T{\mathrm{\Delta}}_{\mathrm{S}0}}\exp (-\frac{{\mathrm{\Delta}}_{\mathrm{S}0}}{{k}_{\mathrm{B}}T})$$ (valid for $$T\ll T_{\text{c}}$$) seems to match the measurement sufficiently well, where $$N_0 = 10^{47}$$ $$\text{J}^{-1} \: \text{m}^{-3} \approx 1.74 \times 10^4\umu \text{eV}^{-1} \umu \text{m}^{-3}$$ is the single spin electronic density of states at the Fermi level. [ 7,13 ]…”

“…[ 6‐9 ] This process affects the population of quasiparticles, in addition to pair‐breaking process induced by cosmic rays, [ 10 ] higher order microwave harmonics, and stray infrared radiation. [ 11‐13 ] These non‐equilibrium quasiparticles are one limiting factor on superconducting resonator Q i and qubit coherence, which can reduce both the qubit relaxation time $$(T_1^{\text{Qubit}})$$ and the coherence time $$(T_2^{\text{Qubit}})$$. [ 14 ]…”