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

R. P. Budoyo, J. B. Hertzberg, C. J. Ballard, K. D. Voigt, Z. Kim, J. R. Anderson, C. J. Lobb, and F. C. Wellstood

Phys. Rev. B 93, 024514 – Published 19 January 2016

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.

Received 11 September 2015
Revised 26 December 2015

DOI:https://doi.org/10.1103/PhysRevB.93.024514

Physics Subject Headings (PhySH)

Thin films

Dilution refrigerator Infrared techniques Superconducting devices

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied Physics

Authors & Affiliations

R. P. Budoyo^*, J. B. Hertzberg^†, C. J. Ballard, K. D. Voigt, Z. Kim^‡, J. R. Anderson, C. J. Lobb, and F. C. Wellstood

Joint Quantum Institute and Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA

^*rbudoyo@umd.edu
^†Present address: IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA.
^‡Present address: Agency for Defense Development, Yuseong, Daejeon 305-600, South Korea.

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 93, Iss. 2 — 1 January 2016

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Diagram showing model for steady-state power flow in the quasiparticles and phonons in a thin-film superconducting resonator under optical illumination.
Reuse & Permissions
Figure 2
(a) Photograph of lumped-element resonator made from 215 nm thick Al film showing meandering inductor $L$ and interdigitated capacitor $C$ . (b) Photograph of resonator chip mounted in cavity, illuminated by light (red spot) supplied by optical fiber in bottom of the cavity. (c) Transmission $| S_{21} |^{2}$ vs frequency showing $L C$ resonance at $f = 6.720$ GHz. (d) Schematic of measurement setup showing microwave drive line between VNA and port 1 of 3D cavity, the $L C$ resonator, microwave readout line between port 2 of 3D cavity and VNA, and the 780 nm illumination line.
Reuse & Permissions
Figure 3
(a) Inverse quality factor $1 / Q$ and (b) resonance frequency $f_{r}$ of the $L C$ resonator vs applied rf power $P_{rf}$ (measured at the cavity input) at listed temperatures $T$ with no illumination. Filled circles are data. Dashed curves were calculated from the nonequilibrium simulation with a background illumination giving an effective phonon source temperature $T_{eff} = 236$ mK.
Reuse & Permissions
Figure 4
(a) Simulated quasiparticle distribution $f (E)$ as a function of normalized quasiparticle energy $E / Δ$ for three drive powers $P_{rf}$ , with $T_{b} = 25$ mK and $T_{eff} = 236$ mK. Blue is for $P_{rf} = - 65$ dBm, red is for $P_{rf} = - 55$ dBm, and yellow is for $P_{rf} = - 45$ dBm. For comparison, green dashed curve shows the Fermi-Dirac distribution at 236 mK. (b) Simulated phonon distribution $n (Ω)$ as a function of normalized phonon energy $Ω / Δ$ . Green dashed curve is the Bose-Einstein distribution at 236 mK. Other colored curves are for the same rf powers as in (a). (c) Detailed view of (a) for $E$ between $Δ$ and $2 Δ$ .
Reuse & Permissions
Figure 5
Inverse quality factor $1 / Q$ of resonator as a function of applied rf power $P_{rf}$ for different illumination intensities $I_{opt}$ at base temperature. Closed circles are data and dashed curves were found from the nonequilibrium simulation with $T_{b} = 25$ mK. For each optical intensity, the effective temperature $T_{eff}$ was varied in the simulation to find the best fit.
Reuse & Permissions
Figure 6
(a) Effective temperature $T_{eff}$ as a function of optical illumination intensity $I_{opt}$ . Each closed black circle is extracted from a fit to the $1 / Q$ vs $P_{rf}$ data, as in Fig. 5. The green curve is a fit of the circles to Eq. (12). (b) Inverse quality factor $1 / Q$ and (c) fractional frequency shift $- δ f_{r} / f_{r}$ as a function of $I_{opt}$ for $P_{rf} = - 65$ dBm (blue) and $- 45$ dBm (red). In each plot, the closed circles are measured and the solid curves are from the nonequilibrium simulation with $T_{b} = 25$ mK and using the $T_{eff}$ fit values shown in (a). For comparison, the dotted curves are from the nonequilibrium simulation assuming simple heating with $T_{b} = T_{eff}$ from fit in (a) and no illumination $(P_{opt} = 0)$ .
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics