# Effects of Optical Illumination on Superconducting Quantum Devices

Understanding the effect of optical illumination incident on a superconducting quantum device is important in building a hybrid quantum system where a superconducting qubit is coupled to optically trapped atoms. I report measurements of two different types of superconducting devices illuminated by 780 nm light, one of the wavelengths needed for trapping atoms in the proposed hybrid system.

I illuminated an isolated thin-film superconducting Al lumped-element microwave resonator and observed the resonator quality factor and resonance frequency as a function of illumination intensity and microwave power in the 25 mK to 300 mK temperature range. The resonator was mounted in a 3d Al cavity with a TE101 mode frequency of 7.501 GHz and an external Q of 1.3 × 105. The external quality factor of the LC resonator to the microwave input and output lines was Qe ≈ 5 × 109. At the base temperature of 25 mK, the resonance frequency of the LC resonator was 6.720 GHz and the highest quality factor Q was about 2 × 106. Two optical fibers were inserted into the cavity and provided 780 nm light to illuminate the resonator.

The optically-induced microwave loss increased with increasing illumination intensity but decreased with increasing microwave power. The variation in Q due to microwave power was very similar to the behavior expected for loss from a distribution of two-level systems. Although this behavior may suggest the presence of optically activated two-level systems, I found that the loss is better explained by the presence of nonequilibrium quasiparticles generated by the illumination and excited by the microwave drive. I described a model of the system that assumes that optical absorption creates an effective source of phonons with energy higher than twice the superconducting gap and solved the coupled quasiparticle-phonon rate equations. I fit the simulation results to the measurements and found good agreement with the observed dependence of the resonator quality factor and frequency shift on temperature, microwave power, and optical illumination.

I then fabricated an Al/AlOx/Al transmon qubit with a |g⟩ → |e⟩ transition frequency of 5.10 GHz and studied the qubit transition frequency and relaxation time as a function of illumination intensity and temperature between 10 mK and 265 mK. The qubit was mounted in a 3d Al cavity with bare TE101 mode frequency of 6.127 GHz and coupled to the cavity with coupling strength g/2π ≈ 119 MHz. One optical fiber was inserted into the cavity and provided 780 nm light to illuminate the resonator.

Qubit relaxation showed non-exponential behavior which I fit to a quasiparticle fluctuation model with two characteristic times, where T1,q is the shorter characteristic time and T1,r is the longer characteristic time. I found T1,q ≈ 1.2 μs and T1,r ≈ 25 μs at 10 mK and no optical illumination. Both relaxation times decreased with increasing illumination intensity. Initially T1,q slightly increased while T1,r decreased with increasing temperature. Above about 130 mK, the relaxation measurements revealed a simple exponential with a single decay time T1.

For comparison, I described a model for the expected frequency shift and relaxation time due to quasiparticle tunneling through the Josephson junction, with the quasiparticle distribution given by the nonequilibrium distribution induced by the illumination. While the quasiparticle simulation predicted the general qualitative behavior of the frequency shift and relaxation time, there were some significant discrepancies with the data. This suggests the model needs to be extended, for example by including a different gap in the two superconductor layers forming the junction, and by taking into account other possible sources of loss and decoherence.

Examination of the quasiparticle model reveals approaches to reducing optically-induced loss and improving the relaxation time of superconducting quantum devices, including shielding the device from background radiation as well as high energy phonons, minimizing the kinetic inductance ratio to reduce the sensitivity of loss and frequency shift to temperature changes and optical illumination, and using quasiparticle traps.

Dissertation Committee Chair: Prof. Frederick Wellstood

Committee:

Dr. Christopher Lobb

Dr. Steven Anlage

Dr. Luis Orozco

Dr. John Cumings

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