We propose a protection scheme of a superconducting microwave resonator to realize a hybrid quantum system, where cold neutral atoms are coupled with a single microwave photon through magnetic dipole interaction at an interface inductor. The evanescent field atom trap, such as a waveguide/nanofiber atom trap, brings both surface-scattered photons and absorption-induced broadband blackbody radiation which result in quasiparticles and a low quality factor at the resonator. A proposed multiband protection layer consists of pairs of two dielectric layers and a thin nanogrid conductive dielectric layer above the interface inductor. We show numerical simulations of quality factors and reflection/absorption spectra, indicating that the proposed multilayer structure can protect a lumped-element microwave resonator from optical photons and blackbody radiation while maintaining a reasonably high quality factor.
We propose a protection scheme of a superconducting microwave resonator to realize a hybrid quantum system, where cold neutral atoms are coupled with a single microwave photon through magnetic dipole interaction at an interface inductor. The evanescent field atom trap, such as a waveguide/nanofiber atom trap, brings both surface-scattered photons and absorption-induced broadband blackbody radiation which result in quasiparticles and a low quality factor at the resonator. A proposed multiband protection layer consists of pairs of two dielectric layers and a thin nanogrid conductive dielectric layer above the interface inductor. We show numerical simulations of quality factors and reflection/absorption spectra, indicating that the proposed multilayer structure can protect a lumped-element microwave resonator from optical photons and blackbody radiation while maintaining a reasonably high quality factor.
The high critical magnetic field and relatively high critical temperature of niobium nitride (NbN) make it a promising material for applications in superconducting quantum technology. However, NbN-based superconducting circuits are sensitive to decoherence sources such as two-level system (TLS) defects. Here, we numerically and experimentally investigate NbN superconducting microwave coplanar waveguide resonator arrays with a 100 nm thickness, capacitively coupled to a common coplanar waveguide on a silicon chip. We observe that the resonators' internal quality factor (Qi) decreases fromQi∼1.07×106in a high power regime toQi∼1.36×105in the single photon regime. Data from this study is consistent with the TLS theory, which describes the TLS interactions in resonator substrates and interfaces. Moreover, we study the temperature (T) dependence ofQiand resonance frequencyfrof coplanar waveguide resonators to characterize the quasiparticle density. We observe that the increase in kinetic inductance at higher temperatures is the main reason for the frequency shift. Finally, we measurefrandQiin response to in-plane magnetic fields (B∣∣). We verify thatQistays well above104up toB∣∣= 240 mT in the photon number<nph>= 1.8 atT= 100 mK. Our results could enable the development of robust microwave superconducting circuits for circuit quantum electrodynamics at high magnetic fields required for hybrid superconducting-semiconducting quantum circuits.
I describe my contributions towards a hybrid quantum system that would have coupled 87Rb atoms to a superconducting device. I first discuss my work coupling an optical fiber to a translatable thin-film LC lumped-element superconducting Al microwave resonator operating at 100 mK in a dilution refrigerator. The LC resonators had resonance frequencies f0 of 6.15 GHz, quality factors Q of 1.5 x 105 to 6.5 x 105 at high powers, and were mounted inside a superconducting aluminum 3D cavity with a resonance frequency of 7.5 GHz and Q of 8 x 103. An optical microfiber (60 µm diameter) passed through a hole in the 3D cavity near the LC resonator. The 3D cavity was mounted on an x-z attocube-translation stage that allowed the LC resonator to be moved relative to the fiber. The resonator’s f0 and Q were affected both by the fiber dielectric perturbing the resonator’s electric field and from scattered light from the fiber. I measured both effects as a function of fiber-resonator position. I modeled the resonator’s optical response by accounting for optical production, recombination, and diffusion of quasiparticles and the non-uniform position-dependent illumination of the resonator. Using the model, I extracted key parameters describing quasiparticles in the resonator. The hybrid quantum system requires the 87Rb and LC resonator resonance to be tuned to the same frequency. I describe our LC resonator tuning method which moves a superconducting Al pin into the resonator’s electric field, decreasing the resonator capacitance and increasing its resonance frequency up to 137 MHz. This was done at 15 mK using an attocube translation stage. I also investigated two-level system (TLS) defects in an LC resonator by applying a dc voltage. I describe a model in which the TLS causes a capacitive perturbation to the resonator rather than the ‘standard’ electric-dipole coupling model. I use this model of a capacitive TLS or cTLS, to describe intermittent telegraph noise measured in the transmission S21 through the resonator. I measured shifts in f0 of more than 6 kHz corresponding to a cTLS fluctuating its capacitance contribution by 430 zF.
Abstract Superconducting resonators integrated with germanium (Ge) quantum wells (QWs) offer a promising platform for hybrid quantum devices. Yet, in the most common heterostructure architectures, they have so far been limited by sizable photon losses. Here, we report the fabrication and characterization of microwave resonators patterned in the aluminum (Al) thin film of an in-situ grown superconductor/semiconductor hybrid heterostructure (HS). The semiconductor part of this hybrid HS is grown on a commercial Ge substrate. We consistently achieve internal quality factors Q i > 1000, surpassing previous results on Ge QW heterostructures grown using the concept of a virtual Ge substrate on silicon (Si) substrates. We reach Q i ≈ 49,000 at single-photon occupation and a plateau of Q i ≈ 20,000 at sub-one photon, an order of magnitude larger than any previously reported value of resonators on Ge QW structures at low power. We further characterize the thin Al film forming the resonator, extracting its kinetic inductance and superconducting gap, and studying its magnetic field dependence. Notably, the resonance remains well-defined up to in-plane magnetic fields of 850 mT. A hysteresis emerges in the out-of-plane magnetic field dependence, for both the resonance frequency and the quality factor, indicating an interesting interplay between vortex-and quasiparticle loss mechanisms.
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.
I report measurements of two different types of superconducting devices illuminated by 780 nm light, one of the wavelengths needed in a proposed atom-superconductor hybrid quantum system. I illuminated a thin-film Al lumped-element resonator and observed the resonator quality factor and resonance frequency as a function of illumination intensity, microwave power, and temperature. The resonator was mounted in a 3d aluminum cavity. The variation in optically-induced loss due to microwave power was 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 where optical absorption creates an effective source of phonons and solved the coupled quasiparticle-phonon rate equations. I found good agreement between the simulation and the measured resonator quality factor and frequency shift as a function of temperature, microwave power, and optical illumination. I fabricated a transmon qubit and studied the qubit transition frequency and relaxation time as a function of illumination intensity and temperature. The qubit was mounted in a 3d aluminum cavity and coupled to the cavity forming a Jaynes-Cummings system. Qubit relaxation showed non-exponential behavior that I fit to a quasiparticle fluctuation model with two characteristic times. The transition frequency and both characteristic times decreased with increasing illumination intensity. For comparison, I described a nonequilibrium quasiparticle model for the expected frequency shift and relaxation time due to quasiparticle tunneling through the Josephson junction. 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.
Placing an ensemble of 10;{6} ultracold atoms in the near field of a superconducting coplanar waveguide resonator with a quality factor Q approximately 10;{6}, one can achieve strong coupling between a single microwave photon in the coplanar waveguide resonator and a collective hyperfine qubit state in the ensemble with g_{eff}/2pi approximately 40 kHz larger than the cavity linewidth of kappa/2pi approximately 7 kHz. Integrated on an atomchip, such a system constitutes a hybrid quantum device, which also can be used to interconnect solid-state and atomic qubits, study and control atomic motion via the microwave field, observe microwave superradiance, build an integrated micromaser, or even cool the resonator field via the atoms.