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arXiv:2112.11951 · Physical Review Letters

Tuning Superinductors by Quantum Coherence Effects for Enhancing Quantum Computing

Bo Fan, Abhisek Samanta, Antonio M. Garcı́a-Garcı́a

Published Jan 24, 2023• 17 citations
arXivHTMLPDF

Abstract

Research on spatially inhomogeneous weakly coupled superconductors has recently received a boost of interest because of the experimental observation of a dramatic enhancement of the kinetic inductance with relatively low losses. Here, we study the kinetic inductance and the quality factor of a strongly disordered, weakly coupled superconducting thin film. We employ a gauge-invariant random-phase approximation capable of describing collective excitations and other fluctuations. In line with the experimental findings, we have found that in the range of frequencies of interest, and for sufficiently low temperatures, an exponential increase of the kinetic inductance with disorder coexists with a still large quality factor of ∼10^{4}. More interestingly, on the metallic side of the superconductor-insulator transition, we have identified a range of frequencies and temperatures, T∼0.1T_{c}, where quantum coherence effects induce a broad statistical distribution of the quality factor with an average value that increases with disorder. We expect these findings to further stimulate experimental research on the design and optimization of superinductors for a better performance and miniaturization of quantum devices such as qubit circuits and microwave detectors.

DOI
10.1103/physrevlett.130.047001

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Research on spatially inhomogeneous weakly coupled superconductors has recently received a boost of interest because of the experimental observation of a dramatic enhancement of the kinetic inductance with relatively low losses. Here, we study the kinetic inductance and the quality factor of a strongly disordered, weakly coupled superconducting thin film. We employ a gauge-invariant random-phase approximation capable of describing collective excitations and other fluctuations. In line with the experimental findings, we have found that in the range of frequencies of interest, and for sufficiently low temperatures, an exponential increase of the kinetic inductance with disorder coexists with a still large quality factor of ∼10^{4}. More interestingly, on the metallic side of the superconductor-insulator transition, we have identified a range of frequencies and temperatures, T∼0.1T_{c}, where quantum coherence effects induce a broad statistical distribution of the quality factor with an average value that increases with disorder. We expect these findings to further stimulate experimental research on the design and optimization of superinductors for a better performance and miniaturization of quantum devices such as qubit circuits and microwave detectors.
In this thesis, we first demonstrate that a disordered superconductor with high kinetic inductance can realise a microwave low-loss, non-dissipative circuit element with impedance greater than the quantum resistance. This element, known as a superinductor, can suppress the fluctuations of charge in a quantum circuit.For this purpose, we fabricated and characterised 20 nm thick, 40 nm wide niobium-nitride nanowires and determined the impedance to 6.795 kΩ. We demonstrate internal quality factors Qi = 2.5e4 in nanowire resonators at single photon excitation, which is significantly higher than values reported in devices with similar materials and geometries. Moreover, we show that the dominant dissipation in our nanowires is not an intrinsic property of the disordered films, but can instead be fully understood within the framework of two-level systems.To further characterise these losses, we then explore the geometrical scaling, toward nanowire dimensions, of dielectric losses in superconducting microwave resonators fabricated with the same techniques and from the same NbN thin-film as the nanowire superinductors. For this purpose, we perform an experimental and numerical study of dielectric loss at low temperatures. Using 3D finite-element simulation of the Maxwell--London equations, we compute the geometric filling factors of the lossy regions in our resonator structures and fit the experimental data to determine the intrinsic loss tangents of its interfaces and dielectrics. Finally, we study the effect of two-level systems on the performance of various superconducting quantum circuits. For this purpose, we measure coherence-time fluctuations in qubits and frequency fluctuations in resonators. In all devices, through statistical analysis, we identify the signature of individual Lorentzian fluctuators in the noise. We find that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems. Furthermore, when examining the low-frequency noise of three different types of superconducting resonator - one NbN nanowire, one Al coplanar waveguide, and one Al 3D cavity - we observe a similar power-law dependence of the Lorentzian switching time and amplitude on the circulating power in the resonators, suggesting a common noise mechanism in the three different types of devices.
Disordered superconducting materials with high kinetic inductance are an important resource for generating nonlinearity in quantum circuits and creating high-impedance environments. In thin films fabricated from these materials, the combination of disorder and low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of harnessing these compounds in coherent devices are their proximity to the superconductor-insulator phase transition and the two-level systems located in the disordered structure. Here, we fabricate tungsten silicide wires from quasi-two-dimensional films and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. In this work, we study the dependence of loss on the frequency, disorder, and geometry of the devices, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap. High kinetic inductance materials are widely used in superconducting circuits, yet their loss mechanisms are not fully understood. Here the authors study quantum circuits incorporating nanowires made of quasi-2D disordered superconductor WSi and identify localized quasiparticles as the dominant source of loss.
Quantum decoherence due to charge fluctuations is still a problem in many device architectures for quantum computing or metrology, but it can be suppressed by shunting the circuit with a high microwave impedance, known as a $s\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e$. This study uses the kinetic inductance of a nanowire of disordered superconductor to demonstrate a low-loss, small-capacitance superinductor, in a simpler approach than the traditionally used Josephson-junction arrays. Such nanowire superinductors should have an impact on engineering long-lived superconducting qubits, and quantum-coherent experiments with mesoscopic systems.
We design superinductors made of strongly disordered superconductors for implementation in ``hybrid'' superconducting quantum circuits. The superinductors are fabricated as meandered nanowires made from granular aluminum films. Optimization of the device geometry enables the realization of superinductors with an inductance of approximately 1 $\ensuremath{\mu}\mathrm{H}$ and a self-resonance frequency of over 3 GHz. These compact superinductors are attractive for a wide range of applications, from superconducting circuits for quantum computing to microwave elements of cryogenic parametric amplifiers and kinetic inductance photon detectors.
Superinductors are inductors whose microwave characteristic impedances are greater than the resistance quantum, RQ=h/(2e)2approx6.5kOmegaR_{Q}=h/(2e)^{2}approx6.5kOmegaRQ​=h/(2e)2approx6.5kOmega. They can be implemented using Josephson junction chains and high kinetic inductance nanowires. In this dissertation, we explore applications of superinductors in both implementations in superconducting quantum circuits. The dissertation consists of three parts. In the first part, we discuss the fluxon-parity-protected qubit consisting of a Cooper-pair box (CPB) shunted by a superinductor made of a chain of coupled asymmetric Superconducting Quantum Interference Devices (CASQUIDs). The spectroscopic measurement of a prototype of the fluxon-parity-protected qubit was performed. We observed almost complete suppression of the single fluxon tunneling across the CPB due to the destructive Aharonov-Casher interference when the offset charge on the CPB island was set to eee mod(2eee). A fluxon-parity-protected qubit with a higher superinductance can potentially be used to perform fault-tolerant Clifford gates. In the second part, we studied the microwave losses in high-kinetic-inductance granular Aluminum films using superconducting coplanar-waveguide (CPW) resonators made of the films. We observed that the intrinsic losses in these resonators at low temperatures were limited by resonator coupling to the two-level systems (TLS) in the environment. The demonstrated internal quality factors are comparable with those for CPW resonators made of conventional superconductors. The characterized granular Aluminum films can be used to fabricate superinductors for a wide range of applications in quantum metrology and quantum information processing. In the third part, we discuss the one-dimensional Josephson metamaterial made of a similar structure as the superinductor used in the fluxon-parity-protected qubit. The metamaterial demonstrated strong Kerr nonlinearity with the Kerr constant tunable over a wide range from positive to negative values by the magnetic field. The metamaterial is promising for use as an active medium for quantum-limited Josephson traveling-wave parametric amplifiers.
Superconducting boron-doped silicon is a promising material for integrated silicon quantum devices. In particular, its low electronic density and moderate disorder make it a suitable candidate for the fabrication of large inductances with low losses at microwave frequencies. We study experimentally the electrodynamics of superconducting silicon thin layers using coplanar waveguide resonators, focusing on the kinetic inductance, the internal losses, and the variation of these quantities with the resonator readout power. We report the observation in a doped semiconductor of microwave resonances with internal quality factors of a few thousand. As expected in the BCS framework, superconducting silicon presents a large sheet kinetic inductance in the 50--500-pH range, comparable to that of strongly disordered superconductors. The temperature dependence of the kinetic inductance is well described by the Mattis-Bardeen theory. However, we find an unexpectedly strong nonlinearity in the complex surface impedance that cannot be satisfactorily explained either by depairing or by quasiparticle heating.