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arXiv:1810.03703 · Physical Review Applied

Three-Dimensional Superconducting Resonators at T<20 mK with Photon Lifetimes up to τ=2 s

A. Romanenko, R. Pilipenko, S. Zorzetti, D. Frolov, M. Awida, S. Belomestnykh, Sam Posen, A. Grassellino

Published Mar 12, 2020• 194 citations
arXivHTMLPDF

Abstract

Very-high-quality-factor superconducting radio-frequency cavities developed for accelerators can enable fundamental physics searches with orders of magnitude higher sensitivity, and they can also offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored quantum states in three-dimensional circuit QED architecture. Here we report measurements of multiple accelerator cavities of resonant frequencies of ${f}_{0}=1.3$, 2.6, 5 GHz down to temperatures of about 10 mK and field levels down to a few photons, which reveal very long photon lifetimes up to 2 s, while also further exposing the role of the two-level systems (TLS) in niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by vacuum heat treatments at 340--450 ${}^{\ensuremath{\circ}}\mathrm{C}$.

DOI
10.1103/physrevapplied.13.034032

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Very-high-quality-factor superconducting radio-frequency cavities developed for accelerators can enable fundamental physics searches with orders of magnitude higher sensitivity, and they can also offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored quantum states in three-dimensional circuit QED architecture. Here we report measurements of multiple accelerator cavities of resonant frequencies of f0=1.3{f}_{0}=1.3f0​=1.3, 2.6, 5 GHz down to temperatures of about 10 mK and field levels down to a few photons, which reveal very long photon lifetimes up to 2 s, while also further exposing the role of the two-level systems (TLS) in niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by vacuum heat treatments at 340--450 \ensuremath∘C{}^{\ensuremath{\circ}}\mathrm{C}\ensuremath∘C.
We discuss a potentially dramatic source of quantum decoherence in three-dimensional niobium superconducting resonators and in two-dimensional transmon qubits that utilize oxidized niobium: an aggravation of two-level system (TLS) induced losses driven by vacuum baking at temperatures and durations typically used in transmon qubit fabrication. By coupling RF measurements on cavities with time-of-flight secondary ion mass spectrometry studies on an SRF cavity cutout, we find that modest vacuum baking (150-200~∘^{\circ}∘C for 5~min-11~hrs) produces a partially depleted native niobium oxide which likely contains a large concentration of oxygen vacancies that drive TLS losses. Continued baking is found to eliminate this depleted layer and mediate these additional losses.
We identify a major source of quantum decoherence in three-dimensional superconducting radio-frequency (SRF) resonators and two-dimensional transmon qubits composed of oxidized niobium: oxygen vacancies in the niobium pentoxide, which drive two-level system (TLS) losses. By probing the effect of sequential in situ vacuum-baking treatments on the rf performance of bulk Nb\mathrm{Nb}Nb SRF resonators and on the oxide structure of a representative Nb\mathrm{Nb}Nb sample using TOF SIMS, we find a nonmonotonic evolution of cavity quality factor Q0{Q}_{0}Q0​, which correlates with the interplay of Nb2O5{\mathrm{Nb}}_{2}{\mathrm{O}}_{5}Nb2​O5​ vacancy generation and oxide-thickness reduction. We localize this effect to the oxide itself and present the insignificant role of diffused interstitial oxygen in the underlying Nb\mathrm{Nb}Nb by regrowing the oxide via wet oxidation, which reveals a mitigation of aggravated TLS losses. We hypothesize that such vacancies in the pentoxide serve as magnetic impurities and are a source of TLS-driven rf loss.
Three-dimensional microwave cavities have recently been combined with superconducting qubits in the circuit quantum electrodynamics (cQED) architecture. These cavities should have less sensitivity to dielectric and conductor losses at surfaces and interfaces, which currently limit the performance of planar resonators. We expect that significantly (>10^3) higher quality factors and longer lifetimes should be achievable for 3D structures. Motivated by this principle, we have reached internal quality factors greater than 0.5x10^9 and intrinsic lifetimes of 0.01 seconds for multiple aluminum superconducting cavity resonators at single photon energies and millikelvin temperatures. These improvements could enable long lived quantum memories with submicrosecond access times when strongly coupled to superconducting qubits.
Three-dimensional microwave cavities have recently been combined with superconducting qubits in the circuit quantum electrodynamics architecture. These cavities should have less sensitivity to dielectric and conductor losses at surfaces and interfaces, which currently limit the performance of planar resonators. We expect that significantly (>103) higher quality factors and longer lifetimes should be achievable for 3D structures. Motivated by this principle, we have reached internal quality factors greater than 0.5 × 109 and intrinsic lifetimes of 0.01 s for multiple aluminum superconducting cavity resonators at single photon energies and millikelvin temperatures. These improvements could enable long lived quantum memories with submicrosecond access times when strongly coupled to superconducting qubits.
The high critical magnetic field and relatively high critical temperature of niobium nitride (NbN) make it a promising material candidate for applications in superconducting quantum technology. However, NbN-based devices and 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 from Qi ~ 1.07*10^6 in a high power regime (< nph > = 27000) to Qi ~ 1.36 *10^5 in single photon regime at temperature T = 100 mK. 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 dependence internal quality factor and frequency tuning of the coplanar waveguide resonators to characterise the quasiparticle density of NbN. We observe that the increase in kinetic inductance at higher temperatures is the main reason for the frequency shift. Finally, we measure the resonators' resonance frequency and internal quality factor at single photon regime in response to in-plane magnetic fields B||. We verify that Qi stays well above 10^4 up to B|| = 240 mT in the photon number < nph > = 1.8 at T = 100 mK. Our results may pave the way for realising robust microwave superconducting circuits for circuit quantum electrodynamics (cQED) at high magnetic fields necessary for fault-tolerant quantum computing, and ultrasensitive quantum sensing.
Superconducting radio-frequency (SRF) cavities made using high-purity Niobium can reach single photon lifetimes longer than a second with proper treatment [1]. These resonator cavities are thus an excellent choice for storing higher-dimensional quantum states. However, preparing and manipulating quantum states with larger photon numbers require a nonlinear component, like a transmon, with a sufficiently long coherence time. With the advancement in materials processing enabling coherence times for transmons exceeding hundreds of micro-seconds, one can encode and process quantum information using d-levels (qudit) of the resonators. In this talk, we will present preliminary results showing the control of a single-qudit and our progress toward building multi-qudit architecture using multi-mode SRF cavities.