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arXiv:1509.05811 · Journal of Applied Physics

A frequency and sensitivity tunable microresonator array for high-speed quantum processor readout

Jed D. Whittaker, Loren J. Swenson, Mark H. Volkmann, P. Spear, Fabio Altomare, A. J. Berkley, B. Bumble, P. Bunyk, Peter K. Day, Byeong Ho Eom, R. Harris, Jeremy Hilton, Emile Hoskinson, Mark W. Johnson, Alan W. Kleinsasser, E. Ladizinsky, Trevor M. Lanting, T. Oh, I. Perminov, E. Tolkacheva, J. Yao

Published Jan 7, 2016• 13 citations
arXivHTMLPDF

Abstract

Superconducting microresonators have been successfully utilized as detection elements for a wide variety of applications. With multiplexing factors exceeding 1000 detectors per transmission line, they are the most scalable low-temperature detector technology demonstrated to date. For high-throughput applications, fewer detectors can be coupled to a single wire but utilize a larger per-detector bandwidth. For all existing designs, fluctuations in fabrication tolerances result in a non-uniform shift in resonance frequency and sensitivity, which ultimately limits the efficiency of bandwidth utilization. Here, we present the design, implementation, and initial characterization of a superconducting microresonator readout integrating two tunable inductances per detector. We demonstrate that these tuning elements provide independent control of both the detector frequency and sensitivity, allowing us to maximize the transmission line bandwidth utilization. Finally, we discuss the integration of these detectors in a multilayer fabrication stack for high-speed readout of the D-Wave quantum processor, highlighting the use of control and routing circuitry composed of single-flux-quantum loops to minimize the number of control wires at the lowest temperature stage.

DOI
10.1063/1.4939161

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Superconducting microresonators have been successfully utilized as detection elements for a wide variety of applications. With multiplexing factors exceeding 1000 detectors per transmission line, they are the most scalable low-temperature detector technology demonstrated to date. For high-throughput applications, fewer detectors can be coupled to a single wire but utilize a larger per-detector bandwidth. For all existing designs, fluctuations in fabrication tolerances result in a non-uniform shift in resonance frequency and sensitivity, which ultimately limits the efficiency of bandwidth utilization. Here, we present the design, implementation, and initial characterization of a superconducting microresonator readout integrating two tunable inductances per detector. We demonstrate that these tuning elements provide independent control of both the detector frequency and sensitivity, allowing us to maximize the transmission line bandwidth utilization. Finally, we discuss the integration of these detectors in a multilayer fabrication stack for high-speed readout of the D-Wave quantum processor, highlighting the use of control and routing circuitry composed of single-flux-quantum loops to minimize the number of control wires at the lowest temperature stage.
D-Wave's quantum processors require a high-fidelity readout architecture for fast and accurate operation. The need for cold operation precludes readout methods with significant dissipation. The readout architecture must be scalable and compatible with an inherently dense qubit circuit layout. Superconducting microresonators have been successfully utilized as detection elements for a wide variety of applications, enabling superior multiplexing factors. In existing designs, resonators undergo a non-uniform shift in frequency and sensitivity with fabrication imperfections, ultimately limiting effective bandwidth utilization. We overcome this drawback by implementing independent control of both frequency and sensitivity. We describe the design of the detector and report experimental results demonstrating its resilience to fabrication tolerances.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
We report on the development of a microwave superconducting quantum interference device (SQUID) multiplexer to read out arrays of low-temperature detectors. In this frequency-division multiplexer, superconducting resonators with different frequencies couple to a common transmission line and each resonator couples to a different dissipationless SQUID. We demonstrate multiple designs, with high-Q values (4100–18 000), noise as low as 0.17μΦ0∕Hz, and a naturally linear readout scheme based on flux modulation. This multiplexing approach is compatible with superconducting transition-edge sensors and magnetic calorimeters and is capable of multiplexing more than a thousand detectors in a single transmission line.
Superconducting microresonators have diverse applications, including microwave kinetic inductance detectors, microwave superconducting quantum interference device multiplexers, and superconducting qubits. Arrays of such devices are typically addressed using microwave frequency combs with probe tones matched to individual device resonances. However, resonator frequency collisions caused by wafer non-uniformity and fabrication variations significantly limit usable device yields. In this Letter, we present a technique that mitigates these frequency collisions without the need for ex post facto processing. By leveraging nonlinear kinetic inductance and persistent current in a superconducting loop, we achieve in situ tuning of individual resonator frequencies within an array during device cooldown, effectively resolving frequency collisions in a way that is both highly flexible and reversible. We successfully demonstrate this technique by tuning a small array of four resonators to both identical frequencies and a uniformly spaced frequency comb. This in situ resonator tuning approach provides a universal solution for improving yield and multiplexing density in large resonator arrays, addressing a critical need for scaling up superconducting detector and qubit systems.
We report our development on design, fabrication and characterization of various superconducting resonators for quantum device applications. The resonators are developed mainly for superconducting qubit implementation in circuit QED structure, but future application will include photon detectors such as microwave kinetic inductance detectors. First we designed and fabricated half-wavelength coplanar waveguide resonators with niobium thin film on a high resistive silicon wafer, with the fundamental frequencies in the range of 1 to 8 GHz. The loaded quality factors are controlled by the coupling capacitors, and the lower bound of the intrinsic quality factor was measured down to 7 mK in the dilution refrigerator. We have also designed quarter-wavelength resonators and lumped LC resonators coupled to a transmission line, and characterized multiple resonators in a single transmission line. For the 3D transmon qubit application, we designed and fabricated a rectangular box resonator with superconducting aluminum. The intrinsic quality factor of the box resonator exceeds three million at 7 mK. We will also present basic results on the integration of a transmon qubit in the superconducting resonator.
In this paper, we present a novel design of a compact, double-fold half-wave superconducting coplanar waveguide resonator, specifically optimized for quantum readout. The geometry offers advantages in terms of scalability and allows for a more compact integration of qubits per unit area. The superconducting resonators are designed to operate within a fundamental frequency range of 1.9 GHz$-1.95GHz,utilizingbothgapandfingercouplingschemes.Thedevicesarefabricatedusingthinaluminumfilmsdepositedonhigh−resistivitysapphiresubstrates.Theproposedresonatorsexhibitqualityfactor(Q1.95 GHz, utilizing both gap and finger coupling schemes. The devices are fabricated using thin aluminum films deposited on high-resistivity sapphire substrates. The proposed resonators exhibit quality factor (Q1.95GHz,utilizingbothgapandfingercouplingschemes.Thedevicesarefabricatedusingthinaluminumfilmsdepositedonhigh−resistivitysapphiresubstrates.Theproposedresonatorsexhibitqualityfactor(Q_{L})of2.9×) of 2.9 ×)of2.9× 10^{5}$and 1.58 ×10310^{3}103for gap coupled and finger coupled, respectively, during experimental measurements. The simulated QL_{L}L​of single-folded meander line resonators are found to be 4.65 ×10510^{5}105and 258.42, for gap coupled and finger coupled, respectively, for comparison purpose at similar operating frequency. A comprehensive analysis of the microwave transmission spectra was conducted at cryogenic temperatures near 10 mK, across very high input drive powers ($-30dBmto30 dBm to30dBmto-10dBm).Experimentalresultsshowagoodagreementwithsimulationsandtheequivalentlumpedcircuitmodel.Notably,theproposeddesignreducesthephysicalfootprintoftheresonatordevicesupto28.210 dBm). Experimental results show a good agreement with simulations and the equivalent lumped circuit model. Notably, the proposed design reduces the physical footprint of the resonator devices up to 28.210dBm).Experimentalresultsshowagoodagreementwithsimulationsandtheequivalentlumpedcircuitmodel.Notably,theproposeddesignreducesthephysicalfootprintoftheresonatordevicesupto28.2\%comparedtoconventionalsingle−foldmeanderedstructures.TheresonatorsdemonstratestrongpotentialforintegratingqubitswithreadoutandcontrolbusesinquantuminformationprocessingwhichrequireaQcompared to conventional single-fold meandered structures. The resonators demonstrate strong potential for integrating qubits with readout and control buses in quantum information processing which require a Qcomparedtoconventionalsingle−foldmeanderedstructures.TheresonatorsdemonstratestrongpotentialforintegratingqubitswithreadoutandcontrolbusesinquantuminformationprocessingwhichrequireaQ_{L}$between10410^{4}104−-−10510^{5}105. They also being well-suited for cryogenic detector applications that utilize microwave kinetic inductance detector (MKID) arrays in frequency multiplexing for different coupling schemes.