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arXiv:1903.06193 · Applied Physics Letters

Protecting superconducting qubits from phonon mediated decay

Yaniv Jacob Rosen, Matthew A. Horsley, S. E. Harrison, Eric T. Holland, Allan S. Chang, Tiziana Bond, Jonathan L DuBois

Published May 20, 2019• 18 citations
arXivHTMLPDF

Abstract

For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. Here, we present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgaps block resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We construct a multiscale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state T1 times. We demonstrate that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with theory for resonators operating in the continuous-wave regime and show that improvements in quality factors are expected by up to the enhancement in defect T1 times. Furthermore, we use full master equation simulation to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defect states. We conclude with an exploration of device implementation including tradeoffs between fabrication complexity and qubit performance.

DOI
10.1063/1.5096182

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For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. Here, we present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgaps block resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We construct a multiscale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state T1 times. We demonstrate that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with theory for resonators operating in the continuous-wave regime and show that improvements in quality factors are expected by up to the enhancement in defect T1 times. Furthermore, we use full master equation simulation to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defect states. We conclude with an exploration of device implementation including tradeoffs between fabrication complexity and qubit performance.
Noise within solid-state systems at low temperatures can typically be traced back to material defects. In amorphous materials, these defects are broadly described by the tunneling two-level systems (TLSs) model. TLS have recently taken on further relevance in quantum computing because they dominate the coherence limit of superconducting quantum circuits. Efforts to mitigate TLS impacts have thus far focused on circuit design, material selection, and surface treatments. Our work takes an approach that directly modifies TLS properties. This is achieved by creating an acoustic bandgap that suppresses all microwave-frequency phonons around the operating frequency of a transmon qubit. For embedded TLS strongly coupled to the transmon qubit, we measure a pronounced increase in relaxation time by two orders of magnitude, with the longestT1time exceeding 5 milliseconds. Our work opens avenues for studying the physics of highly coherent TLS and methods for mitigating noise within solid-state quantum devices.
The overhead to construct a logical qubit from physical qubits rapidly increases with the decoherence rate. Current superconducting qubits reduce dissipation due to two-level systems (TLSs) by using large device footprints. However, this approach provides partial protection, and results in a trade-off between qubit footprint and dissipation. This work introduces a new platform using phononics to engineer superconducting qubit-TLS interactions. We realize a superconducting qubit on a phononic bandgap metamaterial that suppresses TLS-mediated phonon emission. We use the qubit to probe its thermalization dynamics with the phonon-engineered TLS bath. Inside the phononic bandgap, we observe the emergence of non-Markovian qubit dynamics due to the Purcell-engineered TLS lifetime of 34 μsμsμs. We discuss the implications of these observations for extending qubit relaxation times through simultaneous phonon protection and miniaturization.
Hybrid systems that couple superconducting qubits to mechanical oscillators facilitate quantum control of individual phonons and have the potential to enable compact, highly coherent quantum memories and processors. To date, however, the utility of hybrid quantum acoustic devices has been limited by poor qubit and mechanical coherence times that are far shorter than those exhibited by their unhybridized counterparts. A primary challenge lies in the intrinsic nature of elastic waves in solids: the slow speed of sound results in a dense spectrum of acoustic modes, making it difficult to prevent spontaneous emission of the qubit into phononic degrees of freedom. In this thesis, I will present a systematic effort to suppress loss mechanisms in superconducting qubits and mechanical resonators in the context of circuit quantum acoustics. I begin by demonstrating the use of crystallographic orientation optimization to passively suppress diffraction losses in narrow-aperture quartz surface acoustic wave (SAW) resonators. We then study the coupling of SAW resonators to a quasi-continuum of bulk acoustic modes; this motivates us to abandon bulk piezoelectric media in favor of bandgap-protected thin-film devices. By fabricating suspended phononic crystal resonators from thin-film quartz, we demonstrate an order-of-magnitude improvement in the coherence of this class of devices. I conclude by presenting preliminary measurements of a new flip-chip architecture for circuit quantum acoustics -- the `acoustic trampoline' -- that is designed to maximize qubit-mechanical coupling while simultaneously suppressing spontaneous acoustic emission.
Abstract In the endeavour to make quantum computers a reality, integrated superconducting circuits have become a promising architecture. A major challene of this approach is decoherence originating from spurious atomic tunneling defects at the interfaces of qubit electrodes, which may resonantly absorb energy from the qubit's oscillating electric field and reduce the qubit's energy relaxation time T1. Here, we show that qubit coherence can be improved by tuning dominating defects away from the qubit resonance using an applied DC-electric field. We demonstrate a method that optimizes the applied field bias and enhances the average qubit T1-time by 23%. We also discuss how local gate electrodes can be implemented in superconducting quantum processors to enable simultaneous in-situ coherence optimization of individual qubits.
Fault-tolerant superconducting quantum processors are limited by decoherence, limited connectivity, and the wiring burden at cryogenic temperatures. This review surveys superconducting “metamaterials”: circuits built from many repeating, deeply subwavelength building blocks (unit cells) that tailor dispersion, impedance, and bandgaps. By shaping the electromagnetic (and sometimes acoustic) environment of a qubit, these networks can suppress or enhance radiative decay and frequency shifts (Purcell and Lamb effects) and provide compact multimode structures for multiplexed control and photon-mediated coupling. We emphasize the practical trade-offs—added dielectric participation, fabrication disorder, and leakage through spurious chip/package modes—and the measurements needed to quantify them. The review summarizes design and tuning approaches (including Josephson/Superconducting Quantum Interference Device implementations), materials and fabrication limits, and mode-resolved benchmarks, and it highlights emerging directions such as topological lattices and engineered reservoirs while clearly separating demonstrations from proposals.
Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently uncorrelated in space and time. In superconducting qubits, this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high-energy phonons in the substrate. These phonons can break Cooper pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential mitigation technique is to place large volumes of normal or superconducting metal on the device, capable of reducing the phonon energy to below the superconducting gap of the qubits. To investigate the effectiveness of this method, we fabricate a quantum device with four nominally identical nanowire-based transmon qubits. On the device, half of the niobium-titanium-nitride ground plane is replaced with aluminum (Al\mathrm{Al}Al), which has a significantly lower superconducting gap. We deterministically inject high-energy phonons into the substrate by voltage biasing a galvanically isolated Josephson junction. In the presence of the small-gap material, we find a factor of 2--5 less degradation in the injection-dependent qubit lifetimes and observe that the undesired excited qubit state population is mitigated by a similar factor. We furthermore turn the Al\mathrm{Al}Al normal with a magnetic field, finding no change in the phonon protection. This suggests that the efficacy of the protection in our device is not limited by the size of the superconducting gap in the Al\mathrm{Al}Al ground plane. Our results provide a promising foundation for protecting superconducting-qubit processors against correlated errors from ionizing radiation.