Fluctuations of Energy-Relaxation Times in Superconducting Qubits

Klimov, Paul V.; Kelly, J.; Chen, Z; Neeley, M.; Megrant, A.; Burkett, Brian; Barends, R.; Arya, Kunal; Chiaro, B.; Chen, Yu; Dunsworth, A.; Fowler, Austin G.; Foxen, Brooks; Gidney, Craig; Giustina, Marissa; Graff, Rob; Huang, Trent; Jeffrey, E.; Lucero, Erik; Mutus, J.; Naaman, Ofer; Neill, C.; Quintana, Chris; Roushan, P.; Sank, D.; Vainsencher, A.; Wenner, J.; White, Theodore C.; Boixo, Sergio; Babbush, Ryan; Smelyanskiy, Vadim; Neven, Hartmut; Martinis, John M.

doi:10.1103/physrevlett.121.090502

Cited by 264 publications

(257 citation statements)

References 31 publications

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“…As a consequence of significantly enhanced coherence times, qubits became sensitive also to weakly coupling defects residing on the surfaces and interfaces of circuit electrodes [9]. Since these are limiting the performance of state-of-the-art circuits [10][11][12], further progress towards scaled-up quantum processors requires strong efforts to prevent the appearance of defects, e.g. by using better materials, improved fabrication procedures [13][14][15][16], and surface treatment to avoid contamination and parasitic adsorbates [17,18].…”

mentioning

confidence: 99%

Resolving the positions of defects in superconducting quantum bits

Bilmes

Megrant

Klimov

et al. 2020

Sci Rep

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Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect's coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.Material defects have a manifold of microscopic origins such as impurities in solids or adsorbates hosted on surfaces [1]. Their detrimental role was identified already in first experiments with superconducting quantum bits (qubits) [2]. Strong interaction with a long-known defect type, charged two-level systems (TLS) [3,4], residing in the tunnel barrier of a qubit's Josephson junction gives rise to avoided level crossings and resonant energy absorption [5]. This form of dielectric loss could be mitigated by reducing the amount of lossy dielectrics, e.g. by incorporating smaller Josephson junctions [6] and by avoiding insulating layers. Another strategy is to enlarge the footprint of device capacitors in order to dilute the electric field induced by the qubit, which excites defects by coupling to their electric dipole moments [7,8]. As a consequence of significantly enhanced coherence times, qubits became sensitive also to weakly coupling defects residing on the surfaces and interfaces of circuit electrodes [9]. Since these are limiting the performance of state-of-the-art circuits [10][11][12], further progress towards scaled-up quantum processors requires strong efforts to prevent the appearance of defects, e.g. by using better materials, improved fabrication procedures [13][14][15][16], and surface treatment to avoid contamination and parasitic adsorbates [17,18]. This endeavor needs to be guided by careful analysis of defect properties such as densities, electric dipole moments, and positions, in order to identify and improve the manufacturing steps that reduce defect formation, and to analyze the microscopic structure of defects.In this Letter, we present a method to extract information on the spatial positions of defects at the profile of the film edge in a qubit circuit. For doing this, we exploit the tunabi...

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confidence: 99%

Resolving the positions of defects in superconducting quantum bits

Bilmes

Megrant

Klimov

et al. 2020

Sci Rep

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“…Due to the material's random structure, TLS resonance frequencies are widely distributed, and those that are near resonance with qubits can dominate qubit energy relaxation [15]. Moreover, the thermal activation of low-energy TLS causes resonance frequency fluctuations of high-energy TLS, resonators, and qubits, which occur on time-scales spanning from milliseconds to hours and days [16][17][18][19][20][21]. For quantum processors, this implies that each qubit needs to be frequently recalibrated, while individual qubits can also become completely unusable due to randomly occurring resonant interaction with fluctuating TLS.…”

Section: Introductionmentioning

confidence: 99%

Electric field spectroscopy of material defects in transmon qubits

Lisenfeld¹,

Bilmes²,

Megrant³

et al. 2019

npj Quantum Inf

120

116

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Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a spectroscopy method to extract the defects' energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only ≈ 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

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“…In particular, the field of superconducting qubits has grown impressively during the last decade 6,7 . In these devices quantum states can live for up to tens of microseconds, while gate times can be as short as tens of nanoseconds [8][9][10][11] . Nevertheless, coherence times need to be further improved by orders of magnitude in order to be able to perform quantum error correction 12,13 with an affordable hardware overhead.One of the main sources of decoherence in superconducting devices at millikelvin temperatures are out of equilibrium quasiparticles (QPs) [14][15][16][17][18][19][20][21][22] , which can be viewed as broken Cooper pairs (CPs).…”

mentioning

confidence: 99%

Phonon traps reduce the quasiparticle density in superconducting circuits

Henriques

Valenti

Charpentier

et al. 2019

Applied Physics Letters

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Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are particularly detrimental in devices with high kinetic inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.Superconducting circuits play a central role in a variety of research and application areas, such as solid state quantum optics 1 , metrology 2,3 , and low temperature detectors 4,5 . In particular, the field of superconducting qubits has grown impressively during the last decade 6,7 . In these devices quantum states can live for up to tens of microseconds, while gate times can be as short as tens of nanoseconds [8][9][10][11] . Nevertheless, coherence times need to be further improved by orders of magnitude in order to be able to perform quantum error correction 12,13 with an affordable hardware overhead.One of the main sources of decoherence in superconducting devices at millikelvin temperatures are out of equilibrium quasiparticles (QPs) [14][15][16][17][18][19][20][21][22] , which can be viewed as broken Cooper pairs (CPs). Quasiparticles can be particularly damaging in high kinetic inductance circuits [23][24][25][26][27] , which are a promising avenue for protected qubits 28 and hybrid superconductingsemiconducting devices [29][30][31] . Proposed mechanisms for CP breaking include stray infrared radiation 32,33 , direct microwave drive 34,35 , and high energy phonons in the device substrate created by environmental or cosmic radioactivity [36][37][38] . The latter is particularly damaging because it gives rise to correlated QP bursts in multiple devices on the same chip 36,39 , possibly resulting in a) Both authors contributed equally b) Electronic

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Fluctuations of Energy-Relaxation Times in Superconducting Qubits

Cited by 264 publications

References 31 publications

Resolving the positions of defects in superconducting quantum bits

Resolving the positions of defects in superconducting quantum bits

Electric field spectroscopy of material defects in transmon qubits

Phonon traps reduce the quasiparticle density in superconducting circuits

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