Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

Paz, Alejandro Pérez; Lebedeva, Irina V.; Tokatly, I. V.; Rubio, Ángel

doi:10.1103/physrevb.90.224202

Cited by 29 publications

(34 citation statements)

References 63 publications

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“…In order to explore TLS arising from more realistic atomic configurations for amorphous Al 2 O 3 , Paz et al (2014) performed molecular dynamics simulations at 'elevated' temperatures of 25K and searched for bistable switching between atomic configurations. The free energy profile was then extracted for these configurations and barrier tunneling and charge dipoles estimated.…”

Section: A Tunneling Atomsmentioning

confidence: 99%

Towards understanding two-level-systems in amorphous solids: insights from quantum circuits

2019

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Amorphous solids show surprisingly universal behaviour at low temperatures. The prevailing wisdom is that this can be explained by the existence of two-state defects within the material. The so-called standard tunneling model has become the established framework to explain these results, yet it still leaves the central question essentially unanswered -what are these two-level defects? This question has recently taken on a new urgency with the rise of superconducting circuits in quantum computing, circuit quantum electrodynamics, magnetometry, electrometry and metrology. Superconducting circuits made from aluminium or niobium are fundamentally limited by losses due to two-level defects within the amorphous oxide layers encasing them. On the other hand, these circuits also provide a novel and effective method for studying the very defects which limit their operation. We can now go beyond ensemble measurements and probe individual defects -observing the quantum nature of their dynamics and studying their formation, their behaviour as a function of applied field, strain, temperature and other properties. This article reviews the plethora of recent experimental results in this area and discusses the various theoretical models which have been used to describe the observations. In doing so, it summarises the current approaches to solving this fundamentally important problem in solid-state physics.

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Section: A Tunneling Atomsmentioning

confidence: 99%

Towards understanding two-level-systems in amorphous solids: insights from quantum circuits

2019

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“…Various microscopic models including dangling bonds, Andreev bound states [5], and Kondo fluctuators [6] have been suggested to explain the origin of TLSs. There is growing evidence [7,8], however, that they are formed by small groups of atoms that are able to tunnel between two energetically almost equivalent configurations. This is most strongly supported by recent experiments where the TLSs' energy splittings were tuned by applying external static strain [9].…”

Section: I: Introductionmentioning

confidence: 99%

Electronic decoherence of two-level systems in a Josephson junction

et al. 2017

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The sensitivity of superconducting qubits allows for spectroscopy and coherence measurements on individual two-level systems present in the disordered tunnel barrier of an Al/AlOx/Al Josephson junction. We report experimental evidence for the decoherence of two-level systems by Bogoliubov quasiparticles leaking into the insulating AlOx barrier. We control the density of quasiparticles in the junction electrodes either by the sample temperature or by injecting them using an on-chip dc-SQUID driven to its resistive state. The decoherence rates were measured by observing the two-level system's quantum state evolving under application of resonant microwave pulses and were found to increase linearly with quasiparticle density, in agreement with theory. This interaction with electronic states provides a noise and decoherence mechanism that is relevant for various microfabricated devices such as qubits, single-electron transistors, and field-effect transistors. The presented experiments also offer a possibility to determine the location of the probed two-level systems across the tunnel barrier, providing clues about the fabrication step in which they emerge. I: INTRODUCTIONWhile superconducting circuits based on Josephson junctions (JJs) rapidly mature towards favorable and applicable qubits for quantum computers [1-3], a major source of their decoherence traces back to spurious material defects that give rise to the formation of low-energy two-level systems (TLSs). On the other hand, sensitivity to tiny perturbations turns JJ qubits into ideal tools to study the properties of TLSs. For example, microwave spectroscopy of JJ phase qubits shows avoided level crossings revealing the TLSs' quantum character as well as their coherent interaction with the qubit [4]. Various microscopic models including dangling bonds, Andreev bound states [5], and Kondo fluctuators [6] have been suggested to explain the origin of TLSs. There is growing evidence [7,8], however, that they are formed by small groups of atoms that are able to tunnel between two energetically almost equivalent configurations. This is most strongly supported by recent experiments where the TLSs' energy splittings were tuned by applying external static strain [9]. TLSs are the source of lowenergy excitations, which are also responsible for the thermal, acoustic, and dielectric properties of glasses at temperatures below 1 K [10,11], which are well studied in bulk materials. Inherent to disordered solids, they are present in surface oxides and insulating layers of any microfabricated device as well as in the tunnel barriers of Josephson junctions.In contrast to traditional measurements performed on glasses that probe huge ensembles of TLSs, the sensitivity of JJ-based qubits allows one to address single TLSs and determine their individual properties. Strain-tuning experiments, e.g., measure a TLS's deformation potential [9] and allow for a detailed analysis of the coherent interaction between two TLSs brought into resonance [12]. In another experiment, the temperatu...

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“…The most commonly adopted tunnel barrier to date is an ultrathin aluminium oxide AlO x layer formed directly on the Al electrode in Josephson junctions. Many phenomenological theories have been proposed to describe the microscopic nature of the TLS in AlO x tunnel barrier in Josephson junctions and try to correlate the noise and decoherence measurements in qubits with the atomic structure models of the junction 4 13 14 15 16 .…”

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confidence: 99%

Atomic structure and oxygen deficiency of the ultrathin aluminium oxide barrier in Al/AlOx/Al Josephson junctions

Zeng

Tran

Tai

et al. 2016

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Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

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Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

Cited by 29 publications

References 63 publications

Towards understanding two-level-systems in amorphous solids: insights from quantum circuits

Towards understanding two-level-systems in amorphous solids: insights from quantum circuits

Electronic decoherence of two-level systems in a Josephson junction

Atomic structure and oxygen deficiency of the ultrathin aluminium oxide barrier in Al/AlOx/Al Josephson junctions

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