Quantum information technology puts stringent demands on the quality of materials and interfaces in the pursuit of increased device coherence. Yet, little is known about the chemical structure and origins of paramagnetic impurities that produce flux/charge noise that causes decoherence of fragile quantum states and impedes the progress toward large-scale quantum computing. Here, we perform high magnetic field electron paramagnetic resonance (HFEPR) and hyperfine multispin spectroscopy on α-Al 2 O 3 , a common substrate for quantum devices. In its amorphous form, α-Al 2 O 3 is also unavoidably present in aluminum-based superconducting circuits and qubits. The detected paramagnetic centers are immanent to the surface and have a well-defined but highly complex structure that extends over multiple hydrogen, aluminum, and oxygen atoms. Modeling reveals that the radicals likely originate from well-known reactive oxygen chemistry common to many metal oxides. We discuss how EPR spectroscopy might benefit the search for surface passivation and decoherence mitigation strategies.
The magnetic stability of E′ centers and the peroxy radical on the surface of α-quartz is investigated with first-principles calculations to understand their role in magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Paramagnetic E′ centers are common in both stoichiometric and oxygen deficient silica and quartz, and we calculate that they are more common on the surface than the bulk. However, we find the surface defects are magnetically stable in their paramagnetic ground state and thus will not contribute to 1/f noise through fluctuation at millikelvin temperatures.
Improving qubit lifetime is essential for fault-tolerant quantum computing. It is widely acknowledged that intricate processing techniques introduce defects at the interfaces and surfaces of superconducting quantum circuits, which can create sources of decoherence. Therefore, a precise understanding of how a material's structure contributes to decoherence is critical for enhancing the performance of superconducting qubits. This talk highlights our recent advancements in understanding surface oxides in resonators and ultrathin aluminum oxide barriers in Josephson junctions (JJs) using transmission electron microscopy and spectroscopy techniques [1-4]. The Superconducting Quantum Materials & Systems Center (SQMS) demonstrated that encapsulating niobium surfaces with a ∼10nm tantalum layer results in 2-5x longer T1 than bare niobium [2]. Our collaborative investigation shows that this improvement is attributed to tantalum's less lossy surface oxide, which contains fewer sub-oxides than niobium oxide. Moreover, the amorphous Ta2O5 shows a bonding nature closer to crystalline form than amorphous Nb2O5, potentially reducing hydrogen diffusion toward the oxide/metal interface [1, 2]. Moreover, we will briefly discuss our preliminary study of the thin film structure under cryogenic conditions down to 20K, conducted using a liquid helium TEM holder in a Thermo Cs-corrected Titan. We also explored the effect of aluminum deposition rates on aluminum oxide microstructure and qubit lifetime. Although increasing the deposition rate from 0.5 Å/s to 5 Å/s significantly improved oxide layer roughness and thickness variation, the qubits’ coherence remained unchanged. This is likely due to Cooper pairs only tunneling through the thinnest parts of the barrier, making them less affected by the grain boundaries in the thin aluminum films. Additionally, we uncovered a stress-induced grain boundary sliding mechanism, which contributes to short-circuit failures in JJs [4]. Our discovery provides vital insight into oxide microstructure and superconducting qubits’ functionality [5].
Reduced transmon qubit
T1 coherence times have been linked to the amorphous oxide layers formed by thin film capacitors during processing. Because Ta or Ta capped Nb capacitors exhibit overall superior qubit performance to those fabricated with Nb capacitors, it has been hypothesized that the amorphous, non-stoichiometric Ta
2O
5−x oxide is less lossy than its Nb
2O
5−x counterpart. The origins of what makes amorphous Ta
2O
5−x less susceptible to accepted decoherence channels is unknown. Here we establish the microscopic features of amorphous Nb
2O
5−x and Ta
2O
5−x using a combination of \textit{ab initio} molecular dynamics and density functional theory calculations. Our simulations establish that oxygen deficiency is less likely to occur in amorphous Ta
2O
5−x than in Nb
2O
5−x for
0≤x≤0.25 and that at a given level of oxygen deficiency the formation of metal Ta-Ta bonds is enhanced. These bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous Nb
2O
5−x. These thermochemical differences quench or highly suppress magnetic moments in amorphous Ta
2O
5−x and eliminate a potential source of quasiparticles and magnetic flux noise. Our calculations also show that hyperfine couplings between Nb nuclei and local magnetic moments in Nb
2O
5−x could form "two-level systems" (TLS) or "two-level fluctuators" (TLF) with energy splittings of 100-1000 MHz or higher. This reveals a new TLS mechanism in amorphous Nb
2O
5−x oxide layers that is unlikely in Ta
2O
5−x. Our work provides fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits, which can be used to guide materials selection and processing.
Effective methods for decoupling superconducting qubits (SQs) from parasitic environmental noise sources are critical for increasing their lifetime and phase fidelity. While considerable progress has been made in this area, the microscopic origin of noise remains largely unknown. In this work, first principles density functional theory calculations are employed to identify the microscopic origins of magnetic noise sources in SQs on an α-Al2O3 substrate. The results indicate that it is unlikely that the existence of intrinsic point defects and defect complexes in the substrate are responsible for low frequency noise in these systems. Rather, a comprehensive analysis of extrinsic defects shows that surface aluminum ions interacting with ambient molecules will form a bath of magnetic moments that can couple to the SQ paramagnetically. The microscopic origin of this magnetic noise source is discussed and strategies for ameliorating the effects of these magnetic defects are proposed.
Quantum computation is a fascinating field that combines novel physics with improvements in computation times and has been rapidly growing in the past few years. Using superconductors to form the qubits has the potential for large-scale computing, if the decoherence inherent in these devices can be understood and reduced. Two-level fluctuators due to defects in the materials are thought to cause changes to the Josephson critical current or the flux through the superconducting loop of a flux qubit, which leads to decoherence in the qubit. Alternatively, defects in the crystal lattice give rise to electron localization which in turn traps spins with random orientations at the substrate/metal interface, again producing decoherence. My work studied the proposed noise mechanisms by using Molecular Beam Epitaxy to fabricate single-crystal niobium-based Josephson junctions. Using RHEED, AFM, and TEM/STEM, I studied the epitaxy of the niobium film at the substrate interface to reduce noise due to crystal defects. I then measured flux noise in these films; the noise in the epitaxial films is lower than the comparative polycrystalline films. Further measurements using ex-situ Josephson junctions and epitaxial niobium loops resulted in the lowest reported flux noise measurements to date. Additionally, EELS measurements made in the course of the STEM analysis of the crystal structure reveal oxygen depletion from the substrate at elevated growth temperatures. This depletion leads to oxygen vacancies in the aluminum oxide substrate, which can in turn lead to charge traps at the substrate/metal interface and hence to decoherence in the qubit. Critical current noise was investigated by changing the oxidation dose of the alumina barrier in more than 50 separate Josephson junctions. Films which were grown with lower oxidation doses have a non-ideal critical current temperature dependence as well as higher critical current noise values, suggesting that insufficient oxygen is being incorporated into these junctions. Finally, I studied the effect of surface cleaning after fabrication for resonators and transmons. Microscopy shows that an oxygen ash and BOE dip together removes all photoresist residue from the chip. Internal-Q measurements do not show a marked improvement due to this cleaning, but the coherence times do improve upon the cleaning step.
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.