Materials for Quantum Technology
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The Balatsky group’s effort in Materials for Quantum Tech centers on understanding and mitigating two-level systems (TLS) that limit coherence in superconducting qubits, primarily within the AFOSR-funded consortium with Rigetti, RMIT, LLNL, and Ames National Labs. The overarching goal is to develop a microscopic “defect playbook” for ultra-stable quantum materials by combining first-principles simulations, machine-learning potentials, many-body theory, and accelerated characterization techniques.
Key Accomplishments
Microscopic Identification of TLS in Amorphous Al₂O₃
- Established causal link between localized Al-O dipole fluctuators in amorphous oxide tunnel barriers and sub-THz TLS behavior.
- Used melt-and-anneal protocols to generate realistic amorphous Al₂O₃ structures interfaced with Al(111).
- Simulated Alternating Bias Assisted Annealing (ABAA) via Car-Parrinello molecular dynamics (Quantum Espresso) with alternating 0.5 V pulses (~2 ps each). Post-pulse relaxation consistently lowers system energy, indicating access to deeper minima.
- Employed MACE-MP-0b3 machine-learned interatomic potentials to compute atom-projected phonon density of states before/after ABAA. Results show systematic shift of vibrational spectrum to higher frequencies, particularly at the oxide–Al interface, supporting removal/suppression of soft-mode TLS candidates (Tyner et al., Adv. Quantum Technol. 2025; Tyner1 arXiv 2025).
2. TLS Impact on Superconducting Transport – Josephson Hotspots
- Extended vibron model of TLS (localized Einstein boson) using many-body perturbation theory and Eliashberg theory.
- Demonstrated that inelastic scattering off a TLS locally amplifies the local density of states and electron-vibron coupling, creating Josephson current “hotspots” within a few nm of the defect.
- Predicted up to 10× modulation of local critical current; effect is stronger in materials with large bulk electron-phonon coupling (e.g., Ta, Nb vs. Al).
- Highlighted non-resonant TLS influence on qubit T₁ even when off-resonance, providing a materials roadmap for coherence improvement (Heath et al., Phys. Rev. Applied 2026).
- Localized Josephson hot spots due to two-level systems JT Heath, AC Tyner, TC Thann, VP Michal, P Krogstrup, MK Svendsen, Physical Review Applied 25 (1), 014022
3. Constructive TLS Engineering for Enhanced Superconductivity
- Introduced TLS/vibron engineering as a third mechanism (beyond disorder or 2D limit) to boost superconducting Tc.
- Added Einstein-boson spectral weight (<1 THz) “by hand” to ab-initio electron-phonon spectral function of thin-film Al.
- Solved linearized Eliashberg equations and analytic Leavens-Carbotte formula, predicting 2–3× Tc enhancement depending on TLS frequency and Al monolayer count (Heath et al., arXiv:2510.23710, under review at PRL).
- Tailoring superconductivity with two-level systems, Joshuah T. Heath and Alexander C. Tyner and S. Pamir Alpay and Peter Krogstrup and Alexander V. Balatsky DOI: https://doi.org/10.1103/zdgk-b66f
4. Accelerated TLS Characterization via Machine Learning
- Developed ML-assisted framework using two-tone spectroscopy and convolutional neural networks (CNNs) for rapid extraction of TLS parameters (frequency, coupling).
- Accelerated Characterization of Two‐Level Systems in Superconducting Qubits Via Machine Learning
A Pathapati, O Mansikkamäki, A Tyner, AV Balatsky, Advanced Quantum Technologies 9 (3), e00868
Broader Context and Impact
These advances build directly on Rigetti’s experimental ABAA breakthrough (2024) and provide the theoretical and computational foundation needed to understand why ABAA works at the atomic scale.
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