Hydrogen bonds in Al2O3 as dissipative two-level systems in superconducting qubits

Area of Science:

• Quantum Computing
• Materials Science
• Condensed Matter Physics

Background:

• Dissipative two-level systems (TLS) are a persistent challenge in glassy solids and a key source of decoherence in quantum computing.
• Resonant absorption by TLSs in dielectrics, such as aluminum oxide (Al2O3), significantly hinders the performance of superconducting qubits.
• The microscopic origin of these TLSs remains largely unestablished, impeding efforts to mitigate their detrimental effects.

Purpose of the Study:

• To identify the microscopic source of TLS resonant absorption in dielectric materials relevant to quantum computing.
• To investigate the role of hydrogen impurities in Al2O3 as potential contributors to TLS phenomena.
• To elucidate the mechanism behind TLS formation and its impact on superconducting qubit coherence.

Main Methods:

• Employed first-principles calculations to model the behavior of hydrogen impurities within the Al2O3 lattice.
• Investigated the formation of hydrogen bonds (O-H...O) and the resulting double-well potential experienced by interstitial hydrogen.
• Calculated the resonant absorption frequencies and qubit-TLS coupling strengths based on the proposed microscopic model.

Main Results:

• Proposed that interstitial hydrogen impurities in Al2O3 are the primary cause of TLS resonant absorption.
• Identified that hydrogen bonding in Al2O3, at specific O-O distances, creates a double-well potential supporting hydrogen tunneling.
• Observed tunneling-induced resonant absorption around 10 GHz and calculated qubit-TLS coupling between 16-20 MHz, aligning with experimental data.

Conclusions:

• Hydrogen impurities in Al2O3 are identified as the dominant source of resonant absorption limiting superconducting qubit performance.
• The quantum mechanical tunneling of hydrogen atoms within a double-well potential explains the observed 10 GHz absorption.
• The findings provide a microscopic understanding of TLS, crucial for developing strategies to improve qubit coherence and advance quantum computing.