Observation of Josephson harmonics in tunnel junctions

Area of Science:

• Solid-state physics
• Quantum computing

Background:

• Superconducting qubits are crucial for quantum computing.
• Aluminum oxide (AlO) tunnel Josephson junctions provide necessary nonlinearity.
• Standard analysis assumes an idealized sinusoidal current-phase relation.

Purpose of the Study:

• To investigate the accuracy of the idealized current-phase relation in AlO Josephson junctions.
• To develop a more accurate model for transmon artificial atoms.
• To explore the impact of Josephson harmonics on qubit performance.

Main Methods:

• Developed a mesoscopic model for tunneling through inhomogeneous AlO barriers.
• Incorporated higher Josephson harmonics into the transmon Hamiltonian.
• Compared computed energy spectra with experimental measurements across multiple samples.

Main Results:

• The standard current-phase relation inaccurately describes transmon energy spectra.
• The mesoscopic model predicts significant contributions from higher Josephson harmonics.
• Including these harmonics yields orders of magnitude better agreement between computed and measured spectra.

Conclusions:

• Higher Josephson harmonics are present and impactful in AlO-based quantum technologies.
• Engineered Josephson harmonics can significantly reduce charge dispersion and errors in transmon qubits.
• This work has implications for advancing quantum computers and parametric amplifiers.