Electronic Decoherence of Two-Level Systems in a Josephson Junction

Abstract

The sensitivity of superconducting qubits allows for spectroscopy and coherence measurements on individual two-level systems present in the disordered tunnel barrier of an $Al/Al{\mathrm{O}}_{\mathrm{x}}/Al$ Josephson junction. We report experimental evidence for the decoherence of two-level systems by Bogoliubov quasiparticles leaking into the insulating $\mathrm{Al}{\mathrm{O}}_{\mathrm{x}}$ 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.

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• On-chip thermometry for microwave optomechanics implemented in a nuclear demagnetization cryostat
• Transmission-line resonators for the study of individual two-level tunneling systems
• Enhancing the Coherence of Superconducting Quantum Bits with Electric Fields
• Rabi oscillations in a superconducting nanowire circuit
• Dynamical Decoupling of Quantum Two-Level Systems by Coherent Multiple Landau-Zener Transitions
• Quantum Computing by Cooling
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• Observation of discrete charge states of a coherent two-level system in a superconducting qubit
• Unfolding system-environment correlation in open quantum systems: Revisiting master equations and the Born approximation
• Identification of soft modes in amorphous Al${}_2$O${}_3$ via first-principles
• On the link between mechanics and thermal properties: mechanothermics
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• Stability of superconducting resonators: motional narrowing and the role of Landau-Zener driving of two-level defects
• Electrical Properties of Selective-Area-Grown Superconductor-Semiconductor Hybrid Structures on Silicon