Coherent manipulation of Andreev states in superconducting atomic contacts (1509.03961v1)

Abstract: Coherent control of quantum states has been demonstrated in a variety of superconducting devices. In all these devices, the variables that are manipulated are collective electromagnetic degrees of freedom: charge, superconducting phase, or flux. Here, we demonstrate the coherent manipulation of a quantum system based on Andreev bound states, which are microscopic quasiparticle states inherent to superconducting weak links. Using a circuit quantum electrodynamics setup we perform single-shot readout of this "Andreev qubit". We determine its excited state lifetime and coherence time to be in the microsecond range. Quantum jumps and parity switchings are observed in continuous measurements. In addition to possible quantum information applications, such Andreev qubits are a testbed for the physics of single elementary excitations in superconductors.

• The paper demonstrates coherent quantum manipulation of Andreev states using circuit QED, achieving microsecond-scale coherence in superconducting atomic contacts.
• It shows precise control of the Andreev transition frequency via magnetic flux tuning, confirmed by single-tone and two-tone spectroscopy.
• The study identifies decoherence sources, including 1/f noise and mechanical vibrations, providing insights for improving superconducting qubit performance.

Coherent Manipulation of Andreev States in Superconducting Atomic Contacts

The paper "Coherent manipulation of Andreev states in superconducting atomic contacts" introduces a significant advancement in the paper of Andreev bound states (ABS) through direct experimental demonstration of coherent quantum manipulation. Focusing on superconducting atomic contacts, the research delineates the characterization and control of a quantum system based on ABS, employing a circuit quantum electrodynamics (cQED) framework.

The experimental architecture incorporates an aluminum loop fabricated via a microfabricated break-junction technique, enabling the creation of atomic-size contacts. These contacts accommodate a limited number of conduction channels, essential for observing Andreev physics. Single-tone and two-tone spectroscopy techniques provide insights into the dynamics of Andreev levels during the experiments.

Key Findings

1. Quantum State Characterization: The characterization of the Andreev qubit reveals an excited state lifetime and coherence time on the order of microseconds. Also, the research observes quantum jumps and parity switching in continuous measurements, highlighting the potential for quantum information applications.
2. Transition Frequency Control: Utilizing the magnetic flux threading the aluminum loop, the phase drop across the contact and the Andreev transition frequency can be precisely controlled. This control is integral for adjusting the transmission probability of the channels, as shown in the robust periodicity and fitting of spectroscopic data.
3. Coherence Times and Decoherence Mechanisms: Notably, Rabi oscillations and Ramsey fringes were used to ascertain relaxation and coherence times, recording values of 4 µs and 38 ns at a flux phase of π, respectively. However, fluctuations, particularly 1/f transmission noise, were identified as primary decoherence sources, with mechanical vibrations contributing significantly.
4. Coupling and Interaction Analysis: The coupling between the resonator and the Andreev quantum dot, under the Jaynes-Cummings model approximation, demonstrates pronounced avoided crossings. This paper elucidates both the expected Purcell relaxation close to degeneracy points and the need for additional phase-independent relaxation mechanisms.

Implications and Future Research

This paper advances the foundational understanding of superconducting qubits, particularly emphasizing microscopically coherent degrees of freedom. The demonstration of coherent control over Andreev states underscores their utility in quantum information processing beyond traditional charge, phase, or flux qubits.

Further exploration of multi-channel contacts could elucidate ways to mitigate decoherence while enhancing coherence times. The Andreev quantum dot's sensitivity to parity changes offers a valuable probe into quasiparticle dynamics, promising improvements in both the performance of qubits and superconducting detectors.

Future research directions may involve the integration of Andreev quantum dots with hybrid superconducting devices and the transition of Andreev states into Majorana states under specific conditions. This could establish new paradigms in topological quantum computing, leveraging the unique properties of Majorana fermions.

In conclusion, this paper contributes a compelling case for Andreev states as a viable quantum computing qubit, proposing experimental frameworks that extend the applicability of superconducting qubits, with implications for both advancing quantum information technologies and deepening the understanding of quantum states in condensed matter physics.