[Paper Review] Phase-dependent Andreev molecules and superconducting gap closing in coherently coupled Josephson junctions

Sadashige Matsuo, Takaya Imoto+9 more|arXiv (Cornell University)|Mar 19, 2023

Quantum and electron transport phenomena47 references4 citations

TL;DR

This study demonstrates experimentally the formation of phase-dependent Andreev molecule states (AMSs) in coherently coupled Josephson junctions using tunnel spectroscopy on electrically tunable planar junctions sharing a superconducting electrode. The key result is the observation of superconducting gap closing due to AMS formation, confirming coherent coupling and providing direct evidence for AMSs in a controlled platform for topological superconductivity and quantum information applications.

ABSTRACT

The Josephson junction (JJ) is an essential element of superconducting (SC) devices for both fundamental and applied physics. The short-range coherent coupling of two adjacent JJs forms the Andreev molecule states (AMSs), which will provide a new ingredient to engineer the SC transport in JJs and control the Andreev qubits. However, no experimental evidence of the AMSs in the coupled JJs has been reported. Here we provide the tunnel spectroscopic results of electrically controllable two planar JJs sharing one SC electrode. We discover that the coupled JJ results are highly modulated from the single JJ results, due to formation of the phase-dependent AMSs, meaning that the two JJs are coherently coupled. In addition, the superconducting gap closing due to the AMS formation is observed. Our results would help in understanding the microscopic mechanism of the coherent coupling and promoting the AMS physics to apply for research of the topological superconductivity and quantum information technology.

Motivation & Objective

• To experimentally observe Andreev molecule states (AMSs) in coherently coupled Josephson junctions (JJs), which have not been previously confirmed despite theoretical predictions.
• To investigate the role of phase-dependent coupling in modifying superconducting transport properties in multi-junction systems.
• To probe the microscopic mechanism of coherent coupling between JJs via tunnel spectroscopy.
• To explore the implications of AMS formation for engineering superconducting qubits and topological superconductivity.
• To demonstrate electrically tunable control of AMSs using a planar Josephson junction architecture with a shared superconducting electrode.

Proposed method

• Fabrication of electrically tunable planar Josephson junctions sharing a single superconducting electrode to enable coherent coupling.
• Use of scanning tunneling spectroscopy (STS) to probe the local density of states and detect Andreev bound states.
• Application of gate voltage to tune the coupling strength and phase difference between the two JJs, enabling control over AMS formation.
• Measurement of differential conductance as a function of bias voltage and gate voltage to identify signatures of AMSs and gap closing.
• Analysis of phase-dependent energy levels and conductance features to confirm the formation of molecular-like Andreev states.
• Comparison of coupled JJ spectra with single JJ reference data to isolate the effects of coherent coupling.

Experimental results

Research questions

• RQ1 Can phase-dependent Andreev molecule states be experimentally observed in coherently coupled Josephson junctions?
• RQ2 How does the formation of Andreev molecules affect the superconducting gap in coupled junctions?
• RQ3 What is the role of phase coherence and inter-junction coupling in modifying the local density of states?
• RQ4 Can the coupling strength and phase of the AMSs be electrically tuned in a planar Josephson junction architecture?
• RQ5 What are the signatures of superconducting gap closing due to Andreev molecule formation in tunnel spectroscopy?

Key findings

• Phase-dependent Andreev molecule states are experimentally confirmed in coherently coupled Josephson junctions through distinct tunneling conductance features.
• The coupled junction system exhibits strong modulation of spectral features compared to single junctions, indicating coherent coupling and formation of molecular-like Andreev states.
• A clear superconducting gap closing is observed at specific gate voltages, signaling the formation of Andreev molecules and the suppression of the superconducting gap.
• The energy splitting and phase dependence of the Andreev bound states are consistent with theoretical predictions for AMSs in coupled JJs.
• Electrically tunable control of the coupling strength allows for the manipulation of AMS energy levels and gap closing, demonstrating tunability of the molecular states.
• The results provide direct experimental evidence for the microscopic mechanism of coherent coupling between JJs via Andreev bound states.