Evidence of topological superconductivity in planar Josephson junctions

Abstract: Majorana zero modes are quasiparticle states localized at the boundaries of topological superconductors that are expected to be ideal building blocks for fault-tolerant quantum computing. Several observations of zero-bias conductance peaks measured in tunneling spectroscopy above a critical magnetic field have been reported as experimental indications of Majorana zero modes in superconductor/semiconductor nanowires. On the other hand, two dimensional systems offer the alternative approach to confine Ma jorana channels within planar Josephson junctions, in which the phase difference {\phi} between the superconducting leads represents an additional tuning knob predicted to drive the system into the topological phase at lower magnetic fields. Here, we report the observation of phase-dependent zero-bias conductance peaks measured by tunneling spectroscopy at the end of Josephson junctions realized on a InAs/Al heterostructure. Biasing the junction to {\phi} ~ {\pi} significantly reduces the critical field at which the zero-bias peak appears, with respect to {\phi} = 0. The phase and magnetic field dependence of the zero-energy states is consistent with a model of Majorana zero modes in finite-size Josephson junctions. Besides providing experimental evidence of phase-tuned topological superconductivity, our devices are compatible with superconducting quantum electrodynamics architectures and scalable to complex geometries needed for topological quantum computing.

• The paper presents evidence for Majorana zero modes by identifying phase-dependent zero-bias conductance peaks in planar Josephson junctions.
• It employs a DC SQUID configuration and tunneling spectroscopy to precisely control and probe the superconducting phase difference.
• The results offer critical insights into tunable topological phases, advancing the potential for fault-tolerant quantum computing.

Evidence of Topological Superconductivity in Planar Josephson Junctions

This paper presents a comprehensive examination of the evidence for topological superconductivity within planar Josephson junctions (JJs) made from an InAs/Al heterostructure. The study leverages the unique properties of planar JJs to probe the presence of Majorana zero modes, topological quasiparticle states that are promising candidates for fault-tolerant quantum computing.

Overview and Methodology

The researchers constructed planar JJs by selectively etching a narrow Al layer above a two-dimensional electron gas (2DEG) formed in InAs. The high mobility of InAs and its strong spin-orbit interaction, combined with the epitaxial Al layer, form the basis of these JJs. By harnessing the transparent interface between InAs and Al, a hard superconducting gap is induced in the InAs region.

Through the integration of these JJs into a superconducting loop, configured to function as a direct-current superconducting quantum interference device (DC SQUID), the study achieves phase bias control by varying the magnetic flux through the SQUID. Tunneling spectroscopy, performed via a laterally coupled quantum point contact (QPC), serves as the primary tool for probing sub-gap states.

Key Findings and Results

A major finding reported is the observation of phase-dependent zero-bias conductance peaks (ZBPs). These ZBPs are indicative of Majorana zero modes and show strong dependency on the superconducting phase difference across the junction, denoted by $\varphi$. When biased to $\varphi \approx \pi$, the critical magnetic field needed for the emergence of these zero-bias peaks is significantly reduced.

The relationship between the ZBPs, magnetic field, and phase difference conforms to a theoretical model predicting Majorana modes in finite-size JJs. Specifically, at increased in-plane magnetic fields, the ZBPs persist over a range of phase differences and chemical potentials, exhibiting stability that aligns with a topological phase.

The study further verifies Majorana signatures by demonstrating that the zero-bias peaks disappear when the magnetic field is applied orthogonally to the junction, another expected behavior for Majorana modes in this system configuration.

Theoretical Implications

The implications of these findings are profound for the field of topological quantum computing. The ability to manage superconducting phase differences as an effective control parameter in driving the system into a topological phase is notable, providing a means to design manipulable and scalable networks essential for topological quantum information processes.

Enhancing the understanding of Majorana modes through these planar JJ systems opens avenues for integrating such junctions in quantum devices, contributing to the realization of braiding operations required for topological fault tolerance in quantum computers.

Future Outlook

This work sets a foundation upon which future explorations can build. Prospective efforts could focus on optimizing fabrication techniques to broaden the coherent length scales and testing materials' properties to push the critical magnetic fields further. Such advancements will support more comprehensive testing of the theoretical models in quantum computing applications, leveraging the major insight that different JJs geometries provide.

In conclusion, this study delivers substantial experimental support for the presence of topological superconductivity in planar Josephson junctions, propelling the field towards more robust forms of quantum computing architectures harnessing Majorana fermions.