Topological Superconductivity in a Phase-Controlled Josephson Junction (1809.03076v1)

Abstract: Topological superconductors can support localized Majorana states at their boundaries. These quasi-particle excitations have non-Abelian statistics that can be used to encode and manipulate quantum information in a topologically protected manner. While signatures of Majorana bound states have been observed in one-dimensional systems, there is an ongoing effort to find alternative platforms that do not require fine-tuning of parameters and can be easily scalable to large numbers of states. Here we present a novel experimental approach towards a two-dimensional architecture. Using a Josephson junction made of HgTe quantum well coupled to thin-film aluminum, we are able to tune between a trivial and a topological superconducting state by controlling the phase difference $\phi$ across the junction and applying an in-plane magnetic field. We determine the topological state of the induced superconductor by measuring the tunneling conductance at the edge of the junction. At low magnetic fields, we observe a minimum in the tunneling spectra near zero bias, consistent with a trivial superconductor. However, as the magnetic field increases, the tunneling conductance develops a zero-bias peak which persists over a range of $\phi$ that expands systematically with increasing magnetic fields. Our observations are consistent with theoretical predictions for this system and with full quantum mechanical numerical simulations performed on model systems with similar dimensions and parameters. Our work establishes this system as a promising platform for realizing topological superconductivity and for creating and manipulating Majorana modes and will therefore open new avenues for probing topological superconducting phases in two-dimensional systems.

• The paper demonstrates that a phase-controlled Josephson junction enables tunable topological superconductivity via Majorana bound states.
• The researchers used spectroscopic measurements showing a pronounced zero-bias peak that marks the transition to a topological phase.
• Numerical tight-binding simulations corroborated the experimental findings, confirming the feasibility of scalable Majorana networks for quantum computing.

Topological Superconductivity in a Phase-Controlled Josephson Junction

The paper, "Topological Superconductivity in a Phase-Controlled Josephson Junction," documents an experimental realization and investigation into two-dimensional (2D) topological superconductivity using a highly controlled platform. This is significant within the research landscape due to ongoing challenges encountered in scaling one-dimensional systems for topological quantum computing.

Summary of Research

Utilizing the unique properties of Majorana bound states (MBS), the paper demonstrates the use of a Josephson junction constructed from an HgTe quantum well interfaced with aluminum superconducting leads. This system stands out due to its capacity for transitioning between trivial and topological superconducting states. The phase difference $\phi$ across the junction, in conjunction with an applied in-plane magnetic field, serves as the control parameters for this transition. The 2D landscape of this system allowed for investigations beyond the limitations of one-dimensional platforms that have traditionally been plagued by scalability and finetuning issues.

Experimental Approach and Results

The authors employ a phase-controlled Josephson junction, where the strategic application of a magnetic field effectively tunes the Zeeman energy. As evidenced by spectroscopic measurements, the tunneling conductance at the junction's edge serves as an indication of its superconducting state. Crucially, the appearance of a zero-bias peak (ZBP) in the tunneling spectrum at elevated magnetic fields aligns with the predicted characteristics of topological superconductivity, specifically attributable to MBS. This zero-bias peak emerges, becoming more pronounced with increased magnetic field strength, which is consistent with theoretical predictions corroborated by quantum mechanical simulations.

The observed ZBP not only marks the topological phase transition but also provides a critical experimental observable linking the physical system to theoretical models. Numerical simulations, based on a tight-binding model, ensconce the experimental results within a robust theoretical framework, suggesting the model aligns well with empirical data.

Implications and Future Directions

This experiment underlines the potential of phase-controlled Josephson junctions as reliable hosts for MBS, presenting a sturdy basis for the development of large-scale networks of Majorana devices. By minimizing the fine-tuning requirement, this two-dimensional platform enhances not only controllability but also the scalability essential for practical quantum computing applications.

The implications are manifold, extending the scope for integration with other quantum systems and materials, thereby enriching the landscape of quantum information processing. Future directions may focus on refining fabrication techniques for such junctions, further decreasing normal reflections, and exploring other 2D materials. The robustness of phase tunability will likely invite enhanced explorations of hybrid quantum systems, fostering advancements toward achieving topological qubits and advancing the quantum computing frontier.

Ultimately, this paper sets an instructive precursor towards harnessing topological superconductivity within flexible and scalable platforms, revealing an enriching spectrum of quantum devices configurable through minor manipulations, pivotal for the advancement of topologically protected quantum computations.