Ballistic Majorana nanowire devices (1603.04069v3)

Abstract: Majorana modes are zero-energy excitations of a topological superconductor that exhibit non-Abelian statistics. Following proposals for their detection in a semiconductor nanowire coupled to an s-wave superconductor, several tunneling experiments reported characteristic Majorana signatures. Reducing disorder has been a prime challenge for these experiments because disorder can mimic the zero-energy signatures of Majoranas, and renders the topological properties inaccessible. Here, we show characteristic Majorana signatures in InSb nanowire devices exhibiting clear ballistic transport properties. Application of a magnetic field and spatial control of carrier density using local gates generates a zero bias peak that is rigid over a large region in the parameter space of chemical potential, Zeeman energy, and tunnel barrier potential. The reduction of disorder allows us to resolve separate regions in the parameter space with and without a zero bias peak, indicating topologically distinct phases. These observations are consistent with the Majorana theory in a ballistic system, and exclude for the first time the known alternative explanations that invoke disorder or a nonuniform chemical potential.

• The paper demonstrates clear Majorana signatures in InSb nanowires by precisely tuning chemical potential, Zeeman energy, and tunnel barrier potential.
• It employs advanced nanofabrication to achieve a hard superconducting gap and preserve quantized conductance under large-bias conditions.
• The study confirms that magnetic field alignment and ballistic conditions are crucial for observing robust, topologically distinct Majorana modes.

Analysis of Ballistic Majorana Nanowire Devices

The paper "Ballistic Majorana Nanowire Devices" presents a significant investigation into the properties and behaviors of Majorana modes within InSb nanowire devices. These devices are coupled with NbTiN superconductors to explore their potential in realizing quantum bits based on Majorana fermions. The objective is to achieve a significant reduction in disorder, which traditionally obscures the detection of Majorana modes by mimicking their zero-energy signatures.

Key Contributions and Findings

The authors demonstrate characteristic Majorana signatures within InSb nanowires exhibiting ballistic transport properties. Through meticulous control of various parameters—chemical potential, Zeeman energy, and tunnel barrier potential—they have resolved distinct zero-bias peaks over expansive regions in parameter space. This indicates topologically distinct phases and marks an advancement by excluding previous disorder-based theories that attempted to explain such observations.

The paper utilized an advanced nanofabrication process ensuring a high-quality interface between the nanowires and the NbTiN superconductors. The devices demonstrated a notably hard superconducting gap with ballistic transport indicative of low disorder levels. The device's ability to retain quantized conductance ($2e^2/h$) for large-bias conditions rectifies previous inconsistencies in zero-bias peak analyses which were often influenced by disorder or quantum dots.

Experimental Insights

The experimental setup was conducted at low temperatures using a dilution refrigerator, and it included the application of magnetic fields along the nanowire axis. The magnetic field not only closed the superconducting gap but also manifested the zero-bias peak consistent with Majorana modes. The persistence of the zero-bias peak over an extended range of magnetic fields and gate voltages strengthens the argument against disorder-induced mimicry.

The paper outlines the role of magnetic field orientation, drawing a clear differentiation between the required external magnetic alignment along the wire’s axis versus parallel to the spin-orbit coupling field. The directional dependency of the zero-bias peak emergence confirms theoretical expectations for the topology-induced transitions in such setups.

Implications and Speculative Projections

This paper holds implications for the broader quest to realize topological qubits and scale quantum computing efforts. Employing ballistic conditions in nanowires addresses critical challenges in maintaining coherent Majorana bounds by controlling factors traditionally attributed to disorder-induced confounding effects.

Further down the line, advancements in reducing dissipation and understanding the interaction dynamics in multi-channel occupations within longer proximitized sections may offer a path towards achieving the ultimate objective of quantized zero-bias peaks—a persistent challenge in current experimental observations.

Conclusion

The paper represents a detailed and methodical approach towards validating the Majorana signatures within nanowire systems. By setting a new standard in reducing disorder, the authors provide a compelling case for the role of clean, ballistic conditions in the accurate detection and control of Majorana modes, thus paving the way for more reliable qubit architectures in the future. Future studies, as proposed, might explore refining the phase diagram with precise modelling including electrostatic and orbital considerations, leading to devices optimized for quantum computation.