Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device (1204.4130v1)

Abstract: We report on the observation of excitation of Majorana fermions in a Nb-InSb nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed between the two Nb contacts by weak Schottky barriers and is thus in the regime of strong couplings to the contacts. Due to the proximity effect, the InSb nanowire segments covered by superconductor Nb contacts turn to superconductors with a superconducting energy gap $\Delta^*$. Under an applied magnetic field larger than a critical value for which the Zeeman energy in the InSb nanowire is $E_z\sim \Delta^*$, the entire InSb nanowire is found to be in a nontrivial topological superconductor phase, supporting a pair of Majorana fermions, and Cooper pairs can transport between the superconductor Nb contacts via the Majorana fermion states. This transport process will be suppressed when the applied magnetic field becomes larger than a second critical value at which the transition to a trivial topological superconductor phase occurs in the system. This physical scenario has been observed in our experiment. We have found that the measured zero-bias conductance for our hybrid device shows a conductance plateau in a range of the applied magnetic field in quasi-particle Coulomb blockade regions.

• The paper reported the detection of Majorana fermions in a Nb-InSb nanowire device via a persistent zero-bias conductance peak above 1.2 T.
• It exploited the superconducting proximity effect and strong spin-orbit coupling in InSb nanowires to induce a topological superconducting phase.
• These findings offer a viable route for engineering fault-tolerant topological qubits in scalable quantum devices.

Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device

The observation and excitement surrounding Majorana fermions have catalyzed extensive research within condensed matter physics, largely due to their potential applications in topological quantum computing. The subject paper, "Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device," reports a significant advancement in the detection of Majorana fermions within a practical solid-state system. This paper represents a critical step in deploying robust quantum devices that harness Majorana states, providing insights into their experimental realization.

Key Aspects of the Study

The authors have engineered a Nb-InSb nanowire system, sandwiched between Nb superconducting contacts, to leverage the interaction of superconductivity and strong spin-orbit coupling in InSb nanowires. These nanowires exhibit a large $g$-factor and significant spin-orbit interaction, lending themselves well to the emergence of Majorana bound states (MBS) within the system when exposed to an external magnetic field. The design is predicated on the superconducting proximity effect where the superconductor Nb induces superconductivity in the nanowire.

Key observations include a zero-bias conductance peak, indicative of Majorana fermions, persisting over a range of applied magnetic fields, which appears robust even as traditional superconductivity vanishes under similar conditions. The zero-bias peak emerges under applied fields exceeding a critical value of 1.2 T, transitioning the system into a nontrivial topological superconducting phase.

Experimental Setup and Data Analysis

The Nb-InSb device was constructed using epitaxially grown zincblende InSb nanowires, favouring the emergence of Majorana modes due to their favorable electronic properties—namely, a small effective mass and a strong spin-orbit coupling. Nb was chosen as the superconductor for its high critical magnetic field and substantial superconducting gap, allowing the system to maintain superconductivity even under high magnetic fields.

Through transport measurements, the authors meticulously scrutinized the device's behavior under varied conditions (e.g., temperature, magnetic field, gate voltage). These measurements revealed distinct signatures associated with Majorana fermions. Notably, the observed transport properties under magnetic field application were consistent with theoretical predictions of Majorana phases, particularly demonstrating a plateau in conductance values around zero-bias.

Theoretical and Practical Implications

The detection of MBS in this hybrid Nb-InSb system underscores the feasibility of realizing topological qubits in solid-state devices, which are pivotal for fault-tolerant quantum computation. The findings bolster theoretical models that predicate Majorana fermions' detection on the interplay of superconductivity and topological insulator behaviors within nanostructures. Also, the zero-bias conductance peak stability shown in the experiments is paramount for engineering reliable quantum bits.

Moving forward, the research lays a foundation for refining device architectures aimed at harnessing Majorana physics for quantum computing applications. The scalability of these nanowire systems offers promising prospects for their integration into larger quantum networks. Future studies could explore the manipulation and braiding of MBS to realize logic gates necessary for robust quantum operations, along with enhancing the interaction of these states with other quantum systems or even leveraging them in exploring non-Abelian statistics.

In conclusion, this paper articulates a coherent experimental strategy and realization for observing Majorana fermions in a solid-state context. It offers compelling empirical evidence supporting their existence and highlights pathways for future investigations in quantum device technologies. The insights gained will undeniably stimulate further research towards realizing topological quantum computation.