Observation of the fractional ac Josephson effect: the signature of Majorana particles (1204.4212v2)

Abstract: Topological superconductors which support Majorana fermions are thought to be realized in one-dimensional semiconducting wires coupled to a superconductor. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a haLLMark of topological matter. When the junction is irradiated with a radio-frequency f in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height hf/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago.

• The paper demonstrates the fractional a.c. Josephson effect as evidence for Majorana fermions in hybrid nanowire junctions.
• It observes a doubling in Shapiro step height—from hf₀/2e to hf₀/e—marking a shift from Cooper pair to Majorana quasiparticle transport.
• The findings bolster topological quantum computing prospects by providing an experimental pathway to confirm Majorana states in superconductors.

Observation of the Fractional a.c. Josephson Effect and the Signature of Majorana Particles

The paper describes an experimental investigation into the fractional a.c. Josephson effect in hybrid semiconductor/superconductor nanowire junctions, which serves as a potential haLLMark of Majorana particles. The research utilizes a superconductor/semiconductor interface within InSb/Nb nanowires to create conditions conducive to Majorana fermion formation—a longstanding theoretical prediction yet to be unambiguously observed in solid-state systems.

Majorana Fermions in Superconducting Systems

Largely inspired by the theoretical insights of Sau et al., the paper focuses on hybrid systems where Majorana fermions are anticipated. These systems rely on the presence of strong spin-orbit (SO) interactions and superconductivity induced by proximity effects. Majorana fermions, non-Abelian anyons, have the intriguing property of being their own antiparticles, predicted to exhibit a modified Josephson relation periodicity—from the conventional $2\pi$ seen with Cooper pairs to $4\pi$ in Majorana modes.

Experimental Setup and Observations

The experimental framework presented in the paper utilizes an InSb nanowire interfaced with Nb, subjected to radio-frequency irradiation. In the absence of an external magnetic field, the system exhibits conventional Shapiro steps with a voltage step height of $\Delta V = hf_0/2e$, corresponding to the movement of charge-$2e$ Cooper pairs. This serves as the baseline for conventional supercurrent behavior.

At high magnetic fields, however, the key observation is a doubling of the Shapiro step height to $\Delta V = hf_0/e$. This doubling is indicative of the supercurrent being carried by charge-$e$ quasiparticles, a signature fundamentally associated with the presence of Majorana fermions. The authors carefully considered alternative explanations for the enhanced zero-bias conductance but emphasized that the observed step doubling strongly aligns with the theoretical predictions for Majorana states.

Implications and Future Directions

The detection of fractional Josephson effects in these hybrid structures holds significant implications for topological quantum computing. As Majorana fermions can facilitate fault-tolerant quantum gates due to their topological properties, such experimental validations pave the way towards practical and resilient quantum computation architectures.

The paper suggests potential improvements in experimental techniques to further solidify the findings. This includes fine-tuning the nanowire configurations, enhancing SEMS with varying superconductor materials, and expanding the range of tested magnetic fields. Additionally, the methodology presents a pathway for expanding the parameter space in which Majorana fermions might be probed, potentially leading to more conclusive corroborations of their existence.

Conclusion

The research presented outlines a comprehensive experimental approach to evidencing the fractional a.c. Josephson effect and provides substantial backing for the hypothesized existence of Majorana particles within these systems. The implications extend far beyond theoretical interest, offering practical avenues for the realization of topologically protected quantum operations. Future explorations may explore unearthing the rich physics of Majorana quasiparticles in higher-dimensional systems, further bridging experimental and theoretical landscapes in quantum materials science.