UC Santa Barbara

Theoretically proposed by Ettore Majorana in 1937, Majorana fermions are a unique class of particles which are their own anti-particles. This concept is realized in Majorana Zero Modes (MZMs), which are quasi-particles bound to zero energy, with no measurable charge and mass. Arising out of topological states of matter, signatures of MZMs were first experimentally observed in 2012.

These quasi-particles are predicted to exhibit non-abelian braiding statistics, allowing them to “remember” whether they were moved clockwise or counterclockwise around each other, forming a braid in time and position. Being their own anti-particles, fusion or annihilation of a pair of MZMs is expected to lead to a different outcome based on how they were braided, making a pair of MZMs the simplest quantum bit or ‘qubit’, forming the basis of Topological Quantum Computation.

As MZMs are indistinguishable from each other and quantum information is encoded in the exchange of MZMs, it is protected from environmental perturbations (noise), referred to as topological protection. Computation based on these qubits is predicted to be fault tolerant and scalable.

This work focusses on the device heterostructure design, Molecular Beam Epitaxy (MBE) growth and low temperature electrical characterization of superconductor-semiconductor hybrid systems hosting MZMs. Low dimensional electron systems (2D quantum wells, 1D nanowires) in semiconductors with strong spin-orbit interaction (e.g. InAs, InAsxSb1-x and InSb), transparently coupled to a superconductor (e.g. Aluminum), have been investigated.

With an emphasis on improving electron mobility, the first demonstration of an InSb quantum well on InSb substrate, as part of this work, showed a record quantum mobility of 50,000 cm2/Vs. Top and bottom gate control of InSb quantum wells on GaSb substrates has also been demonstrated. MBE of InAs and InAsSb nanowires has also been studied, with demonstration of induced superconductivity in InAsSb nanowires. Recent work focused on MBE of near surface InAsSb quantum wells showing gate-controlled depletion and observation of quantized conductance through a Quantum Point Contact.

Lastly, in-vacuo growth of Aluminum partial shells on InSb nanowires, led to the first observation of quantized Majorana conductance, consistent with predictions. While further proof is necessary, these 1D and 2D hybrid systems, coupled with in-situ selective area evaporation of dielectrics, pave the way for realization of complex topological networks and subsequent demonstration of a fault tolerant topological qubit.