Quantum Point Contacts in Cadmium Arsenide Thin Films

Quantum interferometry provides a means of encoding and reading information from qubits based on non-Abelian bound states. An essential component of quantum interferometers is a quantum point contact, which tunes the transmission of single quasiparticle states through a constriction. In addition to interferometry, quantum point contacts are useful for readout operations in flying qubit circuits and as spin filters in the quantum spin Hall state. We demonstrate operation of gate-defined quantum point contacts in the quantum Hall state of cadmium arsenide thin films tuned into different topological and trivial states by magnetic field. Both field and a top gate are used to control the global filling factor of the device, and thereby the number of edge channels flowing into the quantum point contact. A nano-sized split gate is used to form an electrostatic constriction and control the number of transmitted channels. With the global filling factor tuned to the n-type side, we observe integer steps in conductance as individual channels are selectively transmitted through the constriction. Tuning the filling factor through the zeroth Landau level to the p-type side, we observe fractional values of conductance through the constriction. We explain this result as a consequence of charge mixing between electron- and hole-like channels. Lastly, we present experiments of mesa-defined quantum point contacts in the topologically non-trivial state.