Coherent control

Project overview

This project aims to develop techniques for the preparation, manipulation and measurement of quantum states of semiconductor quantum dots. The motivation for this work is both to study the underlying physics, and to implement processes required for quantum information processing and quantum computing. As a result of this, quantum mechanical effects such as entanglement and superposition may be exploited in to provide a massive increase in processing power.

The principle component of a quantum computer is a quantum two-level system that constitutes a qubit. Our work focuses on qubits either from an unoccupied ground state to a single exciton, or the spin-split states of a single semiconductor hole in an in-plane magnetic field. Recently, we demonstrated the complete coherent control of a single hole, which has a long coherence time (≈15 ns), making it a more suitable qubit than the exciton or electron. In our experiments, we use a sequence of ultrafast optical pulses to initialise, control and read-out the spin qubit.

In this paper (Physical Review Letters 108, 017402 (2012)) the long coherence times of single hole spins were demonstrated (≈15 ns), using ps timescale initialisation and readout (photocurrent) techniques (see figures below). The long coherence times, an order of magnitude longer than for electrons due to the weak hyperfine interactions for holes constitute a potentially important result for quantum information processing in solid state systems.

The photocurrent techniques employed have been described in a series of papers from our group (see eg Physical Review Letters 100, 197401 (2008)). They enable preparation of hole spin by circularly polarised pulsed excitation and tunnelling, followed by subsequent pulses to achieve control and readout.

In subsequent work (Applied Physics Letters 102, 181108 (May 2013)), it was shown that by applying pulsed voltages, synchronised with those of the 80MHz repetition rate laser, significant improvements in the sensitivity of the photocurrent techniques could be obtained.