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Oxford University Press eBooks

THE ACOUSTIC-WAVE DRIVEN QUANTUM PROCESSOR

C. H. W. Barnes, M. Pepper

Published Dec 13, 2007• 0 citations
Publisher

Abstract

This chapter reviews the use of surface acoustic waves (SAWs) in an electron-spin based quantum state processor. A SAW-induced electric current of discrete single or controllable numbers of electrons, which can interact and become entangled, is discussed. Variable g-factor materials are proposed as a means of applying electrical control to single-qubit rotation and as a form of optical input and readout. The processing scheme can be used as a component in a quantum communications network.

DOI
10.1093/acprof:oso/9780199238873.003.0004

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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences•Jul 15, 2003•12 citations
This chapter reviews the use of surface acoustic waves (SAWs) in an electron-spin based quantum state processor. A SAW-induced electric current of discrete single or controllable numbers of electrons, which can interact and become entangled, is discussed. Variable g-factor materials are proposed as a means of applying electrical control to single-qubit rotation and as a form of optical input and readout. The processing scheme can be used as a component in a quantum communications network.
Gallium arsenide is piezoelectric, so it is possible to generate coupled mechanical and electrical surface acoustic waves (SAWs) by applying a high-frequency voltage to a transducer on the surface of GaAs. By combining SAWs with existing low-dimensional nanostructures one can create a series of dynamic quantum dots corresponding to the minima of the travelling electric wave, and each dot carries a single electron at the SAW velocity (∼ 2800 m/s). These devices may be of use in developing future quantum information processors, and also offer an ideal environment for probing the quantum mechanical behaviour of single electrons. This thesis describes a numerical and theoretical study of the dynamics of an electron in a range of geometries. The numerical techniques for solving the time-dependent Schr ̈dinger equation with an arbitrary time-dependent potential will be described in Chapter 2, and then applied in Chapter 3 to calculate the transmission of an electron through an Aharonov-Bohm (AB) ring. It will be seen that an important property of the techniques used in this thesis is that they can be easily adapted to study realistic geometries, and we will see features in the AB oscillations which do not arise in simplified analytic descriptions. In Chapter 4, we will then study a device consisting of two parallel SAW channels separated by a controllable tunnelling barrier. We will use numerical simulations to investigate the effect of electric and magnetic fields upon the electron dynamics, and develop an analytic model to explain the simulation results. From the model, it will be apparent that it is possible to use this device to rotate the state of the electron to an arbitrary superposition of the first two eigenstates. We then introduce coherent and squeezed states in Chapter 5, which are ex- cited states of the quantum harmonic oscillator. Coherent and squeezed electronic states may be of use in quantum information processing, and could also arise due to unwanted perturbations in a SAW device. We will discuss how these states can be controllably generated in a SAW device, and also discuss how they could then be detected. In Chapter 6 we describe how to use the motion of a SAW to create a rapidly- changing potential in the frame of the electron, leading to a nonadiabatic excita- tion. The nonadiabatically-excited state oscillates from side to side within a 1D channel on a few-picosecond timescale, and this motion can be probed by placing a tunnelling barrier at one side of the channel. Numerical simulations will be performed to show how this motion can be controlled, and the simulation results will be seen to be in good agreement with recent experimental work performed by colleagues. Finally, we will show that this device can be used to measure the initial state of an electron which is an arbitrary superposition of the first two eigenstates.
Skyrmions are competitive candidates for information-storage units and have great prospect in quantum information processing. We here study coherent control of a quantum skyrmion via surface acoustic wave (SAW) phonons in a piezoelectric SAW cavity. Exact diagonalization of a finite Dzyaloshinskii-Moriya cluster reveals the eigenstates of quantum skyrmion with robust scalar chirality. The underlying lattice-spin symmetry imposes polarization-dependent selection rules for the electromagnetic transitions between eigenstates states. Considering the small mode volume of the SAW phonon easy to reach strong coupling, we derive the interaction Hamiltonian between the quantum skyrmion as a qubit and a single-mode quantized SAW via the electric field induced by piezoelectric effect, and find that the coupling strength at the single-phonon level increases linearly with skyrmion radius. This enables enhanced spin-acoustic coupling for larger skyrmionic textures and for low magnetic dissipation. Our results establish few-spin quantum skyrmions as compact building blocks for hybrid quantum information processing and suggest a route toward more densely integrated on-chip quantum devices.
We propose a scheme to produce spin entangled states for two interacting electrons. One electron is bound in a well in a semiconductor quantum wire and the second electron is transported along the wire, trapped in a surface acoustic wave (SAW) potential minimum. We investigate the conditions for which the Coulomb interaction between the two electrons induces entanglement. Detailed numerical investigation reveals that the two electrons can be fully spin entangled depending on the confinement characteristics of the well and the SAW potential amplitude.
We consider the entanglement of spins for two electrons contributing to the acoustoelectric current driven by a surface acoustic wave (SAW) in two adjacent narrow channels by calculating their exchange energy (J)(J)(J). The channels belong to an acoustic nanocircuit which comprises a network of quasi-one-dimensional pinched-off channels serving as wires along which SAW quantum dots transport electrons. This is motivated by possible practical applications involving quantum information processing and quantum computers. We calculate JJJ as a function of time as the electrons travel side-by-side in the adjacent channels and as a function of the distance between the centers of the channels. The leakage from the state in which the system is prepared, is calculated. The oscillations in the leakage indicate the probability for the electron system to make transitions between the ground and excited states, or for an electron to hop back and forth between channels.