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Novel approach of performing elementary quantum operations using spatial phase

K. Mishima, Michitoshi Hayashi, S. H. Lin

Published Jul 8, 2004• 0 citations
Publisher

Abstract

We propose a new idea of generating and controlling quantum coherences among semiconductor quantum dots by using the spatial phase of one single-mode incident laser pulse. Two or three identical quantum dots are employed as qubit gates. Our strategy is to use the spatial phase of the laser pulse at the different positions of the qubits for quantum operations, rather than to utilize dipole-dipole interactions between them. In this work, Hadamard transformation and creation of entanglements in the bipartite and tripartite systems are examined. We predict that with much simpler experimental setup the spatial phase of the laser pulse makes it possible to perform elementary quantum operations that are infeasible by the present-day quantum computer architecture.

DOI
10.1109/cleopr.2003.1274788

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We propose a new idea of generating and controlling quantum coherences among semiconductor quantum dots by using the spatial phase of one single-mode incident laser pulse. Two or three identical quantum dots are employed as qubit gates. Our strategy is to use the spatial phase of the laser pulse at the different positions of the qubits for quantum operations, rather than to utilize dipole-dipole interactions between them. In this work, Hadamard transformation and creation of entanglements in the bipartite and tripartite systems are examined. We predict that with much simpler experimental setup the spatial phase of the laser pulse makes it possible to perform elementary quantum operations that are infeasible by the present-day quantum computer architecture.
An alternative method is proposed for generating efficiently the entangled states in semiconductor quantum dots by using the spatial phase. The method does not rely on the frequently used dipole–dipole interaction among the qubits at all. It was found that as a result of breaking the selection rule by the spatial phase it enables one to create the entanglement that can never be realized when closely located identical quantum dots are used as qubit gates. In this paper generation of entanglements in two- and three-qubit systems is investigated. The experimental setup based on our approach is much clearer and simpler than that based on the dipole-dipole interaction.
We present a method to enact a deterministic, measurement-free, optically generated controlled-phase gate on two qubits defined by single electrons trapped in large-area quantum dots in a planar microcavity. This method is tolerant to optical quantum dot inhomogeneity, requires only a modest-QQQ planar cavity, employs only a single laser pulse, and allows the integration of many entangled qubits on one semiconductor chip. We present the gate in the contexts of both adiabatic evolution and geometric phases, and calculate the degradation of performance in the presence of both spontaneous emission and cavity loss.
The problems of the design, control, and interaction of the single-electron double-dot charge qubits coherently coupled to the optical cavity mode are studied theoretically. A way to overcome the challenges concerned with the use of classical laser pulses for a qubit state engineering is described, replacing the lasers by a quantized photon field in a semiconductor cavity and a gate voltage. Using this strategy, a simple and efficient scheme is proposed for the creation of highly entangled multiqubit states like the nine-qubit Shor states.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.
A scheme to possibly implement the quantum computer is proposed based on the idea to build up the single qubit from two coupled quantum dots. The spatial separation of the two logic states of the qubit can greatly reduce the decoherence time, which allows one to perform the coherent quantum logic operations at a fairly high ratio. Together with the realization of the universal quantum logic gates, and the proper scheme of quantum measurement, the present proposal can satisfy all the essential criteria for quantum computing.