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AIP conference proceedings

Semiconductor Quantum Dots for Quantum Information Processing: An Optical Approach

M. V. Gurudev Dutt

Published Jan 1, 2005• 0 citations
Publisher

Abstract

Semiconductor quantum dots (QDs) are ideal candidates for quantum information processing, due to their strong resemblance to atomic systems. This paper will show that an optically driven approach is an attractive scheme for quantum computing with QDs. Results will be presented from the experiments with the optical Bloch vector qubit in neutral QDs, demonstrating phase‐locked single qubit rotations and two‐qubit universal control‐NOT logic gates. We will conclude by discussing how the optical approach can be extended to control the electron spin qubit in charged QDs.

DOI
10.1063/1.1993992

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Semiconductor quantum dots (QDs) are ideal candidates for quantum information processing, due to their strong resemblance to atomic systems. This paper will show that an optically driven approach is an attractive scheme for quantum computing with QDs. Results will be presented from the experiments with the optical Bloch vector qubit in neutral QDs, demonstrating phase‐locked single qubit rotations and two‐qubit universal control‐NOT logic gates. We will conclude by discussing how the optical approach can be extended to control the electron spin qubit in charged QDs.
Optically driven semiconductor quantum dots have many features similar to simpler atomic systems but with the potential for much easier development of large scale architectures for novel ideas in quantum optoelectronics. Of current interest is their potential for application to quantum information science. Here, we show the ability to perform arbitrary qubit rotation and conditional quantum logic operations in a single dot as well as showing the ability now to optically manipulate and control the electronic spin of a single electron.
Coherent optically driven semiconductor quantum dot systems are proposed for the implementation of quantum information processing devices. Fundamental quantum computation units, the quantum bits (qubits), are realized and optically manipulated in the exciton and spin bases. In this thesis, consecutive phase-sensitive rotations of a single qubit and their importance in the measurement of the full physical density matrix have been demonstrated experimentally in the exciton system in neutral GaAs/Al0:3Ga0:7As interface fluctuation quantum dots. The experimental work done on the exciton system serves as a foundation for consequent work on the long coherence lifetime spin-based qubits in charged quantum dots. In the spin-based system in the charged GaAs/Al0:3Ga0:7As interface fluctuation quantum dots, we demonstrate the selective excitation from the two spin ground states to the charged exciton (trion) state through phase sensitive control of the spin coherence in an ensemble of three-level spin-trion Lambda systems. This controlled excitation utilized the optically coupled and decoupled states of the electron spin in the presence of coherent transient excitation of the spin ground states in singly charged quantum dots. This progress lays the ground work for achieving complete ultrafast spin rotations.
Implementations of quantum information processing systems based on optically controlled electron spins in semiconductor quantum dots are particulary appealing due to several features. These features include inherent ultrafast gate operation times, reasonably long decoherence times, small optical control power and a natural ability to link to optical fiber communication networks. We will discuss the current state of the art in the experimental implementations of the main elements of semiconductor spin qubits: qubit initialization, single-qubit gates, two-qubit gates, entanglement distribution, projective measurement, quantum memory and indistinguishable single-photon generation.
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.
A quantum-information-processing scheme is proposed with semiconductor quantum dots located in a high-QQQ single-mode QED cavity. The spin degrees of freedom of one excess conduction electron of the quantum dots are employed as qubits. Excitonic states, which can be produced ultrafast with optical operation, are used as auxiliary states in the realization of quantum gates. We show how properly tailored ultrafast laser pulses and Pauli-blocking effects can be used to achieve a universal encoded quantum computing.
A major question for condensed matter physics is whether a solid-state quantum computer can ever be built. Here we discuss two different schemes for quantum information processing using semiconductor nanostructures. First, we show how optically driven coupled quantum dots can be used to prepare maximally entangled Bell and Greenberger-Horne-Zeilinger states by varying the strength and duration of selective light pulses. The setup allows us to perform an all-optical generation of the quantum teleportation of an excitonic state in an array of coupled quantum dots. Second, we give a proposal for reliable implementation of quantum logic gates and long decoherence times in a quantum dots system based on nuclear magnetic resonance (NMR), where the nuclear resonance is controlled by the ground state transitions of few-electron QDs in an external magnetic field. The dynamical evolution of these systems in the presence of environmentally-induced decoherence effects is also discussed.