Quantum Information Processing Using Quantum Dot Spins and Cavity QED

Abstract

The electronic spin degrees of freedom in semiconductors typically have decoherence times that are several orders of magnitude longer than other relevant timescales. A solid-state quantum computer based on localized electron spins as qubits is therefore of potential interest. Here, a scheme that realizes controlled interactions between two distant quantum dot spins is proposed. The effective long-range interaction is mediated by the vacuum field of a high finesse microcavity. By using conduction-band-hole Raman transitions induced by classical laser fields and the cavity-mode, parallel controlled-not operations and arbitrary single qubit rotations can be realized. Optical techniques can also be used to measure the spin-state of each quantum dot. 03.67.Lx, 42.50.Dv, 03.65.Bz Within the last few years, quantum computation (QC) has developed into a truly interdisciplinary field involving the contributions of physicists, engineers, and computer scientists [1]. The seminal discoveries of Shor and others, both in developing quantum algorithms for important problems like prime factorization [2], and in developing protocols for quantum error correction (QEC) [3] and fault-tolerant quantum computation [4], have indicated the desirability and the ultimate feasibility of the experimental realization of QC in various quantum systems.The elementary unit in most QC schemes is a twostate system referred to as a quantum bit (qubit). Since QEC can only work if the decoherence rate is small, it is crucial to identify schemes where the qubits are well isolated from their environment. Ingenious schemes based on Raman-coupled low-energy states of trapped ions [5] and nuclear spins in chemical solutions [6] satisfy this criterion, in addition to providing methods of fast quantum manipulation of qubits that do not introduce significant decoherence. Even though these schemes are likely to provide the first examples of quantum information processing at 5-10 qubit level, they do not appear to be scalable to larger systems containing more than 100 qubits.Here, we propose a new scheme for quantum information processing based on quantum dot (QD) electron spins coupled through a microcavity mode. The motivation for this scheme is threefold: (1) a QC scheme based on semiconductor quantum dot arrays should be scalable to ≥ 100 coupled qubits; (2) recent experiments demonstrated very long spin decoherence times for conduction band electrons in III-V and II-VI semiconductors [7], making electron spin a likely candidate for a qubit; and (3) cavity-QED techniques can provide longdistance, fast interactions between qubits [8]. The QC scheme detailed below relies on the use of a single cavity mode and laser fields to mediate coherent interactions between distant QD spins. As we will show shortly, the proposed scheme does not require that QDs be identical and can be used to carry out parallel quantum logic operations [9].We note that a QC scheme based on electron spins in QDs have been previously proposed [10]: this scheme is based on local exchan...

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“…As we all know, there is an interaction that exists between the system and the external environment, which will make them entangled [97]. If we ignore the external environment system, the entanglement will lead to a decreased coherence of the system itself, which is also named as the decoherent mechanism [98]. The existence of the decoherent mechanism can make the energy exchange between the atoms and the magnetic field weaker until it disappears.…”

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Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source

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