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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Phonon-induced decoherence of optical spin control in a doped semiconductor quantum dot

Anna Grodecka, C. Weber, Paweł Machnikowski, A. Knorr

Published Jun 1, 2007• 0 citations
Publisher

Abstract

This paper studies the carrier-phonon dynamics in the optical control of the spin states in a single doped semiconductor quantum dot by means of a perturbation theory describing the open system evolution under arbitrary driving and a correlation expansion technique. This paper considers the phonon-induced decoherence accompanying a single-qubit gate: an arbitrary rotation between two selected electron spin states performed via stimulated adiabatic Raman transitions and virtual excitation of a trion state. The error of the quantum gate as well as the population and coherence dynamics is calculated depending on the driving duration and detuning and study the possible reduction of the phonon decoherence.

DOI
10.1109/cleoe-iqec.2007.4386799

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This paper studies the carrier-phonon dynamics in the optical control of the spin states in a single doped semiconductor quantum dot by means of a perturbation theory describing the open system evolution under arbitrary driving and a correlation expansion technique. This paper considers the phonon-induced decoherence accompanying a single-qubit gate: an arbitrary rotation between two selected electron spin states performed via stimulated adiabatic Raman transitions and virtual excitation of a trion state. The error of the quantum gate as well as the population and coherence dynamics is calculated depending on the driving duration and detuning and study the possible reduction of the phonon decoherence.
This chapter is devoted to the recent theoretical results on the optical quantum control over charges confined in quantum dots under influence of phonons. We show that lattice relaxation processes lead to decoherence of the confined carrier states. The theoretical approach leading to a uniform, compact description of the phonon impact on carrier dynamics, perturbative in phonon couplings but applicable to arbitrary unperturbed evolution, is described in detail. Next, some applications are presented: phonon damping of Rabi oscillations in quantum dots and phonon-induced error of a single-qubit gate for an excitonic quantum dot qubit as well as for a semiconductor quantum dot spin qubit operated via a STIRAP transfer.
We consider the possibility of using optical vibrational degrees of freedom as an additional resource for controlling the evolution of a charge qubit based on a semiconductor single-electron double quantum dot. The effect of electron – photon – phonon processes on the accuracy of single-qubit operations is investigated. It is shown that, depending on the phonon decay rate and detuning of the electronic transition frequency from the optical phonon frequency, these processes can both facilitate and prevent coherent electron transfer between quantum dots.
We present an all-optical implementation of quantum computation using semiconductor quantum dots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantum dots: exciton decay, hole mixing, and phonon decoherence. We introduce an adiabatic gate procedure that allows one to circumvent these effects and evaluate quantitatively its fidelity.
We analyze the optically driven dynamics of a qubit implemented on a singlet-triplet subspace of two-electron states in a self-assembled quantum dot molecule. We study two possible control schemes based on the coupling to an excited (four-particle) state either by two spectrally separated laser pulses or by a single spectrally broad pulse. We quantitatively characterize the imperfections of the qubit operation resulting from nonadiabatic evolution and from limited spectral selectivity in a real system, as compared to the ideal adiabatic Raman transfer of occupation in the $\mathrm{\ensuremath{\Lambda}}$ system. Next, we study the effects of decoherence induced by the coupling to the phonons of the surrounding crystal lattice and by radiative recombination. As a result, we are able to identify the optimization trade-offs between different sources of errors and indicate the most favorable conditions for quantum control of the singlet-triplet qubit in the two optical control schemes.
Excitonic transitions offer a possible route to ultrafast optical spin manipulation in coupled nanostructures. We perform here a detailed study of the three principal exciton- mediated decoherence channels for optically controlled electron spin qubits in coupled quantum dots: radiative decay of the excitonic state, exciton - phonon interactions, and Landau - Zener transitions between laserdressed states. We consider a scheme for producing an entangling controlledphase gate on a pair of coupled spins which, in its simplest dynamic form, renders the system subject to fast decoherence rates associated with exciton creation during the gating operation. In contrast, we show that an adiabatic approach employing off- resonant laser excitation allows us to suppress all sources of decoherence simultaneously, significantly increasing the fidelity of operations at only a relatively small gating time cost. We find that controlledphase gates accurate to one part in 102 can realistically be achieved with the adiabatic approach, whereas the conventional dynamic approach does not appear to support a fidelity suitable for scalable quantum computation. Our predictions could be demonstrated experimentally in the near future.