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Optics InfoBase Conference Papers

Non-markoffian theory of electron spin decoherence in a single quantum dot

T. Takagahara

Published Jan 1, 2006• 0 citations
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Abstract

A general non-Markoffian theory is developed to calculate the electron spin decoherence time in a single quantum dot. Dependence of the decoherence time on the temperature and magnetic field is clarified and compared with experiments.

Related papers

Non-Markoffian theory of electron spin decoherence in a single quantum dot

T. Takagahara

A general non-Markoffian theory is developed to calculate the electron spin decoherence time in a single quantum dot. Dependence of the decoherence time on the temperature and magnetic field is clarified and compared with experiments.

Conference on Lasers and Electro-Optics•Jan 1, 2006•0 citations

Spin Decoherence in Quantum Dots

D. D. Bhaktavatsala Rao, V. Ravishankar, V. Subrahmanyam

The decoherence of a spin‐1/2 particle (qubit) interacting with a nuclear spin bath in quantum dots has been studied using an isotropic Heisenberg model. An effective magnetic field (a feature of unitary evolution) and an effective temperature (a non‐unitary feature) describe the effective dynamics of the qubit. The decoherence time scale is calculated as a function of the nuclear bath spin distribution and the nuclear polarizations in various spin channels. The averaged auto correlation function of the qubit spin, averaged over all possible initial states of the qubit, depends on the nuclear spin distribution, but is insensitive to the nuclear polarizations.

AIP conference proceedings•Jan 1, 2006•0 citations

Decoherence of coupled electron spins via nuclear spin dynamics in quantum dots

Wen Yang, Ren‐Bao Liu

In double quantum dots, the exchange interaction between two electron spins renormalizes the excitation energy of pair flips in the nuclear spin bath, which in turn modifies the non-Markovian bath dynamics. As the energy renormalization varies with the static Overhauser field mismatch between the quantum dots, the electron singlet-triplet decoherence resulting from the bath dynamics depends on sampling of nuclear spin states from an ensemble, leading to the transition from superexponential decoherence in single-sample dynamics to power-law decay under ensemble averaging. In contrast, the decoherence of a single electron spin in one dot is essentially the same for different choices of the nuclear spin configuration.

arXiv:0707.2529•Physical Review B•Feb 5, 2008•17 citations

Decoherence and fidelity of single-electron spin states in quantum dots: effects of nuclear hyperfine coupling and double occupancy

A. K. Rajagopal, S. D. Mahanti, Sudhanshu Shekhar Jha

The decoherence and fidelity of spin states in a localized single-electron quantum dot in the presence of a dc magnetic field, arising either from the nuclear hyperfine interaction within the dot or due to its coupling with another localized quantum dot, are examined in detail. A general framework for determining the time evolution of the reduced density matrix. for a single dot is presented, which is exact up to the second order in interaction with any reservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, and approximate estimates of coherence decay time are made when the nuclear spins are either polarized or unpolarized and the internal dynamics of nuclear spins is determined mainly by the nuclear magnetic dipole-dipole interaction. The fidelity of a pure unperturbed electronic one-qubit spin state is obtained as a function of time, which is exact even on a very short timescale of logic gate operations. The time variation of the fidelity of the same one-qubit state on the localized dot as a part of the direct product with another one-qubit state on another localized dot arising because of coupling between these quantum dots is also calculated in this paper. In this case, we include both the single-particle tunnelling between the dots as well as the direct and exchange Coulomb interactions, including on-site Coulomb repulsion. This allows for the double occupation of a single dot. It is found that the loss of fidelity of such two-qubit states due to double occupancy and additional phase errors in the presence of appreciable dot-dot coupling can become a more severe limiting factor than that due to the hyperfine interaction in individual dots.

Journal of Physics Condensed Matter•Nov 13, 2006•0 citations

Non-Markovian decoherence dynamics of electrons in a double quantum dot system

Matisse Wei-Yuan Tu, Wei-Min Zhang, Hsi‐Sheng Goan +1 more

We develop a non‐perturbative theory for describing decoherence dynamics of electron charges in the double quantum dot gated by electrodes. We extend the Feynman‐Vernon’s influence functional theory to fermionic environments and derive an exact master equation for the reduced density matrix of electrons in the double dot for a general spectral density at arbitrary temperature and bias.

AIP conference proceedings•Jan 1, 2008•0 citations

Non-Markovian decoherence theory for a double-dot charge qubit

Matisse Wei-Yuan Tu, Wei-Min Zhang

In this paper, we develop a nonperturbation theory for describing decoherence dynamics of electron charges in a double quantum dot gated by electrodes. We extend the Feynman-Vernon influence functional theory to fermionic environments and derive an exact master equation for the reduced density matrix of electrons in the double dot for a general spectral density at arbitrary temperature and bias. We then investigate the decoherence dynamics of the double-dot charge qubit with backreaction of the reservoirs being fully taken into account. Time-dependent fluctuations and leakage effects induced from the dot-reservoir coupling are explicitly explored. The charge qubit dynamics from the Markovian to non-Markovian regime is systematically studied under various manipulating conditions. The decay behavior of charge qubit coherence and the corresponding relaxation time ${T}_{1}$ and dephasing time ${T}_{2}$ are analyzed in detail.

arXiv:0809.3490•Physical Review B•Dec 19, 2008•235 citations
The decoherence of a spin‐1/2 particle (qubit) interacting with a nuclear spin bath in quantum dots has been studied using an isotropic Heisenberg model. An effective magnetic field (a feature of unitary evolution) and an effective temperature (a non‐unitary feature) describe the effective dynamics of the qubit. The decoherence time scale is calculated as a function of the nuclear bath spin distribution and the nuclear polarizations in various spin channels. The averaged auto correlation function of the qubit spin, averaged over all possible initial states of the qubit, depends on the nuclear spin distribution, but is insensitive to the nuclear polarizations.
In double quantum dots, the exchange interaction between two electron spins renormalizes the excitation energy of pair flips in the nuclear spin bath, which in turn modifies the non-Markovian bath dynamics. As the energy renormalization varies with the static Overhauser field mismatch between the quantum dots, the electron singlet-triplet decoherence resulting from the bath dynamics depends on sampling of nuclear spin states from an ensemble, leading to the transition from superexponential decoherence in single-sample dynamics to power-law decay under ensemble averaging. In contrast, the decoherence of a single electron spin in one dot is essentially the same for different choices of the nuclear spin configuration.
The decoherence and fidelity of spin states in a localized single-electron quantum dot in the presence of a dc magnetic field, arising either from the nuclear hyperfine interaction within the dot or due to its coupling with another localized quantum dot, are examined in detail. A general framework for determining the time evolution of the reduced density matrix. for a single dot is presented, which is exact up to the second order in interaction with any reservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, and approximate estimates of coherence decay time are made when the nuclear spins are either polarized or unpolarized and the internal dynamics of nuclear spins is determined mainly by the nuclear magnetic dipole-dipole interaction. The fidelity of a pure unperturbed electronic one-qubit spin state is obtained as a function of time, which is exact even on a very short timescale of logic gate operations. The time variation of the fidelity of the same one-qubit state on the localized dot as a part of the direct product with another one-qubit state on another localized dot arising because of coupling between these quantum dots is also calculated in this paper. In this case, we include both the single-particle tunnelling between the dots as well as the direct and exchange Coulomb interactions, including on-site Coulomb repulsion. This allows for the double occupation of a single dot. It is found that the loss of fidelity of such two-qubit states due to double occupancy and additional phase errors in the presence of appreciable dot-dot coupling can become a more severe limiting factor than that due to the hyperfine interaction in individual dots.
In this paper, we develop a nonperturbation theory for describing decoherence dynamics of electron charges in a double quantum dot gated by electrodes. We extend the Feynman-Vernon influence functional theory to fermionic environments and derive an exact master equation for the reduced density matrix of electrons in the double dot for a general spectral density at arbitrary temperature and bias. We then investigate the decoherence dynamics of the double-dot charge qubit with backreaction of the reservoirs being fully taken into account. Time-dependent fluctuations and leakage effects induced from the dot-reservoir coupling are explicitly explored. The charge qubit dynamics from the Markovian to non-Markovian regime is systematically studied under various manipulating conditions. The decay behavior of charge qubit coherence and the corresponding relaxation time T1{T}_{1}T1​ and dephasing time T2{T}_{2}T2​ are analyzed in detail.