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Journal of Physics Condensed Matter

Decoherence of excitonic qubits in Förster coupled quantum dots

Alagu Thilagam, Max A. Lohe

Published Jul 17, 2008• 8 citations
Publisher

Abstract

We investigate phonon-assisted mechanisms which contribute to the decoherence of excitonic qubits in quantum dot systems coupled by a Förster-type transfer process. Using the Fermi Golden Rule, we derive explicit expressions for the relaxation and dephasing times for excitonic qubits interacting with acoustic phonons via both the deformation potential and piezoelectric coupling. The decoherence times computed using the Fermi Golden Rule are consistent with those estimated using Bloch–Redfield theory at interdot tunnelling energies small enough for Markovian theories to be valid. Our results show that renormalization of interdot tunnelling frequencies due to phonon interactions is influenced mainly by bias and lattice temperature in GaAs/AlGaAs quantum dot systems.

DOI
10.1088/0953-8984/20/31/315205

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We investigate phonon-assisted mechanisms which contribute to the decoherence of excitonic qubits in quantum dot systems coupled by a Förster-type transfer process. Using the Fermi Golden Rule, we derive explicit expressions for the relaxation and dephasing times for excitonic qubits interacting with acoustic phonons via both the deformation potential and piezoelectric coupling. The decoherence times computed using the Fermi Golden Rule are consistent with those estimated using Bloch–Redfield theory at interdot tunnelling energies small enough for Markovian theories to be valid. Our results show that renormalization of interdot tunnelling frequencies due to phonon interactions is influenced mainly by bias and lattice temperature in GaAs/AlGaAs quantum dot systems.
We analyse various phonon-assisted mechanisms which contribute to the decoherence of excitonic qubits in quantum dot systems coupled by the Förster-type transfer process. We show the significant loss of coherence accompanied by dissipation due to charge carrier oscillations between qubit states by using a model of one-phonon assisted Förster-type transfer process. We obtain explicit expressions for the relaxation and dephasing times for excitonic qubits interacting with acoustic phonons via both deformation potential and piezoelectric coupling. We compare the decoherence times of the GaAs/AlGaAs material system with half times of concurrence decay in a 2$\otimes$2 bipartite mixed excitonic qubit system. We extend calculations to determine the influence of phonon mediated interactions on the non-unitary evolution of Berry phase in quantum dot systems.
We investigate the influence of phonon mediated interactions on the non-unitary evolution of geometrical phases in excitonic qubits formed in quantum dots entangled by pure Förster coupling. We consider decoherence to occur via acoustic phonons interacting through the deformation potential and piezoelectric coupling mechanisms. The influences of bath temperature, external electric field, quantum dot size and interdot distance on the evolution of geometrical phases are examined for the GaAs/AlGaAs material system. We extend the theory to determine the effect of dynamic decoupling employing ultrafast π-pulses on the evolution of geometrical phases in the presence of dephasing processes.
We study decoherence of a quantum dot charge qubit due to coupling to piezoelectric acoustic phonons in the Born--Markov approximation. After including appropriate form factors, we find that phonon decoherence rates are one to two orders of magnitude weaker than was previously predicted. We calculate the dependence of the QQQ factor on lattice temperature, quantum dot size, and interdot coupling. Our results suggest that mechanisms other than phonon decoherence play a more significant role in current experimental setups.
We investigate the dynamics of a double quantum dot charge qubit which is coupled to piezoelectric acoustic phonons, appropriate for GaAs heterostructures. At low temperatures, the phonon bath induces the non-Markovian dynamical behavior of the oscillations between the two charge states of the double quantum dot. Upon applying the numerically exact quasiadiabatic propagator path-integral scheme, the reduced density matrix of the charge qubit is calculated, thereby avoiding the Born-Markov approximation. This allows a systematic study of the dependence of the QQQ factor on the lattice temperature, on the size of the quantum dots, as well as on the interdot coupling. We calculate the QQQ factor for a recently realized experimental setup and find that it is two orders of magnitudes larger than the measured value, indicating that the decoherence due to phonons is a subordinate mechanism.
The dynamics of charge qubit in a double quantum dot coupled to phonons is investigated theoretically in terms of a perturbation treatment based on a unitary transformation. The dynamical tunneling current is obtained explicitly. The decoherence induced by acoustic phonons is analyzed in detail after a derivation of spectral density. It is shown that the contribution from deformation potential coupling is comparable to that from piezoelectric coupling in small dots and large tunneling rates. A possible decoupling mechanism is predicted.