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Physical Review B

Pure dephasing of double-quantum-dot charge qubits in freestanding slabs

Ying-Yen Liao, Y. N. Chen

Published Apr 1, 2010• 7 citations
Publisher

Abstract

We study phonon-induced pure dephasing in a double-quantum-dot qubit that is embedded in a freestanding slab. The dispersion relations are evaluated to probe the phonon characteristics of the slab. The pure dephasing factor in the off-diagonal elements of the density matrix is calculated. Numerical results demonstrate that time evolution of this factor exhibits a nonmonotonic feature, resulting from the phonon confinement effect. Significant contribution from individual phonon mode is analyzed in detail. We also study effects of temperature and double-dot position on the pure dephasing. In particular, two remarkable oscillations are observed due to phonon van Hove singularities.

DOI
10.1103/physrevb.81.153301

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We study phonon-induced pure dephasing in a double-quantum-dot qubit that is embedded in a freestanding slab. The dispersion relations are evaluated to probe the phonon characteristics of the slab. The pure dephasing factor in the off-diagonal elements of the density matrix is calculated. Numerical results demonstrate that time evolution of this factor exhibits a nonmonotonic feature, resulting from the phonon confinement effect. Significant contribution from individual phonon mode is analyzed in detail. We also study effects of temperature and double-dot position on the pure dephasing. In particular, two remarkable oscillations are observed due to phonon van Hove singularities.
This study elucidates the theory of phonon-induced decoherence of a double dot charge qubit that is embedded inside a suspended semiconductor slab. The influences of the lattice temperature, width of the slab, and positions of the dots on the decoherence are analyzed. Numerical results indicate that the decoherence in the slab system is weaker than that in a bulk environment. In particular, the decoherence is markedly suppressed by the inhibition of the electron-phonon coupling. Such a system with low decoherence may be useful for manipulating the qubits.
We investigate the decoherence of a charge qubit in a double quantum dot which is formed in a free-standing semiconductor slab. The dots are spatially arranged to align along a particular direction. Confined phonons in the slab have a remarkable influence on the dynamics of a qubit system. We show that the decoherence of such a qubit strongly depends on the positions of the dots. By varying the tunnel coupling, the decoherence differs significantly in laterally and vertically coupled quantum dots. A robust qubit is further achieved due to suppression of electron-phonon coupling. Moreover, the coherent evolution of the system is analyzed by modulating the interdot direction.
In a parabolic quantum dot (QD), the exciton vacuum state and the exciton ground state may be employed as a two-level quantum system——a qubit. The pure dephaseing due to deformation potential exciton-bulk longitudinal acoustic phonon (LAP) interaction for the exciton qubit is investigated by using the reduce density matrix. The dependence of dephasing factor on time and the confinement length of the QD and temperature is calculated. It is shown that the pure dephasing factor increases quickly with increasing time when time is smaller than 2.5ps, and the dephasing time is of the order of ps. Even at absolute zero temperature, exciton-LAP interaction still causes excitonic dephasing. The pure dephasing factor increases with increasing temperature quickly when temperature is higher than 3K. At the same time, it is found that the confinement length of the QD has important effects on pure dephasing of qubit, the smaller the quantum size, the faster the pure dephasing is. The results show that using appropriate QD size at low temperature and using ultra-fast laser technology can effectively suppress pure dephasing due to exciton-LAP interaction.
The spin of an electron confined into a lateral semiconductor quantum dot has been proposed as a possible physical realization of a qubit. While the spin has the advantage of large decoherence times, operations with more than one qubit will necessarily involve orbital degrees of freedom, namely, charge, which is much more prone to decoherence. There are also alternative quantum dot qubit proposals that are entirely based on charge. We have used a realistic model to quantify the limitations imposed by acoustic phonons on the operation of quantum dot-based qubits. Our treatment includes essential aspects of the setup geometry, wave function profile and materials characteristics. The time dependence of the qubit density matrix is the presence of a phonon bath solved within the Born-Markov approximation. We find that the inclusion of geometric form factors makes the phonon-induced decoherence rates in double dot charge qubits nearly one order of magnitude lower than estimates previously in the literature. Moreover, our theoretical prediction for the quality factor of coherent charge oscillations based on phonon decoherence are higher than the values recently observed experimentally. This allows us to conclude that phonons are not the primary source of decoherence in double quantum dot qubits.