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Superlattices and Microstructures

The decoherence time of anisotropic quantum dot qubit: Effects of electric field, electron–phonon interactions, and impurities

Shi‐Hua Chen

Published Apr 7, 2021• 0 citations
Publisher
DOI
10.1016/j.spmi.2021.106884

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Selecting the double-parameter asymmetric Gaussian (AG) potential to describe the confinement effect of electrons in a quantum dot, the ground state and the first excited state energy eigenvalues and eigenfunctions of the three-body interaction system that are composed of the electrons, the impurity and the longitudinal optical phonon are derived by using the Lee-Low-Pines unitary transformation and the Pekar-type variational method, and the two-level structure required for a qubit is constructed. The influences of material parameters such as the dispersion coefficient, dielectric constant (DC) ratio, and electron-phonon coupling (EPC) constant on the probability density and the oscillation period of electron in the AG potential qubit are investigated. Based on the Fermi gold rule and the even-order approximation, the effects of the DC ratio, the dispersion coefficient and the EPC constant on the qubit decoherence are studied. And then the influences of the dispersion coefficient, the DC ratio and the EPC constant on the phase rotation manipulation of the qubit sphere are discussed. Numerical results show that the dispersion coefficient, the DC ratio and EPC constant of the medium have both advantages and disadvantages for the formation and information storage of qubits. The probability density of electrons in quantum dot qubits decreases with DC ratio increasing and exhibits significant oscillations as the well width of the AG potential decreases; the oscillation period of the qubit decreases with the well depth of the AG potential or the DC ratio increasing; the decoherence time increases with DC ratio or dispersion coefficient increasing; the phase rotation quality factor increases with DC ratio or dispersion coefficient increasing. Using the double-parameter AG potential to describe the confinement of electrons in quantum dot will better reflect the quantization properties of qubit. Increasing the dispersion coefficient or the DC ratio of the material is beneficial to not only the phase rotation manipulation of the qubit sphere, but also improving the coherence of the quantum dot qubit. The results of this paper can be used for reference in the experimental work on the constructing and manipulating of the quantum dot qubits.

Acta Physica Sinica•Jan 1, 2019•5 citations
The electromagnetic-field dependence of the ground and the first excited-state (GFES) energy eigenvalues and eigenfunctions of the strong-coupling polaron in a quantum dot (QD) was studied for various QD thicknesses by using the variational method of the Pekar type (VMPT). On this basis, we construct a qubit in the quantum dot (QQD) by taking a two-level structure of the polaron as the carrier. The results of numerical calculations indicate that the oscillation period of the qubit, {itT}{in0}, increases with increasing the thickness of the quantum dot (TQD) {itL}, but decreases with increasing the cyclotron frequency of the magnetic field (CFMF) ω{in{itc}}, electric-field strength {itF}, and electron-phonon coupling strength (EPCS) α. The probability density of the qubit |Ψ({itρ}, {itz}, {itt})|{su2} presents a normal distribution of the electronic transverse coordinate ρ, significantly influenced by the TQD and effective radius of the quantum dot (ERQD) {itR}{in0}, and shows a periodic oscillation with variations in the electronic longitudinal coordinate {itz}, polar angle φ and time {itt}. The decoherence time τ and the quality factor {itQ} of the free rotation increase with increasing the CFMF ω{in{itc}}, dispersion coefficient η, and EPCS α, but decrease with increasing the electric-field strength {itF}, TQD {itL}, and ERQD {itR}{in0}. The TQD is an important parameter of the qubit. Theoretically, the target, which is to regulate the oscillation period, decoherence time and quality factor of the free rotation of the qubit, can be achieved by designing different TQDs and regulating the strength of the electromagnetic field.
The decoherence dynamics of two charge qubits in a double quantum dot is investigated theoretically. We consider the quantum dynamics of two interacting electrons in a vertically coupled quantum dot driven by an external electric field. We derive the equations of motion for the density matrix, in which the presence of an electron confined in the double dot represents one qubit. A Markovian approach to the dynamical evolution of the reduced density matrix is adopted. We evaluate the concurrence of two qubits in order to study the effect of acoustic phonons on the entanglement. We also show that the disentanglement effect depends on the double dot parameters and increases with the temperature.
Selecting the double-parameter asymmetric Gaussian (AG) potential to describe the confinement effect of electrons in a quantum dot, the ground state and the first excited state energy eigenvalues and eigenfunctions of the three-body interaction system that are composed of the electrons, the impurity and the longitudinal optical phonon are derived by using the Lee-Low-Pines unitary transformation and the Pekar-type variational method, and the two-level structure required for a qubit is constructed. The influences of material parameters such as the dispersion coefficient, dielectric constant (DC) ratio, and electron-phonon coupling (EPC) constant on the probability density and the oscillation period of electron in the AG potential qubit are investigated. Based on the Fermi gold rule and the even-order approximation, the effects of the DC ratio, the dispersion coefficient and the EPC constant on the qubit decoherence are studied. And then the influences of the dispersion coefficient, the DC ratio and the EPC constant on the phase rotation manipulation of the qubit sphere are discussed. Numerical results show that the dispersion coefficient, the DC ratio and EPC constant of the medium have both advantages and disadvantages for the formation and information storage of qubits. The probability density of electrons in quantum dot qubits decreases with DC ratio increasing and exhibits significant oscillations as the well width of the AG potential decreases; the oscillation period of the qubit decreases with the well depth of the AG potential or the DC ratio increasing; the decoherence time increases with DC ratio or dispersion coefficient increasing; the phase rotation quality factor increases with DC ratio or dispersion coefficient increasing. Using the double-parameter AG potential to describe the confinement of electrons in quantum dot will better reflect the quantization properties of qubit. Increasing the dispersion coefficient or the DC ratio of the material is beneficial to not only the phase rotation manipulation of the qubit sphere, but also improving the coherence of the quantum dot qubit. The results of this paper can be used for reference in the experimental work on the constructing and manipulating of the quantum dot qubits.