Zhongyuan Zhou

Relaxation and decoherence in a resonantly driven qubit

Relaxation and decoherence of a qubit coupled to the environment and driven by a resonant ac field are investigated by analytically solving the Bloch equation of the qubit. We found that the decoherence of a driven qubit can be decomposed into intrinsic and field-dependent decoherence. The intrinsic decoherence time equals the decoherence time of the qubit in a free decay while the field-dependent decoherence time is identical with the relaxation time of the qubit in driven oscillation. Analytical expressions of the relaxation and decoherence times are derived and applied to study a microwave-driven SQUID flux qubit. The results are in excellent agreement with those obtained from directly numerically solving the master equation. The relations between the relaxation and decoherence times of a qubit in free decay and driven oscillation can be used to extract the decoherence and thus dephasing times of the qubit by measuring its population evolution in free decay and resonantly driven oscillation.

A time-dependent momentum-space density functional theoretical approach for electron transport dynamics in molecular devices

We propose a time-dependent density functional theoretical (TDDFT) approach in momentum (P) space for the study of electron transport in molecular devices under arbitrary biases. The basic equation of motion, which is a time-dependent integrodifferential equation obtained by Fourier transform of the time-dependent Kohn-Sham equation in spatial coordinate (R) space, is formally exact and includes all the effects and information of the electron transport in the molecular devices. The electron wave function is calculated by solving this equation in a finite P-space volume. This approach is free of self-energy function and memory term related to the electrodes in the R space and beyond the wide-band limit (WBL). The feasibility and power of the approach are demonstrated by the calculation of current through one-dimensional systems. Copyright c EPLA, 2009

Rapid Optimization of Working Parameters of Microwave-Driven Multilevel Qubits for Minimal Gate Leakage

We propose an effective method to optimize the working parameters (WPs) of microwave-driven quantum gates implemented with multilevel qubits. We show that by treating transitions between each pair of levels independently, intrinsic gate errors due primarily to population leakage to undesired states can be determined by spectroscopic properties of the qubits and minimized by choosing proper WPs. The validity and efficiency of the approach are demonstrated by applying it to optimize the WPs of two coupled rf SQUID flux qubits for controlled-not operation. The result of this independent transition approximation (ITA) is in good agreement with that of dynamic method (DM). The ratio of the speed of ITA to that of DM scales exponentially as 2(n) when the number of qubits n increases.

Quantum dynamics of a microwave driven superconducting phase qubit coupled to a two-level system

We present an analytical and comprehensive description of the quantum dynamics of a microwave resonantly driven superconducting phase qubit coupled to a microscopic two-level system (TLS), covering a wide range of the external microwave field strength. Our model predicts several interesting phenomena in such an ac driven four-level bipartite system including anomalous Rabi oscillations, high-contrast beatings of Rabi oscillations, and extraordinary two-photon transitions. Our experimental results in a coupled qubit-TLS system agree quantitatively very well with the predictions of the theoretical model.

Entanglement dynamics of a superconducting phase qubit coupled to a two-level system

We report the observation and quantitative characterization of driven and spontaneous oscillations of quantum entanglement, as measured by concurrence, in a bipartite system consisting of a macroscopic Josephson phase qubit coupled to a microscopic two-level system. The data clearly show the behavior of entanglement dynamics such as sudden death and revival, and the effect of decoherence and ac driving on entanglement.

Spin-dependent localized Hartree-Fock density-functional approach for the accurate treatment of inner-shell excitation of closed-shell atoms

A spin-dependent localized Hartree-Fock density-functional approach is presented for the efficient and accurate treatment of inner-shell excited states of atomic systems. The approach is applied to the calculation of the total and excitation energies of inner-shell excited states of several closed-shell atomic systems: Be,

Inner-shell excitation of open-shell atoms: a spin-dependent localized Hartree–Fock density-functional approach

{\mathrm{B}}^{+}

Unified approach for universal quantum gates in a coupled superconducting two-qubit system with fixed always-on coupling

, Ne, and Mg. The predicted results are in overall good agreement with available experimental and other ab initio theoretical data. In addition, results for highly excited inner-shell states are presented.

Quantum entanglement and controlled logical gates using coupled SQUID flux qubits

We present a spin-dependent localized Hartree–Fock (SLHF) densityfunctional approach for the treatment of inner-shell excited states of openshell atomic systems. In this approach, the electron spin-orbitals and singleSlater-determinant energies of an electronic configuration are computed by solving the Kohn–Sham (KS) equation with SLHF exchange potential. The multiplet energy of an inner-shell excited state is evaluated from the singleSlater-determinant energies in terms of Slater’s diagonal sum rule. Based on this procedure, we perform calculations of the total and excitation energies of inner-shell excited states of open-shell atomic systems: Li, B, Ne + ,N e 2+ ,N e 3+ and Na. In the calculation, the electron correlation effect is taken into account via the correlation potential and energy functional of Perdew and Wang (PW) or of Lee, Yang and Parr (LYP). The calculated results are in good agreement with the available experimental and otherab initio theoretical data. In addition, new results for highly excited inner-shell states are also presented.

Time-dependent localized Hartree-Fock density-functional linear response approach for photoionization of atomic excited states

We demonstrate that in a coupled two-qubit system any single-qubit gate can be decomposed into 0-controlled and 1-controlled two-qubit gates which can be implemented by manipulations analogous to that used for a controlled NOT CNOT gate. Based on this we present a unified approach to implement universal single-qubit and two-qubit gates in a coupled two-qubit system with fixed always-on coupling. This approach requires neither supplementary circuit or additional physical qubits to control the coupling nor extra hardware to adjust the energy level structure. The feasibility of this approach is demonstrated by numerical simulation of single-qubit gates and creation of two-qubit Bell states in rf-driven inductively coupled two superconducting quantum interference device flux qubits with realistic device parameters and constant always-on coupling.