arXiv subjects

Shih-I Chu

Publications and source records attributed to Shih-I Chu.

18 recordsLinked to original sources

arXiv2014

A new time-frequency method to reveal quantum dynamics of atomic hydrogen in intense laser pulses: Synchrosqueezing Transform

This study introduces a new adaptive time-frequency (TF) analysis technique, synchrosqueezing transform (SST), to explore the dynamics of a laser-driven hydrogen atom at an {\it ab initio} level, upon which we have demonstrated its versatility as a new viable venue for further exploring quantum dynamics. For a signal composed of oscillatory components which can be characterized by instantaneous frequency, the SST enables rendering the decomposed signal based on the phase information inherited in the linear TF representation with mathematical support. Compared with the classical type TF methods, the SST clearly depicts several intrinsic quantum dynamical processes such as selection rules, AC Stark effects, and high harmonic generation.

physics.data-an↗

arXiv2014

Possible realization of entanglement, logical gates and quantum information transfer with superconducting-quantum-interference-device qubits in cavity QED

We present a scheme to achieve maximally entangled states, controlled phase-shift gate, and SWAP gate for two superconducting-quantum-interference-device (SQUID) qubits, by placing SQUIDs in a microwave cavity. We also show how to transfer quantum information from one SQUID qubit to another. In this scheme, no transfer of quantum information between the SQUIDs and the cavity is required, the cavity field is only virtually excited and thus the requirement on the quality factor of the cavity is greatly relaxed.

quant-ph↗

arXiv2012

Coherent control of atomic spin currents in a double well

We propose an experimental feasible method for controlling the atomic currents of a two-component Bose-Einstein condensate in a double well by applying an external field to the atoms in one of the potential wells. We study the ground-state properties of the system and show that the directions of spin currents and net-particle tunneling can be manipulated by adiabatically varying the coupling strength between the atoms and the field. This system can be used for studying spin and tunneling phenomena across a wide range of interaction parameters. In addition, spin-squeezed states can be generated. It is useful for quantum information processing and quantum metrology.

quant-ph↗

arXiv2011

Steady-state entanglement in a double-well Bose-Einstein condensate through coupling to a superconducting resonator

We consider a two-component Bose-Einstein condensate in a double-well potential, where the atoms are magnetically coupled to a single-mode of the microwave field inside a superconducting resonator. We find that the system has the different dark-state subspaces in the strong- and weak-tunneling regimes, respectively. In the limit of weak tunnel coupling, steady-state entanglement between the two spatially separated condensates can be generated by evolving to a mixture of dark states via the dissipation of the photon field. We show that the entanglement can be faithfully indicated by an entanglement witness. Long-lived entangled states are useful for quantum information processing with atom-chip devices.

quant-ph↗

arXiv2010

Exploration of the memory effect on the photon-assisted tunneling via a single quantum dot: A generalized Floquet theoretical approach

The generalized Floquet approach is developed to study memory effect on electron transport phenomena through a periodically driven single quantum dot in an electrode-multi-level dot-electrode nanoscale quantum device. The memory effect is treated using a multi-function Lorentzian spectral density (LSD) model that mimics the spectral density of each electrode in terms of multiple Lorentzian functions. For the symmetric single-function LSD model involving a single-level dot, the underlying single-particle propagator is shown to be related to a 2 x 2 effective time-dependent Hamiltonian that includes both the periodic external field and the electrode memory effect. By invoking the generalized Van Vleck (GVV) nearly degenerate perturbation theory, an analytical Tien-Gordon-like expression is derived for arbitrary order multi- photon resonance d.c. tunneling current. Numerically converged simulations and the GVV analytical results are in good agreement, revealing the origin of multi- photon coherent destruction of tunneling and accounting for the suppression of the staircase jumps of d.c. current due to the memory effect. Specially, a novel blockade phenomenon is observed, showing distinctive oscillations in the field-induced current in the large bias voltage limit.

cond-mat.mes-hall↗

arXiv2009

Description of electron transport dynamics in molecular devices: A time-dependent density functional theoretical approach in momentum space makes it simple

We propose a first-principles time-dependent density functional theoretical (TDDFT) approach in momentum (P) space for quantitative study of electron transport in molecular devices under arbitrary biases. In this approach, the basic equation of motion is a time-dependent integrodifferential equation obtained by Fourier transform of the time-dependent Kohn-Sham (TDKS) equation in spatial coordinate (R) space. It is formally exact and includes all the effects and information of the electron transport in molecular devices. The electron wavefunction is calculated by solving this equation in a closed finite P-space volume. This approach is free of self-energy function and memory term and beyond the wide-band limit (WBL). The feasibility and power of the approach are demonstrated by the calculation of current through one-dimensional (1D) systems.

cond-mat.mes-hall↗

arXiv2009

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

We present a time-dependent localized Hartree-Fock density-functional linear response approach for the treatment of photoionization of atomic systems. This approach employs a spin-dependent localized Hartree-Fock (SLHF) exchange potential to calculate electron orbitals and kernel functions, and thus can be used to study the photoionization from atomic excited states. We have applied the approach to the calculation of photoionization cross sections of Ne ground state. The results are in agreement with available experimental data and have comparable accuracies with other ab initio theoretical results. We have also extended the approach to explore the photoionization from Ne excited states and obtained some new results for the photoionization from outer-shell and inner-shell excited states.

quant-ph↗

arXiv2007

Decoherence of a driven multilevel quantum system interacting with a multi-bath reservoir

A general theory is presented for the treatment of decoherence of a multilevel quantum system (with many degrees of freedom) interacting with multi-bath reservoir and driven by ac fields. In this approach, the system is described by a reduced density operator and the multi-bath reservoir is characterized by a number of spectral densities. The reduced density operator is governed by the master equation in which the effect of ac driving fields and the leakage to non-computational states are included. The theory is applied to the study of decoherence of a two-dimensional (2D) SQUID flux qubit coupled to control and readout circuits. The predicted results are in very good agreement with available experimental results in the absence of driving fields and with the analytic results of a dissipative two-level system in the presence of weak driving fields. The relaxation and decoherence times versus the parameters and temperature of the control and readout circuits are also explored in details to facilitate the optimization of the 2D SQUID qubit.

quant-ph↗

arXiv2007

Relaxation and decoherence in a resonantly driven qubit

Relaxation and decoherence of a qubit coupled to environment and driven by a resonant ac field are investigated by analytically solving Bloch equation of the qubit. It is found that the decoherence of a driven qubit can be decomposed into intrinsic and field-dependent ones. The intrinsic decoherence time equals to the decoherence time of the qubit in 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 by 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.

cond-mat.other↗

arXiv2006

Inner-shell excitation of open-shell atoms: A spin-dependent localized Hartree-Fock density-functional calculation

The spin-dependent localized Hartree-Fock (SLHF) density-functional approach is extended to the treatment of the inner-shell excited-state calculation of open-shell atomic systems. In this approach, the electron spin-orbitals in an electronic configuration are obtained by solving Kohn-Sham (KS) equation with SLHF exchange potential and the Slater's diagonal sum rule is used to evaluate the multiplet energy of an inner-shell excited state from the single-Slater-determinant energies of the electronic configurations involved. This approach together with the correlation potentials and energy functionals proposed by Perdew and Wang's (PW) or Lee, Yang, and Parr's (LYP) have been used to calculate the total and excitation energies of inner-shell excited states of open-shell atomic systems: Li, B, Ne^+, Ne^{2+}, Ne^{3+}, and Na. The results with the PW and LYP energy functionals are in overall good agreement with each other and also with available experimental and other ab initio theoretical data. Some new results for highly excited inner-shell states are presented.

physics.atom-ph↗

arXiv2006

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

We present a spin-dependent localized Hartree-Fock (SLHF) density-functional approach for the treatment of the inner-shell excited-state calculation of atomic systems. In this approach, the electron spin-orbitals in an electronic configuration are obtained first by solving Kohn-Sham (KS) equation with SLHF exchange potential. Then a single-Slater-determinant energy of the electronic configuration is calculated by using these electron spin-orbitals. Finally, a multiplet energy of an inner-shell excited state is evaluated from the single-Slater-determinant energies of the electronic configurations involved in terms of Slater's diagonal sum rule. This procedure has been used to calculate the total and excitation energies of inner-shell excited states of close-shell atomic systems: Be, B^+, Ne, and Mg. The correlation effect is taken into account by incorporating the correlation potentials and energy functionals of Perdew and Wang's (PW) or Lee, Yang, and Parr's (LYP) into calculation. The calculated results with the PW and LYP energy functionals are in overall good agreement with each other and also with available experimental and other ab initio theoretical data. In addition, we present some new results for highly excited inner-shell states.

physics.atom-ph↗

arXiv2006

High-Order Harmonic Generation in Laser-Irradiated Homonuclear Diatomics: The Velocity Gauge Version of Molecular Strong-Field Approximation

The generation of high harmonics in laser-irradiated light homonuclear diatomics (H2+, N2 and O2) compared to that in atomic counterparts (of nearly identical binding energy) is studied within the velocity gauge version of conventional strong-field approximation. The applied strong-field approach (we alternatively developed earlier to incorporate rescattering effects beyond the conventional saddle-point approximation) is currently extended to molecular case by means of supplement the standard linear combination of atomic orbitals} and molecular orbitals method. The associated model proved to adequately reproduce a general shape and detailed structure of molecular harmonic spectra, which demonstrate a number of remarkable distinctive differences from respective atomic spectra calculated under the same laser pulses. The revealed differences are found to be strongly dependent on internuclear separation and also very sensitive to the orbital and bonding symmetry of contributing molecular valence shell. In particular, the model correctly predicts the behavior of high-frequency plateau (both for its extent and even details of structure) in molecular harmonic spectra at small (nearly equilibrium) and large internuclear separations. In addition, for some group of harmonics, the harmonic emission rates were ascertained to dominate by contribution from inner molecular shells of higher binding energy and different orbital symmetry compared to the outermost molecular orbital normally predominantly contributing.

physics.atom-ph↗

arXiv2005

Efficient many-party controlled teleportation of multi-qubit quantum information via entanglement

We present a way to teleport multi-qubit quantum information from a sender to a distant receiver via the control of many agents in a network. We show that the original state of each qubit can be restored by the receiver as long as all the agents collaborate. However, even if one agent does not cooperate, the receiver can not fully recover the original state of each qubit. The method operates essentially through entangling quantum information during teleportation, in such a way that the required auxiliary qubit resources, local operation, and classical communication are considerably reduced for the present purpose.

quant-ph↗

arXiv2005

A unified approach to realize universal quantum gates in a coupled two-qubit system with fixed always-on coupling

We demonstrate that in a coupled two-qubit system any single-qubit gate can be decomposed into two conditional two-qubit gates and that any conditional two-qubit gate can be implemented by a manipulation analogous to that used for a controlled two-qubit 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 SQUID flux qubits with realistic device parameters and constant always-on coupling.

quant-ph↗

arXiv2005

Rapid optimization of working parameters of microwave-driven multi-level qubits for minimal gate leakage

We propose an effective method to optimize the working parameters (WPs) of microwave-driven quantum logical gates implemented with multi-level physical 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 estimated accurately from spectroscopic properties of the qubits and minimized by choosing appropriate 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 (CNOT) operation. The result of this independent transition approximation (ITA) is in good agreement with that of dynamic method (DM). Furthermore, the ratio of the speed of ITA to that of DM scales exponentially as 2^n when the number of qubits n increases.

quant-ph↗

arXiv2003

Efficient scheme for quantum entanglement, quantum information transfer, and quantum gate with three-level SQUID qubits in cavity QED

A novel scheme is proposed for realizing quantum entanglement, quantum information transfer and a set of universal quantum gates with superconducting-quantum-interference-device (SQUID) qubits in cavity QED. In the scheme, the two logical states of a qubit are the two lowest levels of the SQUID. An intermediate level of the SQUID is utilized to facilitate coherent control and manipulation of quantum states of the qubits. The method presented here does not create finite intermediate-level population or cavity-photon population during the operations. Thus, decoherence due to spontaneous decay from the intermediate levels is minimized and the requirement on the quality factor of the cavity is greatly loosened.

quant-ph↗

arXiv2002

Error prevention scheme with two pairs of qubits

A scheme is presented for protecting one-qubit quantum information against decoherence due to a general environment and local exchange interactions. The scheme operates essentially by distributing information over two pairs of qubits and through error prevention procedures. In the scheme, quantum information is encoded through a decoherence-free subspace for collective phase errors and exchange errors affecting the qubits in pairs; leakage out of the encoding space due to amplitude damping is reduced by quantum Zeno effect. In addition, how to construct decoherence-free states for n-qubit information against phase and exchange errors is discussed.

quant-ph↗

arXiv2002

Error correction code for protecting three-qubit quantum information against erasures

We present a quantum error correction code which protects three quantum bits (qubits) of quantum information against one erasure, i.e., a single-qubit arbitrary error at a known position. To accomplish this, we encode the original state by distributing quantum information over six qubits which is the minimal number for the present task (see reference [1]). The encoding and error recovery operations for such a code are presented. It is noted that the present code is also a three-qubit quantum hidded information code over each qubit. In addition, an encoding scheme for hiding $n$-qubit quantum information over each qubit is proposed.

quant-ph↗