arXiv subjects

Pochung Chen

Publications and source records attributed to Pochung Chen.

46 records · Page 3Linked to original sources

arXiv2008

Universal dynamics of quantum spin decoherence in a spin bath

We systematically investigate the universal spin decoherence dynamics of a localized electron in an arbitrary nuclear spin bath, which can be even far away from equilibrium due to the weak nuclear-lattice interaction. We show that the electron spin relaxation dynamics (as well as spin pure dephasing and Hahn echo decay) can {\it always} have a universal behavior as long as the initial state is composed of a sufficiently large amount of spin eigenstates. For a given system, the pattern of the universal dynamics depends on the complicated initial condition only via a {\it single} parameter, which measures the amount of phase coherence between different spin eigenstates in the initial state. Our results apply even when the number of the involved nuclei is not large, and therefore provide a solid foundation in the comparison of theoretical/numerical results with the experimental measurement. As an example, we also show numerical results for systems of noninteracting spin bath in zero magnetic field regime, and discuss the features of universal decoherent dynamics.

cond-mat.mes-hall↗

arXiv2008

Induced decoherence and entanglement by interacting quantum spin baths

The reduced dynamics of a single or two qubits coupled to an interacting quantum spin bath modeled by a XXZ spin chain is investigated. By using the method of time-dependent density matrix renormalization group (t-DMRG), we go beyond the uniform coupling central spin model and evaluate nonperturbatively the induced decoherence and entanglement. It is shown that both decoherence and entanglement strongly depend on the phase of the underlying spin bath. We show that in general, spin baths can induce entanglement for an initially disentangled pair of qubits. Furthermore, when the spin bath is in the ferromagnetic phase, because qubits directly couple to the order parameter, the reduced dynamics shows oscillatory type behavior. On the other hand, only for paramagnetic and antiferromagnetic phases, initially entangled states suffer from the entanglement sudden death. By calculating concurrence, the finite disentanglement time is mapped out for all phases in the phase diagram of the spin bath.

cond-mat.str-el↗

arXiv2006

Dynamical decoupling induced renormalization of the non-Markovian dynamics

In this work we develop a numerical framework to investigate the renormalization of the non-Markovian dynamics of an open quantum system to which dynamical decoupling is applied. We utilize a non-Markovian master equation which is derived from the non-Markovian quantum trajectories formalism. It contains incoherent Markovian dynamics and coherent Schrödinger dynamics as its limiting cases and is capable of capture the transition between them. We have performed comprehensive simulations for the cases in which the system is either driven by the Ornstein-Uhlenbeck noise or or is described by the spin-boson model. The renormalized dynamics under bang-bang control and continuous dynamical decoupling are simulated. Our results indicate that the renormalization of the non-Markovian dynamics depends crucially on the spectral density of the environment and the envelop of the decoupling pulses. The framework developed in this work hence provides an unified approach to investigate the efficiency of realistic decoupling pulses. This work also opens a way to further optimize the decoupling via pulse shaping.

quant-ph↗

arXiv2005

Continuous Dynamical Decoupling with Bounded Controls

We develop a theory of continuous decoupling with bounded controls from a geometric perspective. Continuous decoupling with bounded controls can accomplish the same decoupling effect as the bang-bang control while using realistic control resources and it is robust against systematic implementation errors. We show that the decoupling condition within this framework is equivalent to average out error vectors whose trajectories are determined by the control Hamiltonian. The decoupling pulses can be intuitively designed once the structure function of the corresponding SU(n) is known and is represented from the geometric perspective. Several examples are given to illustrate the basic idea. From the physical implementation point of view we argue that the efficiency of the decoupling is determined not by the order of the decoupling group but by the minimal time required to finish a decoupling cycle.

quant-ph↗

arXiv2005

Quantum shape effects on Zeeman splittings in semiconductor nanostructures

We develop a general method to calculate Zeeman splittings of electrons and holes in semiconductor nanostructures within the tight-binding framework. The calculation is carried out in the electron-hole picture and is extensible to the excitonic calculation by including the electron-hole Coulomb interaction. The method is suitable for the investigation of quantum shape effects and the anisotropy of the g-factors. Numerical results for CdSe and CdTe nanostructures are presented.

cond-mat.other↗

arXiv2003

Theory of Quantum Optical Control of Single Spin in a Quantum Dot

We present a theory of quantum optical control of an electron spin in a single semiconductor quantum dot via spin-flip Raman transitions. We show how an arbitrary spin rotation may be achieved by virtual excitation of discrete or continuum trion states. The basic physics issues of the appropriate adiabatic optical pulses in a static magnetic field to perform the single qubit operation are addressed.

cond-mat↗

arXiv2002

Optical RKKY Interaction between Charged Semiconductor Quantum Dots

We show how a spin interaction between electrons localized in neighboring quantum dots can be induced and controlled optically. The coupling is generated via virtual excitation of delocalized excitons and provides an efficient coherent control of the spins. This quantum manipulation can be realized in the adiabatic limit and is robust against decoherence by spontaneous emission. Applications to the realization of quantum gates, scalable quantum computers, and to the control of magnetization in an array of charged dots are proposed.

cond-mat↗

arXiv2001

Theory of Fast Quantum Control of Exciton Dynamics in Semiconductor Quantum Dots

Optical techniques for the quantum control of the dynamics of multiexciton states in a semiconductor quantum dot are explored in theory. Composite bichromatic phase-locked pulses are shown to reduce the time of elementary quantum operations on excitons and biexcitons by an order of magnitude or more. Analytic and numerical methods of designing the pulse sequences are investigated. Fidelity of the operation is used to gauge its quality. A modified Quantum Fourier Transform algorithm is constructed with only Rabi rotations and is shown to reduce the number of operations. Application of the designed pulses to the algorithm is tested by a numerical simulation.

cond-mat.mes-hall↗

arXiv2001

Control of Spin Dynamics of Excitons in Nanodots for Quantum Operations

This work presents a step furthering a new perspective of proactive control of the spin-exciton dynamics in the quantum limit. Laser manipulation of spin-polarized optical excitations in a semiconductor nanodot is used to control the spin dynamics of two interacting excitons. Shaping of femtosecond laser pulses keeps the quantum operation within the decoherence time. Computation of the fidelity of the operations and application to the complete solution of a basic quantum computing algorithm demonstrate in theory the feasibility of quantum control.

cond-mat.mes-hall↗

arXiv2000

Theory of Coherent Optical Control of Exciton Spin Dynamics in a Semiconductor Quantum Dot

We use the spin-polarized excitons in a single quantum dot to design optical controls for basic operations in quantum computing. We examine the ultrafast nonlinear optical processes required and use the coherent nonlinear optical responses to deduce if such processes are physically reasonable. The importance and construction of an entangled state of polarized exciton states in a single quantum dot is explained. We put our proposal in perspective with respect to a number of theoretical suggestions of utilizing the semiconductor quantum dots.

cond-mat.mes-hall↗