arXiv subjects

H. C. Fu

Publications and source records attributed to H. C. Fu.

11 recordsLinked to original sources

arXiv2017

Chiral topological excitons in the monolayer transition metal dichalcogenides

We theoretically investigate the chiral topological excitons emerging in the monolayer transition metal dichalcogenides, where a bulk energy gap of valley excitons is opened up by a position dependent external magnetic field. We find two emerging chiral topological nontrivial excitons states, which exactly connects to the bulk topological properties, i.e., Chern number =2. The dependences of the spectrum of the chiral topological excitons on the width of the magnetic field domain wall as well as the magnetic filed strength are numerically revealed. The chiral topological valley excitons are not only important to the excitonic transport due to prevention of the backscattering, but also give rise to the quantum coherent control in the optoelectronic applications.

cond-mat.mes-hall↗

arXiv2013

Indirect controllability of degenerate quantum systems with quantum accessor

Complete controllability of degenerate quantum system using quantum accessor modeled as a qubit chain with nearest neighborhood coupling is investigated. Sufficient conditions on the length of accessor and the way of coupling between controlled system and accessor are obtained. General approach to arbitrary finite system is presented and two and three level degenerate systems are investigated in detail.

quant-ph↗

arXiv2013

Control protocol of finite dimensional quantum systems using alternating square pulse

Control protocol to drive finite dimensional quantum systems to an arbitrary target state using square pulses is proposed explicitly. It is a multi-cycle control process and in each cycle we apply square pulses to cause single or a few transitions between energy levels. Systems with equal energy gaps except the first one, four dimensional system with equal first and third energy gaps and different second energy gap, and systems with all equal energy gaps of dimension three, are investigated in detail. The control parameters, the interaction time between systems and control fields and free evolution times between cycles, are connected with the probability amplitudes of target states via trigonometric functions and are determined analytically.

quant-ph↗

arXiv2012

A control protocol of finite dimensional quantum systems using square pules

The control protocols of two types of finite dimensional quantum systems are proposed. The feasibility of each protocol is possible and an arbitrary target state can be achieved from initial state by a constant field. The control parameters which are time periods of interaction between systems and control fields in each cycles are connected with the probability amplitudes of target states via trigonometric functions and can be determined analytically.

quant-ph↗

arXiv2012

A control protocol of finite dimensional quantum systems

An exact and analytic control protocol of two types of finite dimensional quantum systems is proposed. The system can be drive to an arbitrary target state using cosine classical fields in finite cycles. The control parameters which are time periods of interaction between systems and control fields in each cycles are connected with the probability amplitudes of target states via triangular functions and can be determined analytically.

quant-ph↗

arXiv2010

Quantum decoherence in a hybrid atom-optical system of a one-dimensional coupled-resonator waveguide and an atom

Decoherence for a one-dimensional coupled-resonator waveguide with a two-level system inside one of resonators, induced by their interaction with corresponding environments, is investigated. Each environment is modeled as a continuum of harmonic oscillators. By finding the eigenstates of the hybrid system, which is the dressed state of the hybrid system, we calculate the lifetime of one excitation, which characterizes the existence of quantum coherence in such hybrid system and the basic quantum nature.

quant-ph↗

arXiv2010

Complete controllability of finite quantum systems with two-fold energy level degeneracy

Complete controllability of finite dimensional quantum systems with energy level degeneracy is investigated using two different approaches. One approach is to apply a weak constant field to eliminate the degeneracy and then control it using techniques developed for non-degenerate quantum systems. Conditions for the elimination of degeneracy are found and the issue of influence of relaxation time of constant external field to the target state are addressed through the fidelity. Another approach is to control the degenerate system by a single control field directly. It is found that the system with two-fold degenerate excited states and non-degenerate ground state are completely controllable except for the two-level system. Conditions of complete controllability are found for both systems with different energy gaps and with equal energy gaps.

quant-ph↗

arXiv2009

Indirect quantum control for finite-dimensional coupled systems

We present a new analysis on the quantum control for a quantum system coupled to a quantum probe. This analysis is based on the coherent control for the quantum system and a hyperthesis that the probe can be prepared in specified initial states. The results show that a quantum system can be manipulated by probe state-dependent coherent control. In this sense, the present analysis provides a new control scheme which combines the coherent control and state preparation technology.

quant-ph↗

arXiv2008

Indirect control of quantum system via accessor: pure coherent control without system excitation

A pure indirect control of quantum systems via quantum accessor is investigated. In this control scheme, we do not apply any external classical excitation fields on the controlled system and we control a quantum system via a quantum accessor and classical control fields control the accessor only. Complete controllability is investigated for arbitrary finite dimensional quantum systems and exemplified by 2 and 3 dimensional systems. The scheme exhibits some advantages; it uses less qubits in accessor and does not depend on the energy-level structure of the controlled system.

quant-ph↗

arXiv2007

Indirect control with quantum accessor: coherent control by initial state preparation

This is the second one in our series of papers on indirect quantum control assisted by quantum accessor. In this paper we propose and study a new class of indirect quantum control(IDQC) scheme based on the initial states preparation of the accessor. In the present scheme, after the initial state of the accessor is properly prepared, the system is controlled by repeatedly switching on and off the interaction between the system and the accessor. This is different from the protocol of our first paper, where we manipulate the interaction between the controlled system and the accessor. We prove the controllability of the controlled system for the proposed indirect control scheme. Furthermore, we give an example with two coupled spins qubits to illustrate the scheme, the concrete control process and the controllability.

quant-ph↗

arXiv2007

Indirect control with quantum accessor (I): coherent control of multi-level system via qubit chain

Indirect controllability of an arbitrary finite dimensional quantum system (N-dimensional qudit) through a quantum accessor is investigated. Here, The qudit is coupled to a quantum accessor which is modeled as a fully controllable spin chain with nearest neighbor (anisotropic) XY-coupling. The complete controllability of such indirect control system is investigated in detail. The general approach is applied to the indirect controllability of two and three dimensional quantum systems. For two and three dimensional systems, a simpler indirect control scheme is also presented.

quant-ph↗