arXiv subjects

Lian-Ao Wu

Publications and source records attributed to Lian-Ao Wu.

At least 109 records · Page 6Linked to original sources

arXiv2012

Non-perturbative Dynamical Decoupling Control: A Spin Chain Model

This paper considers a spin chain model by numerically solving the exact model to explore the non-perturbative dynamical decoupling regime, where an important issue arises recently (J. Jing, L.-A. Wu, J. Q. You and T. Yu, arXiv:1202.5056.). Our study has revealed a few universal features of non-perturbative dynamical control irrespective of the types of environments and system-environment couplings. We have shown that, for the spin chain model, there is a threshold and a large pulse parameter region where the effective dynamical control can be implemented, in contrast to the perturbative decoupling schemes where the permissible parameters are represented by a point or converge to a very small subset in the large parameter region admitted by our non-perturbative approach. An important implication of the non-perturbative approach is its flexibility in implementing the dynamical control scheme in a experimental setup. Our findings have exhibited several interesting features of the non-perturbative regimes such as the chain-size independence, pulse strength upper-bound, noncontinuous valid parameter regions, etc. Furthermore, we find that our non-perturbative scheme is robust against randomness in model fabrication and time-dependent random noise.

quant-ph↗

arXiv2012

Fermionic Stochastic Schrödinger Equation and Master Equation: An Open System Model

This paper considers the extension of the non-Markovian stochastic approach for quantum open systems strongly coupled to a fermionic bath, to the models in which the system operators commute with the fermion bath. This technique can also be a useful tool for studying open quantum systems coupled to a spin-chain environment, which can be further transformed into an effective fermionic bath. We derive an exact stochastic Schrödinger equation (SSE), called fermionic quantum state diffusion (QSD) equation, from the first principle by using the fermionic coherent state representation. The reduced density operator for the open system can be recovered from the average of the solutions to the QSD equation over the Grassmann-type noise. By employing the exact fermionic QSD equation, we can derive the corresponding exact master equation. The power of our approach is illustrated by the applications of our stochastic approach to several models of interest including the one-qubit dissipative model, the coupled two-qubit dissipative model, the quantum Brownian motion model and the N-fermion model coupled to a fermionic bath. Different effects caused by the fermionic and bosonic baths on the dynamics of open systems are also discussed.

quant-ph↗

arXiv2011

General Quantum State Swap: an XY model analysis

We consider an exact state swap, defined as the swap between two quantum states |A> and |B> in the Hilbert space of a quantum system. We show that, given an arbitrary Hamiltonian dynamics, there is a straightforward approach to calculating the probability of the occurrence of an exact state swap, by employing an exchange operator P_{AB}. For a given dynamics, the feasibilities of proposed quantum setups, such as quantum state amplifications and transfers can be evaluated. These setups are only distinguished by different forms of P_{AB}, which easily lead to innovative designs of quantum setups or devices. We illustrate the method with the isotropic XY model, whose unnoticed features are revealed.

quant-ph↗

arXiv2011

Theory of quantum energy transfer in spin chains: From superexchange to ballistic motion

Quantum energy transfer in a chain of two-level (spin) units, connected at its ends to two thermal reservoirs, is analyzed in two limits: (i) In the off-resonance regime, when the characteristic subsystem excitation energy gaps are larger than the reservoirs frequencies, or the baths temperatures are low. (ii) In the resonance regime, when the chain excitation gaps match populated bath modes. In the latter case the model is studied using a master equation approach, showing that the dynamics is ballistic for the particular chain model explored. In the former case we analytically study the system dynamics utilizing the recently developed Energy-Transfer Born-Oppenheimer formalism [Phys. Rev. E {\bf 83}, 051114 (2011)], demonstrating that energy transfers across the chain in a superexchange (bridge assisted tunneling) mechanism, with the energy current decreasing exponentially with distance. This behavior is insensitive to the chain details. Since at low temperatures the excitation spectrum of molecular systems can be truncated to resemble a spin chain model, we argue that the superexchange behavior obtained here should be observed in widespread systems satisfying the off-resonance condition.

cond-mat.mes-hall↗

arXiv2011

Geometric phases in dressed state quantum computation

Geometric phases arise naturally in a variety of quantum systems with observable consequences. They also arise in quantum computations when dressed states are used in gating operations. Here we show how they arise in these gating operations and how one may take advantage of the dressed states producing them. Specifically, we show that that for a given, but arbitrary Hamiltonian, and at an arbitrary time τ, there always exists a set of dressed states such that a given gate operation can be performed by the Hamiltonian up to a phase ϕ. The phase is a sum of a dynamical phase and a geometric phase. We illustrate the new phase for several systems.

quant-ph↗

arXiv2011

Quantum effects in thermal conduction: Nonequilibrium quantum discord and entanglement

We study the process of heat transfer through an entangled pair of two-level system, demonstrating the role of quantum correlations in this nonequilibrium process. While quantum correlations generally degrade with increasing the temperature bias, introducing spatial asymmetry leads to an intricate behavior: Connecting the qubits unequally to the reservoirs one finds that quantum correlations persist and increase with the temperature bias when the system is more weakly linked to the hot reservoir. In the reversed case, linking the system more strongly to the hot bath, the opposite, more natural behavior is observed, with quantum correlations being strongly suppressed upon increasing the temperature bias.

quant-ph↗

arXiv2011

Fast ground-state cooling of mechanical resonator with time-dependent optical cavities

We propose a feasible scheme to cool down a mechanical resonator (MR) in a three-mirror cavity optomechanical system with controllable external optical drives. Under the Born-Oppenheimer (BO) approximation, the whole dynamics of the mechanical resonator and cavities is reduced to that of a time-dependent harmonic oscillator, whose effective frequency can be controlled through the optical driving fields. The fast cooling of the MR can be realized by controlling the amplitude of the optical drives. Significantly, we further show that the ground-state cooling may be achieved via the three-mirror cavity optomechanical system without the resolved sideband condition.

quant-ph↗

arXiv2011

Nondeterminstic ultrafast ground state cooling of a mechanical resonator

We present an ultrafast feasible scheme for ground state cooling of a mechanical resonator via repeated random time-interval measurements on an auxiliary flux qubit. We find that the ground state cooling can be achieved with \emph{several} such measurements. The cooling efficiency hardly depends on the time-intervals between any two consecutive measurements. The scheme is also robust against environmental noises.

quant-ph↗

arXiv2011

Heisenberg Spin Bus as a Robust Transmission Line for Perfect State Transfer

We study the protocol known as quantum state transfer for a strongly coupled antiferromagnetic spin chain or ring (acting as a spin bus), with weakly coupled external qubits. By treating the weak coupling as a perturbation, we find that perfect state transfer (PST) is possible when second order terms are included in the expansion. We also show that PST is robust against variations in the couplings along the spin bus and between the bus and the qubits. As evidence of the quantum interference which mediates PST, we show that the optimal time for PST can be smaller with larger qubit separations, for an even-size chain or ring.

quant-ph↗

arXiv2011

Chaos can act as a decoherence suppressor

We propose a strategy to suppress decoherence of a solid-state qubit coupled to non-Markovian noises by attaching the qubit to a chaotic setup with the broad power distribution in particular in the high-frequency domain. Different from the existing decoherence control methods such as the usual dynamics decoupling control, high-frequency components of our control are generated by the chaotic setup driven by a low-frequency field, and the generation of complex optimized control pulses is not necessary. We apply the scheme to superconducting quantum circuits and find that various noises in a wide frequency domain, including low-frequency $1/f$, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubits to a Duffing oscillator as the chaotic setup. Significantly, the decoherence time of the qubit is prolonged approximately $100$ times in magnitude.

quant-ph↗

arXiv2010

Quantum and classical correlations in the one-dimensional XY model with Dzyaloshinskii-Moriya interaction

We study the effect of Dzyaloshinskii-Moriya (DM) interaction on pairwise quantum discord, entanglement, and classical correlation in the anisotropic XY spin-half chain. Analytical expressions for both quantum and classical correlations are obtained from the spin-spin correlation functions. We show that these pairwise quantities exhibit various behaviors in relation to the relative strengths of the DM interaction, the anisotropy and the magnetic intensity. We observe non-analyticities of the derivatives of both quantum and classical correlations with respect to the magnetic intensity at the critical point, with consideration of the DM interaction.

quant-ph↗

arXiv2010

Measurement-based quantum phase estimation algorithm for finding eigenvalues of non-unitary matrices

We propose a quantum algorithm for finding eigenvalues of non-unitary matrices. We show how to construct, through interactions in a quantum system and projective measurements, a non-Hermitian or non-unitary matrix and obtain its eigenvalues and eigenvectors. This proposal combines ideas of frequent measurement, measured quantum Fourier transform, and quantum state tomography. It provides a generalization of the conventional phase estimation algorithm, which is limited to Hermitian or unitary matrices.

quant-ph↗

arXiv2010

Quantum heat transfer: A Born Oppenheimer method

We develop a Born-Oppenheimer type formalism for the description of quantum thermal transport along hybrid nanoscale objects. Our formalism is suitable for treating heat transfer in the off-resonant regime, where e.g., the relevant vibrational modes of the interlocated molecule are high relative to typical bath frequencies, and at low temperatures when tunneling effects dominate. A general expression for the thermal energy current is accomplished, in the form of a generalized Landauer formula. In the harmonic limit this expression reduces to the standard Landauer result for heat transfer, while in the presence of nonlinearities multiphonon tunneling effects are realized.

cond-mat.mes-hall↗

arXiv2010

Repeated measurements and nuclear spin polarization

We study repeated (noncontinuous) measurements on the electron spin in a quantum dot and find that the measurement technique may lead to a different met$ or mechanism to realize nuclear spin polarization. While it may be used in any case, the method is aimed at the further polarization, providing that nuclear spins have been polarized by the existent electrical or optical methods. The feasibility of the method is analyzed. The existing techniques in electron spin measurements are applicable to this scheme. The repeated measurements \emph{deform} the structures of the nuclear wave function and can also serve as $\emph{gates}$ to manipulate nuclear spins.

quant-ph↗

arXiv2010

Perfect Function Transfer in two- and three- dimensions without initialization

We find analytic models that can perfectly transfer, without state initializati$ or remote collaboration, arbitrary functions in two- and three-dimensional interacting bosonic and fermionic networks. We elaborate on a possible implementation of state transfer through bosonic or fermionic atoms trapped in optical lattices. A significant finding is that the state of a spin qubit can be perfectly transferred through a fermionic system. Families of Hamiltonians, both linear and nonlinear, are described which are related to the linear Boson model and that enable the perfect transfer of arbitrary functions. This includes entangled states such as decoherence-free subsystems enabling noise protection of the transferred state.

quant-ph↗

arXiv2010

Casimir Invariants for Systems Undergoing Collective Motion

Dicke states are states of a collection of particles which have been under active investigation for several reasons. One reason is that the decay rates of these states can be quite different from a set of independently evolving particles. Another reason is that a particular class of these states are decoherence-free or noiseless with respect to a set of errors. These noiseless states, or more generally subsystems, can avoid certain types of errors in quantum information processing devices. Here we provide a method for calculating invariants of systems of particles undergoing collective motions. These invariants can be used to determine a complete set of commuting observables for a class of Dicke states as well as identify possible logical operations for decoherence-free/noiseless subsystems. Our method is quite general and provides results for cases where the constituent particles have more than two internal states.

quant-ph↗

arXiv2009

Dressed Qubits in Nuclear Spin Baths

We present a method to encode a \textit{dressed} qubit into the product state of an electron spin localized in quantum dot and its surrounding nuclear spins via a dressing transformation. In this scheme, the hyperfine coupling and a portion of nuclear dipole dipole interaction become logic gates, while they are the sources of decoherence in electron spin qubit proposals. We discuss errors and corrections for the dressed qubits. Interestingly, the effective Hamiltonian of nuclear spins is equivalent to a pairing Hamiltonian, which provides the microscopic mechanism to protect dressed qubits against decoherence.

quant-ph↗

arXiv2009

Universality in Exact Quantum State Population Dynamics and Control

We consider an exact population transition, defined as the probability of finding a state at a final time being exactly equal to the probability of another state at the initial time. We prove that, given a Hamiltonian, there always exists a complete set of orthogonal states that can be employed as time-zero states for which this exact population transition occurs. The result is general: it holds for arbitrary systems, arbitrary pairs of initial and final states, and for any time interval. The proposition is illustrated with several analytic models. In particular we demonstrate that in some cases, by tuning the control parameters a \textit{complete} transition might occur, where a target state, vacant at $t=0$, is fully populated at time $τ$.

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