arXiv subjects

D. Z. Xu

Publications and source records attributed to D. Z. Xu.

15 recordsLinked to original sources

arXiv2016

Spontaneous Decoherence of Coupled Harmonic Oscillators Confined in A Ring

We study the spontaneous decoherence of the coupled harmonic oscillators confined in a ring container, where the nearest-neighbor harmonic potentials are taken into consideration. Without any external symmetry breaking field or surrounding environment, the quantum superposition state prepared in the relative degrees of freedom gradually loses its quantum coherence spontaneously. This spontaneous decoherence is interpreted by the hidden couplings between the center-of-mass and relative degrees of freedoms, which actually originates from the symmetries of the ring geometry and corresponding nontrivial boundary conditions. Especially, such spontaneous decoherence completely vanishes at the thermodynamical limit because the nontrivial boundary conditions become trivial Born-von Karman boundary conditions when the perimeter of the ring container tends to infinity. Our investigation shows that a thermal macroscopic object with certain symmetries has chance to degrade its quantum properties even without applying an external symmetry breaking field or surrounding environment.

quant-ph↗

arXiv2014

Synchro-thermalization of composite quantum system

We study the thermalization of a composite quantum system consisting of several subsystems, where only a small one of the subsystem contacts with a heat bath in equilibrium, while the rest of the composite system is contact free. We show that the whole composite system still can be thermalized after a relaxation time long enough, if the energy level structure of the composite system is connected, which means any two energy levels of the composite system can be connected by direct or indirect quantum transitions. With an example where an multi-level system interacts with a set of harmonic oscillators via non-demolition coupling, we find that the speed of relaxation to the global thermal state is suppressed by the multi-Franck-Condon factor due to the displacements of the Fock states when the degrees of freedom is large.

quant-ph↗

arXiv2014

Non-canonical statistics of finite quantum system

The canonical statistics describes the statistical properties of an open system by assuming its coupling with the heat bath infinitesimal in comparison with the total energy in thermodynamic limit. In this paper, we generally derive a non-canonical distribution for the open system with a finite coupling to the heat bath, which deforms the energy shell to effectively modify the conventional canonical way. The obtained non-canonical distribution reflects the back action of system on the bath, and thus depicts the statistical correlations through energy fluctuations.

cond-mat.stat-mech↗

arXiv2013

Collective effects of multi-scatterer on coherent propagation of photon in a two dimensional network

We study the collective phenomenon in the scattering of a single-photon by one or two layers of two-level atoms. By modeling the photon dispersion with a two-dimensional (2D) coupled cavity array, we analytically derive the scattering probability of a single-photon. It is discovered that in the case with one layer of atoms, the atomic collective excitation leads to a shift for the single-photon scattering spectrum. Such a shift is related to the density of the atomic ensemble. For the case with two layers of atoms, an inter-layer effective coupling appears and induces an electromagnetic-induced-transparency-like phenomenon for the single-photon scattering. Our result provides a new scheme of analyzing photon coherent transport in 2D and may help to understand the current experiments about the high energy photon scattering by the layer nuclei material.

quant-ph↗

arXiv2013

Recoil effects of a motional scatterer on single-photon scattering in one dimension

The scattering of a single photon with sufficiently high energy can cause a recoil of a motional scatterer. We study its backaction on the photon's coherent transport in one dimension by modeling the motional scatterer as a two-level system, which is trapped in a harmonic potential. While the reflection spectrum is of a single peak in the Lamb-Dicke limit, multi-peaks due to phonon excitations can be observed in the reflection spectrum as the trap becomes looser or the mass of the two-level system becomes smaller.

quant-ph↗

arXiv2013

Experimental quantum Zeno effect in NMR with entanglement-based measurement

We experimentally demonstrate a new dynamic fashion of quantum Zeno effect in nuclear magnetic resonance systems. The frequent measurements are implemented through quantum entanglement between the target qubit(s) and the measuring qubit, which dynamically results from the unitary evolution of duration $τ_{m}$ due to dispersive-coupling. Experimental results testify the presence of "the critical measurement time effect", that is, the quantum Zeno effect does not occur when $τ_{m}$ takes the some critical values, even if the measurements are frequent enough. Moreover, we provide a first experimental demonstration of an entanglement preservation mechanism based on such dynamic quantum Zeno effect.

quant-ph↗

arXiv2013

Coherent control of single photons in the cross resonator arrays via the dark state mechanism

We study the single photon transfer in a hybrid system where the normal modes of two coupled resonator arrays interact with two transition arms of a ?-type atom localized in the intersectional resonator. It is found that, due to the Fano-Feshbach effect based on the dark state of the ?-type atom, the photon transfer in one array can be well controlled by the bound state of the photon in the other array. This conceptual setup could be implemented in some practical cavity QED system to realize a quantum switch for single photon.

quant-ph↗

arXiv2012

Exact Non-Markovian Master Equation and Dispersive Probing of Non-Markovian Process

For a bosonic (fermionic) open system in a bath with many bosons (fermions) modes, we derive the exact non-Markovian master equation in which the memory effect of the bath is reflected in the time dependent decay rates. In this approach, the reduced density operator is constructed from the formal solution of the corresponding Heisenberg equations. As an application of the exact master equation, we study the active probing of non-Markovianity of the quantum dissipation of a single boson mode of electromagnetic (EM) field in a cavity QED system. The non-Markovianity of the bath of the cavity is explicitly reflected by the atomic decoherence factor.

quant-ph↗

arXiv2012

Superradiant quantum phase transition in a circuit QED system: a revisit from a fully microscopic point of view

In order to examine whether or not the quantum phase transition of Dicke type exists in realistic systems, we revisit the model setup of the superconducting circuit QED from a microscopic many-body perspective based on the BCS theory with pseudo-spin presentation. By deriving the Dicke model with the correct charging terms from the minimum coupling principle, it is shown that the circuit QED system can exhibit superradiant quantum phase transition in the limit N\rightarrow\infty. The critical point could be reached at easiness by adjusting the extra parameters, the ratio of Josephson capacitance C_{J} to gate capacitance C_{g}, as well as the conventional one, the ratio of Josephson energy E_{J} to charging energy E_{C}.

quant-ph↗

arXiv2011

Dynamics of Quantum Zeno and Anti-Zeno Effects in Open System

We provide a general dynamical approach for the quantum Zeno and anti-Zeno effects in an open quantum system under repeated non-demolition measurements. In our approach the repeated measurements are described by a general dynamical model without the wave function collapse postulation. Based on that model, we further study both the short-time and long-time evolutions of the open quantum system under repeated non-demolition measurements, and derive the measurement-modified decay rates of the excited state. In the cases with frequent ideal measurements at zero-temperature, we re-obtain the same decay rate as that from the wave function collapse postulation (Nature {\bf 405}, 546 (2000)). The correction to the ideal decay rate is also obtained under the non-ideal measurements. Especially, we find that the quantum Zeno and anti-Zeno effects are possibly {\it enhanced} by the non-ideal natures of measurements. For the open system under measurements with arbitrary period, we generally derive the rate equation for the long-time evolution for the cases with {\it arbitrary} temperature and noise spectrum, and show that in the long-time evolution the noise spectrum is effectively tuned by the repeated measurements. Our approach is also able to describe the quantum Zeno and anti-Zeno effects given by the phase modulation pulses, as well as the relevant quantum control schemes.

quant-ph↗

arXiv2011

Coherent excitation transferring via dark state in light-harvesting process

We study the light absorption and energy transferring in a donor-acceptor system with a bionic structure. In the optimal case with uniform couplings, it is found that the quantum dynamics of this seemingly complicated system is reduced as a three-level system of $Λ$-type. With this observation, we show that the dark state based electromagnetically-induced transparency (EIT) effect could enhance the energy transfer efficiency, through a quantum interference effect suppressing the excited population of the donors. We estimate the optimal parameters of the system to achieve the maximum output power. The splitting behavior of maximum power may be used to explain the phenomenon that the photosynthesis systems mainly absorb two colors of light.

physics.bio-ph↗

arXiv2010

Quantum Maxwell's Demon in Thermodynamic Cycles

We study the physical mechanism of Maxwell's Demon (MD) helping to do extra work in thermodynamic cycles, by describing measurement of position, insertion of wall and information erasing of MD in a quantum mechanical fashion. The heat engine is exemplified with one molecule confined in an infinitely deep square potential inserted with a movable solid wall, while the MD is modeled as a two-level system (TLS) for measuring and controlling the motion of the molecule. It is discovered that the the MD with quantum coherence or on a lower temperature than that of the heat bath of the particle would enhance the ability of the whole work substance formed by the system plus the MD to do work outside. This observation reveals that the role of the MD essentially is to drive the whole work substance being off equilibrium, or equivalently working with an effective temperature difference. The elaborate studies with this model explicitly reveal the effect of finite size off the classical limit or thermodynamic limit, which contradicts the common sense on Szilard heat engine (SHE). The quantum SHE's efficiency is evaluated in detail to prove the validity of second law of thermodynamics.

quant-ph↗

arXiv2010

Decoherence-Based Quantum Zeno Effect in a Cavity-QED System

We present a decoherence-based interpretation for the quantum Zeno effect (QZE) where measurements are dynamically treated as dispersive couplings of the measured system to the apparatus, rather than the von Neumann's projections. It is found that the explicit dependence of the survival probability on the decoherence time quantitatively distinguishes this dynamic QZE from the usual one based on projection measurements. By revisiting the cavity-QED experiment of the QZE [J. Bernu, et al., Phys. Rev. Lett, 101, 180402 (2008)], we suggest an alternative scheme to verify our theoretical consideration that frequent measurements slow down the increase of photon number inside a microcavity due to the nondemolition couplings with the atoms in large detuning.

quant-ph↗

arXiv2010

Dimerization-assisted energy transport in light-harvesting complexes

We study the role of the dimer structure of light-harvesting complex II (LH2) in excitation transfer from the LH2 (without a reaction center (RC)) to the LH1 (surrounding the RC), or from the LH2 to another LH2. The excited and un-excited states of a bacteriochlorophyll (BChl) are modeled by a quasi-spin. In the framework of quantum open system theory, we represent the excitation transfer as the total leakage of the LH2 system and then calculate the transfer efficiency and average transfer time. For different initial states with various quantum superposition properties, we study how the dimerization of the B850 BChl ring can enhance the transfer efficiency and shorten the average transfer time.

physics.bio-ph↗

arXiv2008

Photonic Feshbach Resonance

Hermann Feshbach predicted fifty years ago that when two atomic nuclei are scattered within an open entrance channel-- the state observable at infinity, they may enter an intermediate closed channel -- the locally bounded state of the nuclei. If the energy of a bound state of in the closed channel is fine-tuned to match the relative kinetic energy, then the open channel and the closed channel "resonate", so that the scattering length becomes divergent. We find that this so-called Feshbach resonance phenomenon not only exists during the collisions of massive particles, but also emerges during the coherent transport of massless particles, that is, photons confined in the coupled resonator arrays \cite{lzhou08}. We implement the open and the closed channels inside a pair of such arrays, linked by a separated cavity or a tunable qubit. When a single photon is bounded inside the closed channel by setting the relevant physical parameters appropriately, the vanishing transmission appears to display this photonic Feshbach resonance. The general construction can be implemented through various experimentally feasible solid state systems, such as the couple defected cavities in photonic crystals. The numerical simulation based on finite-different time-domain(FDTD) method confirms our conceive about physical implementation.

quant-ph↗