D. Z. Xu

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 unexcited states of a bacteriochlorophyll (BChl) are modeled by a quasispin. 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.

Coherent excitation transfer via the dark-state channel in a bionic system

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.

Quantum anti-Zeno effect without wave function reduction

We study the measurement-induced enhancement of the spontaneous decay for a two-level subsystem, where measurements are treated as couplings between the excited state and an auxiliary state rather than the von Neumanns wave function reduction. The photon radiated in a fast decay of the atom, from the auxiliary state to the excited state, triggers a quasi-measurement, as opposed to a projection measurement. Our use of the term “quasi-measurement” refers to a “coupling-based measurement”. Such frequent quasi-measurements result in an exponential decay of the survival probability of atomic initial state with a photon emission following each quasi-measurement. Our calculations show that the effective decay rate is of the same form as the one based on projection measurements. The survival probability of the atomic initial state obtained by tracing over all the photon states is equivalent to that of the atomic initial state with a photon emission following each quasi-measurement.

Quantum Maxwell's demon in thermodynamic cycles.

We study the physical mechanism of Maxwells demon (MD), which helps do extra work in thermodynamic cycles with the heat engine. This is exemplified with one molecule confined in an infinitely deep square potential with a movable solid wall. The MD is modeled as a two-level system (TLS) for measuring and controlling the motion of the molecule. The processes in the cycle are described in a quantum fashion. It is discovered that a MD with quantum coherence or one at a temperature lower than the molecules heat bath can enhance the ability of the whole working substance, formed by the heat engine plus the MD, to do work outside. This observation reveals that the essential role of the MD is to drive the whole working substance off equilibrium, or equivalently, to work between two heat baths with different effective temperatures. The elaborate studies with this model explicitly reveal the effect of finite size off the classical limit or thermodynamic limit, which contradicts common sense on a Szilard heat engine (SHE). The quantum SHEs efficiency is evaluated in detail to prove the validity of the second law of thermodynamics.

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 photons 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.

Dispersive-coupling-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 Neumanns 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.

Noncanonical statistics of a finite quantum system with non-negligible system-bath coupling

D. Z. Xu, Sheng-Wen Li, X. F. Liu, and C. P. Sun State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Science, Beijing 100190, China Beijing Computational Science Research Center, Beijing 100084, China Department of Mathematics, Peking University, Beijing 100871, China Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

Collective effects of multiscattering on the coherent propagation of photons 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 coupled cavity array (2D CCA), we analytically derive the scattering probability of a single photon. We find that the translational symmetry of the atomic distribution leads to many important effects in the single-photon scattering. In the case with one layer of atoms, the atomic collective Lamb shift is related to the photonic density of states (DOS) of a 1D CCA, rather than the photonic DOS of a 2D CCA. As a result, the photon is effectively not cattered by the atoms when the incident momentum of the photon takes some special values. In the case with two layers of atoms, an effective coupling between two layers appears and induces an electromagnetically induced transparency like phenomenon. Our work provides a scheme of analyzing photon coherent transport in 2D and may help to understand recent experiments related to high-energy photon scattering by layered nuclei material.

Master equation and dispersive probing of a non-Markovian process

For a bosonic (fermionic) open system in a bath with many bosonic (fermionic) 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 the non-Markovianity of the quantum dissipation of a single bosonic mode of an electromagnetic field in a cavity-QED system. The non-Markovianity of the bath of the cavity is explicitly reflected by the atomic decoherence factor. DOI: 10.1103/PhysRevA.87.012110

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 Λ-type atom localised in the intersectional resonator. It is found that, due to the Fano-Feshbach effect based on the dark state of the Λ-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 realise a quantum switch for single photon.