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Phonon-assisted excitation energy transfer between detuned pigments in photosynthetic system has been identified as a potential design principle for optimizing exciton migration on next-generation materials. Based on a detuned dimer system consisting of a donor and an acceptor modeled by two two-level systems with different excitation energy, in which one of them is coupled to a high-energy vibrational mode and driven by a Gaussian-type pulse, we show the important role of the high-energy vibrational mode on both energy absorption and transfer in the detuned dimer model. It is found that, for driven pulse with fixed width, the efficiency of energy absorption and transfer varies with the parameters of the high-energy vibrational mode in different patterns, but there exist optimal parameters for both energy absorption and transfer. In addition, the narrower the width of the driven pulse is, the higher both the efficiency of energy absorption and transfer are.

Hyperentanglement has a higher information density than conventional single-degree-of-freedom entanglement, which has attracted much attention due to its fascinating applications in quantum communication. However, since the inevitable interactions between quantum entangled systems and the environment will drive hyperentangled systems into less hyperentangled states or even mixed hyperentangled states, the efficiency and security of quantum communication will be greatly depressed. The currently existing distillation protocols are not universal, i.e., they only work for the lossy hyperentangled states in specific systems. In this paper, based on local positive-operator-valued measures (POVMs), we present a distillation protocol for lossy hyperentangled photonic Bell and Greenberger-Horne-Zeilinger (GHZ) states. The intrinsic property of our protocol is twofold, i.e., the POVM nature of the protocol guarantees that the protocol is a universal one, and the distillation operation on only one of the two degrees of freedom (DOFs) can enhance the fidelity of the system in both DOFs. Furthermore, in the implementation level, our hyperentanglement distillation protocol (HEDP) has other two merits: no auxiliary local entanglement resources and sophisticated single-photon detectors are required, and only one copy of the lossy state will be operated in each distillation round, which show that our HEDPs are relatively simple and feasible.

Manipulation of non-Markovian dynamics of open quantum systems has become an important topic in the field of quantum information since quantum non-Markovianity has been proved to be a potential quantum resource. Here, focusing on a simple yet well-known model of a two-level system coupled to bosonic fields, we study how to control the non-Markovian dynamics of an open quantum system using quantum-jump-based feedback control. Numerical simulations show that the quantum-jump-based feedback control can be used to enhance the measure of non-Markovianity. We find that the non-Markovianity can further be optimized by choosing appropriate feedback amplitude. These results may trigger potential applications in exploring non-Markovian effect for future quantum technology and quantum memory.

We present an entanglement swapping process for unknown nonmaximally entangled photonic states, where the standard Bell-state measurement is replaced by a three-step quantum walk-like state discrimination process, i.e., the practically nontrivial coupling element of two photons is replaced by manipulating their trajectories, which will greatly enrich the dynamics of the coupling between photons in realizing quantum computation, and reduce the integration complexity of optical quantum processing. In addition, the output state can be maximally entangled, which allows for entanglement concentration as well., Comment: 16pages, 2 figures

We propose schemes for preparing W and GHZ photonic states entangled in polarization via quantum walks, where the trajectory exchanging of identical photons plus postselection induces the coupling between their polarization degrees of freedom. Being different from the polarization beam splitter-based approach, the indistinguishability of identical photons provides an alternative for coupling photons, which is more phase stable and may induce richer actions than the traditional polarization beam splitter on input photons. Our schemes demonstrate that this new coupling mechanism for photons is originated from the fact that trajectory-exchanging operation plus postselection can extract entangled states from the initially product states, and this new coupling mechanism may find considerable potential applications in both quantum communication and quantum computation.

We propose a scheme to enhance entanglement from amplitude damping or correlated amplitude damping decoherence. We show that entanglement sudden death time can be prolonged by the initial single-qubit operation combined with local filtering operation. For the amplitude damping channel case, we give the optimal single-qubit operation for arbitrary pure state [Formula: see text]. For the correlated amplitude damping channel case, we find that single-qubit operation on the initial state can not only enhance the final entanglement but also avoid entanglement sudden death. Compared to the previous schemes, the optimal operations and local filtering operations used in our scheme are independent with the decay parameters of the environment.

A scheme for implementing the direct measurement of Concurrence is given in a cavity–quantum dot system. The scenario not only can directly measure the Concurrence of two-spin pure entangled state, but also suitable for the case of mixed state. More importantly, all of the operations are of geometric nature, which depend on the cavity-state-free evolution and can be robust against random operation errors. Our scheme provided an alternative method for directly measuring the degree of entanglement in solid-state system.

Efficient preparation of W and GHZ states encoded in various degrees of freedom of quantum particles is vital in quantum information science. So far, most of the studies have focused on polarization encoded photonic W and GHZ states. In this paper, we focus on W- and GHZ-class entangled coherent states, and propose schemes to fuse small W- and GHZ-entangled coherent states into larger ones. Based on successive detuned interactions between optical modes and an ancilla atom, an ( N + M − 2 ) -mode entangled coherent W state can be probabilistically prepared from an N-mode and an M-mode entangled coherent W states. This fusion scheme applies to entangled coherent GHZ states too, and it can succeed in a deterministic way. The ancilla atom only interacts with a single optical mode, which avoids the problem of synchronizing many atoms in the previous cavity QED based fusion schemes. The detuning property of the interaction makes the current fusion scheme more feasible that the ones based on resonant atom–light interactions. In addition, the two levels of the ancilla atom for encoding quantum information are two degenerate ground states, and the excited state is adiabatically eliminated during the fusion process, so the atomic decay from excited states does not affect the quality of the fusion process.

We investigate the effects of the directions of Dzyaloshinskii—Moriya (DM) interaction vector and magnetic field on the quantum discord in the pure DM model. For different directions of DM vector, we find that there are different optimal parameter components of magnetic field. Moreover, we find that the optimal parameter components rules are the same for the Hamiltonian H1 and H2. According to the rules, for a certain axial DM vector, we can get the maximal quantum discord by adjusting the direction of the external magnetic field, which is feasible under the current experimental technology.

It is known that the inevitable interaction of the entangled qubits with their environments may result in the degradation of quantum correlation. We study the decoherence of two remote qubits under general local single- and two-sided amplitude-damping channel (ADC). By using concurrence, quantum discord and Clauser–Horne–Shimony–Holt (CHSH) inequality, we find that the relation between the residual quantum correlations and the initial ones are different. Recently, Wang et al. [Int. J. Theor. Phys. 54 (2015) 5] showed that there exist a set of partially entangled states that are more robust than maximally entangled states in terms of the residual quantum correlation measured by concurrence, fully entangled fraction and quantum discord, respectively. Here we find that both in single- and two-sided ADC, only the evolution of CHSH inequality with the initial parameter is proportional to that of the initial nonlocality. That means the initial state with maximally nonlocality will retain its role in the evolution. It implies that the evolution of nonlocality may reveal the characteristics of quantum state better. Furthermore, we discuss the evolutions of the three different quantum measurements with the initial parameter under generalized amplitude damping channel (GADC) and find that they are all proportional to that of the initial state.

Excitation energy transfer (EET) plays a vital role in many areas of physics and biology processes. Here we address the role of quantum-jump-based feedback control in the efficiency of EET through a chain model. Usually, the decoherence caused by dissipative noise is detrimental to the transfer efficiency. We demonstrate that feedback control can always enhance the efficiency of EET and the dependence of different feedback controls is also discussed in detail. In addition, we investigate the strategy to enhance the efficiency of EET in the Fenna–Matthews–Olson complex as a prototype for larger photosynthetic energy transfer systems.

The quantum network model is widely used to describe the dynamics of excitation energy transfer in photosynthesis complexes. Different from the previous schemes, we explore a specific network model, which includes both light-harvesting and energy transfer process. Here, we define a rescaled measure to manifest the energy transfer efficiency from external driving to the sink, and the external driving fields are used to simulate the energy absorption process. To study the role of initial state in the light-harvesting and energy transfer process, we assume the initial state of the donors to be two-qubit and three-qubit entangled states, respectively. In the two-qubit initial state case, we find that the initial entanglement between the donors can help to improve the absorption and energy transfer process for both the near-resonant and large-detuning cases. For the case of three-qubit initial state, we can see that the transfer efficiency will reach a larger value faster in the tripartite entanglement case compared to the bipartite entanglement case.

In this paper, we propose two new qubit-loss-free (QLF) fusion schemes for W states in cavity QED system. Resonant interactions between atoms and single cavity mode constitute the main fusion mechanism, with which atomic $$|W_{n+m}\rangle $$ and $$|W_{n+m+q}\rangle $$ states can be generated, respectively, from a $$|W_{n}\rangle $$ and a $$|W_{m}\rangle $$ and from a $$|W_{n}\rangle $$ , a $$|W_{m}\rangle $$ and a $$|W_{q}\rangle $$ , by detecting the cavity mode. The QLF property of the schemes makes them more efficient and simpler than the currently existing ones, and fewer intermediate steps and memory resources are required for generating a target large-scale W state. Furthermore, the fusion of atomic states can be realized via the detection on cavity mode rather than the much complicated atomic detection, which makes our schemes feasible. In addition, the analyses of the optimal resource cost and the experimental feasibility indicate that the present schemes are simple and efficient, and maybe implementable within the current experimental techniques.

We present two schemes for transforming bipartite non-maximally entangled states into a W state in cavity QED system, by using highly detuned interactions and the resonant interactions between two-level atoms and a single-mode cavity field. A tri-atom W state can be generated by adjusting the interaction times between atoms and the cavity mode. These schemes demonstrate that two bipartite non-maximally entangled states can be merged into a maximally entangled W state. So the scheme can, in some sense, be regarded as an entanglement concentration process. The experimental feasibility of the schemes is also discussed.

We consider entanglement swapping with two copies of a class of two-qubit X-states which do not violate Clauser---Horne---Shimony---Holt (CHSH) inequality or exceed the maximal classical teleportation fidelity. It is shown that there exists a class of initial states for which that the final states violate CHSH inequality or exceed the maximal classical channel for some results of Bell measurements. This implies that, by using entanglement swapping, it is possible to increase the violation of CHSH inequality or teleportation fidelity probabilistically. We also demonstrate that the activation of violating CHSH inequality does not represent the increase in entanglement.

Protecting entanglement from decoherence has attracted more and more attention recently. Amplitude damping is a typical decoherence mechanism. If we detect the environment to guarantee no excitation escapes from the system, the amplitude damping is modified into a weak measurement of the system state. In this paper, based on local pulse series, we propose a scheme for protecting tripartite entanglement against decaying caused by weak-measurement-induced damping. Unlike previous bipartite state protection schemes, we consider three different situations: A series of unitary operations are applied on all of the three qubits, on two of the three qubits, and on only one qubit. The results show that this protocol can protect remote tripartite entanglement with a wide range of unitary operations. For the case of GHZ state, when the uniform pulses are applied on all qubits or on two qubits, the tripartite entanglement can be fixed around the entanglement of the initial state. Moreover, in the W state case, if a train of uniform pulses is applied on two qubits, we can see that the bipartite entanglement can be enhanced to the maximum with the third qubit being traced out. We also generalize our scheme to the cases of the superposition and mixture of GHZ and W states, and the numerical simulation shows that our protection scheme still works fine. The most distinct advantage of this entanglement protection scheme is that there is no need for the users to synchronize their operations. The fluctuations of the time interval between two adjacent local unitary operations, the operation parameters, and the pulse duration are all taken into consideration. All these advantages suggest that our scheme is much simpler and feasible, which may warrant its experimental realization.

In this paper, we study how to fuse two W states into a larger W state in a cavity quantum electrodynamic system. Two atoms are collided within a detuned vacuum cavity mode. One atom is from the first N-atom W state system, and the other is from the second M-atom W state system. The two atoms will be detected after flying out of the cavity mode. If only one excitation is detected between the two atoms, an (N + M − 2)-atom W state can be generated with some probability. If no excitation is detected between the two atoms, the remaining (N − 1)-atom W state and (M − 1)-atom W state can be re-fused using the same procedure, continuing until two excitations are detected between the two atoms. Using this procedure, the chance of entanglement of the two W states is as large as possible. A feasibility analysis indicates that this scheme can be implemented using existing experimental technologies.

Most of the existing distillation protocols only work for binary-input binary-output nonlocal boxes (two-dimensional boxes), and they cannot be generalized to the binary-input multi-output nonlocal boxes (high-dimensional boxes) in a trivial way. We will design some comparator-based protocols to distill high-dimensional nonlocal boxes. Our protocols are more powerful and universal than the previous ones in the sense that they can distill the arbitrary-dimensional boxes rather than the limited two-dimensional ones. The initial nonlocalities and the wiring manners between the boxes are two main factors deciding the distillation efficiency.

We present a two-copy-based protocol for directly measuring the concurrence of two-photon polarization entangled mixed states (Collins–Gisin class state and the more complicated bipartite mixed entangled state—Werner class state) without quantum state tomography. The quantum circuit designed for directly measuring concurrence can be realized in an optical system. Our protocol works without the sophisticated controlled-NOT gate, which makes it much simpler than the previous ones. Because all the operations used here are local, the scheme can be used for directly measuring remote mixed entanglement too.

We study the Bell-nonlocality dynamics of three remote atoms, two of which are trapped in one single-mode cavity and the third atom is trapped in another remote single-mode cavity. The interactions between the atoms and the cavity modes are studied via Tavis Cummings and Jaynes Cummings models. Here, the two single-mode cavities are introduced to simulate two different enviroments of the three atoms. The tripartite nonlocal correlations are studied in terms of the Svetlichny inequality and the WWZB inequality, respectively. The results show that the tripartite Bell-nonlocality sudden death will occur for the W state and GHZ state initial conditions. The detailed results demonstrate that the tripartite nonlocality of GHZ state is more robust than that of W state when suffering from the effect of environments.

We propose a scheme for reconstructing an unknown two-particle mixed state by means of unambiguous state discrimination. In this protocol, an ancillary particle is introduced for distinguishing four nonorthogonal states. The discrimination process is performed by a bipartite unitary operation on the two-particle system and the ancilla followed by a von Neumann measurement on the ancilla. Then the original two-particle system is measured in mutually unbiased bases. Consequently, the two-particle mixed state can be reconstructed. Furthermore, the total number of measurements in this protocol is less than that of the standard quantum tomography, thus the quantum resources is saved. Additionally, the nonorthogonal states discrimination and mutually unbiased measurement can be experimentally achieved, therefore our protocol may be realized with the current technology.

In unambiguous state discrimination, the measurement results consist of the error-free results and an inconclusive result, and an inconclusive result is conventionally regarded as a useless remainder from which no information about initial states is extracted. In this paper, we investigate the problem of extracting remaining information from an inconclusive result, provided that the optimal total success probability is determined. We present three simple examples. An inconclusive answer in the first two examples can be extracted partial information, while an inconclusive answer in the third one cannot be. The initial states in the third example are defined as the highly symmetric states.

Based on resonant interactions between atoms and cavity modes, we have designed a scheme for expanding atomic entangled W states. By sending one of the atoms in a N-atom W state and an auxiliary excited-state atom into an vacuum cavity, we have obtained a (N ? 1)-atom W state by detecting the cavity field after interaction. The feasibility analysis indicates that this scheme may be implementable in near future.

We present an entanglement distillation scheme for enhancing remote two-photon polarization entanglement of mixed states. Although the main idea of the current scheme is based on Gisin׳s work (Phys. Lett. A 210 (1996) 151 [21] ), there are new advantages in our new scheme, which are guaranteed by the nondemolition measurement of photonic state and the re-distillation of the garbage states. This entanglement distillation scheme not only can enhance the remote entanglement of mixed states, but also can expand two-photon entangled states to four-photon entangled states. So this scheme is an apparently feasible way for preparing multi-photon entangled states. The main idea is based on the principle of the cross-Kerr nonlinearity and the parity-check measurements (a nondemolition measurement) on photonic states. Two distant users Alice and Bob first start with one shared but less entangled photon pair, and with the help of local auxiliary photons, parity-check measurements and classical communication they can get a four-photon highly entangled states with a high success probability. For the fail result, although the garbage state is less entangled than the initial one, there is still entanglement in it. So these garbage states can be re-collected and distilled again instead of being discarded. In this sense, we can see that this protocol has a high yield, and the fidelity (with respect to the Bell state) of the initial state is not required to be bigger than 1/2 (a common threshold of the standard entanglement purification theory). In addition, post-selection measurements on the entangled photons are not needed here because of the nondemolition measurement. The nondemolition character of the measurement allows further processing of the resulting states. These advantages make the current scheme more feasible within the current technology.

We present a practical and general scheme of remote state preparation in the presence of the classical non-Markovian noises, where one of the quantum channels becomes a mixed state. The noises can be modelled as the so-called Ornstein-Uhlenbeck processes. This remote state preparation scheme is more practical than the pure state case in quantum information processing. The fidelity of the remote state preparation is (1 + e −4f(t))/2 and the success probability is 1/2.

We study the dynamics of geometric measure of quantum discord (GMQD) under the influences of two local phase damping noises. Consider the two qubits initially in arbitrary X-states, we find the necessary and sufficient conditions for which GMQD is unaffected for a finite period. It is further shown that such results also hold for the non-Markovian dephasing process., Comment: 4 pages, 2 figures

We present a protocol for directly measuring the concurrence of a two-photon polarization entangled pure or mixed state without prior quantum state tomography. By parity-check measurements and simple operations on two copies of the two-photon polarization entangled pure state, the concurrence is encoded in the total probability of picking up the odd parity states from the signal states. This protocol makes use of highly efficient homodyne detection, and it could be feasible in the near future with the help of the weak cross-Kerr nonlinearity. Moreover, our protocol can be used in a distributed fashion to directly determine the entanglement of remote states, which may find its important applications in quantum communication.

Fusion technique plays a key role in the preparation of large-scale $W$ states, but the currently existing fusion schemes are suffering from the qubit loss problem, i.e., the number of the output entangled qubits is smaller than the sum of numbers of the input entangled qubits, which will inevitably decrease the fusion efficiency and increase the number of fusion steps as well as the requirement of quantum memories. In this paper, we design a fusion mechanism to fuse two small-size $W$ states into a large-scale $W$ state without qubit loss, and thus we call it qubit-loss-free (QLF) fusion mechanism. This QLF fusion mechanism is based on a two-outcome positive-operator valued measurement on two qubits extracting from two small-size $W$ states, and works for both pure and mixed $W$ states. The QLF nature of this fusion mechanism clearly increases the final size of the obtained $W$ state, and greatly reduces the number of fusion steps as well as the requirement of quantum memories required to achieve a $W$ state of a target size, so it is more efficient and feasible than the currently existing fusion schemes. There is no complete failure output in our QLF fusion scheme, and all the garbage states are recyclable. Two example schemes are proposed to realize this QLF fusion mechanism in a cavity quantum electrodynamics system and three-qubit Heisenberg XYZ model, respectively, which demonstrates the possibility of the physical realization of this QLF fusion mechanism.

We present a deterministic scheme for generating large-scale atomic W states in cavity QED system via a simple expansion mechanism, which is realized only by a detuned interaction between two identical atoms and a vacuum cavity mode. With the presented scheme, a W-type Bell pair can be created and an n-atom W state can be expanded to a 2n-atom W state with a unit probability of success in principle. No multi-atom gates, quantum memories or quantum non-demolition measurements are required, greatly simplifying the experimental realization of the scheme. The feasibility analysis shows that our expansion scheme can be implemented with state-of-the-art technologies. Our scheme enables advances not only in quantum information and communication but also in quantum thermodynamics, where atomic W states plays a crucial role., Comment: 5 pages, 1 color figure