Detection of emitter-resonator coupling strength in quantum Rabi model via an auxiliary resonator

In this paper, we propose a theoretical scheme to detect the emitter-resonator coupling strength in the ultra-strong coupling regime in the quantum Rabi model via introducing an auxiliary resonator. We demonstrate the total system as a two-mode Rabi model and obtain the ground state by the transformed rotating wave approximation, which is shown to be superior to the usually applied rotating wave approximation. Here, the coupling strength is detected by monitoring the average excitation number in the auxiliary resonator and the sensitivity of the detection scheme is discussed analytically.Comment: 5 pages, accepted by PR

Coherence enhanced quantum metrology in a nonequilibrium optical molecule

We explore the quantum metrology in an optical molecular system coupled to two environments with different temperatures, using a quantum master equation beyond secular approximation. We discover that the steady-state coherence originating from and sustained by the nonequilibrium condition can enhance quantum metrology. We also study the quantitative measures of the nonequilibrium condition in terms of the curl flux, heat current and entropy production at the steady state. They are found to grow with temperature difference. However, an apparent paradox arises considering the contrary behaviors of the steady-state coherence and the nonequilibrium measures in relation to the inter-cavity coupling strength. This paradox is resolved by decomposing the heat current into a population part and a coherence part. Only the latter, coherence heat current, is tightly connected to the steady-state coherence and behaves similarly with respect to the inter-cavity coupling strength. Interestingly, the coherence heat current flows from the low-temperature reservoir to the high-temperature reservoir, opposite to the direction of the population heat current. Our work offers a viable way to enhance quantum metrology for open quantum systems through steady-state coherence sustained by the nonequilibrium condition, which can be controlled and manipulated to maximize its utility. The potential applications go beyond quantum metrology and extend to areas such as device designing, quantum computation and quantum technology in general.Comment: The errors are corrected, analytical results are added, acceptec by New J. Phy

Cooperation percolation in spatial prisoner's dilemma game

The paradox of cooperation among selfish individuals still puzzles scientific communities. Although a large amount of evidence has demonstrated that cooperator clusters in spatial games are effective to protect cooperators against the invasion of defectors, we continue to lack the condition for the formation of a giant cooperator cluster that assures the prevalence of cooperation in a system. Here, we study the dynamical organization of cooperator clusters in spatial prisoner's dilemma game to offer the condition for the dominance of cooperation, finding that a phase transition characterized by the emergence of a large spanning cooperator cluster occurs when the initial fraction of cooperators exceeds a certain threshold. Interestingly, the phase transition belongs to different universality classes of percolation determined by the temptation to defect $b$. Specifically, on square lattices, $1<b<4/3$ leads to a phase transition pertaining to the class of regular site percolation, whereas $3/2<b<2$ gives rise to a phase transition subject to invasion percolation with trapping. Our findings offer deeper understanding of the cooperative behaviors in nature and society

Phase controlled single-photon nonreciprocal transmission in a one-dimensional waveguide

We study the controllable single-photon scattering via a one-dimensional waveguide which is coupled to a two-level emitter and a single-mode cavity simultaneously. The emitter and the cavity are also coupled to each other and form a three-level system with cyclic transitions within the zero- and single-excitation subspaces. As a result, the phase of emitter-cavity coupling strength serves as a sensitive control parameter. When the emitter and cavity locate at the same point of the waveguide, we demonstrate the Rabi splitting and quasidark-state--induced perfect transmission for the incident photons. More interestingly, when they locate at different points of the waveguide, a controllable nonreciprocal transmission can be realized and the non-reciprocity is robust to the weak coupling between the system and environment. Furthermore, we demonstrate that our theoretical model is experimentally feasible with currently available technologies.Comment: 11 pages, 8 figures,Accepted by Phys. Rev.

Single-photon scattering and bound states in an atom-waveguide system with two or multiple coupling points

In this paper, we investigate the single-photon scattering and bound states in a one-dimensional coupled-resonator waveguide which couples to a single artificial giant atom with two or more coupling points. When the atom couples to the waveguide via two resonators, the single-photon reflection rate is characterized by either Breit-Wigner or Fano line shapes. When the atom couples to the waveguide via multiple resonators, we numerically show how the destructive interference effect leads to a complete single-photon reflection. We also find a phase transition phenomena for the multi-resonator coupling case, which reveals that the upper bound state only exists when the atom-waveguide coupling strength is above a critical value.Comment: 8 pages, 7 figures, Accepted by Phys. Rev.

Decentralized Approximate Newton Methods for Convex Optimization on Networked Systems

In this paper, a class of Decentralized Approximate Newton (DEAN) methods for addressing convex optimization on a networked system are developed, where nodes in the networked system seek for a consensus that minimizes the sum of their individual objective functions through local interactions only. The proposed DEAN algorithms allow each node to repeatedly perform a local approximate Newton update, which leverages tracking the global Newton direction and dissipating the discrepancies among the nodes. Under less restrictive problem assumptions in comparison with most existing second-order methods, the DEAN algorithms enable the nodes to reach a consensus that can be arbitrarily close to the optimum. Moreover, for a particular DEAN algorithm, the nodes linearly converge to a common suboptimal solution with an explicit error bound. Finally, simulations demonstrate the competitive performance of DEAN in convergence speed, accuracy, and efficiency

Steady-state entanglement and coherence of the coupled qubit system in equilibrium and nonequilibrium environments

We investigate analytically and numerically the steady-state entanglement and coherence of two coupled qubits each interacting with a local boson or fermion reservoir, based on the Bloch-Redfield master equation beyond the secular approximation. We find that there is non-vanishing steady-state coherence in the nonequilibrium scenario, which grows monotonically with the nonequilibrium condition quantified by the temperature difference or chemical potential difference of the two baths. The steady-state entanglement in general is a non-monotonic function of the nonequilibrium condition as well as the bath parameters in the equilibrium setting. We also find that weak inter-qubit coupling and high base temperature or chemical potential of the baths can strongly suppress the steady-state entanglement and coherence, regardless of the strength of the nonequilibrium condition. On the other hand, the energy detuning of the two qubits, when used in a compensatory way with the nonequilibrium condition, can lead to significant enhancement of the steady-state entanglement in some parameter regimes. In addition, the qubits typically have a stronger steady-state entanglement when coupled to fermion baths exchanging particle with the system than boson baths exchanging energy with the system under similar conditions. We also discussed the possible experimental realization of measuring the steady state entanglement and coherence for coupled qubits systems in nonequilibrium environments. These results offer some general guidelines for optimizing the steady-state entanglement and coherence in the coupled qubit system and may find potential applications in quantum information technology.Comment: 21 pages, 9 figures, comments are welcomed, Accepted by Phys. Rev.

Quantum interferometry for rotation sensing in an optical microresonator

We theoretically propose a scheme to perform rotation sensing in a Whispering-gallery-mode resonator setup. With the assistance of a large detuned two-level atom, which induces the effective coupling between clockwise and counterclockwise propagating modes in the resonator, we realize an effective interferometry with SU(2) algebraic structure. By studying the quantum Fisher information of the system, we find that the estimate accuracy for the angular velocity of the rotation can achieve and even break the Heisenberg limit in linear and nonlinear setup, respectively. The high performance of quantum metrology is proved to be associated with the state compressibility during the time evolution. We hope that our investigation will be useful in the design of a quantum gyroscope based on spinning resonators

Ghost imaging for an occluded object

Imaging for an occluded object is usually a difficult problem, in this letter, we introduce an imaging scheme based on computational ghost imaging, which can obtain the image of a target object behind an obstacle. According to our theoretical analysis, once the distance between the object and the obstacle is far enough, one can obtain the image of the object by using ghost imaging technique. The wavelength of the light source also affects the quality of the reconstructed image. In addition, if the bucket detector is placed far away from the obstacle, a tiny point-like detector without collecting lens can be applied to realize the imaging. These theoretical results above have been verified with our numerical simulations. Furthermore, the robustness of this imaging scheme is also investigated.Comment: 12 pages, 6 figure

The quasi-dark state and quantum interference in Jaynes-Cummings model with a common bath

Within the capacity of current experiments, we design a composite atom-cavity system with a common bath, in which the decay channels of the atom and the cavity mode interfere with each other. When the direct atom-cavity coupling is absent, the system can be trapped in a quasi-dark state (the coherent superposition of excited states for the atom and the cavity mode) without decay even in the presence of the bath. When the atom directly couples with the cavity, the largest decay rate of the composite system will surpass the sum of the two subsystems while the smallest decay rate may achieve 0. This is manifested in the transmission spectrum, where the vacuum Rabi splitting shows an obvious asymmetric character.Comment: 8 pages, accepted by PR