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We propose a scheme that optimizes entanglement in nanomechanical resonators through quantum state transfer of squeezed fields assisted by radiation pressure. The system is driven by red-detuned laser fields, which enable simultaneous cooling of the mechanical resonators while facilitating the transfer of quantum states in a weak coupling, sideband-resolved regime. The mechanical entanglement is quantified using logarithmic negativity within the bipartite Gaussian states of the two mechanical modes. Our results show that the degree of mechanical entanglement is strongly influenced by several key parameters, including the parametric phase and nonlinear gain of the non-degenerate OPA, the squeezing strength of the injected squeezed vacuum reservoir, optomechanical cooperativity (controlled by laser drive power), and the mechanical bath temperature (phonon thermal excitation). Additionally, our results indicate that careful tuning of these parameters can enhance entanglement robustness, suggesting that this optomechanical scheme holds promise for applications in quantum sensing and information processing., Comment: 10 pages, 7 figures

We address the dynamical bipartite entanglement in an opto-electro-mechanical system that involves a three-level atom. The system consists of a degenerate three-level atom, a mechanical resonator, an optical cavity, and a microwave cavity. By utilizing the linearization approximation and nonlinear quantum-Langevin equations, the dynamics of the system are analyzed, and the bipartite entanglement is evaluated using the logarithmic negativity. The research findings indicate that the entanglement between each subsystem increases with the atom injection rate, suggesting that a higher atom injection rate leads to enhanced information transmission between the subsystems. Additionally, it is observed that the correlation between subsystems increases with an increase in the coupling rate. Moreover, the study demonstrates that the correlation between each subsystem decreases as temperature rises. We also show that the degree of tripartite entanglement diminishes with increasing atomic decay rates. The results highlight the positive impact of three-level atoms on the bipartite entanglement in an opto-electro-mechanical system. Consequently, such electro-optomechanical systems can offer a framework for optomechanical information transfer., Comment: 11 pages, 7 figures

We address quantum metrology in critical spin chains with anisotropy and Dzyaloshinskii-Moriya (DM) interaction, and show how local and quasi-local measurements may be exploited to characterize global properties of the systems. In particular, we evaluate the classical (magnetization) and quantum Fisher information of the relevant parameters for the density matrix of a single spin and that of a pair of spins ranging from nearest to sixth-nearest neighbors, to the limiting case of very distant spins. Our results allow us to elucidate the role of the different parameters and to individuate the optimal working regimes for the precise characterization of the system, also clarifying the effects of correlations on the estimation precision., Comment: 12 pages, 10 figures

We investigate the role of nonlinearity via optical parametric oscillator on the entropy production rate and quantum correlations in a hybrid optomechanical system. Specifically, we derive the modified entropy production rate of an optical parametric oscillator placed in the optomechanical cavity which is well described by the two-mode Gaussian state. We find a dramatic deviation in the irreversibility and quantum mutual information for small detuning. Our analysis shows that the system irreversibility can be reduced by choosing the appropriate phase of the self-induced nonlinearity. We further demonstrate that the nonlinearity effect persist for a reasonable range of cavity decay rate., Comment: 8 pages 4 figures

In a hybrid quantum system composed of two quantum wells placed inside a cavity with a moving end mirror pumped by bichromatic coherent light, we address the formation of squeezed states of a mechanical resonator. The exciton mode and mechanical resonator interact indirectly via microcavity fields. Under the conditions of the generated coupling, we predict squeezing of the mechanical-mode beyond the resolved side-band regime with existing experimental parameters. Finally, we show that the robustness of this squeezing against thermal fluctuations is important for practical applications of such systems., Comment: 6 pages, 4 figures

We address multiparameter quantum estimation for coherently driven nonlinear Kerr resonators in the presence of loss. In particular, we consider the realistic situation in which the parameters of interest are the loss rate and the nonlinear coupling, whereas the amplitude of the coherent driving is known and externally tunable. Our results show that this driven-dissipative model is asymptotically classical, i.e. the Uhlmann curvature vanishes, and the two parameters may be jointly estimated without any additional noise of quantum origin. We also find that the ultimate bound to precision, as quantified by the quantum Fisher information (QFI), increases with the interaction time and the driving amplitude for both parameters. Finally, we investigate the performance of quadrature detection, and show that for both parameters the Fisher information oscillates in time, repeatedly approaching the corresponding QFI.

We propose a scheme to improve magnon-photon-phonon entanglement in cavity magnomechanics using coherent feedback loop. In addition, we prove that the steady state and dynamical state of the system is a genuine tripartite entanglement state. We use the logarithmic negativity as the witness of quantum correlations to quantify the entanglement of all bipartite subsystem, and genuine tripartite entanglement via the nonzero minimum residual contangle, in steady and dynamical regime. We consider the feasible experiment parameters to realize the tripartite entanglement. We show that the entanglement can be significantly improved with coherent feedback using a suitable tuning of the reflective parameter of the beam splitter. The entanglement is robust against the thermal effects. Our proposal scheme to improve the entanglement can be of interest to applications in quantum information., Comment: 7 pages and 5 figures

We address propagation and entanglement of Gaussian states in optical media characterised by non-trivial spectral densities. In particular, we consider environments with a finite bandwidth and show that in the low temperature regime: i) secular terms in the master equation may be neglected; ii) attenuation (damping) is strongly suppressed; iii) the overall diffusion process may be described as a Gaussian noise channel with variance depending only on the bandwidth. We find several regimes where propagation is not much detrimental and entanglement may be protected form decoherence.

We address the non-Markovian entanglement dynamics for bimodal continuous variable quantum systems interacting with two independent structured reservoirs. We derive an analytical expression for the entanglement of formation without performing the Markov and the secular approximations. We observe a variety of qualitative features such as entanglement sudden death, dynamical generation, and protection for two types of Lorentzian spectral densities, assuming the two modes initially excited in a twin-beam state. Our quantitative analysis shows that these cases with different reservoir spectrum, the environmental temperature and the initial amount of entanglement differ significantly in these qualitative features., Comment: 9 pages, 10 figures. arXiv admin note: text overlap with arXiv:0910.2342 by other authors

Active optical media leading to interaction Hamiltonians of the form $ H = \tilde{\lambda}\, (a + a^{\dagger})^{\zeta}$ represent a crucial resource for quantum optical technology. In this paper, we address the characterization of those nonlinear media using quantum probes, as opposed to semiclassical ones. In particular, we investigate how squeezed probes may improve individual and joint estimation of the nonlinear coupling $\tilde{\lambda}$ and of the nonlinearity order $\zeta$. Upon using tools from quantum estimation, we show that: i) the two parameters are compatible, i.e. the may be jointly estimated without additional quantum noise; ii) the use of squeezed probes improves precision at fixed overall energy of the probe; iii) for low energy probes, squeezed vacuum represent the most convenient choice, whereas for increasing energy an optimal squeezing fraction may be determined; iv) using optimized quantum probes, the scaling of the corresponding precision with energy improves, both for individual and joint estimation of the two parameters, compared to semiclassical coherent probes. We conclude that quantum probes represent a resource to enhance precision in the characterization of nonlinear media, and foresee potential applications with current technology.

The production and manipulation of quantum correlation protocols will play a central role where the quantum nature of the correlation can be used as a resource to yield properties unachievable within a classical framework is a very active and important area of research. In this work, we provide a description of a measure of correlation strength between quantum systems, especially for multipartite quantum systems., Comment: 14 pages, 3 figures

We address quantum $M$-ary phase-shift keyed (PSK) communication channels in the presence of phase diffusion, and analyze the use of probabilistic noiseless linear amplifiers (NLA) to enhance performance of coherent signals. We consider both static and dynamical phase diffusion and assess the performances of the channel for ideal and realistic phase receivers. Our results show that NLA employed at the stage of signal preparations is a useful resource, especially in the regime of weak signals. We also discuss the interplay between the use of NLA, and the memory effects occurring with dynamical noise, in determining the capacity of the channel., Comment: to appear in JOSAB

We address an experimental scheme to analyze the optical bistability and the entanglement of two movable mirrors coupled to a two-mode laser inside a doubly resonant cavity. With this aim we investigate the master equations of the atom-cavity subsystem in conjunction with the quantum Langevin equations that describe the interaction of the mirror cavity. The parametric amplification-type coupling induced by the two-photon coherence on the optical bistability of the intracavity mean photon numbers is found and investigated. Under this condition, the optical intensities exhibit bistability for all large values of cavity laser detuning. We also provide numerical evidence for the generation of strong entanglement between the movable mirrors and show that it is robust against environmental thermalization., Comment: 12 pages, 11 figures

We study theoretically the Bose-Einstein condensation in periodic polariton lattices in presence of a localized reservoir. Using the Schr\"odinger and Boltzmann equations, we find analytically the parameters determining the choice of the bandgap for condensation on the defect states.

We study Klein tunneling in polariton graphene. We show that the photonic spin-orbit coupling associated with the energy splitting between TE and TM photonic modes can be described as an emergent gauge field. It suppresses the Klein tunnelling in small energy range close to the Dirac points. Thanks to polariton spin-anisotropic interactions, polarized optical pumping allows to create potential barriers acting on a single polariton spin. We show that the resulting spin-dependent Klein tunneling can be used to create a perfectly transmitting polarization rotator operating at microscopic scale.

We address quantum communication channels based on phase modulation of coherent states and analyze in details the effects of static and dynamical (stochastic) phase diffusion. We evaluate mutual information for an ideal phase receiver and for a covariant phase-space-based receiver, and compare their performances by varying the number of symbols in the alphabet and/or the overall energy of the channel. Our results show that phase communication channels are generally robust against phase noise, especially for large alphabets in the low energy regime. In the presence of dynamical (non-Markovian) noise the mutual information is preserved by the time correlation of the environment, and when the noise spectra is detuned with respect to the information carrier, revivals of mutual information appears., Comment: 7 pages, 5 figures

We address quantitatively the relationship between the nonlinearity of a mechanical resonator and the nonclassicality of its ground state. In particular, we analyze the nonclassical properties of the nonlinear Duffing oscillator (being driven or not) as a paradigmatic example of a nonlinear nanomechanical resonator. We first discuss how to quantify the nonlinearity of this system and then show that the nonclassicality of the ground state, as measured by the volume occupied by the negative part of the Wigner function, monotonically increases with the nonlinearity in all the working regimes addressed in our study. Our results show quantitatively that nonlinearity is a resource to create nonclassical states in mechanical systems., Comment: 6 pages; 7 figures; RevTeX4-1

We address the quantification of nonlinearity for quantum oscillators and introduce two measures based on the properties of the ground state rather than on the form of the potential itself. The first measure is a fidelity-based one, and corresponds to the renormalized Bures distance between the ground state of the considered oscillator and the ground state of a reference harmonic oscillator. Then, in order to avoid the introduction of this auxiliary oscillator, we introduce a different measure based on the non-Gaussianity (nG) of the ground state. The two measures are evaluated for a sample of significant nonlinear potentials and their properties are discussed in some detail. We show that the two measures are monotone functions of each other in most cases, and this suggests that the nG-based measure is a suitable choice to capture the anharmonic nature of a quantum oscillator, and to quantify its nonlinearity independently on the specific features of the potential. We also provide examples of potentials where the Bures measure cannot be defined, due to the lack of a proper reference harmonic potential, while the nG-based measure properly quantify their nonlinear features. Our results may have implications in experimental applications where access to the effective potential is limited, e.g., in quantum control, and protocols rely on information about the ground or thermal state., Comment: 8 pages, 5 figures, published version

We address estimation of one-parameter unitary gates for qubit systems and seek for optimal probes and measurements. Single- and two-qubit probes are analyzed in details focusing on precision and stability of the estimation procedure. Bayesian inference is employed and compared with the ultimate quantum limits to precision, taking into account the biased nature of Bayes estimator in the non asymptotic regime. Besides, through the evaluation of the asymptotic a posteriori distribution for the gate parameter and the comparison with the results of Monte Carlo simulated experiments, we show that asymptotic optimality of Bayes estimator is actually achieved after a limited number of runs. The robustness of the estimation procedure against fluctuations of the measurement settings is investigated and the use of entanglement to improve the overall stability of the estimation scheme is also analyzed in some details., Comment: 10 pages, 5 figures