Beam shifts controlled by orbital angular momentum in a guided-surface plasmon resonance structure with a four-level atomic medium.

We proposed a scheme to realize tunable giant Goos-Hänchen (GH) and Imbert Fedorov (IF) shifts of the Laguerre-Gauss (LG) beam on a guided-wave surface plasmon resonance (GWSPR) structure backed by a coherent atomic medium with the spontaneously generated coherence (SGC) effect. The orbital angular momentum carried by the incident LG beam can be applied to enhance and control IF shifts but is not beneficial to GH shifts. However, in the presence of SGC effect in the atomic medium, both GH and IF shifts can be simultaneously enhanced and well controlled. With the SGC effect, the linear absorption of the atomic medium vanishes, while the nonlinear absorption of that can be significantly enhanced and controlled by the trigger field, which contributes to controlling of the beam shifts. In particular, the direction of GH shifts can be switched by the Rabi frequency of the trigger field, which can be interpreted as the result of a competition between the inherent damping and the radiative damping corresponding to the nontrivial change in the loci of the reflection coefficients. This scheme provides an effective method to flexibly control and enhance the beam shifts, so it has potential applications in integrated optics, optical sensors, etc.

Quantum-improved phase estimation with a displacement-assisted SU(1,1) interferometer.

By performing two local displacement operations (LDOs) inside an SU(1,1) interferometer, called as the displacement-assisted SU(1,1) [DSU(1,1)], both the phase sensitivity based on homodyne detection and quantum Fisher information (QFI) with and without photon losses are investigated in this paper. In this DSU(1,1) interferometer, we focus our attention on the extent to which the introduced LDO affects the phase sensitivity and the QFI, even in the realistic scenario. Our analyses show that the estimation performance of DSU(1,1) interferometer is always better than that of SU(1,1) interferometer without the LDO, especially for the phase precision of the former in the ideal scenario closer to the Heisenberg limit via the increase of the LDO strength. Different from the latter, the robustness of the former can be also enhanced markedly by regulating and controlling the LDO. Our findings would open an useful view for quantum-improved phase estimation of optical interferometers.

Phase estimation of an SU(1,1) interferometer with a coherent superposition squeezed vacuum in a realistic case.

The phase sensitivity of SU(1,1) interferometer is investigated using a coherent state and an m-coherent superposition squeezed vacuum states as inputs and the intensity detection. Photon-subtraction, photon-addition and photon superposition are three special cases. Both ideal and realistic cases are considered. It is shown that the coefficient s of coherent superposition can modulate the performance of phase sensitivity, especially in a small squeezing region. Even in the presence of photon losses, the three-kind of non-Gaussian operations can achieve the improvement of measure precision, and the photon addition presents the best robustness compared to the photon subtraction and coherent superposition. For small squeezing, the first-order non-Gaussian operation may be the most preferred in improving phase sensitivity if considering the limitations of experimental conditions. Our results may be helpful for the practical application of quantum information.

Evaluating the quantum Ziv-Zakai bound for phase estimation in noisy environments.

In the highly non-Gaussian regime, the quantum Ziv-Zakai bound (QZZB) provides a lower bound on the available precision, demonstrating the better performance compared with the quantum Cramér-Rao bound. However, evaluating the impact of a noisy environment on the QZZB without applying certain approximations proposed by Tsang [Phys. Rev. Lett.108, 230401 (2012)10.1103/PhysRevLett.108.230401] remains a difficult challenge. In this paper, we not only derive the asymptotically tight QZZB for phase estimation with the photon loss and the phase diffusion by invoking the variational method and the technique of integration within an ordered product of operators, but also show its estimation performance for several different Gaussian resources, such as a coherent state (CS), a single-mode squeezed vacuum state (SMSVS) and a two-mode squeezed vacuum state (TMSVS). In this asymptotically tight situation, our results indicate that compared with the SMSVS and the TMSVS, the QZZB for the CS always shows the better estimation performance under the photon-loss environment. More interestingly, for the phase-diffusion environment, the estimation performance of the QZZB for the TMSVS can be better than that for the CS throughout a wide range of phase-diffusion strength. Our findings will provide an useful guidance for investigating the noisy quantum parameter estimation.