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Determining if a quantum state is separable or entangled is a complex problem. Proposed schemes usually employ the von Neumann entropy, the positivity of the partial transpose or various inequalities. Determining the separability of the output of an optical system is an even more involved task. In this paper, we find necessary conditions for the output separability of a SU(2) system. We first discuss some previously reported criteria, showing their limitations. We then introduce our covariance-based criterion and assess its performance. For a system having one input port in the vacuum state, we arrive at a very simple criterion for the output separability. For pure input states, we introduce an artifice to easily assess the output separability with the help of the two-parameter quantum Fisher information. Applications of the proposed criterion are given by evaluating the output separability of a non-balanced beam splitter, fed by a number of input states. [ABSTRACT FROM AUTHOR]

The paper presents a classical and quantum description of the diffraction of two light beams incident on an ultrasonic wave at a positive and negative Bragg angle. The quantum description of the interaction with the ultrasonic wave was carried out for the two-photon NOON state. Study of diffraction of entangled pair of photon was performed in Mach–Zehnder interferometer wherein an output beam splitter is replaced by ultrasonic wave. It has been shown theoretically and experimentally that when a pair of photons generated from a parametric down-conversion was incident on two input ports of Mach–Zehnder interferometer with an acousto-optical beam splitter, then two-photon beats are observed at the output of the interferometer. The phenomenon of two-photon beats with the double frequency of the ultrasonic wave is the result of the Doppler effect on the ultrasonic wave. Time-correlated single-photon counting method was used to register the phenomenon of two-photon beats. The method of calibrating the interferometer with the use of an additional light source was presented in detail, which guarantees the observation of interaction of two-photon NOON state with ultrasonic wave. [ABSTRACT FROM AUTHOR]

In this paper, we study a system consisting of two spatially separated cavities, where each cavity contains a magnon mode of YIG sphere coupled to a microwave cavity mode via a linear beam splitter interaction. The two cavities are driven by two-mode squeezed vacuum field. In (Yu et al. in J. Phys. B: At. Mol. Opt. Phys. 53:065402, 2020), it has been investigated about the logarithmic negativity as a measure of quantum entanglement between two magnon modes versus various system parameters. Motivated by this, we will look at two different types of quantum correlations (i.e., entanglement and discord) in two-mode Gaussian subsystems (cavity–cavity modes and magnon–magnon modes). We analyze the robustness of these correlations with respect to the physical and environmental parameters—temperature, squeezing and the cavity–magnon coupling—of the two studied subsystems. For this, we use the Gaussian Bures distance to quantify entanglement and the Gaussian geometric discord (GGD) to quantify correlations beyond entanglement. The entanglement of the two bi-mode subsystems proves to be more sensitive to thermal noise. In particular, under the effect of temperature, the magnon–magnon entanglement degrades much more than the cavity–cavity entanglement. In addition, the GGD is found to be more robust—in both subsystems—against thermal noise, and it can be detected even for high values of temperatures. Also, we show that nonzero quantum correlations can be captured even when entanglement vanishes completely in the two studied subsystems. Finally, two different types of entanglement transfer (i.e., light → light and light → matter) have been observed in the studied system. [ABSTRACT FROM AUTHOR]

The pulse length and off-resonance errors in an atom interferometry caused by the laser parameters drift, the atomic velocity, and the electromagnetic field environment will degrade the performance of the atom interferometry. To improve the accuracy of an atom interferometry, these errors should be eliminated or compensated. In this paper, we proposal an approach to optimize the splitting and mirror Gaussian pulses to improve the tolerance for the interferometry environment. The numerical results show that not only the fidelity of the atomic state obtained under the interaction of a Gaussian pulse with atoms is higher than that gotten under the rectangular pulse atomic interaction, but also the fidelity under the interaction of the optimized Gaussian pulse is the highest and the bandwidth of the fidelity is the largest. Furthermore, the Gaussian pulse sequence consisting of optimized Gaussian pulses can effectively improve the interference fringe contrast of the atom interferometry. [ABSTRACT FROM AUTHOR]

It is of great significance to measure tiny difference frequency precisely, which has important application value for Doppler velocity measurement. However, the systematic error caused by non-ideal detectors during measurements limits the improvement of precision, and the systematic error cannot be reduced by increasing the number of measurements. Aiming at the systematic error, the weak value amplification scheme can reduce it effectively. This paper proposes a precise measurement scheme for tiny difference frequency between Fock state photons based on weak value amplification technology. Meanwhile, we use the theory of error to discuss the measurement precision of this scheme, and finish simulation research on the theoretical basis. Simulation results show that weak value amplification can reduce the systematic error of the measurement system and improve the response sensitivity of non-ideal detectors. By adjusting the transmission and reflection coefficients of the beam splitter in our scheme, the response sensitivity of difference frequency can improve 1000 times compared with that of classical schemes. When the systematic error of the measurement system is the main source of error in the measurement process, the weak value amplification scheme can obtain higher precision. [ABSTRACT FROM AUTHOR]