Conventional interferometers usually utilize beam splitters for wave splitting and recombination. These interferometers are widely used for precision measurement. Their sensitivity for phase measurement is limited by the shot noise, which can be suppressed with squeezed states of light. Here we study a new type of interferometer in which the beam splitting and recombination elements are parametric amplifiers. We observe an improvement of 4.1±0.3 dB in signal-to-noise ratio compared with a conventional interferometer under the same operating condition, which is a 1.6-fold enhancement in rms phase measurement sensitivity beyond the shot noise limit. The improvement is due to signal enhancement. Combined with the squeezed state technique for shot noise suppression, this interferometer promises further improvement in sensitivity. Furthermore, because nonlinear processes are involved in this interferometer, we can couple a variety of different waves and form new types of hybrid interferometers, opening a door for many applications in metrology.

We construct an interferometer with parametric amplifiers as beam splitters. Because of the gain in the parametric amplifiers, the maximum output intensity of the interferometer can be much bigger than the input intensity as well as the intensity inside the interferometer (the phase sensing intensity). We find that the fringe intensity depends quadratically on the intensity of the phase sensing field at high gain. This type of nonlinear interferometer has better sensitivity than the traditional linear interferometer made of beam splitters with the same phase sensing intensity.

Using a nondegenerate four-wave mixing process in hot rubidium vapor, we demonstrate a compact diode-laser-pumped system for the generation of intensity-difference squeezing down to 8 kHz with a maximum squeezing of -7 dB. To the best of our knowledge, this is the first demonstration of kilohertz-level intensity-difference squeezing using a semiconductor laser as the pump source. This scheme is of interest for experiments involving atomic ensembles, quantum communications, and precision measurements. The diode-laser-pumped system would extend the range of possible applications for squeezing due to its low cost, ease of operation, and ease of integration.

We experimentally demonstrate the creation of two correlated beams generated by a nondegenerate four-wave-mixing amplifier at λ=795 nm in hot rubidium vapor. We achieve intensity difference squeezing at frequencies as low as 1.5 kHz which is so far the lowest frequency to observe squeezing in an atomic system. The squeezing spans from 5.5 to 16.5 MHz with a maximum squeezing of -5 dB at 1 MHz. We can control the squeezing bandwidth by changing the pump power. Both low frequency and controllable bandwidth squeezing show great potential in sensitivity detection and precise control of the atom optics measurement.

-In this study, we investigate the phonon antibunching effect in a coupled nonlinear micro/nanoelectromechanical system (MEMS/NEMS) resonator at a finite temperature. In the weak driving limit, the optimal condition for phonon antibunching is given by solving the stationary Liouville-Von Neumann master equation. We show that at low temperature, the phonon antibunching effect occurs in the regime of weak nonlinearity and mechanical coupling, which is confirmed by analytical and numerical solutions. We also find that thermal noise can degrade or even destroy the antibunching effect for different mechanical coupling strengths. Furthermore, a transition from strong antibunching to bunching for phonon correlation has been observed in the temperature domain. Finally, we find that a suitably strong driving in the finite-temperature case would help to preserve an optimal phonon correlation against thermal noise.Introduction. -Quantum state transfer and storage are crucial in quantum information processing. To date, the photon has been the information carrier most commonly used to transfer and store quantum information, and it has the advantage of high velocity, robustness to different environments, and good integrability. However, phonons, which are vibrational modes of mechanical resonators, can be maintained for a very long time before being eventually damped, and they have the ability to interact with a wide range of quantum systems, such as electric, magnetic, and optical systems. Therefore, phonons also have promising potential as quantum information carriers [1][2][3].In quantum phononic networks, the nonclassical states of phonons or of a single phonon are important elements. Many methods to prepare nonclassical states of phonons or a single phonon have been proposed. For example, a single-phonon Fock state is prepared by two-phonon damping [4], a non-Gaussian state of a mechanical resonator is generated by performing measurements [5], and a single phonon is produced by the heralded measurement of the Stokes photon in cavity optomechanics [6,7].

Two four-wave mixing processes have been employed to experimentally construct a nonlinear interferometer [Jing et al., Appl. Phys. Lett. 99, 011110 (2011)], which has a better phase sensitivity than the traditional linear interferometer. For its applications in quantum measurement, interference fringe visibility can significantly affect the quantum detection efficiency. In this letter, we study how various parameters, such as the pump power, the one-photon detuning, and the two-photon detuning, influence the visibility of nonlinear interferometer. We find that the visibility greater than 0.9 can be achieved for large range of system parameters.

Based on the conditional measurement and beam splitters, multi-photon catalysis is introduced to each mode of even entangled coherent states (EECSs). The resulted states belong to a kind of Laguerre polynomials excited EECSs. Then the entanglement properties of resulted state are examined according to EPR correlation, concurrence and the teleportation fidelity. For the symmetrical beam splitters, these three quantities can be improved by multi-photon catalysis (⩾3) and only EPR correlation can be enhanced by zero-photon catalysis. Our optimal results over transmissivities of beam splitters show that the unit concurrence can be achieved for all non-zero photon catalysis which indicates that the optimized output approaches Belltype state. In addition, two single-photon catalysis and zero-photon catalysis present better performance for the improvement of both the EPR correlation and the teleportation fidelity in small and large amplitude regions, respectively. Particularly, it is interesting to notice that both of them can be optimized to be constant for zero-photon catalysis when the amplitude exceeds a certain threshold.

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