Engineering the Frequency Spectrum of Bright Squeezed Vacuum via Group Velocity Dispersion in an SU(1,1) Interferometer

Nonlinear Interferometry for Quantum-Enhanced Measurements of Multiphoton Absorption

Multiphoton absorption is of vital importance in many spectroscopic, microscopic, or lithographic applications. However, given that it is an inherently weak process, the detection of multiphoton absorption signals typically requires large field intensities, hindering its applicability in many practical situations. In this Letter, we show that placing a multiphoton absorbent inside an imbalanced nonlinear interferometer can enhance the precision of multiphoton cross section estimation with respect to strategies based on photon-number measurements using coherent or even squeezed light directly transmitted through the medium. In particular, the power scaling of the sensitivity with photon flux can be increased by 1 order compared with transmission measurements of the sample with coherent light, such that the measurement precision at any given intensity can be greatly enhanced. Furthermore, we show that this enhanced measurement precision is robust against experimental imperfections leading to photon losses, which usually tend to degrade the detection sensitivity. We trace the origin of this enhancement to an optimal degree of squeezing which has to be generated in a nonlinear SU(1,1) interferometer.

Phase Sensitivity Improvement in Correlation-Enhanced Nonlinear Interferometers

Interferometers are widely used as sensors in precision measurement. Compared with a conventional Mach–Zehnder interferometer, the sensitivity of a correlation-enhanced nonlinear interferometer can break the standard quantum limit. Phase sensitivity plays a significant role in the enhanced performance. In this paper, we review improvement in phase estimation technologies in correlation-enhanced nonlinear interferometers, including SU(1,1) interferometer and SU(1,1)-SU(2) hybrid interferometer, and so on, and the applications in quantum metrology and quantum sensing networks.

Two-Colour Spectrally Multimode Integrated SU(1,1) Interferometer

Multimode integrated interferometers have great potential for both spectral engineering and metrological applications. However, the material dispersion of integrated platforms constitutes an obstacle that limits the performance and precision of such interferometers. At the same time, two-colour nonlinear interferometers present an important tool for metrological applications, when measurements in a certain frequency range are difficult. In this manuscript, we theoretically developed and investigated an integrated multimode two-colour SU(1,1) interferometer operating in a supersensitive mode. By ensuring the proper design of the integrated platform, we suppressed the dispersion, thereby significantly increasing the visibility of the interference pattern. The use of a continuous wave pump laser provided the symmetry between the spectral shapes of the signal and idler photons concerning half the pump frequency, despite different photon colours. We demonstrate that such an interferometer overcomes the classical phase sensitivity limit for wide parametric gain ranges, when up to 3×104 photons are generated.

Deterministic distribution of multipartite entanglement in a quantum network by continuous-variable polarization states

Quantum network plays a vitally important role in the practical application of quantum information, which requires the deterministic entanglement distribution among multiple remote users. Here, we propose a feasible scheme to deterministically distribute quadripartite entanglement by continuous-variable (CV) polarization states. The quantum server prepares the quadripartite CV polarization entanglement and distributes them to four remote users via optical fiber. In this way, the measurement of CV polarization entanglement is local oscillation free, which makes the long distance entanglement distribution in commercial optical fiber communication networks possible. Furthermore, both the Greenberger-Horne-Zeilinger-like (GHZ-like) and cluster-like polarization entangled states can be distributed among four users by controlling the beam splitter network in quantum server, which are confirmed by the extended criteria for polarization entanglement of multipartite optical modes. The protocol provides the direct reference for experimental implementation and can be directly extended to quantum network with more users, which is essential for a metropolitan quantum network.

Degenerate four-wave-mixing as a low-power source of squeezed light

Squeezed light is a quantum resource that can improve the sensitivity of optical measurements. However, existing sources of squeezed light generally require high powers and are not amenable to portability. Here we theoretically investigate an alternative technique for generating squeezing using degenerate four-wave-mixing in atomic vapors. We show that by minimizing excess noise, this technique has the potential to generate measurable squeezing with low powers attainable by a small diode laser. We suggest experimental techniques to reduce excess noise and employ this alternative nonlinear optical process to build a compact, low-power source of squeezed light.

Absolute sensitivity of phase measurement in an SU(1,1) type interferometer

Absolute sensitivity is measured for the phase measurement in an SU(1,1) type interferometer, and the results are compared to that of a Mach-Zehnder interferometer operated under the condition of the same intra-interferometer intensity. The interferometer is phase locked to a point with the largest quantum noise cancellation, and a simulated phase modulation is added in one arm of the SU(1,1) interferometer. Both the signal and noise level are estimated at the same frequency range, and we obtained 3 dB improvement in sensitivity for the SU(1,1) interferometer over the Mach-Zehnder interferometer. Our results demonstrate a direct phase estimation and may pave the way for practical applications of a nonlinear interferometer.

Broadband bright twin beams and their upconversion

We report on the observation of broadband (40 THz) bright twin beams through high-gain parametric downconversion in an aperiodically poled lithium niobate crystal. The output photon number is shown to scale exponentially with the pump power and not with the pump amplitude, as in homogeneous crystals. Photon number correlations and the number of frequency/temporal modes are assessed by spectral covariance measurements. By using sum-frequency generation on the surface of a non-phase-matched crystal, we measure a cross-correlation peak with the temporal width of 90 fs.