Applications of correlation and quantum entanglement to optical measurement

Akito Kawasaki, Hector Brunel, Ryuhoh Ide +10 more

Entanglement is a fundamental resource for various optical quantum information processing (QIP) applications. To achieve high-speed QIP systems, entanglement should be encoded in short wavepackets. Here we report the real-time observation of ultrafast optical Einstein–Podolsky–Rosen correlation at a picosecond timescale in a continuous-wave system. Optical phase-sensitive amplification using a 6-THz-bandwidth waveguide-based optical parametric amplifier enhances the effective efficiency of 70-GHz-bandwidth homodyne detectors, mainly used in 5G telecommunication, enabling its use in real-time quantum state measurement. Although power measurement using frequency scanning, such as an optical spectrum analyser, is not performed in real time, our observation is demonstrated through the real-time amplitude measurement and can be directly used in QIP applications. The observed Einstein–Podolsky–Rosen states show quantum correlation of 4.5 dB below the shot-noise level encoded in wavepackets with 40 ps period, equivalent to 25 GHz repetition—103 times faster than previous entanglement observation in continuous-wave systems. The quantum correlation of 4.5 dB is already sufficient for several QIP applications, and our system can be readily extended to large-scale entanglement. Moreover, our scheme has high compatibility with optical communication technology such as wavelength-division multiplexing, and femtosecond-timescale observation is also feasible. Our demonstration is a paradigm shift in accelerating accessible quantum correlation—the foundational resource of all quantum applications—from the nanosecond to picosecond timescales, enabling ultrafast optical QIP. Optical Einstein–Podolsky–Rosen correlation is observed on a picosecond timescale in a continuous-wave system. By introducing waveguide optical parametric amplifiers and balanced detectors, the quantum correlation 4.5 dB below the shot-noise level is observed.

S. Ya. Kilin, Леонід Петрович Яценко

Jun Jia, Túlio Brito Brasil, Maimouna Bocoum +6 more

Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.

F. J. Duarte

David S. Simon, Gregg S. Jaeger, Alexander V. Sergienko

Maksim A. Smirnov, A. F. Khairullin, Sergey A. Moiseev

Abstract Quantum-entangled two-photon fields (biphotons) are attracting increasing interest in practical applications based on high photon correlation. Using a tunable fiber optic biphoton source, it was demonstrated that the performance of a quantum optical system at various wavelengths can be evaluated by correlation measurements. It was established that the results of such measurements, particularly those related to the efficiency of the detectors used, depend significantly on the optical losses in the system.