Polarization Entanglement distribution between two remote locations: first stage of an experiment to study the foundations of Quantum Mechanics

N. SANTOS; C. BONAZZOLA; M.AGUERO; NONAKA M.; A. HNILO; A. VILLANUAVA; M. KOVALSKY

Pucón

Congreso; IX Reunión Iberoamericana de Optica y XII Reunión Iberoamericana de Optica, Láseres y Aplicaciones; 2016

Centro de Optica y Fotonica, Universidad de Concepción

A remarkable feature of Quantum Mechanics is its contradiction with the intuitive notions of Locality and Realism. Experiments verifying the violation of Bell?s inequalities are the main tools to decide whether QM or LR is valid in the Nature. Our Project consists on testing whether there are transient deviations from the QM predictions, when the Bell measurement is performed in a time shorter than the one the light needs to cover the spatial spread of the entangled state. It is then necessary to transmit entangled pairs of photons to remote stations, and to get time-resolved records of the detections. The techniques developed for this experiment are also useful for Quantum Key Distribution. In this work we report preliminary results, which include time-resolved measurement of entanglement, practical methods to calculate the number of accidental coincidences in the pulsed pump regime, and techniques to transmit entanglement through optical fibers. The entangled photons are generated at the efficiency maximum of silicon photodiodes (710 nm) by pumping a set of two crossed BBO-I crystals with the third harmonic of a diode-pumped actively Q-switched Nd:YVO4 laser emitting well-separated pulses (ns duration at a repetition of kHz). The laser was designed and built in our lab. The pulsed regime has intrinsic problems arising from the simultaneous arrival of signal (entangled) and noise photons to the detectors, which make unsuitable the methods developed to calculate the number of accidental coincidences for the continuous-wave or mode-locked pump regimes. These problems are analyzed, and new methods are derived and checked in the practice. These new methods are useful not only to measure entanglement, but also in every situation where extracting the number of valid two-photon coincidences from noisy data generated by such a pulsed process is required. When a photon propagates through an optical fiber, its polarization state changes due to birefringence. Yet, it is possible recovering the initial state by applying controlled mechanical stress on the fiber. We present an experimental study of birefringence compensation in a 50 m coil of 3.5 μm core SM fiber by using manual controllers and an innovative device to help their adjustment. We test the performance of the system with two optical sources: a diode laser at 635 nm and a LED at the wavelength of the entangled photon pairs, as a first step before using entangled pairs.