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First experimental test of Bell inequalities performed using a non-maximally entangled state

Istituto Elettrotecnico, Nazionale Galileo Ferraris, Str Delle Cacce, I- Torino, E. Predazzi

Published Jan 1, 2000• 1 citation
OpenAlex

Abstract

We report on the realisation of a new test of Bell inequalities using thesuperposition of type I parametric down conversion produced in two differentnon-linear crystals pumped by the same laser, but with different polarisation.The produced state is non-maximally entangled. We discuss the advantages andthe possible developments of this configuration.

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•Jan 1, 2015•0 citations
We report on the realisation of a new test of Bell inequalities using thesuperposition of type I parametric down conversion produced in two differentnon-linear crystals pumped by the same laser, but with different polarisation.The produced state is non-maximally entangled. We discuss the advantages andthe possible developments of this configuration.
We report on a test of Bell inequalities using a non-maximally entangled state, which represents an important step in the direction of eliminating the detection loophole. The experiment is based on the creation of a polarisation entangled state via the superposition, by use of an appropriate optics, of the spontaneous fluorescence emitted by two non-linear crystals driven by the same pumping laser. The alignment has profitably taken advantage from the use of an optical amplifier scheme, where a solid state laser is injected into the crystals together with the pumping laser. In principle a very high total quantum efficiency can be reached using this configuration and thus the final version of this experiment can lead to a resolution of the detection loophole, we carefully discuss the conditions which must be satisfied for reaching this result.
In the last few years the parametric down-conversion of light has attracted the attention of the researchers in foundations of quantum physics. The photons that come out from the non-linear crystal are highly correlated in many variables. One of the most important consequences of entanglement is the possibility of violating the so called Einstein locality expressed in Bell's inequality. In general factorizable state vectors do not violate Bell's inequality, but we will show that the factorizable state vector produced with a beam-splitter set on the paths of parametric down-conversion photons violates a new inequality that can be deduced from local realism. This inequality is more stringent than those used up to now to test the locality.
The purpose of this project is to set up a system for creating entangled photon pairs for use in various quantum mechanical experiments, including the testing of Bell’s inequalities which demonstrates the fundamentally indeterministic nature of quantum mechanics. The photons are created by pumping a beta barium borate crystal with 405 nm laser light then, due to a process called type 1 spontaneous parametric down conversion, a single pump photon can be converted into two 810 nm photons in an entangled state. In the same lab, a similar set up has already been achieved using a 474 nm pump laser, but by using a shorter wave length, a higher detection rate can be achieved. Two collimators are set up to collect the down converted photons and send them to two photon counting modules. The collimators need to be positioned precisely to detect any down converted photons because they emerge from the crystal in a narrow beam that is at an angle with respect to the pump beam and only a miniscule fraction of pump photons are down converted. The angle at which the down converted photons emerge is a function of how the crystal is prepared and the polarization orientation of the pump beam. It was calculated using a program by National Institute of Standards and Technology.