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arXiv:quant-ph/0206166 · Physical Review A

Experimental preparation of the Werner state via spontaneous parametric down-conversion

Yong-Sheng Zhang, Yun‐Feng Huang, Chuan‐Feng Li, Guang‐Can Guo

Published Dec 26, 2002• 65 citations
arXivHTMLPDF

Abstract

We present an experiment for preparing a Werner state via spontaneous parametric down-conversion and controlled decoherence of photons in this paper. In this experiment two independent $\ensuremath{\beta}$-barium borate crystals are used to produce down-conversion light beams, which are mixed to prepare the Werner state.

DOI
10.1103/physreva.66.062315

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Physical Review A•Aug 27, 2004•67 citations
We present an experiment for preparing a Werner state via spontaneous parametric down-conversion and controlled decoherence of photons in this paper. In this experiment two independent \ensuremathβ\ensuremath{\beta}\ensuremathβ-barium borate crystals are used to produce down-conversion light beams, which are mixed to prepare the Werner state.
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.
We report a Bell state preparation experiment using polarization-entangled photon pairs which are produced by spontaneous parametric down conversion of a 405 nm pulse laser beam inside two orthogonally oriented 0.1 mm beta-barium borate nonlinear crystals with Type-I phased matching. The degree and phase of all four Bell states are easily tunable. With two-photon interference visibilities in excess of 85%, we observe that the Bell parameter S for any of the four Bell states exceeds the classical limit in the Clauser–Horne–Shimony–Holt form of Bell inequality by five standard deviations.
A parametric source of polarization-entangled photon pairs with striking spatial characteristics is reported. The distribution of the output electromagnetic k\mathbf{k}k modes excited by spontaneous parametric down-conversion and coupled to the output detectors can be very broad. Using these states realized over a full entanglement ring output distribution, the nonlocal properties of the generated entanglement have been tested by standard Bell measurements and by Ou-Mandel interferometry. A ``mode-patchwork'' technique based on the quantum superposition principle is adopted to synthesize in a straightforward and reliable way any kind of mixed state, of large conceptual and technological interest in modern quantum information. Tunable Werner states and maximally entangled mixed states have indeed been created by this technique and investigated by quantum tomography. A study of the entropic and nonlocal properties of these states has been undertaken experimentally and theoretically, by a unifying variational approach.