Experimental Detection of Entanglement with Polarized Photons

We report on the first experimental realization of the entanglement witness for polarization entangled photons. It represents a recently discovered significant quantum information protocol which is based on few local measurements. The present demonstration has been applied to the so-called Werner states, a family of ”mixed” quantum states that include both entangled and non entangled states. These states have been generated by a novel high brilliance source of entanglement which allows to continuously tune the degree of mixedness. Typeset using REVTEX 1 One of the main issues of modern technology is the manipulation of information, its transmission, processing, storing, and computing, with an increasingly high demand of speed, reliability and security. Quantum Physics has recently opened the way to the realization of radically new information-processing devices, with the possibility of guaranteed secure cryptographic communications, and of huge speedups of some computational tasks. In this respect quantum entanglement represents the basis of the exponential parallelism of future quantum computers [1], of quantum teleportation [2–5] and of some kinds of cryptographic communications [6,7]. In practical realizations, however, entanglement is degraded by decoherence and dissipation processes that result from unavoidable couplings with the environment. Since entanglement is an expensive resource—it cannot be distributed between distant parties by classical communication means—it becomes crucial to be able to detect it efficiently, with the minimum number of measurements. Several methods have been proposed to assess the presence of entanglement for different types of quantum systems [8–13]. In particular, the socalled method of ”entanglement witness” is a simple and efficient protocol that uses only a few local measurements [13]. In the present paper we report the first experimental implementation of an entanglement witness for polarizationentangled photons. The entangled photon state is generated by a new general method of spontaneous parametric downconversion (SPDC) and the entanglement is detected by only three independent quantum measurements. Precisely, we implement experimentally the method of Ref. [10] for a pair of polarized photons that can be in any of the so-called Werner states [14], a family of ”mixed” quantum states that include both entangled and non entangled states. The key resource of our apparatus consists of a new, universal high-brilliance source of bi-partite entangled states spanning the Hilbert space H1 ⊗H2, where dim(H1) = dim(H1) = 2. These states can be generated as either “pure” or ”mixed”, with a complete control of the degree of mixedeness [15,16]. In particular, here we generate the Werner states ρW = p|Ψ−〉〈Ψ−|+ 1− p 4 1l4 , (1)