The Stokes-parameter operators and the associated Poincaré sphere, which describe the quantum-optical polarization properties of light, are defined and their basic properties are re-

We report on the generation of a continuous variable Einstein-Podolsky-Rosen (EPR) entanglement using an optical fibre interferometer. The Kerr nonlinearity in the fibre is exploited for the generation of two independent squeezed beams. These interfere at a beam splitter and EPR entanglement is obtained between the output beams. The correlation of the amplitude (phase) quadratures are measured to be 4.0 ± 0.2 (4.0 ± 0.4) dB below the quantum noise limit. The sum criterion for these squeezing variances 0.80 ± 0.03 < 2 verifies the nonseparability of the state. The product of the inferred uncertainties for one beam (0.64 ± 0.08) is well below the EPR limit of unity.Since the original proposal of a Gedankenexperiment intending to show the incompleteness of quantum mechanics in 1935 [1], a number of schemes for generating the Einstein-Podolsky-Rosen (EPR) entanglement have been realized. The schemes range from the production of gamma-ray pairs from positron-electron annihilations [2], to proton pairs [3], to pairs of low-energy photons from atomic radiative cascade [4] and more recently to schemes involving optical parametric processes [5]. Most of these initial experiments utilized the entanglement as originally intended: to test the validity of quantum mechanics via either the violation of Bell inequality [4] or the demonstration of the EPR paradox [6]. Following the proposals of a myriad of quantum information schemes in recent years where entanglement is regarded as a basic requisite, the subject matter has experienced a resurgence of interest. The purposes of entanglement generation are now shifting to that of quantum information applications. Amongst these applications are the realization of quantum teleportation, the implementation of dense coding, quantum cryptography and other quantum communication schemes [7]. In view of these changing needs, it is desirable to explore simpler and more reliable alternatives for the generation of EPR entanglement.In this letter, we report on what is to our knowledge the first generation of EPR entanglement of photons that does not rely on any pair production process, such as those in the above-mentioned examples. Instead, the Kerr (χ (3) ) nonlinearity of an optical fibre is utilized to produce two amplitude squeezed beams, with the nonlinear interaction on each beam uncoupled to the other. To create the EPR entanglement, no additional nonlinear interaction is required. Instead, the amplitude squeezed beams are made to interfere at a 50/50 beam splitter [8]. In this vein sum squeezing is obtained for the amplitude quadratures and difference squeezing for the phase quadratures. The signs of these correlations are interchanged compared to those achieved in other systems. This fact may be of importance in applications involving the opto-mechanical coupling of radiation pressure [9]. Apart from the simplicity of our scheme, it also has the potential advantage of being integrable into existing fibre-optics communication networks.

We present an experimental demonstration of a quantum key distribution protocol using coherent polarization states. Post selection is used to ensure a low error rate and security against beam-splitting attacks even in the presence of high losses. Signal encoding and readout in polarization bases avoids the difficult task of sending a local oscillator with the quantum channel. This makes our setup robust and easy to implement. A shared key was established for losses up to 64%

We experimentally demonstrate a protocol for entanglement distribution by a separable quantum system. In our experiment, two spatially separated modes of an electromagnetic field get entangled by local operations, classical communication, and transmission of a correlated but separable mode between them. This highlights the utility of quantum correlations beyond entanglement for the establishment of a fundamental quantum information resource and verifies that its distribution by a dual classical and separable quantum communication is possible. [6,7]. Furthermore, it extends our abilities to process information. Here, entanglement is used as a resource which needs to be shared between remote parties. However, entanglement is not the only manifestation of quantum correlations. Notably, separable quantum states can also be used as a shared resource for quantum communication. The experiment presented in this Letter highlights the quantumness of correlations in separable mixed states and the role of classical information in quantum communication by demonstrating entanglement distribution using merely a separable ancilla mode.The role of entanglement in quantum information is nowadays vividly demonstrated in a number of experiments. A pair of entangled quantum systems shared by two observers enables us to teleport [8] quantum states between them with a fidelity beyond the boundary set by classical physics. Concatenated teleportations [9] can further span entanglement over large distances [10] which can be subsequently used for secure communication [11]. An a priori shared entanglement also allows us to double the rate at which information can be sent through a quantum channel [12] or one can fuse bipartite entanglement into larger entangled cluster states that are "hardware" for quantum computing [13]. * contributed equally to this workThe common feature of all entangling methods used so far is that entanglement is either produced by some global operation on the systems that are to be entangled or it results from a direct transmission of entanglement (possibly mediated by a third system) between the systems. Even entanglement swapping [9,14], capable of establishing entanglement between the systems that do not have a common past, is not an exception to the rule because also here entanglement is directly transmitted between the participants.However, quantum mechanics admits conceptually different means of establishing entanglement which are free of transmission of entanglement. Remarkably, the creation of entanglement between two observers can be disassembled into local operations and the communication of a separable quantum system between them [15]. The impossibility of entanglement creation by LOCC is not violated because communication of a quantum system is involved. The corresponding protocol exists only in a mixed-state scenario and obviously utilizes fewer quantum resources in comparison with the previous cases because communication of only a discordant [16][17][18] separable quantum system is required.In this Le...