Path Entangled Photons from Parametric Down-Conversion

Path entangled photon states can be used to overcome classical limits on the accuracy of interferometric measurements such as the diffraction limit. These states are superpositions of finding $n$ photons in one out of two (or more) paths. Using stimulated parametric down-conversion (PDC), we propose a method for generating heralded multiphoton path entanglement, without applying post-selection. PDC is relatively easy to produce compared to pure Fock states as demanded by other proposals. By a special coincidence detection at one down-converted arm, the photons of the second arm non-locally bunch into the desired state. Entanglement in photon number is created between two polarization modes rather than two paths. A polarization beam-splitter and a $\lambda $/2 waveplate can translate between the two representations. The experimental generation of a two-photon path entangled state was detected by observing interference at half the photon wavelength. The scheme is generally extendable to higher photon numbers. \newline [1] M.J. Holland and K. Burnett, ``Interferometric detection of optical phase shifts at the Heisenberg limit'', Phys. Rev. Lett. \textbf{71}, 1355 (1993). \newline [2] P. Kok, H. Lee and J.P. Dowling, ``Creation of large-photon-number path entanglement conditioned on photodetection'', Phys. Rev. A \textbf{65}, 052104 (2002).