Systems for preparing entangled states of light from biphotons on the basis of linear optical elements and projective measurements are considered. Specific features of quantum correlations of the appearing states can be detected in the proposed detection schemes.
Quantum entanglement is known as a unique quantum feature that cannot be obtained by classical physics. Over the last several decades, however, such an understanding on quantum entanglement might have confined us in a limited world of weird quantum mechanics. Unlike a single photon, a definite phase relation between paired photons is the key to understanding quantum features. Recently, an intuitive approach to the otherwise mysterious quantum features has emerged and shined a light on coherence manipulations of product-basis superposition via selective measurements. Here, a coherence manipulation is presented to excite polarization-path correlation using Poisson-distributed coherent photons for a classically excited joint-phase relation of independent local parameters. For this, linear optics is used for the preparation of the polarization-basis randomness, and a gated heterodyne detection technique is adopted for the selective measurement of polarization bases. As a result, the nonlocal quantum feature is now coherently understood in a deterministic way.
We experimentally demonstrate a simple classical-field optical heterodyne method which employs postselection to reproduce the polarization correlations of a four-particle entangled state. We give a heuristic argument relating this method to the measurement of multiple quantum fields by correlated homodyne detection. We suggest that using multiple classical fields and postselection, one can reproduce the polarization correlations obtained in quantum experiments which employ multiple single-photon sources and linear optics to prepare multiparticle entangled states. Our experimental scheme produces four spatially separated beams which are separately detected by mixing with four independent optical local oscillators (LO) of variable polarization. Analog multiplication of the four beat signals enables projection onto a four-particle polarization-state basis. Appropriate band pass filtering is used to produce a signal proportional to the projections of the maximally entangled four-field polarization state, $\ensuremath{\mid}{H}_{1})\ensuremath{\mid}{H}_{2})\ensuremath{\mid}{H}_{3})\ensuremath{\mid}{H}_{4})+\ensuremath{\mid}{V}_{1})\ensuremath{\mid}{V}_{2})\ensuremath{\mid}{V}_{3})\ensuremath{\mid}{V}_{4})$, onto the product of the four LO polarizations. Since the data from multiple observers is combined prior to postselection, this method does not constitute a test of nonlocality. However, we reproduce the polarization correlations of the 32 elements in the truth table from the quantum mechanical Greenberger-Horne-Zeilinger experiments on the violation of local realism. We also demonstrate a form of classical entanglement swapping in a four-particle basis.
Bell state measurements, in which two quantum bits are projected onto a maximally entangled state, are an essential component of quantum information science. We propose and experimentally demonstrate the projection of two quantum systems with three states (qutrits) onto a generalized maximally entangled state. Each qutrit is represented by the polarization of a pair of indistinguishable photons-a biphoton. The projection is a joint measurement on both biphotons using standard linear optics elements. This demonstration enables the realization of quantum information protocols with qutrits, such as teleportation and entanglement swapping.