Entanglement and visibility at the output of a Mach-Zehnder interferometer

We study the entanglement between the two beams exiting a Mach-Zehnder interferometer fed by a couple of squeezed-coherent states with arbitrary squeezing parameter. The quantum correlations at the output are functions of the internal phase shift of the interferometer, with the output state ranging from a totally disentangled state to a state whose degree of entanglement is an increasing function of the input squeezing parameter. A couple of squeezed vacuums at the input lead to maximum entangled state at the output. The fringe visibilities resulting from measuring the coincidence counting rate or the squared difference photocurrent are evaluated and compared to each other. Homodynelike detection turns out to be preferable in almost all situations, with the exception of the very-low-signal regime. @S1050-2947~99!06102-8# The notion of entanglement is an essential feature of quantum mechanics, and is strictly connected with the nonlocal character of the theory. A two-part physical system prepared in an entangled state is described by a nonfactorizable density matrix. This gives rise to partial or total correlation between the outcomes of measurements performed on the two parts, even though the parts may be so far apart that no effects resulting from one measurement can reach the other part within the light cone. Sources of entangled states are required for fundamental tests of quantum mechanics, as well as for applications such as quantum computation and communication @1#, and teleportation @2,3#. In recent years, entangled photon pairs had been used to test nonlocality of quantum mechanics @4‐7# by Bell inequality @8#. In practice, all the available sources of two-mode entangled states are based on the process of spontaneous down conversion, taking place in x (2) nonlinear crystals @9#. Recently, it has been demonstrated that a beam splitter can split an incident photon into two correlated secondary photons @10,11#. However, such process occurs at a very low rate, and thus it is of no interest in practical applications.