George Khoury

Quantum entanglement of a large number of photons

A bipartite multiphoton entangled state is created through stimulated parametric down-conversion of strong laser pulses in a nonlinear crystal. It is shown how detectors that do not resolve the photon number can be used to analyze such multiphoton states. Entanglement of up to 12 photons is detected using both the positivity of the partially-transposed density matrix and a newly derived criteria. Furthermore, evidence is provided for entanglement of up to 100 photons. The multiparticle quantum state is such that even in the case of an overall photon collection and detection efficiency as low as a few percent, entanglement remains and can be detected.

Multiphoton Path Entanglement by Nonlocal Bunching

Multiphoton path entanglement is created without applying postselection, by manipulating the state of stimulated parametric down-conversion. A specific measurement on one of the two output spatial modes leads to the nonlocal bunching of the photons of the other mode, forming the desired multiphoton path entangled state. We present experimental results for the case of a heralded two-photon path entangled state and show how to extend this scheme to higher photon numbers.

Nonlinear interferometry via fock-state projection

We use a photon-number-resolving detector to monitor the photon-number distribution of the output of an interferometer, as a function of phase delay. As inputs we use coherent states with mean photon number up to seven. The postselection of a specific Fock (photon-number) state effectively induces high-order optical nonlinearities. Following a scheme by Bentley and Boyd [Opt. Express 12, 5735 (2004).10.1364/OPEX.12.005735], we explore this effect to demonstrate interference patterns a factor of 5 smaller than the Rayleigh limit.

Optimal generation of pulsed entangled photon pairs

We experimentally investigate a double-pass parametric down-conversion scheme for producing pulsed, polarization-entangled photon pairs with high visibility. The amplitudes for creating photon pairs on each pass interfere to compensate for distinguishing characteristics that normally degrade two-photon visibility. The result is a high-flux source of polarization-entangled photon pulses that does not require spectral filtering. We observe quantum interference visibility of over 95% without the use of spectral filters for 200 fs pulses, and up to 98.1% with 5 nm bandwidth filters.

Experimental phase detection at the quantum limit with coherent state interferometry

We experimentally demonstrate phase sensitivity at the standard quantum limit independent of phase using Bayesian phase estimation. This technique is unbiased and saturates the Cramer-Rao lower bound representing the optimum technique using coherent states.

Subwavelength interferometry using path entanglement and photon-number resolving detectors

We demonstrate subwavelength interference fringes using two methods. The first preselects path entangled states from parametric downconversion, and the second uses multiphoton detection of classical light.

Path entangled photons from parametric down-conversion

Using stimulated parametric down-conversion, we propose a method for generating multiphoton path entanglement. By a special coincidence detection at one down-converted arm, the photons of the second arm non-locally bunch into the desired state. Experimental demonstration for the case of two photons is presented, as well as how to extend the scheme to any number of photons.