A. Z. Khoury

Topological phase for spin-orbit transformations on a laser beam

We investigate the topological phase associated with the SO(3) representation in terms of maximally entangled states. An experimental demonstration of this topological phase is provided for polarization and spatial mode transformations of a laser beam.

Orbital angular momentum exchange in an optical parametric oscillator

We present a study of orbital angular momentum transfer from pump to down-converted beams in a type-II optical parametric oscillator. Cavity and anisotropy effects are investigated and demonstrated to play a central role in the transverse mode dynamics. While the idler beam can oscillate in a Laguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in the signal beam that prevents the resonance of such a mode.

Conservation of orbital angular momentum in stimulated down-conversion

We report on an experiment demonstrating the conservation of the orbital angular momentum in stimulated down-conversion. It has been demonstrated that the orbital angular momentum is not transferred to the individual beams of the spontaneous down-conversion. It is also known that it is conserved when twin photons are taken individually. We observe the conservation law for an individual beam of the down-conversion through cavity-free stimulated emission.

Entanglement in the above-threshold optical parametric oscillator

We investigate entanglement in the above-threshold optical parametric oscillator, both theoretically and experimentally, and discuss its potential applications to quantum information. The fluctuations measured in the subtraction of signal and idler amplitude quadratures are Δ2p−=0.50(1), or −3.01(9) dB, and in the sum of phase quadratures they are Δ2q+=0.73(1), or −1.37(6) dB. A detailed experimental study of the noise behavior as a function of pump power is presented, and the discrepancies with theory are discussed.

Quantum teleportation in the spin-orbit variables of photon pairs

We propose a polarization to orbital angular momentum teleportation scheme using entangled photon pairs generated by spontaneous parametric down-conversion. By making a joint detection of the polarization and angular momentum parity of a single photon, we are able to detect all the Bell states and perform, in principle, perfect teleportation from a discrete to a continuous system using minimal resources. The proposed protocol implementation demands experimental resources that are currently available in quantum optics laboratories.

Topological defects in moiré fringes with spiral zone plates

We present a study of spatial structures created by superposition of spiral zone plates used for generating optical beams with phase singularities. Moiré fringes are observed that show topological defects similar to those appearing in interference patterns of optical vortices. A brief theoretical discussion is included that supports the similarities between the two phenomena. Our results may lead to interesting applications to digital information processing by optical means.

Quantum and classical separability of spin-orbit laser modes

In this work we investigate the quantum noise properties of polarization vortices in connection with an intensity based Clauser-Horne-Shimony-Holt inequality for their spin-orbit separability. We evaluate the inequality for different input quantum states and the corresponding intensity fluctuations. The roles played by coherence and photon number squeezing provide a suitable framework for characterizing pure state spin-orbit entanglement. Structural inseparability of the spin-orbit mode requires coherence, an issue concerning either classical or quantum descriptions. In both cases, it can be witnessed by violation of this intensity based CHSH inequality. However, in the quantum domain, entanglement requires both coherence and reduced photon number fluctuations.

Environment-induced entanglement with a single photon

We propose an all-optical setup, which couples different degrees of freedom of a single photon, to investigate entanglement generation by a common environment. The two qubits are represented by the photon polarization and Hermite-Gauss transverse modes, while the environment corresponds to the photon path. For an initially two-qubit separable state, the increase of entanglement is analyzed as the probability of an environment-induced transition ranges from 0 to 1. An entanglement witness that is invariant throughout the evolution of the system yields a direct measurement of the concurrence of the two-qubit state.

Observation of Image Transfer and Phase Conjugation in Stimulated Down-Conversion

We observe experimentally the transfer of angular spectrum and image formation in the process of stimulated parametric down-conversion. Images and interference patterns can be transferred from either the pump or the auxiliary laser beams to the stimulated down-converted one. The stimulated field propagates as the complex conjugate of the auxiliary laser. The phase conjugation is observed through intensity pattern measurements.

Measurement of the degree of polarization entanglement through position interference

We produce polarization entangled states with variable degree of entanglement for twin photons. Entanglement in polarization is coupled to entanglement in position that produces transverse coincidence interference fringes. We show both theoretically and experimentally that we can use the position interference pattern to measure the polarization degree of entanglement. The coupling between polarization and transverse degrees of freedom are demonstrated to be useful in the manipulation of the entanglement of the twin photons.