Experimental optimal cloning of four-dimensional quantum states of photons.

Optimal quantum cloning is the process of making one or more copies of an arbitrary unknown input quantum state with the highest possible fidelity. All reported demonstrations of quantum cloning have so far been limited to copying two-dimensional quantum states, or qubits. We report the experimental realization of the optimal quantum cloning of four-dimensional quantum states, or ququarts, encoded in the polarization and orbital angular momentum degrees of freedom of photons. Our procedure, based on the symmetrization method, is also shown to be generally applicable to quantum states of arbitrarily high dimension-or qudits-and to be scalable to an arbitrary number of copies, in all cases remaining optimal. Furthermore, we report the bosonic coalescence of two single-particle entangled states.

The polarizing Sagnac interferometer: a tool for light orbital angular momentum sorting and spin-orbit photon processing.

In this paper we show that an optical setup based on a polarizing Sagnac interferometer combined with a Dove prism can be used as a convenient general-purpose tool for the generation, detection and sorting of spin-orbit states of light. This device can work both in the classical and in the quantum single-photon regime, provides higher sorting efficiency and extinction ratio than usual hologram-fiber combinations, and shows much higher stability and ease of alignment than Mach-Zehnder interferometer setups. To demonstrate the full potential of this setup, we also report some demonstrative experiments of several possible applications of this setup.

Polarization control of single photon quantum orbital angular momentum states.

The orbital angular momentum of photons, being defined in an infinite-dimensional discrete Hilbert space, offers a promising resource for high-dimensional quantum information protocols in quantum optics. The biggest obstacle to its wider use is presently represented by the limited set of tools available for its control and manipulation. Here, we introduce and test experimentally a series of simple optical schemes for the coherent transfer of quantum information from the polarization to the orbital angular momentum of single photons and vice versa. All our schemes exploit a newly developed optical device, the so-called "q-plate", which enables the manipulation of the photon orbital angular momentum driven by the polarization degree of freedom. By stacking several q-plates in a suitable sequence, one can also have access to higher-order angular momentum subspaces. In particular, we demonstrate the control of the orbital angular momentum m degree of freedom within the subspaces of |m| = 2h and |m| = 4h per photon.

Femtosecond field ion emission by surface optical rectification.

We show that a model based on the surface optical rectification effect associated with the nonlinear response of free electrons may explain quantitatively, without adjustable parameters, all the observed features of the ultrafast laser-assisted field-ion emission from metal tips. Moreover, the same model provides also a plausible explanation for the low-fluence ultrafast laser ablation recently observed in metal surfaces and nanoparticles. We further test our model with experiments of ultrafast laser-assisted field-ion emission from tungsten tips in the tomographic atom probe.

Nonlinear dynamics induced in liquid crystals in the presence of the orbital and spin angular momentum of light.

We studied the dynamical effects induced in a homeotropic nematic film when a normally incident circularly polarized light beam with an elliptical intensity profile is used. A three-dimensional dynamical model shows that, besides the spin, the orbital angular momentum of photons also plays a role in the reorientation process. Our measurements fairly reproduce the main dynamical features predicted by the model in the near threshold region. The model, however, does not work, as it is, at higher incident laser power where chaotic director rotation was reported [A. Vella, A. Setaro, B. Piccirillo, E. Santamato, Phys. Rev. E 67, 051704 (2003)].

On-off intermittency in chaotic rotation induced in liquid crystals by competition between spin and orbital angular momentum of light.

We observed on-off intermittency in the chaotic rotation induced by a cw laser beam in a thin liquid crystal film where the spin and the orbital angular momentum of light compete in reorienting the sample. We found that the azimuthal angle phi(t) of the molecular director increased linearly in time on large time scales but, occasionally, it exhibited large fluctuations about its average value omega(0)t, so that its angular velocity phi;(t) undergoes an on-off intermittent motion. The intermittent signal omega(t)=phi;(t)-omega(0) obeyed the scaling laws of on-off intermittency, including the symmetry between laminar and burst phases. The chaotic rotations were observed only when the spin and the angular momentum of light were transferred simultaneously to the sample.

Coupled-mode approach to the nonlinear dynamics induced by an elliptically polarized laser field in liquid crystals at normal incidence.

A coupled-mode theory is presented to describe the dynamics of the molecular director induced by an elliptically polarized light plane wave normally incident onto a homeotropic liquid crystal film. The model provides a set of time ordinary differential equations for the lowest two modes of the system while the influence of the higher-order twist modes is accounted for by means of the adiabatic approximation. The resulting dynamics is complex above the reorientation threshold, according to the intensity and polarization of the incident light, rotating, oscillating, or steady states may settle. The dynamical regimes have been studied as functions of external parameters. The agreement with the experimental data was very good.

Orbital and spin photon angular momentum transfer in liquid crystals.

All-optical angular control of the molecular alignment in liquid-crystal films is demonstrated using a laser beam having an elliptically shaped intensity profile. The material birefringence is unimportant, as proven by the fact that good alignment is obtained with unpolarized light. This raises the possibility of achieving optical angular control of transparent isotropic bodies. A general theoretical approach, based on light and matter angular momentum conservation, shows that the optical alignment is due to the internal compensation between the transfer of the orbital and the spin part of angular momentum of the incident photons to the material.

Theory for a new full-vectorial beam-propagation method in anisotropic structures.

We present a theoretical model of light-beam propagation in anisotropic and inhomogeneous dielectric structures obtained as a direct extension of the scalar fast-Fourier-transform beam-propagation method. We solve Maxwell's equations in a generalized geometrical optics approximation, in which the reflected fields are neglected. This is a full-vectorial model because it accounts for the polarization effects that are due to both the anisotropy and the inhomogeneity of the medium.