Sergei Slussarenko

Tunable liquid crystal q-plates with arbitrary topological charge

Using a photoalignment technique with a sulphonic azo-dye as the surfactant aligning material, we fabricated electrically tunable liquid crystal q-plates with topological charge 0.5, 1.5 and 3 for generating optical vortex beams with definite orbital angular momentum (OAM) 1,3 and 6 per photon (in units of ¯h), respectively. We carried out several tests on our q-plates, including OAM tomography, finding excellent performances. These devices can have useful applications in general and quantum optics.

Polarization pattern of vector vortex beams generated by q-plates with different topological charges

We describe the polarization topology of the vector beams emerging from a patterned birefringent liquid crystal plate with a topological charge q at its center (q-plate). The polarization topological structures for different q-plates and different input polarization states have been studied experimentally by measuring the Stokes parameters point-by-point in the beam transverse plane. Furthermore, we used a tuned q=1/2-plate to generate cylindrical vector beams with radial or azimuthal polarizations, with the possibility of switching dynamically between these two cases by simply changing the linear polarization of the input beam.

Complete experimental toolbox for alignment-free quantum communication

We developed a complete experimental toolbox to prove the communication possibility through the feasibility demonstration of a cryptographic-key distribution protocol, distribution of entanglement, and violation of a Bell inequality, all in alignment-free settings, i .e . by rotating the reference frame of the receiver.

Photonic polarization gears for ultra-sensitive angular measurements

Quantum metrology bears a great promise in enhancing measurement precision, but is unlikely to become practical in the near future. Its concepts can nevertheless inspire classical or hybrid methods of immediate value. Here we demonstrate NOON-like photonic states of m quanta of angular momentum up to m=100, in a setup that acts as a ‘photonic gear’, converting, for each photon, a mechanical rotation of an angle θ into an amplified rotation of the optical polarization by mθ, corresponding to a ‘super-resolving’ Malus’ law. We show that this effect leads to single-photon angular measurements with the same precision of polarization-only quantum strategies with m photons, but robust to photon losses. Moreover, we combine the gear effect with the quantum enhancement due to entanglement, thus exploiting the advantages of both approaches. The high ‘gear ratio’ m boosts the current state of the art of optical non-contact angular measurements by almost two orders of magnitude.

Photon spin-to-orbital angular momentum conversion via an electrically tunable q-plate

Exploiting electro-optic effects in liquid crystals, we achieved real-time control of the retardation of liquid-crystal-based q-plates through an externally applied voltage. Electro-optic q-plates can be operated as electrically driven converters of photon spin into orbital angular momentum, enabling a variation of the orbital angular momentum probabilities of the output photons over a time scale of milliseconds.

Quantum walks and wavepacket dynamics on a lattice with twisted photons

A discrete quantum walk occurs in the orbital angular momentum space of light, both for a single photon and for two simultaneous photons. The “quantum walk” has emerged recently as a paradigmatic process for the dynamic simulation of complex quantum systems, entanglement production and quantum computation. Hitherto, photonic implementations of quantum walks have mainly been based on multipath interferometric schemes in real space. We report the experimental realization of a discrete quantum walk taking place in the orbital angular momentum space of light, both for a single photon and for two simultaneous photons. In contrast to previous implementations, the whole process develops in a single light beam, with no need of interferometers; it requires optical resources scaling linearly with the number of steps; and it allows flexible control of input and output superposition states. Exploiting the latter property, we explored the system band structure in momentum space and the associated spin-orbit topological features by simulating the quantum dynamics of Gaussian wavepackets. Our demonstration introduces a novel versatile photonic platform for quantum simulations.

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.

Polarization-controlled evolution of light transverse modes and associated Pancharatnam geometric phase in orbital angular momentum

We present an easy, efficient, and fast method to generate arbitrary linear combinations of light orbital angular-momentumeigenstates � =± 2startingfromalinearlypolarizedTEM00 laserbeam.Themethodexploits the spin-to-orbital angular-momentum conversion capability of a liquid-crystal-based q plate and a Dove prism inserted into a Sagnac polarizing interferometer. The nominal generation efficiency is 100%, being limited only by reflection and scattering losses in the optical components. When closed paths are followed on the polarization Poincar´ e sphere of the input beam, the associated Pancharatnam geometric phase is transferred unchanged to the orbital angular momentum state of the output beam.

Tunable supercontinuum light vector vortex beam generator using a q-plate.

Spatially coherent multicolored optical vector vortex beams were created using a tunable liquid crystal q-plate and a supercontinuum light source. The feasibility of the q-plate as a tunable spectral filter (switch) was demonstrated, and the polarization topology of the resulting vector vortex beam was mapped. Potential applications include multiplexing for broadband high-speed optical communication, ultradense data networking, and super-resolution microscopy.

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.