Lorenzo Marrucci

Free-space quantum key distribution by rotation-invariant twisted photons

Twisted photons are photons carrying a well-defined nonzero value of orbital angular momentum (OAM). The associated optical wave exhibits a helical shape of the wavefront (hence the name) and an optical vortex at the beam axis. The OAM of light is attracting a growing interest for its potential in photonic applications ranging from particle manipulation, microscopy, and nanotechnologies to fundamental tests of quantum mechanics, classical data multiplexing, and quantum communication. Hitherto, however, all results obtained with optical OAM were limited to laboratory scale. Here, we report the experimental demonstration of a link for free-space quantum communication with OAM operating over a distance of 210xa0m. Our method exploits OAM in combination with optical polarization to encode the information in rotation-invariant photonic states, so as to guarantee full independence of the communication from the local reference frames of the transmitting and receiving units. In particular, we implement quantum key distribution, a protocol exploiting the features of quantum mechanics to guarantee unconditional security in cryptographic communication, demonstrating error-rate performances that are fully compatible with real-world application requirements. Our results extend previous achievements of OAM-based quantum communication by over 2 orders of magnitude in the link scale, providing an important step forward in achieving the vision of a worldwide quantum network.

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.

Spin-to-Orbital Angular Momentum Conversion and Spin-Polarization Filtering in Electron Beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.

Test of mutually unbiased bases for six-dimensional photonic quantum systems

In quantum information, complementarity of quantum mechanical observables plays a key role. The eigenstates of two complementary observables form a pair of mutually unbiased bases (MUBs). More generally, a set of MUBs consists of bases that are all pairwise unbiased. Except for specific dimensions of the Hilbert space, the maximal sets of MUBs are unknown in general. Even for a dimension as low as six, the identification of a maximal set of MUBs remains an open problem, although there is strong numerical evidence that no more than three simultaneous MUBs do exist. Here, by exploiting a newly developed holographic technique, we implement and test different sets of three MUBs for a single photon six-dimensional quantum state (a “qusix”), encoded exploiting polarization and orbital angular momentum of photons. A close agreement is observed between theory and experiments. Our results can find applications in state tomography, quantitative wave-particle duality, quantum key distribution.

Deterministic qubit transfer between orbital and spin angular momentum of single photons

In this work we experimentally implement a deterministic transfer of a generic qubit initially encoded in the orbital angular momentum of a single-photon to its polarization. Such a transfer of quantum information, which is completely reversible, has been implemented adopting an electrically tunable q-plate device and a Sagnac interferometer with a Dove prism. The adopted scheme exhibits high fidelity and low losses.

Orbital angular momentum for quantum information processing

The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report some recent results regarding the adoption of higher dimensional quantum states encoded in the polarization and orbital angular momentum for quantum information and cryptographic processing.

Generation of tunable entanglement and violation of a Bell-like inequality between different degrees of freedom of a single photon

We demonstrate a scheme to generate noncoherent and coherent correlations, i.e., a tunable degree of entanglement, between degrees of freedom of a single photon. Its nature is analogous to the tuning of the purity (first-order coherence) of a single photon forming part of a two-photon state by tailoring the correlations between the paired photons. Therefore, well-known tools such as the Clauser-Horne-Shimony-Holt (CHSH) Bell-like inequality can also be used to characterize entanglement between degrees of freedom. More specifically, CHSH inequality tests are performed, making use of the polarization and the spatial shape of a single photon. The four modes required are two polarization modes and two spatial modes with different orbital angular momentum.

Light confinement via periodic modulation of the refractive index

We investigate, both theoretically and numerically, light confine- ment in dielectric structures with a transverse refractive index distribution pe- riodically modulated in the longitudinal coordinate. We demonstrate that light can be guided even in the balanced limit when the average refractive index con- trast vanishes in the direction of propagation, a dynamic trapping phenomenon analogous to the Kapitza effect in quantum mechanics. Finally, with reference to segmented waveguides with an unbalanced index modulation, we address the interplay of dynamic and static confinements.

Photonic Quantum Information Applications of Patterned Liquid Crystals

In this paper we review recent results we obtained in the field of photonic quantum information that were made possible by the introduction of patterned non-uniform liquid crystal cells known as “q-plates”: (i) the generation of entangled states of polarization and orbital angular momentum of a photon; (ii) the transfer of a qubit of quantum information from the spin to the orbital angular momentum of photons and vice versa; (iii) the Hong-Ou-Mandel coalescence in the same outgoing mode of a beam-splitter of two photons having nonzero orbital angular momentum; (iv) the universal optimal quantum cloning of orbital-angular-momentum-encoded qubits.

Quantum interference by coherence transfer from spin to orbital angular momentum of photons

The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report the characterization in the quantum regime of a recently introduced optical device, known as q-plate. Exploiting the spin-orbit coupling that takes place in the q-plate, it is possible to transfer coherently the information from the polarization to the orbital angular momentum degree of freedom, and viceversa. Hence the q-plate provides a reliable bi-directional interface between polarization and orbital angular momentum. As a first paradigmatic demonstration of the q-plate properties, we have carried out the first experimental Hong-Ou-mandel effect purely observed in the orbital angular momentum degree of freedom.