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.
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.
Light orbital angular momentum (OAM) has been recognized as a new promising resource for classical and quantum information applications. In contrast to the spin angular momentum, the OAM is an inherently multidimensional. Thus, the information can be encoded in the higher-dimensional OAM alphabets. Recently, Marrucci et al. have invented a new device named q-plate (QP), made of liquid crystal cell patterned in such a way to introduce a topological charge q at the transverse plane, which is able to generate a well-defined values of photon OAM. My research has been aimed at investigating the physics of the QP, of the optical fields that it generates and of its possible applications for optical communication and quantum information. We studied both theoretically and experimentally a novel set of non-orthogonal but over-complete paraxial modes, named Hypergeometric-Gaussian modes, that is typically associated with OAM-carrying optical fields. We have also found the light propagation kernel inside the QP and we have shown analytically that if small losses due to reflection, absorption, and scattering are neglected, the QP can convert the photon spin into OAM with up to 100 \% efficiency. We implemented a technique to control the QP optical retardation by tuning the QP temperature. At the optimal temperature, the QP can generate a beam with up to 97\% efficiency. Moreover, the OAM state generated by QP can be rotated easily in the 2D OAM Hilbert space by proper manipulation of the input polarization state, a fact which opens a new way of beam shape controlling in MHz scale. We also performed a novel way to encode and read two bits information on the 4D OAM space by using only one QP. Finally, we experimentally demonstrated the transfer of quantum information from spin to OAM and vice versa, including the case of bi-photon states having quantum correlations. Furthermore, by exploiting these quantum information transfer devices, we demonstrated the Hong-Ou-Mandel effect and the optimal quantum cloning with OAM-carrying photons.
The optical "spin-orbit" coupling occurring in a suitably patterned nonuniform birefringent plate known as a "q plate" allows entangling the polarization of a single photon with its orbital angular momentum (OAM). This process, in turn, can be exploited for building a bidirectional "spin-OAM interface," capable of transposing the quantum information from the spin to the OAM degree of freedom of photons and vice versa. Here, we experimentally demonstrate this process by single-photon quantum tomographic analysis. Moreover, we show that two-photon quantum correlations such as those resulting from coalescence interference can be successfully transferred into the OAM degree of freedom.
The rotational properties of a light beam are controlled by its spin and orbital angular momentum (SAM and OAM). The q-plate, a liquid crystal device that can give rise to a coupling of these two quantities, was introduced a few years ago, leading to several applications in classical and quantum photonics. Very recently, in particular, a specific kind of q-plate was used to generate rotational-invariant states of single photons, which were then employed for performing a demonstration of quantum key distribution without the need for establishing a common reference frame between the transmitting and the receiving units. This result may find applications in future satellite-based quantum communication. By a similar approach, photonic states having a strongly enhanced rotational sensitivity, as opposed to rotational invariance, can be generated by using q-plates with very high topological charge. Photons in these states can be obtained starting from light having a uniform linear polarization and, after a physical rotation, can be converted back into light having uniformly linear polarization. As a result, one obtains linearly polarized light whose polarization plane rotates by an angle that is proportional to the angle of physical rotation between the generation and detection stages, with a very large proportionality constant. This effect of rotational amplification, which we named “photonic gear”, leads to a sort of “super-resolved Malus’ law”, potentially useful for measuring mechanical angles with very high precision.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report a series of optical schemes adopted for the manipulation of quantum states encoded in the orbital angular momentum exploiting the q-plate device. We further demonstrate the capability of generating hybrid entangled states as well as quantum states in dimension higher than two. Indeed we present the manipulation of ququart states encoded in the polarization and orbital angular momentum, which have been adopted for new quantum cryptographic protocols
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.
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.