The observation of extreme dynamics within quantum simulators based on photonic circuits is typically precluded by optical losses, exponentially increasing with the system depth or, equivalently, with the number of optical components. This is a natural consequence of the standard approach to photonic simulations of quantum dynamics, where the complexity of the setup grows with the extension of the evolution in time. By focusing on simple protocols of discrete-time quantum walks, we show that it is possible to compress homogeneous evolutions within only three liquid-crystal metasurfaces, encompassing up to a few hundreds of time steps. By exploiting spin-orbit effects, these devices implement space-dependent polarization transformations that mix circularly polarized optical modes carrying quantized transverse momentum, mimicking the target quantum dynamics with high efficiency and accuracy. Being extremely versatile, our compact platform will pave the way to the simulations of extreme regimes of more exotic dynamics.
Engineering single-photon states endowed with orbital angular momentum (OAM) is a powerful tool for quantum information photonic implementations. Indeed, due to its unbounded nature, OAM is suitable for encoding qudits, allowing a single carrier to transport a large amount of information. Most of the experimental platforms employ spontaneous parametric down-conversion processes to generate single photons, even if this approach is intrinsically probabilistic, leading to scalability issues for an increasing number of qudits. Semiconductor quantum dots (QDs) have been used to get over these limitations by producing on-demand pure and indistinguishable single-photon states, although only recently they have been exploited to create OAM modes. Our work employs a bright QD single-photon source to generate a complete set of quantum states for information processing with OAM-endowed photons. We first study hybrid intraparticle entanglement between OAM and polarization degrees of freedom of a single photon whose preparation was certified by means of Hong–Ou–Mandel visibility. Then, we investigate hybrid interparticle OAM-based entanglement by exploiting a probabilistic entangling gate. The performance of our approach is assessed by performing quantum state tomography and violating Bell inequalities. Our results pave the way for the use of deterministic sources for the on-demand generation of photonic high-dimensional quantum states.
The angular momentum of a light beam in the paraxial limit can be split into spin and orbital components. Only recently, optical processes involving a conversion of angular momentum from one form to another and related phenomena were conceived and experimentally demonstrated. I will more specifically focus on the effects generated by optical devices named q-plates and their ensuing generalizations, which have proved to be extremely convenient tools for controlling the phase and polarization structure of light beams. Several applications of these devices have been demonstrated in classical photonics and in quantum optics during the last years. In this presentation, after introducing the main concepts of spin-orbit optical couplings and the underlying physics, I will review a selection of recent applications.
We present a device based on liquid crystal and via Pancharatnam-Berry phase to generate Poincare beams by the coherent collinear superposition of two Free-Form Dark Hollow (FFDH) beams. We generate beams with spatially-variable polarization encoded on their cross section showing disclinations in the azimuth orientation and mappings of the Poincare sphere onto the transverse mode. We report generated beams characterized by nonuniform rotation rate of the local polarization azimuth in different polarization configurations, radial and azimuthal, lemon and star disclinations, and other richer and complex higher-order disclinations, by using tailored space-varying-axis plates based on liquid crystals.
The capability to engineer and characterize high dimensional states has become a crucial request in the quantum information field. The quantum walk dynamics proved to be a suitable resource for developing general quantumstate engineering protocols. Here, we experimentally verified the flexibility of an engineering protocol based on a one-dimensional quantum walk in the Orbital Angular Momentum (OAM). Although this degree of freedom has found several applications in the quantum information field, extract the information stored in them appears to be difficult. Therefore, we employ machine learning protocols to classify and characterize particularly structured beams endowed with a not uniform distribution of the polarization on the transverse plane. Moreover, we prove that by modeling the engineering process through a refined model it is possible to improve the performances of measurement techniques such as holographic projection and machine-learning based classification. These results represent a further investigation in the manipulation and detection of OAM modes coupling the photonics platforms with machine-learning protocols.
We present newly conceived liquid-crystal-based retardation waveplates in which the optic axis distribution has a “superelliptically” symmetric azimuthal structure with a topological charge q superimposed. Such devices, named superelliptical q-plates, act as polarization-to-spatial modes converters that can be used to produce optical beams having peculiar spiral spectra. These spectra reflect the topological charge of the optic axis distribution as well as the symmetry properties of the underlying superellipse. The peculiar capability of q-plates of producing optical modes entangled with respect to spin and orbital angular momentum is here extended to superelliptical q-plates in order to create more complex optical modes structurally inseparable with respect to polarization and spatial degrees of freedom. Such superelliptical modes can play a crucial role in studying polarization singularities or to develop polarization metrology.
A spatially incoherent white light optical vortex is generated using a tunable liquid crystal q-plate and white lamp source. This work investigates the propagation of incoherent vector vortex to the far field, and makes comparisons with a coherent optical vortex at a particular wavelength. The contrast ratio between the vortex’s ring and core darkness is determined, and the polarization of the vortex s mapped. For the keywords, select up to 8 key terms for a search on your manuscript's subject.
Tuning vector vortex in spatially coherent multicolored beam is studied. A wavelength filter based on the tunable modal properties of light is experimentally demonstrated, and the polarization topology of optical beam profile is mapped. A hybrid mode-wavelength division multiplexing (HMWDM) scheme is proposed. In this scheme information is encoded in the wavelength of light, and the spatial mode and polarization modulation about the optical mode is used to turn on and off frequency channels. This scheme is applicable to increase information capacity of light and in high resolution microscopy.
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.
A thin light beam, such as that emitted by a laser, may possess a hidden rotational structure, invisible to the naked eye. This structure is rooted in the electromagnetic wave nature of light and it takes two distinct forms, which may be dubbed spin and twist. Spin is associated with the rotation of the electric and magnetic fields oscillating within the optical wave—i.e., the circular polarization of light. Twist instead occurs in light waves having a helical-shaped (or twisted) wavefront and an optical vortex located at the beam axis. When a free material particle absorbs light having spin or twist, it is itself made to spin—in other words, the light exerts a rotational form of radiation pressure, showing that this kind of light carries angular momentum. More specifically, the angular momentum of light having circular polarization is named as spin angular momentum (SAM), while that associated with a spiral wavefront is called orbital angular momentum (OAM). The OAM of light has recently been attracting much attention for its possible technological applications in the areas of particle manipulation, optical sensing, and classical and quantum optical communication.
Qudits, the d-dimensional extension of qubits, open new perspectives in several fields, from fundamental quantum mechanics to quantum cryptography. Although photon polarization is a privileged choice for qubits encoding, it is not suitable for the physical realization of qudits. However, in order to realize multidimensional quantum systems, other degrees of freedom of single photons such as path or orbital angular momentum are available. When two or more degrees of freedom are exploited simultaneously we refer to "hybrid encoding". It is possible for instance to encode information in a four dimensional (ququart) hybrid space spanned by polarization and a bidimensional orbital angular momentum subspace of a single photon. Here we present how high dimensional hybrid systems can be exploited to overcome a major limitation of quantum communication: the need of a shared reference frame. Indeed the joint action of polarization and orbital angular momentum of hybrid ququarts can be exploited to realize quantum communication without a shared reference frame. We experimentally showed that, by using a proper subspace of hybrid ququart space, it is possible to perform any quantum communication protocol and violate CHSH inequalities without any information about the reference frame orientation of the two parties (except the direction of propagation of the photons). Such feature could find application in satellite based communication schemes.
Liquid crystals (LC) are particularly well suited for the manipulation of the angular momentum of light. Only recently, the possibility of coupling the LCs and the so-called orbital angular momentum (OAM) of light has been identified. OAM is associated with a light beam having a helical wavefront and an optical vortex at its axis. A singular-patterned LC plate, named “q-plate”, can be used for generating and controlling helical beams of light, or “vortex beams”. The qplate can be also used in the quantum regime, for controlling the OAM of single photons, leading to several applications in the quantum information field.
The angular momentum of light can be split into spin and orbital components. Only recently several optical processes
involving a conversion of angular momentum from one form to another were conceived and experimentally
demonstrated. We will briefly review these processes, and then survey some applications we have demonstrated of these
spin-orbit effects in classical and quantum optics. Finally, we will show that analogous spin-orbit effects can be
conceived for electron beams, leading to a theoretical proposal for a high-efficiency electron spin filter. If it will prove to
be practical, this device could lead to a spin-polarized electron microscopy.
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.
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.
I review recent results on a novel method for generating helical waves of visible light based on inhomogeneous
birefringent plates made of a suitably patterned liquid crystal. These devices, dubbed "q-plates", act on the
light wave by converting its spin angular momentum into orbital angular momentum, an optical process not
envisioned before. The output helical wave can be easily and rapidly switched between opposite wavefront
helicities by switching the input polarization with standard electro-optics devices. The process can be cascaded,
so that rapid switching can take place among multiple values of the wavefront helicity. More generally, patterned
liquid-crystal devices similar to those realized for generating helical beams may be used for shaping the optical
wavefront in any prescribed way, with the possibility of dynamical polarization multiplexing between conjugate
wavefronts. This is an application of the Pancharatnam-Berry phase principle, allowing the realization of a novel
kind of optical elements for wavefront shaping. Potential developments in the fields of optical communication
and quantum computation are briefly discussed.
A survey of recent results of experiments aimed at understanding the basic mechanism of the photoinduced reorientation phenomena in dye-doped liquid crystals is presented. In particular, I shall focus on experiments based on the isotopic substitution of hydrogen atoms with deuterium in dye molecules, which have shown an unexpected enhancement of the photoinduced reorientation effect by a factor two. The isotopic substitution also changes the dye excited state lifetime and orientational diffusion times. These results are in good agreement with the model proposed for the effect, and confirm the hypothesis that the active photoexcited state in the photoinduced reorientation phenomena is simply the first-excited singlet electronic state of dye molecules.
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