In this paper, we address the use of a calcite crystal-based local detector for the discrimination of the orbital angular momentum of quantum radiation produced by parametric down-conversion, using only a portion of the beam. Specifically, we propose and experimentally demonstrate that discrimination can be achieved by exploiting the introduction of a precise and controlled spatial shift between two replicas of the state within the crystals. This approach utilizes a robust and intrinsically stable monolithic configuration, obviating the need for feedback mechanisms or thermal drift compensation. Our method offers a promising avenue for enhancing the reliability and efficiency of quantum communication systems, and we believe that this technology could significantly advance the development of quantum communication techniques, where information encoding is based on orbital angular momentum, or spatially distributed orbital angular momentum detection.
In this paper, we address the use of a calcite crystal-based local detector for the discrimination of the orbital angular momentum of quantum radiation produced by parametric down-conversion, using only a portion of the beam. Specifically, we propose and experimentally demonstrate that discrimination can be achieved by exploiting the introduction of a precise and controlled spatial shift between two replicas of the state within the crystals. This approach utilizes a robust and intrinsically stable monolithic configuration, obviating the need for feedback mechanisms or thermal drift compensation. Our method offers a promising avenue for enhancing the reliability and efficiency of quantum communication systems, and we believe that this technology could significantly advance the development of quantum communication techniques, where information encoding is based on orbital angular momentum, or spatially distributed orbital angular momentum detection.
We propose an innovative monolithic interferometer to distinguish the topological charge of radiation carrying orbital angular momentum. Remarkably, our method requires to access only a small portion of the entire wavefront. The proposed scheme relies on a monolithic birefringent crystal, and as such it is intrinsically stable and does not require any feedback or thermal drift compensation. An experimental setup has been realized to prove the effectiveness of the proposed method down to the photon counting regime.
The bandwidth of any communication system, classical or quantum, is limited by the number of orthogonal states in which the information can be encoded. Quantum key distribution systems available commercially rely on the two-dimensional polarisation state of photons. Quantum computation has also been largely designed on the basis of qubits. However, a photon is endowed with other degrees of freedom, such as orbital angular momentum (OAM). OAM is an attractive basis to be used for quantum information because it is discrete and theoretically infinite-dimensional. This promises a higher information capacity per photon which can lead to more complex quantum computation protocols and more security and robustness for quantum cryptography. Entanglement of OAM naturally arises from spontaneous parametric down-conversion (SPDC). However, any practical experiment utilising the innately high-dimensional entanglement of the orbital angular momentum (OAM) state space of photons is subject to the modal capacity of the detection system. Only a finite subset of this space is accessible experimentally. Given such a constraint, we show that the number of measured, entangled OAM modes in photon pairs generated by SPDC can be increased by tuning the phase-matching conditions in the SPDC process. We achieve this by tuning the orientation angle of the nonlinear crystal generating the entangled photons.
In the present dissertation we describe the generation of a Gaussian bipartite entangled state in which the two subsystems are multi-distinguishable thanks to the fact that they have different polarizations and carry an opposite amount of orbital angular momentum along the propagation direction.
Polarization entangled states are produced by using an optical parametric oscillator as spontaneous parametric down conversion source. This is able to produce a bipartite state consisting of collinear thermal crossed polarized modes exhibiting entanglement. These two modes have the same frequency and constitute a continuous variable bipartite entangled system in which each of the modes can be labeled by the polarization degree of freedom. Once these two polarization entangled states are produced, the bipartite state is endowed with an additional degree of freedom constituted by orbital angular momentum, that makes possible to further distinguish between these two co-propagating modes. The two-dimensional Hilbert polarization space is mapped into the orbital angular momentum one by means of Gaussian operations i.e. physical trasformations that preserve Gaussianity. In order to achieve this task we used a liquid crystal optical device called q-plate. This device is able to couple polarization and orbital angular momentum degrees of freedom by making the passing through beam acquire orbital angular momentum that depends on the topological charge q of the device and on the polarization of the incoming beam. Moreover, the setup enginereed to achieve this task is also capable of generating squeezed vortex beams i.e. single mode states for which the quadrature noise is below the standard quantum limit.
After producing the bipartite entangled state it enters the characterizaton stage. Its quantumness is investigated by witnessing both squeezing and non classical correlations via balanced optical homodyne, a phase sensitive detection method that permits, by measuring the electromagnetic field quadratures statistics, to reconstruct the state of the system. In order to retrieve the quantum state with a high fidelity, homodyne detector needs to be optimized. This, besides imposing stringent conditions on the optical components involved in the setup, forces to improve the mode matching between the signal and the local oscillator that plays a central role in determining the overall detector efficiency. Although homodyne is a consolidated detection scheme, herein it is proposed an innovative extension of this technique to structured modes that opens the doors for homodyning directly in the orbital angular momentum space. The very central role, in this detection method, is played by interference between the signal under investigation and a strong coherent reference beam called local oscillator. Besides behaving as an amplifier for the quadrature under scrutiny, local oscillator also acts as a spatial and frequency filter, selecting for the measure the part of the signal that shares with it the same spatio-temporal properties. Hence, in order to ensure interference, in case of a signal carrying orbital angular momentum a further effort is required. Indeed, also the local oscillator has to be in the same helical mode, in particular the two beams have to transport the same amount of orbital angular momentum along the propagation direction. So by suitably designing the overall experimental setup it is possible to homodyne the bipartite entangled state carrying orbital angular momentum directly in this degree of freedom Hilbert space. Once the covariance matrix of the bipartite Gaussian state has been reconstructed, thanks to Gaussianity, it is possible to assess entanglement
between the vortex modes by means of entanglement criteria based on covariance matrix elements such as the Peres-Horodecki-Simon (PHS) and the Duan ones.
Maximally entangled photon pairs with a spatial degree of freedom is a potential way for realizing high-capacity quantum computing and communication. However, methods to generate such entangled states with high quality, high brightness, and good controllability are needed. Here, a scheme is experimentally demonstrated that generates spatially maximally entangled photon pairs with an orbital angular momentum degree of freedom via spontaneous parametric down-conversion in a nonlinear crystal. Compared with existing methods using postselection, the technique can directly modulate the spatial spectrum of down-converted photon pairs by engineering the input pump beam. In addition, the relative phase between spatially entangled photon pairs can be easily manipulated by preparing the relative phase of input pump states.
The OAM of light provides a new resource to explore quantum physics in a d-dimensional Hilbert space, beyond the two dimensional Hilbert space generated by the polarization state of the photons. The implementation of such a ddimensional quantum channel requires the generation of arbitrary engineered entangled states, thus controlling the OAM of the entangled photons is of paramount importance for many applications. Here we address the orbital angular momentum, i.e. the spatial shape, of photons generated in SPDC in non collinear geometries, when the interacting waves can exhibit Poynting vector walk off. The spatial shape depends on the interplay between the state ellipticity caused by the nonlinear geometry, and the Poynting vector walk off. The importance of both effects is dictated by the relationship between three characteristics lengths: the length of the nonlinear crystal, the walk off length and the non collinear length. The effects described here are relevant to current experiments, especially for the implementation of quantum information protocols based on spatially encoded information. Finally, the consideration of new geometries for SPDC, more specifically, highly non collinear configurations, will lead us to the discussion of the relationship between the OAM of the classical beam that pumps the nonlinear crystal, and the quantum OAM of the down converted photons. Regarding experimental measurements related to this issue, it is of great
importance to make a clear distinction between the measurement of locally paraxial light beams in a suitable transverse frame, and the description of the global down conversion process, which is not necessarily paraxial.
Photon pairs generated from spontaneous parametric down-conversion are a well-established method to realize entangled bipartite photonic systems. Laguerre-Gaussian modes, which carry orbital angular momentum (OAM), are commonly exploited to engineer high-dimensional entangled quantum states. For Hilbert spaces with dimension d>2, maximally entangled states (MESs) help to improve the capacity and security of quantum communication protocols, among several other promising features. However, the direct generation of MESs in well-defined high-dimensional subspaces of the infinite OAM basis has remained a challenge. Here, we formalize how the spatial distribution of the pump beam and the nonlinear profile of the crystal can be simultaneously utilized to generate MESs without additional spatial filtering of OAM modes within a subspace. We illustrate our approach with maximally entangled qutrits (d=3) and ququints (d=5).