Obtaining Bright Squeezed Vacuum in Orbital Angular Momentum Modes Using a Non-Linear Interferometer

Summary: Light beams with helical or spiral wavefronts carry orbital angular momentum (OAM). OAM beams can be used in a large range of applications. Beyond the optical range, for instance, there is the possibility to generate bright x-rays carrying OAM components and helical wavefronts can be sued in radio frequency communications or in astronomy, where an antenna array can be used to detect them. In optical trapping, the transfer of OAM is used for the optical manipulation of particles. The OAM beams also have applications in the communications field. Superpositions of helical beams have been used to perform information encoding over large distances. The OAM modes can also be used to realize multidimensional entanglement in quantum key distribution (QKD).

The bright squeezed vacuum (BSV) is a state of light which presents non-classical correlations and manifests macroscopic entanglement. This light is characterized by its large number of photons per mode. Hence, the BSV is a promising source for the information encoding in the quantum domain. The BSV can be generated by means of a strongly pumped non-linear crystal in a process called parametric down-conversion (PDC).

Because of the fundamental features of the OAM modes and the BSV light mentioned above, it is interesting to prepare the BSV in OAM modes. The quantum information encoding can be performed using the orthogonality of the OAM modes prepared in the BSV state. It is also interesting to generate a mode of BSV carrying a single value of OAM.

In this project, the generation of BSV radiation in OA; modes is performed by means of two identical non-linear crystals. The constructive and destructive interference of the radiation from the two crystals provides OAM-types modes. By separating the crystals, the interference pattern changes and therefore, the generated modes change. However, the radiation can always be decomposed in terms of OAM modes. The main objective in this work is to study the OAM characteristics of this light under the described conditions. For this purpose, two different methods are used to measure the OAM components of the BSV radiation. In the first method, the measurement of the auto-correlation function provides the effective number of modes. In the second method, the diffraction of the beam with the phase modulated by means of a spatial light modulator (SLM) is used to measure the content of each mode in the superposition or mixture of OAM modes.

From the measurements performed in this project, the most important observations are summarized as follows: The effective number of modes is recued with the increase of the pump power for both measurements. As the pump power increases from 40mW to 90mW, the effective number of modes reduces from 10.11 to 3.338. The modal decomposition showed that all the modes have ring-shape but different azimuthal components. Having those results, the next step is to filter the modes without losing the non-classical features of the BSV. This can be performed by the use of additional refractive optical elements which convert OAM states into transvers momentum states. In recent experiments, these elements have been used and the losses were not too high for the applications considered. For this work, the loses should be carefully calibrated and reduced as much as possible. An evaluation of the performance of this method will be done in future experiments.