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Frontiers in Optics

OAM Sorting of Light at Low Light Levels Using an Interferometric Method

Ashan Ariyawansa, Edward J. Figueroa, Thomas G. Brown

Published Jan 1, 2020• 0 citations
Publisher

Abstract

The orbital angular momentum (OAM) provides a unique way of adding a degree of freedom to the information content of a photon. We show a method to sort the OAM content of a collimated laser beam at very low light levels by using spatial interference between the OAM beam and a strong local oscillator field. The resulting interferogram is analyzed in the Fourier domain. By further numerically filtering, the OAM content of the beam can be distinguished with a one-count standard deviation consistent with the quantum noise floor.

DOI
10.1364/fio.2020.fw4a.4

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The orbital angular momentum (OAM) provides a unique way of adding a degree of freedom to the information content of a photon. We show a method to sort the OAM content of a collimated laser beam at very low light levels by using spatial interference between the OAM beam and a strong local oscillator field. The resulting interferogram is analyzed in the Fourier domain. By further numerically filtering, the OAM content of the beam can be distinguished with a one-count standard deviation consistent with the quantum noise floor.
We present a method of photon orbital angular momentum selection at very low light levels using spatial interference between a strong local oscillator field and a weak beam. By using Fourier phase recovery techniques familiar in classical interferometry, we can experimentally obtain a quantum-limited Q distribution with a standard deviation consistent with the quantum noise floor. Further, by projecting the complex Fourier peak on a Laguerre–Gauss basis, we can distinguish states of different orbital angular momentum with high fidelity for small numbers of counts per acquisition frame. The noise equivalent photoelectron count for this measurement is 10 − 5 counts per pixel per frame.
The orbital angular momentum (OAM) carried by optical beams is a useful quantity for encoding information. This form of encoding has been incorporated into various works ranging from telecommunications to quantum cryptography, most of which require methods that can rapidly process the OAM content of a beam. Among current state-of-the-art schemes that can readily acquire this information are so-called OAM sorters, which consist of devices that spatially separate the OAM components of a beam. Such devices have found numerous applications in optical communications, a field that is in constant demand for additional degrees of freedom, such as polarization and wavelength, into which information can also be encoded. Here, we report the implementation of a device capable of sorting a beam based on its OAM and polarization content, which could be of use in works employing both of these degrees of freedom as information channels. After characterizing our fabricated device, we demonstrate how it can be used for quantum communications via a quantum key distribution protocol.
We analyze, and test experimentally, a method of imaging at very low light levels using spatial interference between a strong local oscillator field and a weak beam. By using Fourier phase recovery techniques familiar in classical interferometry and by correcting the inhomogeneities in the reference field, we can represent the quadrature components of a given spatial mode in the complex plane. Further, by post selection filtering (convolution and sampling), the method can distinguish photons of different orbital angular momentum with a one-count standard deviation consistent with the quantum noise floor.
Orbital angular momentum (OAM) of light is a promising degree of freedom for quantum communication and quantum computation. However, efficient methods to transform and measure OAM modes are still not available readily. Here, we propose a method for full quantum state tomography for arbitrary OAM states of light. The key component in our scheme is the helicity sorter, a device that can sort OAM modes of a light beam based on their Helicity. We present two setups to implement the Helicity sorted, one requires O(N)\mathcal{O}(N)O(N) number of optical elements for 2N2^N2N OAM modes, and the other uses spatial light modulators and is independent of the number of OAM modes. Ours is the first method to provide the efficient and scalable full quantum state tomography of the OAM modes and can be extremely important for quantum communication and quantum computation.
The existing methods for measuring the orbital-angular-momentum (OAM) spectrum suffer from issues such as poor efficiency, strict interferometric stability requirements, and too much loss. Furthermore, most techniques inevitably discard part of the field and measure only a post-selected portion of the true spectrum. Here, we propose and demonstrate an interferometric technique for measuring the true OAM spectrum of optical fields in a single-shot manner. Our technique directly encodes the OAM-spectrum information in the azimuthal intensity profile of the output interferogram. In the absence of noise, the spectrum can be fully decoded using a single acquisition of the output interferogram, and, in the presence of noise, acquisition of two suitable interferograms is sufficient for the purpose. As an important application of our technique, we demonstrate measurements of the angular Schmidt spectrum of the entangled photons produced by parametric down-conversion and report a broad spectrum with the angular Schmidt number 82.1.