Sorting quantum systems efficiently

TL;DR

This paper introduces a universal, efficient scheme for sorting quantum systems based on any $d$-dimensional degree of freedom, with a practical implementation for orbital angular momentum of photons.

Contribution

It presents a general, single-particle, universal quantum sorting scheme with 100% theoretical efficiency, applicable to various quantum degrees of freedom.

Findings

Proposed a universal quantum sorter scheme for any $d$-dimensional degree of freedom.

Designed an efficient OAM sorter using a multi-path interferometer.

Scheme is suitable for integration into photonic chips.

Abstract

Measuring the state of a quantum system is a fundamental process in quantum mechanics and plays an essential role in quantum information and quantum technologies. One method to measure a quantum observable is to sort the system in different spatial modes according to the measured value, followed by single-particle detectors on each mode. Examples of quantum sorters are polarizing beam-splitters (PBS) -- which direct photons according to their polarization -- and Stern-Gerlach devices. Here we propose a general scheme to sort a quantum system according to the value of any $d$-dimensional degree of freedom, such as spin, orbital angular momentum (OAM), wavelength etc. Our scheme is universal, works at the single-particle level and has a theoretical efficiency of 100\%. As an application we design an efficient OAM sorter consisting of a single multi-path interferometer which is suitable for a photonic chip implementation.