---
abstract: 'We demonstrate the capability to discretize the frequency spectrum of broadband energy-time entangled photons by means of a spatial light modulator to encode qudits in various bases. Exemplarily, we implement three different discretization schemes, namely frequency bins, time bins and Schmidt modes. Entangled qudits up to dimension $d=4$ are then revealed by two-photon interference experiments with visibilities violating a $d$-dimensional Bell inequality.'
address: 'Institute of Applied Physics, University of Bern, CH-3012 Bern, Switzerland'
author:
- 'B Bessire, C Bernhard, T Feurer and A Stefanov'
title: 'Versatile shaper-assisted discretization of energy-time entangled photons'
---

Introduction
============

Entanglement [@horodecki2009] is a unique feature of quantum theory having no analogue in classical physics. Spontaneous parametric down-conversion (SPDC) has been used as a source of entangled photon pairs for more than two decades [@ghosh1987] and provides an efficient way to generate non-classical states of light for fundamental tests of nature [@zeilinger1999; @genovese2005], for quantum information processing [@gisin2002; @gisin2007; @kok2007] or for quantum metrology [@giovannetti2011]. Entanglement between two photons emitted by SPDC can occur in one or several (hyperentanglement [@barreiro2005]) possible degrees of freedom of light, namely polarization, transverse momentum and energy. Polarization entanglement [@kwiat1999] has been used in many experiments even involving 