Opto-mechanical design and assembly of a Sagnac configuration of Entangled Photon Source for space applications: preliminary results
Preview: Francisco Jose Torcal-Milla 1*, Eusebio Bernabeu 1, Laura Peñate 2, Demetrio Lopez 2, Fabian Steinlechner 3 and Valerio Pruneri 3,4 1 Optics Department, Universidad Complutense de Madrid, Av. Complutense S.N. 28040, Madrid (Spain) 2 ALTER TECHNOLOGY TÜV NORD (ATN), Madrid, Spain 3 ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain 4 ICREA-Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
Entangled photon pairs are a key resource for a number of applications of quantum physics, such as quantum teleportation  and quantum cryptography . A vital step towards the global-scale implementation of such applications via ground-to-satellite and inter-satellite links is the development of robust, space-proof entangled photon sources (EPS). In a simple description, entangled photons can be considered a pair of particles which are intimately correlated. In fact they are part of the same quantum state: any measurement on one of them affects the state of the other. Photon pairs can be entangled in a number of degrees of freedom (e.g. time, energy, momentum, polarization), whereby polarizationentanglement is the most established for applications in free-space optics. In this case, both photons are anti-correlated in their polarizations, i.e. measuring one particular polarization for one photon yields a perpendicular polarization state for the partner photon – the two photons are entangled irrespectively of their spatial separation (Einstein referred to this as spooky action at a distance). To date the best-developed method for the generation of photon pairs is spontaneous parametric down-conversion (SPDC) in second-order nonlinear crystals. In the SPDC process, a high energy pump beam (=405nm) spontaneously can produce a pair of photons at lower energy (=810nm).
The efficient generation of photon pairs requires very strict characteristics on the illuminating beam, such e.g. its size, shape, and polarization state. For these photon pairs to be entangled in their polarizations there must be a superposition of two distinct pair-generation possibilities each with orthogonal polarized photon pairs. To achieve this with a single nonlinear crystal we have chosen a set-up based on Sagnac interferometer . Thus, beams must travel equal optical paths along both sides and focus on the crystal at the same point.