Simple synchronization of independent picosecond photon sources for quantum communication experiments O. Landry † , 1 J.A.W. van Houwelingen, 1 P. Aboussouan, 2 A. Beveratos, 3 S. Tanzilli, 2 H. Zbinden, 1 and N. Gisin 1 $^{1}$Group of Applied Physics, University of Geneva, rue de l’Ecole-de-M´ edecine 20,1211, Gen` eve, Switzerland $^{2}$Laboratoire de Physique de la Mati` ere Condens´ ee, CNRS UMR 6622, Universit´ e de Nice Sophia-Antipolis, Parc Valrose, 06108 Nice Cedex 2, France $^{3}$Laboratoire de Photonique et Nanostructures LPN-CNRS UPR-20, Route de Nozay, 91460 Marcoussis, France (Dated: December 11, 2018) An essential requirement for the future of quantum communication is the capability to create undistinguishable photons at distant locations. The main challenge is to control the various sources of timing jitter in order to conserve temporal undistinguishability. We report on a system that enables a large tolerance against such jitter by using synchronized sources of photons with coherence lengths on the order of centimeters. PACS numbers: 03.67.Hk,42.81.-i,42.65.Re Quantum repeaters (QR) [1–3] are devices that use sources of entanglement [4–10], quantum memories [11– 14] and partial Bell state analyzers [15–17] (BSA) to per- form cascaded entanglement swappings over a large dis- tance between photons that never interacted (Fig. 1). One particular difficulty is the synchronization of in- dependent sources of entangled pairs of photons. Par- tial BSA functions through perfect undistinguishability of two photons at a beamsplitter. If the two photons ar- rive at different times at the beamsplitter the BSA does not succeed, and therefore neither does the entanglement swapping. Several methods have been conceived to en- sure temporal undistinguishability. Previous experiments have achieved this using a fem- tosecond laser as a common pump [18, 19]. However this Spectral filtering of the photon pairs increases their co- herence length and relaxes the synchronization require- ments, as shown in Fig. 2. In turn, this requires a narrow pump to conserve energy-time entanglement. If pulsed operation is not necessary, narrowly filtered con- tinuous sources can be used and post-selected entangle- ment swapping resistant to path length fluctuations has been demonstrated [25, 26]. method is not applicable to realistic implementations of a quantum repeater which must have independent nodes. The simple duplication of femtosecond lasers on different locations is difficult to achieve as the sources must be syn- chronized to a better precision than their pulse lengths. For a typical Ti-Sapphire pulse length of 200 fs (with co- herence length of the same order), this corresponds to 40 µ m. Path length fluctuation of underground fibers, which is on the order of 10 − 5 K − $^{1}$, or 1 cm/(km · K), would de- stroy indistinguishability at the BSA even in the pres- ence of perfect sychronization [20, 21]. Such experiments have nevertheless been performed in the laboratory us- ing either cavities with a common element [22] or actively controlled cavities with fast electronics [23, 24]. FIG. 2: Effects of a displacement on the overlap between two wavepackets. The top two pictures show a clear reduction of the overlapping area (black) of two identical but displaced wavepackets when their displacement is increased. The bot- tom two pictures show a less important reduction when the wavepackets are wider. FIG. 1: Graphic representation of a quantum repeater of total length l . The squares represent quantum memories and the stars photon pair sources. At midpoint between the sources a beamsplitter performs a simple BSA for which synchroniza- tion is required. $^{†}$email:olivier.landry@unige.ch arXiv:1001.3389v2 [quant-ph] 9 Mar 2010 1. INTRODUCTION