In this article we present independent synchronised sources of pulsed picosecond length entangled photons. One laser is an electrically pumped pulsed diode laser which can be triggered externally. This is a simpler and less expensive solution than the usually used modelocked lasers. The second laser is a modelocked Ti-Sapphire in the picosecond regime. We demonstrate first that our lasers are synchronized at a distance of 2.2 km with a timing jitter of less than 60 ps. Then we create down-converted photons using these lasers and additional filtering, and we perform a Hong-Ou-Mandel (HOM) dip to measure the degree of undistinguishability of these photons. We demonstrate that long distance synchronization does not affect undis- tinguishability. It is important to have simple, easily deployable en- tanglement sources. With this in mind we developed a source based on a pulsed laser diode, instead of the com- monly used modelocked lasers (Fig. 3). The diode laser (PicoQuant LDH-P-C-W-1550 with PDL-800-B driver) produces pulses of light with a FWHM of about 20 ps at a wavelength of 1550 nm with 20 µ W mean power with 12.3 mW peak power at a repetition rate of 76 MHz. A higher output power is possible but this in- creases the pulse length. In order to amplify the sig- nal three Er-doped fiber amplifiers are used. First, the light passes two pre-amplifiers (one home-made, the other model EAD-60-C from IPG). The amplifiers are operated in the linear regime to minimize spontaneous amplified emission (ASE). The output is filtered with a 1 nm spec- tral width Fiber Bragg Grating (FBG) in order to remove the residual ASE, after which it has a mean power of 1 mW. Afterwards the light is once more amplified by 25.4 dB using the main amplifier (Keopsys high-power ampli- fier). The resulting pulses have a measured mean power of 350 mW. These pulses are then frequency-doubled by a periodically poled lithium niobate (PPLN) crystal which generates 2 mW mean power or 1.0 · 10 8 photons/pulse with 1.2 W peak power of the desired light at 775 nm with a spectral width of 0.4 nm. Before we can test the synchronization we must per- form an auto-correlation measurement (Fig. 4) to estab- lish the pulse lengths of both pump lasers. We separate the beams in two parts, one of which passes through an optical delay line. Both beams are then recombined at a PBS, such that the time delay between the horizon- tal and vertical polarizations of the output mode of the PBS is controlled by the delay. This output mode is sent through a beta-barium borate (BBO) crystal cut for type II sum frequency generation of two 775 nm pho- tons of perpendicular polarizations to a single 387.5 nm photon. Such generation is only possible if the input modes arrive at the same time at the PBS. The gener- ated blue light is separated from the other wavelengths with a prism and measured using a silicon photon count- We use two different links between the master and the slave. As a first step, we direct a small part of the light emitted from the modelocked laser to a high-speed In- GaAs photodetector (Thorlabs, DET01CFC). The out- put signal is directly used to externally trigger the diode laser. In a quantum repeater setup such as that of Fig. 1, many sources will need to be synchronized such that the produced photons are in the same temporal mode at the beamsplitters. We demonstrate here synchronization of the diode laser in slave mode triggered by a signal emitted by a MIRA modelocked laser (Coherent) generating 5 picosecond long pulses with a wavelength of 775nm and a bandwidth of 1.1 nm. 3. SOURCE SYNCHRONIZATION 2. DIODE LASER SOURCE 2 FIG. 3: Schematic of the diode based pulsed picosecond laser. The light from a pulsed diode laser is amplified in several stages. After this it is frequency doubled and the remaining pump photons are spatially filtered using a dispersion prism. FIG. 4: Setup for correlation measurements. For cross- correlations two laser pulses from different sources are com- bined at a polarization beamsplitter (PBS). For autocorre- lation measurements these pulses originate from the same laser. Frequency doubling is only possible if both pulses pass through the crystal at the same time. DiodeGLYPH<.notdef>based pulsedGLYPH<.notdef>laser Mode- lockedGLYPH<.notdef>laser Trigger D auto-correlationGLYPH<.notdef>only BBO PC DelayGLYPH<.notdef>line PBS Si-APD Diode Laser Pre- amplifier Main amplifier FBG PPLN 1 5 0 n m 775GLYPH<.notdef>nm