deletions | additions       diff --git a/section_Experimental_Setup_An_optimal__.tex b/section_Experimental_Setup_An_optimal__.tex  index ad5ad03..615f99a 100644  --- a/section_Experimental_Setup_An_optimal__.tex  +++ b/section_Experimental_Setup_An_optimal__.tex 
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Thanks to the BS phasematching flexibility, there are no limitations on the amount of detuning [Mechin2006a] between pumps and signal, the only fundamental parameter being ωZDW (rather then β(3) ), easily tunable via dispersion engineering.  We operate using a dispersion-shifted fiber Vistacor from Corning inc.. Although (Vistacor, Corning): although  the fiber is not optimized for nonlinear interactions ( γ ~ 3 W/km), a sufficiently long spool makes up for the reduced nonlinear parameter. Measurement of the dispersion measures , with λZDW $\lambda_{ZDW}  = 1420nm 1420 $ nm  that corresponds to a ωZDW =1330 THz ,the $\omega_{ZDW} = 1330 $ THz, with the signal/idler and the pumps placed respectively in the  O-Band (1260 nm - 1320 nm ) and the C-Band (1530 nm - 1565 nm) ΔΩ ~ 120 THz . This is an attractive configuration because it enables the large detuning needed for a low-noise operation, while still operating at wavelengths where off-the-shelf equipment is available. For $\delta\omega  = 5 $  THz (about 6.5 nm at 1560 (~6.5  nm for pumps  and 4.3 ~4.3  nm at 1300 nm) for the signal/idler)  the acceptance bandwidth is ( FWHM).   Fig. 3. Vistacor dispersion 
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  Fig. 4. Experimental setup. LD Laser Diode. EDFA Erbium Doped Fiber Amplifier. WDM Wavelenght Division Multiplexer. DSF Dispersion Shifted Fiber. SpAPD. Single photon avalanche photodioide. PPLN Periodiacally Poled Lithium Niobate Crystal.  To generate the  pump fields, we use temperature stabilized laser diodes (QPhotonics) (DFB, QPhotonics)  that are current modulated via a pulse generator, producing pulses of duration τ = 1-10 ns and peak power 5 mW. The pumps are amplified with cascaded c-band C-band  erbium-doped fiber amplifier (EDFA). The last EDFA (Keopsys) is optimized for high power pulsed amplification at low duty cycle (output P out = 30 dBm at 100/1 duty cycle). Both pulses are temporally separated when traversing the EDFA to avoid mutual nonlinear effects in the gain medium, and synchronized afterward using an unbalance combination of 1551.7 nm fiber wavelength division multiplexers (WDM).A free space filtering setup removes higher frequency noise (low-pass filter, cut-off 1550 nm) and selects one polarization with a combination of λ/2- and λ/4- waveplates, and a polarizing beam splitter.  Signal and pumps are coupled together using O-Band/C-band WDM, temporally synchronized and injected in the nonlinear fiber. At the end of the interaction, second WDM removes most The polarization  of the pumps, and the signal each field  is sent independlty controlled  to ensure parallel polarization in  the detection stage. nonlinear fiber.  The 100 m of nonlinear fiber is spooled and placed in a Styrofoam box that operates as a cryostat. cryogenic container.  Signal losses through the setup are as low as 2.6 dB, due to splices, connectors and WDMs. Since the second WDM removes > 30dbB of pump power, we consider most of the Raman noise generated between the two WDMs, and we take care of placing as much amount of fiber as possible in the cryostat. At the end of the interaction, a second WDM removes most of the pumps, and the signal is sent to the detection stage.  Before detection, the signal has to be carefully filtered of all residual pump photons and thermal noise: we used a combination of a fiber-based 1300 nm/1550 nm pass/reject filter and a free-space filtering setup (3 dB losses) composed of a lowpass sharp-edge filter (1300-Semrock). A fiber-based WDM (passband 1290 ± 6.5 nm nominal) can be used to separate the signal (1300-Semrock)  andidler fields at approximately 1283.3 nm and 1278.5 nm. For some of the measurements, we selected  a narrow band(0.5 nm) using a  grating filter (2.6 dB), or (0.5 nm, 2.6 dB). Alternatively  we used a Dispersion Compensating Module (DCM D =-1200nm/km = -1200nm/km  at 1300 nm, 7.6 dB losses) for spectral characterization of the entire 1260-1300 nm band,  using time of arrival information to recover the wavelength. For detection, we utilized superconductive nanowire single photon detector SNSPD, with nominal q.e. ~70% and ~200 dark counts per second in free-running operation. Temporal measurements are performed using a Time Tagging Module (TTM) with internal resolution 83.5 ? ps. The time delay between triggering and detection is about 1200 ns.  An external cavity laser (OSICS), tunable between 1260 nm - 1340 nm, is used both for testing purposes and, in combination with a tunable attenuator, attenuator and a elecro-optical modulator,  to generate a weak coherent signals. To generate single photons we use a source based on spontaneous parametric downconversion in a PPLN crystal. In a 10 mm long LiN crystal, a CW pump at 543 nm generates photon pairs (phasematching achieved via temperature tuning): 940-nm photons are detected by a Si APD to herald the presence of 1283-nm photons and generate a synchronization signal that is used trigger the pulse generator. The marginal bandwidth of the signal photons is larger > 10 nm, to match it to the acceptance bandwith of BS-FWM, (unless stated, at 1283.8 with 0.5 nm FWHM), both heralding and signal photons are spectrally filtered, the former with an holographic .5 nm filter (??), the latter with a the  free space tunable grating setup.   Fig. 5. Characterization of the noise dependency on temperature. Total noise photon generated vs. temperature.