deletions | additions       diff --git a/section_Experimental_Setup_An_optimal__.tex b/section_Experimental_Setup_An_optimal__.tex  index 4636290..d0f9229 100644  --- a/section_Experimental_Setup_An_optimal__.tex  +++ b/section_Experimental_Setup_An_optimal__.tex 
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In both cases, noise is reduced the farther we place the signal from the pumps: because of to the BS phasematching flexibility, there are no limitations on the amount of detuning $\Delta\Omega$ \cite{M_chin_2006} between pumps and signal, the only fundamental parameter being $\omega_{ZDW}$ (rather than $\beta^{(3)}$ ), easily tunable via dispersion engineering.  We operate using a dispersion-shifted fiber (Vistacor, Corning): although the fiber is not optimized for nonlinear interactions ( $\gamma \simeq 3$ W/km), a sufficiently long spool makes up for the reduced nonlinear parameter. Measurement of the dispersion (shown in figure \ref{fig:dispersion}) measures ?, ? with $\lambda_{ZDW} = 1420 $ nm that corresponds to a $\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) for a $\Delta \Omega ~ 120$ 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 (~6.5 nm for pumps and ~4.3 nm for the signal/idler) the calculated acceptance bandwidth is ( ? (?  FWHM). The experimental setup is depicted in figure \ref{fig:setup}: to generate the pump fields, we use temperature stabilized laser diodes that are current modulated via a pulse generator, producing pulses of duration τ $\tau  = 1-10 1-10$  ns and peak power 5 $5$  mW. The pumps are amplified with cascaded 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). 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 $1551.7$  nm fiber wavelength division multiplexers (WDM). Signal and pumps are coupled together using O-Band/C-band WDM, temporally synchronized and injected in the nonlinear fiber. The polarization of each field is independently controlled to ensure parallel polarization in the nonlinear fiber.   The 100 $~100$  m of nonlinear fiber is spooled and placed in a cryogenic container. Signal losses through the setup are as low as 2.6 $2.6$  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) at 1310 nm (Semrock)  and a narrow band grating filter (0.5 nm, 2.6 dB). Alternatively we used a Dispersion Compensating Module (DCM D $D  = -1200nm/km -1200nm$  at 1300 $1300$  nm, 7.6 $7.6$  dB losses) for spectral characterization of the entire 1260-1300 $1260-1300$  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\%. $~70\%$ at $1300$ nm  and ~200 $~200$  dark counts per second in free-running operation. Temporal measurements are performed using a Time Tagging Module (TTM) (TTM, Roithner Lasertechnik)  with internal resolution 83.5 ? $82.3$  ps. The time delay between triggering and detection is about 1200 ns. An external cavity laser (OSICS), tunable between 1260 $1260$  nm - 1340 and $1340$  nm, is used both for testing purposes and, in combination with a tunable 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 $543$  nm generates photon pairs (phasematching achieved via temperature tuning): 940-nm photons are detected by a Si APD (Perkin-Elmer)  to herald the presence of 1283-nm $1283$-nm  photons and generate a synchronization signal that is used trigger the pulse generator. The marginal bandwidth of the signal photons is larger > 10 than $10$  nm, and  to match it to the acceptance bandwith of BS-FWM, (unless stated, centered  at 1283.8 $1284.45$ nm  with 0.5 $\delta\omega_{BS} = 0.5$  nm FWHM), both heralding and signal photons are spectrally filtered, the former with an holographic .5 nm filter (??), the latter with the free space tunable grating setup. We monitor the noise at one of the outputs while the fiber is changing temperature. If we collect the full signal setup, where contributions come over 12 $12$  nm bandwidth of the WDM, we can observe the temperature dependency (fig 5). The reduction of noise is limited to 2 orders of magnitude, which is expected because of the amount of fiber not placed in the cryostat (about 1 $1$  meter over 100 $100$  m of fiber in the cooler). Taking losses into account, we calculate the probability of generating a photon of noise is, while being already extremely low compared with other BS demonstrations ( P = demonstrations, about  1 photon of noise  per gate in highest efficiency reported [Clark2013a]) \cite{Clark_2013}),  can be additionally filtered both temporally and spectrally to match the acceptance bandwidth.