deletions | additions       diff --git a/section_Experimental_Setup_An_optimal__.tex b/section_Experimental_Setup_An_optimal__.tex  index cc26394..aae37ca 100644  --- a/section_Experimental_Setup_An_optimal__.tex  +++ b/section_Experimental_Setup_An_optimal__.tex 
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An optimal choice of nonlinear medium and operating regime enables BS frequency translation in the quantum regime. There are two major design decisions that affect the choice of medium. First, one must fulfill the phase matching for a given set of pumps and signals. In fact, given the selectivity of the process, a very specific dispersion profile must be used for any given configuration. The second design decision is the balance between nonlinearity and dispersion properties: an optimal ratio between the amount of pump power required for full conversion and the acceptance bandwidth, as well as the minimum frequency separation $\Delta \omega$ for which cascaded BS becomes prominent. In order to maximize the former with respect to the latter, it is convenient to operate at large dispersion values . As a reference, the demonstrations reported above showed translation over Δ ω > 10 nm.  As mentioned, one obstacle for most quantum implementation of BS is technical noise, in the form of spontaneous FWM and Raman noise \cite{Lefrancois_2015}. Modulation instability (purple in figure 2) \ref{fig:tech_noise})  is a competing FWM process, its gain profile given by $G_{MI}$ where $g_{MI} = $ and $\kappa_{MI}$ are respectively the MI wavevector and linear phasematching term for a frequency detuning $\delta \omega$ from the a strong pump. Modulation instability affects BS in multiple ways, either by depleting the pump and generating spurious sidebands, or amplifying the vacuum fluctuations and generating pairs of energy correlated photons. It can be managed placing the pump on the normal dispersion, where parametric gain is forbidden and the bandwidth for pair generation is minimized, as well as detuning the signal far from the pump frequencies. Spontaneous and Stimulated Raman Scattering are processes that couple light with the thermal phonons of the medium. Scattering strength depends on the density of occupied states as well as the Raman spectrum . The probability of a spontaneous Stokes scattering (i.e. the photon losing energy) is given by while for anti-Stokes it is   The spectrum depends on the material: amorphous materials have a broad spectrum (i.e. for glass it extends to about 40 THz [Stolen1973]), while crystalline materials have strong, sharp features. Both processes become less probable for very large detuning, though the anti-Stoke probability depends exponentially on the temperature when (6 THz for room temperature), so that cooling the fiber can further reduce the noise by several order of magnitude (red 300 K and blue 90 K on figure 2) [Li2004, Takesue2005].     Fig. 2. Technical noise contributions to BS for large detuning. Spontaneus pairs generation (purple) . Spontaneus Raman scattering at room temperature (red) and at 76 K (blue)  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 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 = 1420nm that corresponds to a ωZDW =1330 THz ,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 = 5 THz (about 6.5 nm at 1560 nm and 4.3 nm at 1300 nm) the acceptance bandwidth is ( FWHM).