Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n(2)≃10(-7) cm(2) W(-1), almost 9 orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.

We report the experimental observation of scalar and cross-phase modulation instabilities by pumping a highly birefringent photonic crystal fiber in the normal dispersion regime at 45° to its principal polarization axes. Five sideband pairs (two scalar and three vector ones) are observed simultaneously in the spontaneous regime, four of which have a large frequency shift from the pump, in the range 79-93 THz. These results are in excellent agreement with phase-matching arguments and numerical simulations.

Photocount statistics are an important tool for the characterization of electromagnetic fields, especially for fields with an irrelevant phase. In the microwave domain, continuous rather than discrete measurements are the norm. Using a novel approach, we recover discrete photon stastistics from the cumulants of a continuous distribution of field quadrature measurements. The use of cumulants allows the separation between the signal of interest and experimental noise. Using a parametric amplifier as the first stage of the amplification chain, we extract useful data from up to the sixth cumulant of the continuous distribution of a coherent field, hence recovering up to the third moment of the discrete statistics associated with a signal with much less than one average photon.Introduction. Photon statistics measurements provide a wealth of information on the state of the electromagnetic field. For instance, Glauber's theory of optical coherence [1] is solely based on correlations between multiple photon measurements. As the discrete nature of the interaction between light and matter is essentially a quantum feature, statistical distributions can also characterize the classicality of photonic states. For instance, single photon states [2, 3] exhibit sub-Poisson photocount distributions that are not predicted by classical theories [4]. These states are not just of theoretical interest, as they feature prominently in proposals for the development of quantum computation [5] and quantum communication networks [6].With the advent of circuit QED [7], there is currently a great interest in quantum states of the electomagnetic field in the microwave domain. From early on, predictions have been made on the specific type of photon statistics that can be expected in mesoscopic conductors [8][9][10]. Recently, purely quantum photonic states have been demonstated in the microwave domain, using superconducting devices [11,12]. Entanglement has also been demonstrated in the GHz range using normal conductors [13,14]. The detection of single photons in the microwave domain remains a challenge, but schemes have been proposed for the extraction of photocounts from continuous measurements with linear detectors [15]. Experiments have already been performed to specifically extract discrete statistics from continuous measurements [16][17][18], although they focused on the calculation of centered moments of the continuous distributions, rather than the cumulants, as is the case herein.In this Letter, we derive simple formulas linking the cumulants [19] of the continuous variable (CV) distribution of field quadrature measurements to the centered moments of the photocount statistics. The latter fully characterize the state of the electromagnetic field when its phase is either not well defined or irrelevant. The additive nature of cumulants is especially important in the quantum regime, when we reconstruct photon statistics for signals with much less than one photon per mea-

We demonstrate a source of photon pairs with widely separated wavelengths, 810 and 1548 nm, generated through spontaneous four-wave mixing in a microstructured fiber. The second-order autocorrelation function g((2))(0) was measured to confirm the nonclassical nature of a heralded single-photon source constructed from the fiber. The microstructured fiber presented herein has the interesting property of generating photon pairs with wavelengths suitable for a quantum repeater able to link free-space channels with fiber channels, as well as for a high-quality telecommunication wavelength heralded single photon source. It also has the advantage of potentially low-loss coupling into standard optical fiber. These reasons make this photon pair source particularly interesting for long-distance quantum communication.

The effect of birefringence in 2-fold-symmetric microstructured optical fibers on the phase matching conditions for four-wave mixing is analyzed. The three general types of four-wave mixing are considered. General features are obtained through analytic expansions of phase-matching formulas. Three commonly used designs of fibers are analyzed numerically. Particular designs allow the generation of specified wavelengths, supercontinuum or entangled photons.