Quantum and classical photon correlation in four wave mixing and CARS spectroscopy

1. Quantum and classical photon correlation in four wave mixing and CARS spectroscopy  Rafi Z. Vered, Yelena Ben-Or, Michael Rosenbluhand AviPe’erDepartment of Physics and BINA Center for Nano-technology, Bar-Ilan University, Ramat-Gan 52900, Israel Introduction: Experimental concept: In degenerate Four Wave Mixing (FWM), a pump field at frequency is converted through a third-order non linearity, to another pair of frequencies (signal and idler) so that energy and momentum are conserved ,. For the energy flows from the pump to signal and idler. For the energy flows from signal and idler to pump. Dispersive window 2nd FWM 1st FWM A dispersive window imposes a spectrally modulated relative phase, causing interference fringes to appear on the FWM spectrum. Classically, FWM is an amplification process, and its fringe contrast is expected to be , where the gain is dictated by the pump intensity during the 2nd FWM pass. Quantum mechanically, this two-photon interference can reveal the quantum nature of the signal-idler light in a simple way. FWM is governed by phase. The pump, signal and idler relative phase relationship () dictate the direction of energy flow (from pump to signal/idler light or vice versa). Pump beam Two-photon BS Two-photon BS FWM beam Pump Laser Ti:Sapphire Dispersive window Experimental setup: PCF 1st attenuator 2nd attenuator 6ps pump pulses at 784nm enter a 12 cm long photonic crystal fiber (PCF), with zero dispersion at 783nm, generating signal-idler pairs over a broad frequency range. After the first pass through the fiber the pump and the signal-idler pairs are reflected back for a second pass through the fiber. In between, a dispersive window modulates the spectral phase of the signal-idler pairs compared to the pump, causing the appearance of spectral fringes. The resulting FWM spectrum is measured on a high resolution spectrometer. Dichroic BS Spectrometer Abstract: We demonstrate two-photon interference with correlated photon pairs produced by FWM. We explore the quantum-classical transition of the light by observing the loss dependence of the interference contrast for various pump intensities. Isolator Classically, these two scenarios are equivalent, but quantum mechanically, they are very different, as pump attenuation before the first FWM reduces the bi-photon flux, but leaves their correlations intact, whereas attenuation between the passes (loss) hampers the correlation severely. Thus, the fringe contrast in the first scenario is considerably higher than in the second scenario, providing a measure of the non-classicality of the light. First scenario: attenuating the pump before the fiber. Second scenario: attenuating the pump and FWM between the passes. Results: Fringe contrast Signal spectrum (attenuation before the fiber) Signal spectrum (attenuation between passes) (a) (b) Contrast Classical regime Quantum regime Pump Intensity (mW) The measured fringe contrast as a function of the pump power after the 2nd pass scanned down from 150mW (average power) in two ways, either by attenuating the pump before both passes (red) or by attenuation between the passes (blue). Up to 60mW pump power (close to the calculated value, due to time correlation measurements[1])a clear difference between the two scenarios is obvious, marking the transition between the quantum regime of single bi-photons at low pump powers and the multi-photon semi-classical regime at high powers. [1] Rafi Z. Vered, Michael Rosenbluh, and AviPe’er, "Two-photon correlation of broadband-amplified spontaneous four-wave mixing", Phys. Rev. A 86, 043837 (2012). (a) Measured spectral interference when attenuation is applied before the fiber. (b) The same interference when attenuation is applied between the passes through the fiber. Both graphs present the same four measurable lowest pump intensities. Summary:A simple two-photon interference method has been demonstrated for investigating the quantum correlation of broadband bi-photons generated by FWM. Due to the high gain of our PCF fiber, we fully observe the transition between quantum and classical regimes. The collinear, in fiber arrangement makes the experimental configuration inherently robust to phase fluctuations and does not require any phase locking to observe a stable fringe pattern, thereby considerably simplifying the measurement.We expect this method to be useful as an additional tool in the quantum optics and quantum information toolbox.