Pulse Compression to Enhance Photon-Pairs Brightness in Hollow-Core Photonic Crystal Fiber Based Source

Abstract

The advancement of quantum technologies [1] heavily relies on the efficient generation and manipulation of the photon-pairs. Among the numerous methods available for photon-pairs generation, spontaneous four-wave mixing (SFWM) in photonic crystal fibers [2] has emerged as a promising technique due to its inherent advantages in scalability and integration within existing telecommunication infrastructures. Inhibited-coupling hollow-core photonic crystal fibers (IC-HCPCFs) filled with inert gases provide a unique platform for SFWM-based photon- pairs generation, offering several features including entanglement control [3], integrability into telecommunication networks and also Raman-free generation [4] which minimizes noise and enhances the purity of the generated photon-pairs resulting in record coincidence-to-accidental ratio (CAR) superior to 104[5]. Building upon these advantages, we have recently focused on enhancing the photon-pairs brightness. In our previous work, we demonstrated significant improvements in brightness through optimization of fiber parameters such length, effective area and dispersion [5]–[6] on one hand and the use of a GHz-repetition rate femtosecond laser [7] on another hand to get closer to the MHz-level. One of the remaining parameters to further improve the generation efficiency$(\eta)$of IC-HCPCF based photon-pair source, and thereby conclude our investigation, is the pump pulse duration$(\tau_{\mathrm{p}})$. Since$\eta$is inversely proportional to the pump pulse duration [8], shorter pulses are expected to yield larger brightness. In this work, we developed a homemade pulse compressor [9] based on a hybrid IC-HCPCF design [10], which was integrated as the first stage of our photon-pair generation set-up as shown in Fig. 1a. By engineering the fiber dispersion and by controlling the optical nonlinearities through the pressure of the filling gas (argon), we achieved a self-compression of input pulses from an ultrashort pulse laser (~280 fs pulse width,1030 nm wavelength, 2 MHz repetition rate) down to ~35 fs. Fig. 1b shows the pulse duration of the compressor output over an input energy range of$0.01 \mu \mathrm{J}$to$0.7 \mu \mathrm{J}$. The energy transmission efficiency of the compressor was measured to exceed 80%. This compressor enables tunable pulse durations ranging from 280 fs down to 35 fs. As expected, the compression was accompanied by a spectral broadening from 8 nm to 41 nm (gaussian fit FWHM), primarly driven by self-phase modulation (see Fig. 1c). The compressed pulses were subsequently coupled into the photon-pair generation fiber (1 m of tubular lattice HCPCF with a 40 μm core, filled with xenon at 3.2 bar). As expected, when the pulse duration is reduced, the brightness increase scales as$1 / \tau_{\mathrm{p}}$(see Fig. 1d). For the shortest input duration of 35 fs, the brightness was enhanced by a factor of 7.7, which is in agreement with the expected value. Additionally, we observed a slight improvement in the CAR attributed to the increase in the number of coincidences (see Fig. 1e). Finally, the structuring of the spectral pump has led to a change in the joint spectral intensity (JSI) pattern (see inset of Fig. 1d). The implication of this change on the photon-correlation is under investigation.