Narrowband heralded single photons via Bragg grating inscription in germanium-doped photonic crystal fiber

Abstract

We present a fiber-based source of narrowband heralded single photons in the telecoms C-band. Photon pairs were generated by spontaneous four-wave mixing in photonic crystal fiber (PCF) with a germanium-doped region incorporated into its core for enhanced photosensitivity. A fiber Bragg grating (FBG) with a bandwidth of 0.2 nm and contrast of 17.5 dB was UV-written into the PCF to reflect a sub-nanometre slice of the photon-pair spectrum. This allowed narrowband photons to be heralded at the proximal end of the fiber by detection events after the distal end. We present photon counting data with a coincidence-to-accidental ratio of up to 70. Our source demonstrates a viable route to fiber-integrated narrowband heralded single photon sources suitable for coupling to quantum memories and interfacing heterogeneous qubit types.

Figures (6)

• Figure 1: Results of finite element simulations of the GePCF. a) Simulated group-velocity dispersion of the fundamental mode. Inset: GePCF design cross-section. Light grey - pure silica, dark grey - air holes, blue - Ge-doped silica. b) The resulting phase matching contour showing FWM signal wavelength as a function of pump wavelength.
• Figure 2: a) Points show the group velocity dispersion of the fabricated GePCF measured by white-light interferometry. Orange line is a polynomial fit. Inset: Microscope image of the cleaved end facet of the GePCF. Light grey areas are pure silica and dark spots are air holes. The purple region in the middle is the Ge doped core transmitting back-illuminated light. b) Bright-light FWM generated in the GePCF by sub-nanosecond pulses from a 1064 nm microchip laser.
• Figure 3: Measurement of the joint spectral intensity (JSI) by stimulated emission tomography. a) Experimental setup. Pump pulses at 1064 nm co-propagate alongside a tunable C-band seed in the GePCF containing an FBG with stop band at 1556 nm. The GePCF was spliced to HI1060 fiber containing a 1064 nm FBG for pump rejection and a fiber wavelength dividsion multiplexer (WDM) to separate signal, idler, and residual pump fields. The output around 800 nm was monitored with an optical spectrum analyser (OSA) b) The reconstructed JSI. The substructure along lines of constant total energy arise from the structure of the pump spectrum (self phase modulation in the amplification stage of the laser and in the GePCF) as well as structural variation along the length of the GePCF. Note that the resolution of the measurement along the 1550 nm axis is insufficient to show clearly the stop band of the 1556 nm FBG.
• Figure 4: a) Diagram of the FBG writing set up. See text for details. b) Transmission of the GePCF used in this work in the vicinity of its grating stop band.
• Figure 5: Alignment of 800 nm and 1550 nm arms. a) Setup for reverse alignment of 800 nm arm using broadband supercontinuum light (SC fiber). b) Setup for alignment of at 1556 nm reflected by FBG. Supercontinuum is generated in the GePCF and light reflected by the FBG is coupled into the single mode patch cord and OSA. c) Reflected spectrum measured at the OSA with (orange) and without (blue) additional filters to remove background. d) Reflection spectrum from FBG with additional filters plotted on a linear scale.