Highly Efficient Single-Pass Source of Pulsed Single-Mode Twin Beams of Light

We report the realization of a bright ultrafast type II parametric down-conversion source of twin beams free of any spatiotemporal correlations in a periodically poled ${\mathrm{KTiOPO}}_4$ (PP-KTP) waveguide. From a robust, single-pass setup it emits pulsed two-mode squeezed vacuum states: photon-number entangled pairs of single-mode pulses or, in terms of continuous variables quantum optics, pulsed Einstein-Podolsky-Rosen states in the telecom wavelength regime. We verify the single-mode character of our source by measuring Glauber correlation functions $g^{(2)}$ and demonstrate with a pump energy as low as 75 pJ per pump pulse a mean photon number of 2.5.

Figure 1

Experimental setup: (a) PP-KTP waveguide source of two-mode squeezed vacuum states. (b) Fiber spectrometer for JSI measurement. (c) $g^{(2)}$ measurement setup. (d) Mean photon number $⟨n⟩$ measurement.

Figure 2

Two-photon spectral intensities from the setup 1b with pump width below, equal to and above photon pair separability width at 1.95 nm FWHM. Green: 50% intensity. Violet: phase matching width. Blue: pump width. Bright blue: theoretical 50% intensity.

Figure 3

Left: Marginal spectra of signal, idler beams from Fig. 2b. Middle: $g^{(2)}$ values from setup 1c (red) with theory curve (blue) and background corrected theory curve (violet); (a), (b) and (c) mark pump FWHM of the JSI from Fig. 2. Right: Mean photon number from setup 1d (red) with the theoretical gain of a two-mode squeezer (blue) and the linear gain of a highly MM squeezer (violet).