Generation of broadband frequency entangled biphotons for quantum clock synchronization

Abstract

Frequency entangled biphotons based on spontaneous parametric down conversion (SPDC) of nonlinear crystal are widely used in Quantum clock synchronization protocols. The time-correlation width of the entangled source determines the accuracy of the attainable synchronization, which relies on the spectral bandwidth of the generated biphotons. We theoretically investigate the generation of frequency anti-correlated biphotons from chirped periodically-poled potassium titanyl phosphate (C-PPKTP). It is demonstrated that an ultra-broadband entangled biphoton source with a width of 857 nm was obtained, by using a 10 mm-long crystal with a chirping of 9.7 × 10 -6 μm -2, and a cw pumping source with a wavelength of 792 nm. The corresponding time correlation width is only 3.5 fs, which implies feasible clock synchronization accuracy in femtosecond scale. We further demonstrate that the narrowing of the time-correlation width slows down dramatically by increasing the chirping and the length of the nonlinear crystal, which provides a theoretical instruction for us to trade off between the complexity of the crystal fabrication and the sufficiently narrow time-correlation width. © 2012 Springer-Verlag.