We put forward a new method to control the spectra of photon pairs generated in parametric downconversion that allows their spectral properties to be tuned from correlation to anticorrelation, including uncorrelation. The method employs tilted pulses and can be implemented in materials and frequency bands for which conventional methods do not hold.
Juan P. Torres, Macia, F., Carrasco, S., Torner, L. Engineering the frequency correlations of entangled two-photon states by achromatic phase-matching. "Optics letters", Febrer 2005, vol. 30, núm. 3, p. 314-316.