US Patent 8073336 Entanglement-based QKD system with active phase tracking

Entanglement-based QKD systems and methods with active phase tracking and stabilization are disclosed wherein pairs of coherent photons at a first wavelength are generated. Second harmonic generation and spontaneous parametric downconversion are used to generate from the pairs of coherent photons entangled pairs of photons having the first wavelength. Relative phase delays of the entangled photons are tracked using reference optical signals. Classical detectors detect the reference signals while single-photon detectors and a control unit generate a phase-correction signal that maintains the relative phases of phase-delay loops via adjustable phase-delay elements.