Stable generation of quadrature entanglement using a ring interferometer

We propose and demonstrate a scheme to stably generate quadrature-entangled optical pulses using a ring interferometer composed of an optical parametric amplifier and a dispersive media. The entangled light pulses at telecommunication wavelength are generated by combining two squeezed beams. In our scheme, the relative phase between the two beams is kept stable by the ring interferometer and is controllable using dispersive media. The amplitude and phase quadratures of the entangled beams are measured using two time-domain pulsed homodyne detectors. When the relative phase is fixed at π/2, we verify the inseparability of the states by a sufficient criterion ⟨Δ²[X̂_a(ϕ₀)+X̂_b(π−ϕ₀)]⟩+⟨Δ²[X̂_a(ϕ₀^′)−X̂_b(−ϕ₀^′)]⟩=0.64<1 where ϕ₀^′−ϕ₀=π/2.