Observation of Entanglement between Two Light Beams Spanning an Octave in Optical Frequency

We have experimentally demonstrated how two beams of light separated by an octave in frequency can become entangled after their interaction in a ${\chi }^{(2)}$ nonlinear medium. The entangler was a nonlinear optical resonator that was strongly driven by coherent light at the fundamental and second-harmonic wavelengths. An interconversion between the fields created quantum correlations in the amplitude and phase quadratures, which were measured by two independent homodyne detectors. Analysis of the resulting correlation matrix revealed a wave function inseparability of $0.74(1)<1$, thereby satisfying the criterion of entanglement.

Figure 1

Map of entanglement (inseparability) as a function of seed and pump driving fields as based on our OPA model. Darker shading indicates stronger entanglement. Symbols mark where regions have been experimentally probed.

Figure 2

Entanglement generation between the reflected fundamental and second-harmonic light of an OPA.

Figure 3

Inseparability as a function of angle parameter with total input power held at 65 mW. Entanglement is achieved for values $<1$. Solid and dashed curves are from our model with and without GAWBS noise, respectively. Measurements marked by (circles) crosses are (un)corrected for artifacts from the OCR process.

Figure 4

(a) Time-series quadrature data (scaled 50%). (b) Shaded ellipses follow a contour of the resulting probability distribution. Dashed circles mark the quantum noise limit. The quantum correlation in amplitude is evident since the ellipse falls within the circle. (c) Dual quantum correlations are exhibited by the same data with the correlation matrix brought into standard form.