Tailored Photon-Pair Generation in Optical Fibers

We experimentally control the spectral structure of photon pairs created via spontaneous four-wave mixing in microstructured fibers. By fabricating fibers with designed dispersion, one can manipulate the photons’ wavelengths, joint spectrum, and, thus, entanglement. As an example, we produce photon pairs with no spectral correlations, allowing direct heralding of single photons in pure-state wave packets without filtering. We achieve an experimental purity of $(85.9\pm 1.6)\%$, while theoretical analysis and preliminary tests suggest that 94.5% purity is possible with a much longer fiber.

Figure 1

(a) Theoretical phase-matching curve (i.e., wavelength pairs where $\Delta k=0$) (solid lines) for orthogonally polarized SFWM in our fiber. (b) Magnified view of boxed region in (a). The curve is fit to experimentally measured signal (bottom □) and idler (top ○) central wavelengths as a function of pump wavelength. Vertical dashed lines indicate the zero GVD wavelengths. Points where frequency correlated (×) and anticorrelated states (△) can be created are marked. Thin sections of the curve indicate where factorable states are possible.

Figure 2

Fiber spectrum (inset) and polarization HOM experimental setup. Although some Raman background remains at the idler wavelength, the lack of background at the signal ensures that coincidence detection events most likely originate from SFWM.

Figure 3

Measured counts in 100 s for the polarization HOM experiment. (a) Singles counts of APDs $A$ (•), $B$ (×), $C$ (○), and $D$ (△). (b) Twofold coincidence counts $R_{AB}$ (•), $R_{CD}$ (×), $R_{AD}$ (○), and $R_{BC}$ (△). (c) Raw fourfold coincidence counts. (d) Normalized fourfold coincidence counts with theoretical fit corresponding to purity $(82.1\pm 1.6)\%$. Error bars represent propagated errors assuming Poissonian count statistics.

Figure 4

Experimental normalized fourfold coincidences in 200 s with a theoretical fit for 1 m fiber. The fit corresponds to $(85.9\pm 1.6)\%$ purity.