Bright phase-stable broadband fiber-based source of polarization-entangled photon pairs

We describe an achromatic, phase-stable, broadband source of polarization-entangled photon pairs with high spectral brightness that uses four-wave mixing in a fiber Sagnac interferometer. We achieved a polarization-entangled two-photon coincidence rate of $7\mathrm{kHz}$ per $0.5\mathrm{THz}$ $\left(0.9\mathrm{nm}\right)$ of bandwidth per $300\mathrm{\mu }\mathrm{W}$ of average pump power. At this rate, we observed two-photon fringe interference visibilities greater than 97%, over a $10\mathrm{THz}$ $(\approx 21\mathrm{nm})$ spectral range. We measured violations of Bell’s inequality by more than 22 standard deviations for each of the four Bell states in less than $3\text{minutes}$ per state. The high spectral brightness $\left(26\mathrm{kHz}{\mathrm{nm}}^{-1}{\mathrm{mW}}^{-1}\right)$, large tunable wavelength range, single spatial mode, and phase stability make this source a promising candidate for a wide range of quantum-information applications.

Figure 1

(Color online) Schematic of the experimental setup. PMMF, polarization-maintaining microstructure fiber; FC, fiber coupler; PBS, polarizing beam splitter; $\lambda ∕2$, half-wave plate; $\lambda ∕4$, quarter-wave plate; M, mirror; IF, interference filter.

Figure 2

(Color online) Raw coincidence count rates as a function ${\theta }_i$ for four different values of ${\theta }_s$ $(0°,45°,90°,-45°)$ along with fits to ${\sin }^2\left({\theta }_i\right)$ (lines). A $10\mathrm{s}$ integration time was used for each point. ${\lambda }_s=689\mathrm{nm}$ and ${\lambda }_i=800\mathrm{nm}$.

Figure 3

(Color online) Raw coincidence count rates (dots) as a function of ${\theta }_i$ for two different values of ${\theta }_s$ $(0°,45°)$ along with fits to ${\sin }^2\left({\theta }_i\right)$ (lines). A $10\mathrm{s}$ integration time was used for each point. ${\lambda }_s=693\mathrm{nm}$ and ${\lambda }_i=795\mathrm{nm}$.