Polarization Entanglement by Time-Reversed Hong-Ou-Mandel Interference

Sources of entanglement are an enabling resource in quantum technology, and pushing the limits of generation rate and quality of entanglement is a necessary prerequisite towards practical applications. Here, we present an ultrabright source of polarization-entangled photon pairs based on time-reversed Hong-Ou-Mandel interference. By superimposing four pair-creation possibilities on a polarization beam splitter, pairs of identical photons are separated into two spatial modes without the usual requirement for wavelength distinguishability or noncollinear emission angles. Our source yields high-fidelity polarization entanglement and high pair-generation rates without any requirement for active interferometric stabilization, which makes it an ideal candidate for a variety of applications, in particular those requiring indistinguishable photons.

Figure 1

Illustration of the source’s principle. A pair of identical photons in a correlated state is incident on a PBS via either input port 1 (a) or 2 (b). As a consequence of time-reversed HOM interference of orthogonal polarization modes, the photons antibunch into the output ports $1^{\prime }$ and $2^{\prime }$. Superposition of (a) and (b) thus results in a polarization-entangled state. $H$, $V$, $A$, and $D$ represent horizontal, vertical, antidiagonal, and diagonal polarizations, respectively.

Figure 2

Illustration of the crossed-crystal Sagnac source. LD, laser diode; PBS, polarization beam splitter; dHWP, dual-wavelength half-wave plate; HWP, half-wave plate; WP, wave plate; DM, dichroic mirror; ppKTP, type-0 periodically poled potassium titanyl phosphate crystal; TEC, temperature controller; LP, long pass filter; IF, interference filter; POL, polarizer. The top left-hand inset illustrates the 45° orientation of the oven, which ensures that photon pairs are generated either with diagonal or antidiagonal polarizations.

Figure 3

Stability of our source over long time under laboratory conditions. The twofold coincidence counts and visibilities in correlated and anticorrelated $A/D$ bases are stable on the order of hours, indicating its suitability for long-term operation in field experiments.