From polarization entanglement to color entanglement

The strangeness of the quantum world is epitomized by entangled states, whose nonintuitive correlations cannot be mimicked by any classical system. These days experimenters routinely create two-photon states in which the photons’ polarization is entangled. Now, starting with such a state, Sven Ramelow and Lothar Ratschbacher (Institute for Quantum Optics and Quantum Information and University of Vienna) and colleagues have entangled the frequencies of two photons. It’s not the first demonstration of frequency entanglement, but earlier protocols relied on frequency filtering. In the Vienna work, only the two frequencies to be entangled are present in the initial state. The accompanying figure depicts the technique. Initially, the “red” photon in fiber 1 has a definite frequency, as does the “green” photon in fiber 2. The two photons have entangled polarizations—both are either horizontal or vertical. The key step is implemented by a polarizing beamsplitter that shunts the red photon into fiber...