Two Photons into One

Quantum information science opens new opportunities for future information-processing architectures that exploit the fundamental properties of quantum mechanics. Quantum information processing (QIP) has two essential ingredients: elementary quantum logic gates and remote communication of quantum information. Photons are particularly suitable as quantum information carriers, thanks to their long coherence times and the maturity of existing optical communication techniques, and quantum information encoded in single photons is a crucial ingredient for quantum cryptography, entanglement of distant nodes in a quantum network, and quantum sensing [1]. For photonic quantum logic gates, however, a major hurdle is that individual photons do not interact in ordinary linear media. QIP researchers have long sought deterministic photonic quantum logic gates using individual photons interacting in nonlinear media. Thiago Guerreiro and colleagues from the University of Geneva, Switzerland, and Stanford University, California, have used an optical waveguide to observe, for the first time, the nonlinear interaction of individual pairs of telecommunications-wavelength photons [2]. Although the current process is probably too inefficient to be of immediate practical use, these findings may open a route towards integrated QIP platforms.