Firstenberg, OferPeyronel, ThibaultLiang, Qi-YuGorshkov, AlexeyLukin, MikhailVuletić, Vladan2017-08-072013Firstenberg, Ofer, Thibault Peyronel, Qi-Yu Liang, Alexey V. Gorshkov, Mikhail D. Lukin, and Vladan Vuleti?. 2013. “Attractive Photons in a Quantum Nonlinear Medium.” Nature 502 (7469) (September 25): 71–75. doi:10.1038/nature12512.0028-0836http://nrs.harvard.edu/urn-3:HUL.InstRepos:33719343The fundamental properties of light derive from its constituent particles—massless quanta (photons) that do not interact with one another\(^1\). However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications\(^{2,3}\). Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state\(^{4–7}\). We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift8 exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light\(^9\).en-USJournal Article10.1038/nature12512