Two-photon excitation microscopy is used to image living tissue. The fluorescence technique uses infrared light and allows deeper penetration of materials than does traditional fluorescence imaging. But the two-photon process is inefficient when applied to organic materials—from small molecules to proteins—and signals are weak. One solution is to increase the intensity of light directed at a sample, but that may damage the probed material. Another is to use entangled photons, reports a group led by Theodore G. Goodson III of the University of Michigan and Shaul Mukamel of the University of California, Irvine (J. Phys. Chem. Lett. 2013, DOI: 10.1021/jz400851d). Entanglement occurs when a pair of particles interact such that, after they separate, the actions of one affect the other. Goodson and colleagues created entangled photons using ultrafast laser techniques. They found that the entangled photons could enhance two-photon imaging by 10 orders of magnitude over classical two-photon measurements. They also found that some molecules preferentially absorb entangled photons rather than classical photons, on the basis of the molecules’ energy level dynamics. This effect will allow scientists to design materials for selective imaging, Goodson says.