But perhaps most fascinating, they enable the exploration of new quantum states and phases, similar to those explored in strongly correlated particle systems. They can be used to realize deterministic two-qubit optical gates for scalable quantum computing and to produce highly-correlated states for high-precision measurements. These interactions constitute a fundamental tool towards the ultimate control of light fields ‘quantum by quantum’. The realization of these interactions between individual photons could make light behave akin to conventional particles, such as electrons and atoms.Īchieving strong interactions between individual photons has been a goal at the forefront of the quantum-optics research for several decades.

The Kerr effect, for example, where the light itself alters the refraction index of a medium, can be viewed as a medium-mediated, ‘dispersive’ interaction between the photons. In this regime, the particle-like nature of light becomes significant, and the most basic concepts of optics and photonics need to be reexamined. Imagine nonlinear optics pushed to the extreme limit of the quantum world, at the sub-picowatt levels. The rate of outgoing photons is independent of the incoming rate and (in our case) saturates at the level of 1 per microsecond, in analogy to an hourglass. It thus prevents the transmission of more than one photon at a time for a narrow light-beam. 'Attractive photons in a quantum nonlinear medium', Nature (London) 502, 71–75 (2013)ĭissipative photon-photon interaction results in the scattering of photons close together. 'Quantum nonlinear optics with single photons enabled by strongly interacting atoms', Nature (London) 488, 57-60 (2012) In our lab, we realize a medium that pushes nonlinear optics to its extreme (quantum) limit, where the photon-photon interaction is strong on the level of individual photons.Įxperiments in Harvard and MIT have demonstrated a 'photonic hourglass,' through which photons can only pass one-by-one, and a two-photon bound state pertaining to a 'photonic molecule'.

Such interactions exist only within 'nonlinear' optical materials and usually require a huge number of photons to be meaningful. Photons, the fundamental particles of light, do not interact with each other in everyday life.

Photon vs quantum torpedoes