Quantum-optics researchers have been trying to achieve strong inter­actions between individual photons for decades. These interactions constitute a fundamental tool toward the ultimate control of light fields “quantum by quantum.” 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. Also, they enable the exploration of new quantum states and phases, similar to those explored in strongly correlated particle systems. We realized that long-term goal and have observed both dissipative and dispersive interactions between individual photons.

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