Strong nonlinear interactions between photons enable logic operations forboth classical and quantum-information technology. Unfortunately, nonlinearinteractions are usually feeble and therefore all-optical logic gates tend tobe inefficient. A quantum emitter deterministically coupled to a propagatingmode fundamentally changes the situation, since each photon inevitablyinteracts with the emitter, and highly correlated many-photon states may becreated . Here we show that a single quantum dot in a photonic-crystalwaveguide can be utilized as a giant nonlinearity sensitive at thesingle-photon level. The nonlinear response is revealed from the intensity andquantum statistics of the scattered photons, and contains contributions from anentangled photon-photon bound state. The quantum nonlinearity will findimmediate applications for deterministic Bell-state measurements andsingle-photon transistors and paves the way to scalable waveguide-basedphotonic quantum-computing architectures.
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