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We report all-optical switching due to state-filling in quantum dots (QDs). The switching energy is as low as 6 fJ since state-filling requires only 2 electron-hole pairs per QD. The single layer of InAs/InP QDs is inserted within a InGaAsP/InP waveguide which are processed into a Mach-Zehnder interferometric switch (MZI). A 1530-1570 nm probe beam is switched by optical excitation of one MZI-arm from above. By exciting below the InGaAsP bandgap, we prove that the refractive index nonlinearity is entirely due to state-filling in the QDs.