A device includes photon detectors, first photonic modes (coupled with photons sources) for outputting a first set of four photons, second photonic modes to provide a second set of at least four photons to the photon detectors, third photonic modes (coupled with the photon sources) to provide a third set of at least photons to the photon detectors, first couplers coupling modes in the first set of photonic modes to modes in the second set of photonic modes, and second couplers coupling modes of the third set of photonic modes to modes of the second set of photonic modes. The first and second couplers are configured to cause the first photonic modes to output, with a first non-zero probability, a pair of photons in a Bell state when a first number of photons is provided to respective inputs of the first photonic modes and the third photonic modes.