  Exciton-polaritons are hybrid elementary excitations of light and matter
that, thanks to their nonlinear properties, enable a plethora of physical
phenomena ranging from room temperature condensation to superfluidity. While
polaritons are usually exploited in high density regime, evidence of quantum
correlations at the level of few excitations has been recently reported, thus
suggesting the possibility of using these systems for quantum information
purposes. Here we show that integrated circuits of propagating single
polaritons can be arranged to build deterministic quantum logic gates in which
the two-particle interaction energy plays a crucial role. Besides showing their
prospective potential for photonic quantum computation, we also show that these
systems can be exploited for metrology purposes, as for instance to precisely
measure the magnitude of the polariton-polariton interaction at the two-body
level. In general, our results introduce a novel paradigm for the development
of practical quantum polaritonic devices, in which the effective interaction
between single polaritonic qubits may provide a unique tool for future quantum
technologies.
