Boson sampling with optical feedback

This work presents a theoretical model of boson sampling with optical feedback, in which a subset of the interferometer's output modes is looped back into the input modes. If the bosons are injected periodically into the input modes of the interferometer and optical feedback lines' length match the period of injection, it allows for interference between bosons injected at the consequent time iterations. We propose several methods methods for computing the output photon distributions in both output spacial and temporal modes, including not only standard spatiotemporal mode-unfolding technique, but also the Kraus-operator formalism, and a correlation-tensor-based approach. The two latter approaches help us to reveal that for random interferometers this system evolves to a unique stationary state over time. Because of the existence of the stationary state, we introduce new computational problem Stationary Distribution Boson Sampling which appears to be harder than conventional boson sampling problem and contains it as a special case when there are no optical feedback lines. We also show the experimental implementation of the boson sampling with optical feedback in integrated 25-channel linear-optical interferometer. Our results show significant influence of optical feedback lines on the output distribution and paves the way to the demostration of quantum computational advantages with single photons.