Universal photonic quantum computation via time-delayed feedback

Significance Creating large entangled states with photons as quantum information carriers is a central challenge for quantum information processing. Since photons do not interact directly, entangling them requires a nonlinear element. One approach is to sequentially generate photons using a quantum emitter that can induce quantum correlations between photons. Here we show that delayed quantum feedback dramatically expands the class of achievable photonic quantum states. In particular, we show that in state-of-the-art experiments with single atom-like quantum emitters, the most basic form of delayed quantum feedback already allows for creation of states that are universal resources for quantum computation. This opens avenues for quantum information processing with photons using minimal experimental resources. We propose and analyze a deterministic protocol to generate two-dimensional photonic cluster states using a single quantum emitter via time-delayed quantum feedback. As a physical implementation, we consider a single atom or atom-like system coupled to a 1D waveguide with a distant mirror, where guided photons represent the qubits, while the mirror allows the implementation of feedback. We identify the class of many-body quantum states that can be produced using this approach and characterize them in terms of 2D tensor network states.