Scalable controlled-NOT gate for linear optical quantum computing using microring resonators

We propose a scalable version of a Knill-Laflamme-Milburn (KLM) controlled-NOT (CNOT) gate based upon integrated waveguide microring resonators (MRR), vs the original KLM approach using beam splitters. The core element of our KLM CNOT gate is a nonlinear phase-shift gate (NLPSG) using three MRRs, which we examine in detail. We find an expanded parameter space for the NLPSG over that of the conventional version. Whereas in all prior proposals for bulk optical realizations of the NLPSG the optimal operating point is precisely a single zero-dimensional manifold within the parameter space of the device, we find conditions for effective transmission amplitudes which define a set of one-dimensional manifolds in the parameter spaces of the MRRs. This allows for an increased level of flexibility in operation of the NLPSG that allows for the fabrication of dynamically tunable MRR-based devices with high precision and low loss.