Directionally-unbiased multiports and topological states. The goal is to entangle states associated with distinct topological sectors, and to do so in a way that allows this entangled topology to be readily available for information processing and detection. Specifically, linear optics will be used to produce: (i) winding-number-entangled bulk states, and (ii) an entangled pair of error-protected memory registers. To create the states, a source of initial polarization-entangled light is necessary, specifically type-II spontaneous parametric down conversion (SPDC) in a nonlinear crystal. All further processing requires only linear optical elements. Topological invariants characterize global properties of systems and cannot be easily distinguished by localized measurements. This difficulty in measurement traditionally limits their use in many applications. That problem is solved here by linking topology to a more easily-measured variable, polarization. Polarization and winding number will be tightly correlated (and in fact, jointly entangled with each other), but will serve distinct purposes: winding number provides stability against perturbations, while polarization allows easy access and measurement.
Jointly-entangled topologically-protected bulk states. Start with a polarization-entangled photon source, type-II spontaneous parametric down conversion in a nonlinear crystal allowing the two-photon output to be taken as an entangled Bell state. The goal is to convert this into a state of entangled winding number, while keeping polarization entanglement intact to use for control and measurement purposes.