Open-loop and closed-loop control of flying qubits

Overview & Synopsis

We describe two recent techniques, along with related experiments, to control and reduce the noise affecting a photon polarization qubit. The first is based on the open-loop " bang-bang " method, where suitably tailored pulses are implemented on the system to adverse polarization decoherence. This requires only passive elements when the physical system is a photon and the operation are performed in space rather than in time. The second technique is based on a closed-loop " asymmetric feedback ", where some quantities are measured and used for a real-time correction of the system dynamics. This technique necessarily requires an active electronics to work.