Preserving photon qubits in an unknown quantum state with Knill Dynamical Decoupling -- Towards an all optical quantum memory*

The storage of quantum states and its distribution over long distances is essential for emerging quantum technologies such as quantum networks and long distance quantum cryptography. The implementation of polarization-based quantum communication is limited by signal loss and decoherence caused by the birefringence of a single-mode fiber. We investigate the Knill dynamical decoupling scheme, implemented using half-wave plates in a single mode fiber, to minimize decoherence of polarization qubit and show that a fidelity greater than $99\%$ can be achieved in absence of rotation error and fidelity greater than $96\%$ can be achieved in presence of rotation error. Such a scheme can be used to preserve any quantum state with high fidelity and has potential application for constructing all optical quantum memory, quantum delay line, and quantum repeater.