Atomic state preparation using coherent feedback control.

Initializing a qubit in a desired state with high fidelity is important in quantum information. The existing protocols achieve this task by rotating the qubit from a known initial state to the desired state. In atomic systems these rotations require resonant pulses with precise control over phase and shape of the amplitude of the pulse.  Also the atom has to be prepared in a known initial state (typically a ground state).

Here we propose a scheme to prepare an atom in an arbitrary state using coherent control feedback protocol.  In this scheme, the superposition state is obtained by interacting the atom with a sequence of controller photons. The proposed scheme works even with off-resonant pulses and doesn’t require any information about the initial state. We show that the fidelity can be brought close to unity by increasing the number of controller photons. We also discuss possible implementations of this scheme using the existing experimental setups.