I discuss a scheme for preparing two mutually non-interacting fixed qubits in a maximally entangled state. By repeating on- and off-resonant scattering of ancilla qubits, the state of the target qubits is driven from an arbitrary initial state into the singlet state with probability 1. Neither the preparation nor the post-selection of the ancilla spin state is required. The convergence from an arbitrary input state to the unique fixed point (mixing property) is proved rigorously, and its robustness is investigated, by scrutinizing the effects of imperfections in the incident wave of the ancilla, such as mistuning to a resonant momentum, imperfect monochromatization, and fluctuation of the incident momentum, as well as detector efficiency.

K. Yuasa, D. Burgarth, V. Giovannetti, and H. Nakazato,
arXiv:0909.3401 [quant-ph]