A method of purification through Zeno-like measurements has been proposed by Nakazato and co-workers [H. Nakazato, T. Takazawa, and K. Yuasa, Phys. Rev. Lett. 90, 060401 (2003); H. Nakazato, M. Unoki, and K. Yuasa, Phys. Rev. A 70, 012303 (2004)]. A series of frequent measurements on a quantum system is demonstrated to result in the purification of another quantum system in interaction with the former. Inspired by this idea, we have designed a strategy to obtain entangled states through only two measurements. A deterministic purification method is also provided in this article.

We propose to nest 3M layers of explicitly constructed Uhrig's dynamical decoupling sequences of local control operators to achieve universal high-order protection of arbitrary and unknown M-qubit states on a 2 M -dimensional computational subspace. This is the case even if the mechanism responsible for population leakage from the computational subspace is unclear and the actual form of system-environment coupling is not given. Simple numerical experiments are carried out to illustrate our theoretical proposal.

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We propose a scheme to for fast conditional phase shift and creation entanglement of two qubits that interact with a common heat bath. Dynamical decoupling is applied in the scheme so that it works even in the regime of strong interaction between qubits and environment. Our scheme does not request any direct interaction between the two qubits.