A novel method of purification, purification through Zeno-like measurements [H. Nakazato, T. Takazawa, and K. Yuasa, Phys. Rev. Lett. 90, 060401 (2003)], is discussed extensively and applied to a few simple qubit systems. It is explicitly demonstrated how it works and how it is optimized. As possible applications, schemes for initialization of multiple qubits and entanglement purification are presented, and their efficiency is investigated in detail. Simplicity and flexibility of the idea allow us to apply it to various kinds of settings in quantum information and computation, and would provide us with useful and practical methods of state preparation.

A recently proposed purification method, in which the Zeno-like measurements of a subsystem can bring about a distillation of another subsystem in interaction with the former, is utilized to yield entangled states between distant systems. It is shown that the measurements of a two-level system locally interacting with other two spatially separated not coupled subsystems, can distill entangled states from the latter irrespectively of the initial states of the two subsystems.

A neutron-spin experimental test of the quantum Zeno effect (QZE) is discussed from a practical point of view, when the nonideal efficiency of the magnetic mirrors, used for filtering the spin state, is taken into account. In the idealized case the number N of (ideal) mirrors can be indefinitely increased, yielding an increasingly better QZE. By contrast, in a practical situation with imperfect mirrors, there is an optimal number of mirrors, Nopt, at which the QZE becomes maximum: more frequent measurements would deteriorate the performance. However, a quantitative analysis shows that a good experimental test of the QZE is still feasible. These conclusions are of general validity: in a realistic experiment, the presence of losses and imperfections leads to an optimal frequency Nopt, which is in general finite. One should not increase N beyond Nopt. A convenient formula for Nopt, valid in a broad framework, is derived as a function of the parameters characterizing the experimental setup.

We present a novel method for purifying quantum states, i.e. purification through Zeno-like measurements, and show an application to entanglement purijication. = 14)x (41 8 Q~' ( N ) , (2) Journal of Modern Optics

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