Preparation and entanglement purification through two-step measurements

Abstract

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.

Quantum repeater based on cavity QED evolutions and coherent light

“…Using this extended setup in the framework of approach proposed in our previous paper, we have checked that the purified fidelity obtained using four successive purification rounds is slightly smaller as the fidelity F 4 obtained in this paper. However, since one successful purification round is conditioned upon the generation of the ancilla state ( 8) and atomic postselection (15), we obtained a much lower probability of success if compared with P pd derived above. This result leads, in turn, to a dramatically low success probability associated with the entire scheme and almost vanishing repeater rates.…”

“…[16], in which the fidelity of a single low-fidelity entangled pair becomes gradually increased at the cost of all other low-fidelity entangled pairs available. Indeed, in our approach we also produced first the entangled pair (15) or (16) described by the fidelity F 0 that became gradually increased up to F 4 at the cost of entangled pairs associated with the atoms 3, . .…”

Sensitivity of measurement-based purification processes to inner interactions

“…Very recently, generation of long-lived singlet pairs in a nuclear spin ensemble coupled to the electron spins of a Nitrogen Vacancy center in diamond has been proposed [17]. Over the years, generalizations of the repeated-measurement based purification scheme have been proposed: we mention schemes involving iterative operations (not necessarily measurments) [18] and a two-step measurements scheme [19]. It is the case to underline that very recently many papers on purification protocols, in different physical contexts, have appeared in literature witnessing the importance and the actuality of this topic [20,21,22,23].…”