Generation and Purification of Atomic Entangled States

Ming Yang, Zhuo‐Liang Cao

In cavity QED, the atoms would be sent through the sequential arrays of cavities for the generation of multi-cavity entanglement, or several atoms would be sent into the same cavity mode one bye one for the generation of multi-atom entanglement. The complexity of these processes will impose limitations on the experimental feasibility of it. So, following our previous publication [International Journal Of Quantum Information 2, 231 (2004)] we will propose an alternative scheme for the preparation of multi-cavity W state via cavity QED, which uses the geometrical method to do what other authors have proposed previously using sequential arrays of cavities. Due to the impossibility that one quantum system can be isolated from the environment absolutely, the entanglement of the entangled objects will decrease exponentially with the propagating distance of the objects, and the practically available quantum entangled states are all non-maximally entangled states or the more general case–-mixed states. Following our previous publications [Phys. Rev. A 72, 042307 (2005), ibid. 71, 012308 (2005)], we will propose an entanglement generation, concentration and purification scheme for atomic or ionic system, which is mainly based on Cavity QED and linear optical elements. This purification process avoids the controlled-NOT (C-NOT) operations needed in the original purification protocol, which simplifies the whole purification process.

Yong Wook Cheong, Jinhyoung Lee

Thomas Pellizzari, S. A. Gardiner, J. I. Cirac +1 more

Zhuo‐Liang Cao, Ming Yang

In this paper, we propose a novel scheme that can generate two-atom maximally entangled states from pure product states and mixed states using linear optics. Because the scheme can generate pure maximally entangled states from mixed states, we denote it as purification-like generation scheme.

Simon A. Haine, Stuart S. Szigeti

Quantum-enhanced metrology can be achieved by entangling a probe with an auxiliary system, passing the probe through an interferometer, and subsequently making measurements on both the probe and auxiliary system. Conceptually, this corresponds to performing metrology with the purification of a (mixed) probe state. We demonstrate via the quantum Fisher information how to design mixed states whose purifications are an excellent metrological resource. In particular, we give examples of mixed states with purifications that allow (near) Heisenberg-limited metrology and provide examples of entangling Hamiltonians that can generate these states. Finally, we present the optimal measurement and parameter-estimation procedure required to realize these sensitivities (i.e., that saturate the quantum Cram\'er-Rao bound). Since pure states of comparable metrological usefulness are typically challenging to generate, it may prove easier to use this approach of entanglement and measurement of an auxiliary system. An example where this may be the case is atom interferometry, where entanglement with optical systems is potentially easier to engineer than the atomic interactions required to produce nonclassical atomic states.

Igor Bargatin, B. A. Grishanin, Виктор Николаевич Задков