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Entropy

Quantum Entanglement Concentration Based on Nonlinear Optics for Quantum Communications

Yu‐Bo Sheng, Lan Zhou

Published May 16, 2013• 73 citations
Publisher

Abstract

Entanglement concentration is of most importance in long distance quantum communication and quantum computation. It is to distill maximally entangled states from pure partially entangled states based on the local operation and classical communication. In this review, we will mainly describe two kinds of entanglement concentration protocols. One is to concentrate the partially entangled Bell-state, and the other is to concentrate the partially entangled W state. Some protocols are feasible in current experimental conditions and suitable for the optical, electric and quantum-dot and optical microcavity systems.

DOI
10.3390/e15051776

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Entanglement concentration is of most importance in long distance quantum communication and quantum computation. It is to distill maximally entangled states from pure partially entangled states based on the local operation and classical communication. In this review, we will mainly describe two kinds of entanglement concentration protocols. One is to concentrate the partially entangled Bell-state, and the other is to concentrate the partially entangled W state. Some protocols are feasible in current experimental conditions and suitable for the optical, electric and quantum-dot and optical microcavity systems.
We present two realistic entanglement concentration protocols (ECPs) for pure partially entangled photons. A partially entangled photon pair can be concentrated to a maximally entangled pair with only an ancillary single photon with a certain probability, while the conventional ECPs require two copies of partially entangled pairs at least. Our first protocol is implemented with linear optics and the second protocol is implemented with cross-Kerr-nonlinearities. Compared with other ECPs, they do not need to know the accurate coefficients of the initial state. With linear optics, it is feasible with current experiments. With cross-Kerr-nonlinearities, it does not require sophisticated single-photon detectors and can be repeated to get a higher success probability. Moreover, the second protocol can get the higher entanglement transformation efficiency and it may be the most economical protocol by far. Meanwhile, both protocols are more suitable for multiphoton system concentration because they need less operations and classical communications. All these advantages make the two protocols useful in current long-distance quantum communications.
A recent paper [Chuan Wang, Phys. Rev. A \textbf{86}, 012323 (2012)] discussed an entanglement concentration protocol (ECP) for partially entangled electrons using a quantum dot and microcavity coupled system. In his paper, each two-electron spin system in a partially entangled state can be concentrated with the assistance of an ancillary quantum dot and a single photon. In this paper, we will present an optimal ECP for such entangled electrons with the help of only one single photon. Compared with the protocol of Wang, the most significant advantage is that during the whole ECP, the single photon only needs to pass through one microcavity which will increase the total success probability if the cavity is imperfect. The whole protocol can be repeated to get a higher success probability. With the feasible technology, this protocol may be useful in current long-distance quantum communications.
We propose practical schemes for concentrating entanglement of a pair of unknown partially entangled Bell states and three-photon W states with cross-Kerr nonlinearity. In these schemes, utilizing local operations and classical communication, two separated parties can obtain one maximally entangled photon pair from two previously shared partially entangled photon pairs, and three separated parties can obtain one maximally entangled three-photon W state and a maximally entangled cluster state from two identical partially entangled three-photon W states with a certain success probability. Finally, we discuss the influence of sources of error and decoherence on the schemes. The proposed setup only employs some linear optical elements and the cross-Kerr medium, which greatly reduces the difficulty of experimental realization. These schemes are feasible with current experimental technology.
High-dimensional quantum systems offer a number of advantages in larger information capacity, stronger noise resiliency, higher improved efficiency and accuracy over the qubit systems. In quantum communication the maximally entangled states will inevitably become mixed states or less-entangled pure states by the channel noise during the practical transmission or storage. We propose a universal scheme to concentrate nonlocal high-dimensional generalized Bell states with unknown parameters. After the cross-Kerr nonlinearities, XXX-quadrature homodyne measurements, and single-partite projection measurements are performed only at Bob's site, a two-qutrit maximally entangled Bell state can be distilled, while previous entanglement concentration protocols (ECPs) mostly focused on two-level qubit systems. The concentrated partially entangled qubit states, reserved as the by-product are the fascinating resources for some quantum information processing tasks. Moreover, single-qutrit projection measurement, the key ingredient for our ECP with unknown parameters, are completed by using linear optical elements. Additionally, linear optical high-dimensional ECP with known parameters are also designed.
We present two schemes to concentrate unknown partially entangled three-photon W states with weak cross-Kerr nonlinearity. In both schemes, the three distant parities can obtain a maximally entangled W state via local operations and classical communication with a higher success probability that is double that in Xiong and Ye’s scheme [J. Opt. Soc. Am. B28, 2030 (2011)]. The entanglement concentration schemes need only one party’s local operations. Our schemes are relatively simple because fewer linear optics elements are used. In addition, our schemes do not need to use sophisticated single-photon detectors, which makes the scheme more feasible with present techniques.
Maximal entanglement concentration for (n+1)(n+1)(n+1)-qubit states
We propose two schemes for concentration of (n+1)(n+1)(n+1)-qubit entangled states that can be written in the form of (α∣φ0⟩∣0⟩+β∣φ1⟩∣1⟩)n+1(α|φ_{0}\rangle|0\rangle+β|φ_{1}\rangle|1\rangle)_{n+1}(α∣φ0​⟩∣0⟩+β∣φ1​⟩∣1⟩)n+1​ where ∣φ0⟩|φ_{0}\rangle∣φ0​⟩ and ∣φ1⟩|φ_{1}\rangle∣φ1​⟩ are mutually orthogonal nnn-qubit states. The importance of this general form is that the entangled states like Bell, cat, GHZ, GHZ-like, ∣Ω⟩|Ω\rangle∣Ω⟩, ∣Q5⟩|Q_{5}\rangle∣Q5​⟩, 4-qubit cluster states and specific states from the 9 SLOCC-nonequivalent families of 4-qubit entangled states can be expressed in this form. The proposed entanglement concentration protocol is based on the local operations and classical communications (LOCC). It is shown that the maximum success probability for ECP using quantum nondemolition (QND) technique is 2β22β^{2}2β2 for (n+1)(n+1)(n+1)-qubit states of the prescribed form. It is shown that the proposed schemes can be implemented optically. Further it is also noted that the proposed schemes can be implemented using quantum dot and microcavity systems.