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arXiv:0807.3645 · Physical Review A

High-efficiency cluster-state generation with atomic ensembles via the dipole-blockade mechanism

Marcin Zwierz, Pieter Kok

Published Feb 4, 2009• 26 citations
arXivHTMLPDF

Abstract

We demonstrate theoretically a scheme for cluster-state generation, based on atomic ensembles and the dipole-blockade mechanism. In the protocol, atomic ensembles serve as single-qubit systems. Therefore, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultracold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step $Q$-qubit GHZ states with success probability ${p}_{\text{success}}\ensuremath{\sim}{\ensuremath{\eta}}^{Q∕2}$, where $\ensuremath{\eta}$ is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.

DOI
10.1103/physreva.79.022304

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We demonstrate theoretically a scheme for cluster-state generation, based on atomic ensembles and the dipole-blockade mechanism. In the protocol, atomic ensembles serve as single-qubit systems. Therefore, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultracold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step QQQ-qubit GHZ states with success probability psuccess\ensuremath∼\ensuremathηQ∕2{p}_{\text{success}}\ensuremath{\sim}{\ensuremath{\eta}}^{Q∕2}psuccess​\ensuremath∼\ensuremathηQ∕2, where \ensuremathη\ensuremath{\eta}\ensuremathη is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
This paper proposes scalable schemes to generate the Greenberger–Horne–Zeilinger (GHZ) state and the cluster state with atomic ensembles via the dipole blockade mechanism on an atom chip, where the qubit is not carried by a single atom but an atomic ensemble. In the protocols, multiqubit entangled states are determinately prepared. Needlessness for single-photon source further decreases the complexity of the experiment. Based on the present laboratory technique, the schemes may be realized. The achieved results reveal a prospect for large-scale quantum communication and quantum computation.
A scheme is proposed for generating graph-state entanglement between many atomic ensembles. In this scheme, the photons are transferred from the single-"spin" excitations of the atomic ensembles via the dipole blockade mechanism, which act as ancillary qubits and interfere with each other on linear optics apparatus. The quantum-entangled state of the atomic ensembles can be obtained under the conditional detection of the photons. Since the single-"spin" excitations can be deterministically created via strong long-range dipole–dipole interaction and the transferred efficiency is enhanced by the many-atom interference effects, our scheme can work with a high probability of success.
Abstract The central technological appeal of quantum science resides in exploiting quantum effects, such as entanglement, for a variety of applications, including computing, communication and sensing 1 . The overarching challenge in these fields is to address, control and protect systems of many qubits against decoherence 2 . Against this backdrop, optical photons, naturally robust and easy to manipulate, represent ideal qubit carriers. However, the most successful technique so far for creating photonic entanglement 3 is inherently probabilistic and, therefore, subject to severe scalability limitations. Here we report the implementation of a deterministic protocol 4–6 for the creation of photonic entanglement with a single memory atom in a cavity 7 . We interleave controlled single-photon emissions with tailored atomic qubit rotations to efficiently grow Greenberger–Horne–Zeilinger (GHZ) states 8 of up to 14 photons and linear cluster states 9 of up to 12 photons with a fidelity lower bounded by 76(6)% and 56(4)%, respectively. Thanks to a source-to-detection efficiency of 43.18(7)% per photon, we measure these large states about once every minute, which is orders of magnitude faster than in any previous experiment 3,10–13 . In the future, this rate could be increased even further, the scheme could be extended to two atoms in a cavity 14,15 or several sources could be quantum mechanically coupled 16 , to generate higher-dimensional cluster states 17 . Overcoming the limitations encountered by probabilistic schemes for photonic entanglement generation, our results may offer a way towards scalable measurement-based quantum computation 18,19 and communication 20,21 .
In the famous quantum communication scheme developed by Duan et al. [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature (London) 414 (2001) 413 ], the probability of successful generating a symmetric collective atomic state with a single-photon emitted have to be far smaller than 1 to obtain an acceptable entangled state. Based on strong dipole-dipole interaction between two Rydberg atoms, two simultaneous excitations in an atomic ensemble are greatly suppressed, which makes it possible to excite a mesoscopic cold atomic ensemble into a near-ideal singly-excited symmetric collective state accompanied by a signal-photon with near unity success probability.
The generation of various entangled states is an essential task in quantum information processing. Recently, a scheme ( PRA 79, 022304 ) has been suggested for generating Greenberger–Horne–Zeilinger state and cluster state with atomic ensembles based on the Rydberg blockade. Using similar resources as the earlier scheme, here we propose an experimentally feasible scheme of preparing arbitrary four-qubit W class of maximally and non-maximally entangled states with atomic ensembles in a single step. Moreover, we carefully analyze the realistic noises and predict that four-qubit W states can be produced with high fidelity ( F ∼ 0.994) via our scheme.