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International Journal of Quantum Information

GENERATION OF GRAPH-STATE ENTANGLEMENT WITH ATOMIC ENSEMBLES VIA THE DIPOLE BLOCKADE MECHANISM

Hong Xie, MEI-XIANG CHEN

Published Feb 1, 2011• 0 citations
Publisher

Abstract

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.

DOI
10.1142/s0219749911007058

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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.
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 ${p}_{\text{success}}\ensuremath{\sim}{\ensuremath{\eta}}^{Q∕2}$, 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.
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.
We summarize our recent work on quantum information with atomic ensembles. In particular, we describe techniques for storing and manipulating quantum information in collective states of mesoscopic ensembles. Quantum information processing is is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting “dipole blockade” can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. Furthermore, we present schemes for entangling a very large number of atoms of a spinor 1 Bose Einstein condensate based on collisional interaction between atoms. As a final example, we discuss the possiblilty of creating entangled atomic states of a few bosonic atoms in a setup which mimics a “fractional quantum Hall effect” and allows the manipulation of anyonic excitations.
Entanglement between a single photon and a matter qubit is an indispensable resource for quantum repeater and quantum networks. With atomic ensembles, the entanglement creation probability is typically very low to inhibit high-order events. In this paper, we propose and experimentally realize a scheme that creates atom-photon entanglement with an intrinsic efficiency of 50%. We make use of Rydberg blockade to generate two collective excitations, lying in separate internal states. By introducing the momentum degree of freedom for the excitations, and interfering them via Raman coupling, we entangle the two excitations. Via retrieving one excitation, we create the entanglement between the polarization of a single photon and the momentum of the remaining atomic excitation, with a measured fidelity of 0.901(8). The retrieved optical field is verified to be genuine single photons. The realized entanglement may be employed to create entanglement between two distant nodes in a fully heralded way and with a much higher efficiency.
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.
We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting "dipole blockade" can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as nonclassical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.