Functional Quantum Nodes for Entanglement Distribution over Scalable Quantum Networks

Abstract

We demonstrate entanglement distribution between two remote quantum nodes located 3 meters apart [1]. This distribution involves the asynchronous preparation of two pairs of atomic memories and the coherent mapping of stored atomic states into light fields in an effective state of near maximum polarization entanglement. Entanglement is verified by way of the measured violation of a Bell inequality, and can be used for communication protocols such as quantum cryptography. The demonstrated quantum nodes and channels can be used as segments of a quantum repeater, providing an essential tool for robust long-distance quantum communication.In quantum information science [2], distribution of entanglement over quantum networks is a critical requirement, including for metrology [3], quantum computation [4,5] and communication [4,6]. Quantum networks are composed of quantum nodes for processing and storing quantum states, and quantum channels that link the nodes. Significant advances have been made with diverse systems towards the realization of such networks, including ions [7], single trapped atoms in free space [8,9] and in cavities [10], and atomic ensembles in the regime of continuous variables [11].An approach of particular importance has been the seminal work of Duan, Lukin, Cirac, and Zoller (DLCZ) for the realization of quantum networks based upon entanglement between single photons and collective excitations in atomic ensembles [12]. Critical experimental capabilities have been achieved, beginning with the generation of nonclassical fields [13,14] with controlled waveforms [15] and extending to the creation and retrieval of single collective excitations [16,17,18] with high efficiency [19,20]. Heralded entanglement with quantum memory, which is the cornerstone of networks with efficient scaling, was achieved between two ensembles [21]. More recently, conditional control of the quantum states of a single ensemble [22,23,24] and of two distant ensembles [25] has also been implemented, as are likewise required for the scalability of quantum networks based upon probabilistic protocols.Our interest is to develop the physical resources that enable quantum repeaters [6], thereby allowing entanglement-based quantum communication tasks over quantum networks on distance scales much larger than set by the attenuation length of optical fibers, including quantum cryptography [26]. For this purpose, heralded number state entanglement [21] between two remote atomic ensembles is not directly applicable. In- * Current address: Group of Applied Physics, University of Geneva, Geneva, Switzerland † Current address: Departamento de Física, Universidade Federal de Pernambuco, Recife-PE, 50670-901, Brazil stead, DLCZ proposed to use pairs of ensembles (U i , D i ) at each quantum node i, with the sets of ensembles {U i }, {D i } separately linked in parallel chains across the network [12]. Relative to the state of the art in Ref.[21], the DLCZ protocol requires the capability for the independent control of pairs of entangled ensembles...

Mapping photonic entanglement into and out of a quantum memory

“…Atomic ensembles can play the role of such nodes [4]. So far, in the photon counting regime, heralded entanglement between atomic ensembles has been successfully demonstrated via probabilistic protocols [5,6]. However, an inherent drawback of this approach is the compromise between the amount of entanglement and its preparation probability, leading intrinsically to low count rate for high entanglement.…”

“…In the regime of continuous variables, a particularly notable advance has been the teleportation of quantum states between light and matter [14]. For discrete variables with photons taken one by one, important achievements include the efficient mapping of collective atomic excitations to single photons [15,16,17,18,19], the realization of entanglement between a pair of distant ensembles [5,20] and, more recently, entanglement distribution involving two pairs of ensembles [6]. The first step toward entanglement swapping has been made [21] and lightmatter teleportation has been demonstrated [22].…”

“…1d, to obtain a single spatial mode with orthogonal polarizations for the L k , R k fields [6,20]. The diagonal elements of ρ are measured with (λ/2) v set at 0 • so that detection events at D 1 , D 2 are recorded directly for the L k , R k fields.…”

Coupling superconducting qubits via a cavity bus

“…A particular example is trapped ions 10,11 in which a variety of quantum operations and algorithms have been performed using the quantized motion of the ions (phonons) as the bus. Photons are another natural candidate as a carrier of quantum information 12,13 , because they are highly coherent and can mediate interactions between distant objects. To create a photon bus, it is helpful to utilize the increased interaction strength provided by the techniques of cavity quantum electrodynamics, where an atom is coupled to a single cavity mode.…”

Robust quantum repeater with atomic ensembles and single-photon sources

“…A protocol of special importance for long-distance quantum communication with atomic ensemble as local memory qubits and linear optics is proposed in a seminal paper of Duan et al [13]. After that considerable efforts have been devoted along this line [14,15,16,17,18,19,20].In additional to using relatively simple ingredients, DLCZ protocol has built-in entanglement purification and thus is tolerant against photon losses. However, entanglement generation and entanglement connection in the DLCZ protocol is based on a single-photon Mach-Zehnder-type interference, resulting the relative phase in the entangled state of two distant ensembles is very sensitive to path length instabilities [19,21].…”