Robust quantum repeater with atomic ensembles and single-photon sources

Abstract

We present a quantum repeater protocol using atomic ensembles, linear optics and single-photon sources. Two local 'polarization' entangled states of atomic ensembles u and d are generated by absorbing a single photon emitted by an on-demand single-photon sources, based on which high-fidelity local entanglement between four ensembles can be established efficiently through Bell-state measurement. Entanglement in basic links and entanglement connection between links are carried out by the use of two-photon interference. In addition to being robust against phase fluctuations in the quantum channels, this scheme may speed up quantum communication with higher fidelity by about 2 orders of magnitude for 1280 km compared with the partial read (PR) protocol (Sangouard et al., Phys. Rev. A 77, 062301 (2008)) which may generate entanglement most quickly among the previous schemes with the same ingredients. Entanglement plays a fundamental role in quantum information science [1] because it is a crucial requisite for quantum metrology [2], quantum computation [3,4], and quantum communication [3,5]. Because of losses and other noises in quantum channels, the communication fidelity falls exponentially with the channel length. In principle, this problem can be overcome by applying quantum repeaters [5,6,7,8,9,10], in which initial imperfect entangled pairs are established over elementary links, these initial pairs are then purified to high fidelity entanglement and connected through quantum swaps [11,12] with doubled quantum communication length. With the quantum repeater protocol one may generate high fidelity long-distance entanglement with resources increasing only polynomially with communication distance. 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]. Moreover, entanglement generation is created by detecting a single photon from one of two ensembles. The probability of generating one excitation in two ensembles denoted by p is related to the fidelity of the entanglement, leading to the condition p ≪ 1 to guaranty an acceptable quality of the entanglement. But when p → 0, some experimental imperfections such as stray light scattering and detector dark counts will contaminate the entangled state increasingly [20], and subsequent processes including quantum swap and quantum communication become more challenging for finite coherent time of quantum memory [16].In recent papers [19,21], ...

Heralded high-efficiency quantum repeater with atomic ensembles assisted by faithful single-photon transmission

“…We would like to briefly discuss the imperfections of our quantum repeater protocol. The photon loss is the main imperfection, which is also of crucial importance for the previous quantum repeaters with photon interference 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 . The photon loss happens, due to the fiber absorbtion, diffraction, the cavity imperfection, and the inefficiency of the single-photon detectors.…”

“…Besides these protocols based on Mach-Zehnder-type interference, Zhao et al 21 22 proposed a robust quantum repeater protocol based on two-photon Hong-Ou-Mandel-type interference, which relaxes the long-distance stability requirements and suppresses the vacuum component to a constant item. Subsequently, the single-photon sources are embedded to improve the performance of robust quantum repeaters 23 24 25 . In addition, Rydberg blockade effect 26 is used to perform controlled-NOT gate between the two atomic ensembles in the middle node 27 28 , which makes the quantum entanglement swapping operation be performed deterministically.…”

Efficient Excitation of a Symmetric Collective Atomic State with a Single-Photon Through Dipole Blockade

“…But when p → 0, some experimental imperfections such as stray light scattering and detector dark counts will contaminate the entangled state increasingly [22], and subsequent processes including quantum swap and quantum communication become more challenging for finite coherent time of quantum memory [10]. To solve this problem, protocols based on single photon sources [10,23] were suggested.…”

Atomic-ensemble-based quantum repeater against general polarization and phase noise

“…In 2001, Duan et al [2] suggested a relatively simple realization of a quantum repeater (known as the DLCZ type), which utilized atomic ensembles as the quantum memories instead of using individual atoms. Since then, this attractive proposal has inspired a lot of scientific groups to study the atomic-ensemble-based quantum repeater and therefore various kinds of improved protocols have been proposed [3][4][5][6][7][8][9][10].…”