Fast entanglement distribution with atomic ensembles and fluorescent detection

Quantum repeaters based on atomic ensemble quantum memories are promising candidates for achieving scalable distribution of entanglement over long distances. Recently, important experimental progress has been made toward their implementation. However, the entanglement rates and scalability of current approaches are limited by relatively low retrieval and single-photon detector efficiencies. We propose a scheme which makes use of fluorescent detection of stored excitations to significantly increase the efficiency of connection and hence the rate. Practical performance and possible experimental realizations of the new protocol are discussed.

Figure 1

(a) One atomic ensemble with a red (hexagon) and a blue (circle) level makes up each repeater node. At the first step of the protocol, neighboring nodes are entangled with red and blue links established asynchronously. At subsequent steps, distant nodes are connected via entanglement swapping. (b) Atomic level scheme. Encircled levels store spin waves. The transitions (i) and (ii) are employed for entanglement generation and fluorescent detection, respectively.

Figure 2

(a) Entanglement generation with the blue levels. (b) Entanglement connection consisting of a $\pi /2$ pulse between the storage levels (i) followed by fluorescent measurement of their populations (ii).

Figure 3

Ascending curves, left axis: Ratio of rates in the present dual-rail (solid) and single-rail (dashed) schemes with PIR to those of DLCZ and Ref. [5], assuming $L_{\mathrm{att}}=20$ km, $\eta =0.05$, optical depth 100, and efficiencies 0.95 for fluorescent detection in the present scheme and 0.4 for single-photon detection. Steps occur when the numbers of nodes change. Descending curves, right axis: corresponding absolute rates in the new scheme. The final fidelity is 90%.