Long-Distance Distribution of Atom-Photon Entanglement at Telecom Wavelength

Abstract

Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation it will enable for efficient distribution of quantum states. Here we report on the generation and observation of entanglement between a Rb-87 atom and a photon at telecom wavelength over 20 km optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S-band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of 78.5±0.9% over 20 km optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.

Long-lived and multiplexed atom-photon entanglement interface with feed-forward-controlled readouts

“…Considering an MQI that stores 12 spatial and 10 temporal SW qubits, the total number of memory qubits is N m ¼ 120. To minimize transmission losses in fibers, the Stokes photons (795 nm) has to be converted into photons in the telecommunications band 49,[85][86][87] . The lower retrieval efficiency (15%) can be increased using high optical-depth cold atoms 75,76,88,89 or coupling the atoms with an optical cavity 24,39 to enhance the collective interference (Supplementary Note 7).…”

“…In each circle, the duration for the preparation of cold atoms and that for the experimental run of the SWPE generation are 42 ms and 8 ms, respectively, corresponding to a 20-Hz cycle frequency. During preparations, >10 8 atoms of 87 Rb are trapped in a twodimension magneto-optical trap (MOT) for 41.5 ms and further cooled by Sisyphus cooling for 0.5 ms. The cloud of cold atoms has a size of~5 × 2 × 2 mm 3 , a temperature of~100 μK and an optical density of about 14.…”

“…To overcome this key difficulty, we developed a polarization interferometer that encodes multiple Stokes modes correlated with magnetic-field-insensitive SWs into multiplexed photonic qubits. We used an ensemble of laser-cooled 87 Rb atoms as DLCZ-type memory and the approximately collinear configuration to suppress the motion-induced decoherence 23 . The polarization interferometer was formed from two identical beam displacers (BDs).…”

Experimental device-independent quantum key distribution between distant users

“…The CHSH value is found to be S = 2.578(75) using Eq. ( 1) with E 2,0 = −0.599( 41), E 3,0 = −0.664( 36), E 2,1 = 0.618 (39), and E 3,1 = −0.697 (35). The QBERs are given by the correlation data for X = Y , i.e., Q 0 = 0.0781(127) and Q 1 = 0.0777(132), which gives an average error rate of Q = 0.0779(91).…”

“…The two atomic qubits are independently analysed via state-selective ionisation (Fig. 3b) [39]. There, a particular state of the atomic qubit is ionised and leaves the trap depending on the polarisation χ = cos(γ)V + e −iφ sin(γ)H of a read-out laser pulse (γ = α for Alice's and γ = β for Bob's device).…”