We propose a temporally multiplexed version of the Duan-Lukin-Cirac-Zoller (DLCZ) quantum-repeater protocol using controlled inhomogeneous spin broadening in atomic gases. A first analysis suggests that the advantage of multiplexing is negated by noise due to spin-wave excitations corresponding to unobserved directions of Stokes photon emission. However, this problem can be overcome with the help of a moderate-finesse cavity which is in resonance with Stokes photons, but invisible to the anti-Stokes photons. Our proposal promises greatly enhanced quantum repeater performance with atomic gases.
We propose a temporally multiplexed version of the Duan-Lukin-Cirac-Zoller (DLCZ) quantum-repeater protocol using controlled inhomogeneous spin broadening in atomic gases. A first analysis suggests that the advantage of multiplexing is negated by noise due to spin-wave excitations corresponding to unobserved directions of Stokes photon emission. However, this problem can be overcome with the help of a moderate-finesse cavity which is in resonance with Stokes photons, but invisible to the anti-Stokes photons. Our proposal promises greatly enhanced quantum repeater performance with atomic gases.
Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, will benefit from multiplexed photonic quantum memories. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of ^{87}Rb atoms. We employ the Duan-Lukin-Cirac-Zoller protocol where pairs of photons and single collective spin excitations (so-called spin waves) are created in several temporal modes using a train of write pulses. To make the spin waves created in different temporal modes distinguishable and enable selective readout, we control the dephasing and rephasing of the spin waves by a magnetic field gradient, which induces a controlled reversible inhomogeneous broadening of the involved atomic hyperfine levels. We demonstrate that by embedding the atomic ensemble inside a low finesse optical cavity, the additional noise generated in multimode operation is strongly suppressed. By employing feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove nonclassical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported capability is a key element of a quantum repeater architecture based on multiplexed quantum memories.
Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, rely on quantum memories for light [1]. Quantum repeaters can benefit from a modal multiplexing implementation of the memory, essentially scaling up the repeater's throughput [2]. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of 87-Rb atoms (as proposed in [3]). We employ the DLCZ protocol where pairs of photons and single collective spin excitations (so called spin-waves) are created [4]. The latter can then be efficiently transferred into a second single photon. For selective readout, we need to control the dephasing and rephasing of the spin-waves created in different temporal modes. We achieve this by a magnetic field gradient, which induces an inhomogeneous broadening of the involved atomic hyperfine levels [5]. By employing this steering technique, combined with cavity-enhanced noise suppression and feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove non-classical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon-photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported device is a crucial key element of a quantum repeater architecture implementing multiplexed quantum memories. [1] H.-J. Briegel, W. Dür, J. Cirac and P. Zoller; Phys. Rev. Lett. 81 5932 (1998) [2] C. Simon, H. de Riedmatten, M. Afzelius,N. Sangouard, H. Zbinden and N. Gisin; Phys. Rev. Lett. 98 190503 (2007) [3] C. Simon, H. de Riedmatten and M. Afzelius; Phys. Rev. A 82 010304(R) (2010) [4] L. Duan, M. Lukin, J. Cirac and P. Zoller, P; Nature 414 413 (2001) [5] B.Albrecht, P. Farrera, G. Heinze, M. Cristiani and H. de Riedmatten; Phys. Rev. Lett. 115 160501 (2015)
Practical realization of quantum repeaters requires quantum memories with high retrieval efficiency, multi-mode storage capacities, and long lifetimes. Here, we report a high-retrieval-efficiency and temporally multiplexed atom-photon entanglement source. A train of 12 write pulses in time is applied to a cold atomic ensemble along different directions, which generates temporally multiplexed pairs of Stokes photons and spin waves via Duan-Lukin-Cirac-Zoller processes. The two arms of a polarization interferometer are used to encode photonic qubits of 12 Stokes temporal modes. The multiplexed spin-wave qubits, each of which is entangled with one Stokes qubit, are stored in a "clock" coherence. A ring cavity that resonates simultaneously with the two arms of the interferometer is used to enhance retrieval from the spin-wave qubits, with the intrinsic retrieval efficiency reaching 70.4%. The multiplexed source gives rise to a ∼12.1-fold increase in atom-photon entanglement-generation probability compared to the single-mode source. The measured Bell parameter for the multiplexed atom-photon entanglement is 2.21(2), along with a memory lifetime of up to ∼125 µs.
Duan-Lukin-Cirac-Zoller quantum repeater protocol provides a feasible scheme to implement long-distance quantum communication and large-scale quantum networks. The elementary link, namely the entanglement between two atomic ensembles, is a fundamental component of quantum repeater. For practical quantum repeater, it is required that the elementary link can be prepared with high yield and the spin waves stored in atoms can be efficiently converted into photons on demand. However, so far, such quantum repeater link has not been demonstrated in experiments. Here, we demonstrate a proof-of-principle multiplexed quantum repeater link by entangling two temporally multiplexed quantum memory. Compared with a single-mode link, the successful preparation rate of the multiplexed link is increased by one order of magnitude. By using the cavity-enhanced scheme, the on-demand retrieval efficiency of atomic spin waves is improved to 70%, which is beneficial for the subsequent entanglement swapping between adjacent links. The realization of temporally multiplexed quantum repeater link with high retrieval efficiency lays a foundation for the development of practical quantum networks.
A source that can generate atom-photon quantum correlations or entanglement based on a quantum memory is a basic building block of quantum repeaters (QRs). To achieve high entanglement generation rates in ensemble-based QRs, spatial-, temporal-, and spectral-multimode memories are needed. Previous temporal-multimode memories are based on rephasing mechanisms in inhomogeneously broadened media. Here, by applying a train of write pulses in time, with each pulse coming from a different direction, to a homogeneously broadened atomic ensemble to induce Duan-Lukin-Cirac-Zoller-like Raman processes, we prepare up to 19 pairs of modes, namely, one spin-wave mode and one photonic time bin. Spin-wave-photon (i.e., atom-photon) entanglement is probabilistically produced in these mode pairs. Based on the stored spin-wave modes together with feedforward-controlled readout, we build a temporally multiplexed source and then demonstrate an 18.8-fold increase in the probability of the generation of spin-wave-photon entanglement compared to the sources that use individual modes. The measured Bell parameter for the multiplexed source is $2.30\ifmmode\pm\else\textpm\fi{}0.02$, and the memory lifetime is 30 \ensuremath{\mu}s.
We propose a quantum repeater protocol which builds on the well-known Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L. M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)10.1038/35106500], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multimode memories based on the principle of photon echo, using solids doped with rare-earth-metal ions. The use of multimode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.