We present a multiplexed quantum repeater protocol based on an ensemble of laser-cooled and trapped rubidium atoms inside an optical ring cavity. We have already demonstrated strong collective coupling in such a system and have constructed a multiplexing apparatus based on a two-dimensional acousto-optical deflector. Here, we show how this system could enable a multiplexed quantum repeater using collective excitations with non-trivial spatial phase profiles (spinwaves). Calculated entanglement generation rates over long distances reveal that such a multiplexed ensemble-cavity platform is a promising route towards long distance quantum entanglement and networking.
We present a multiplexed quantum repeater protocol based on an ensemble of laser-cooled and trapped rubidium atoms inside an optical ring cavity. We have already demonstrated strong collective coupling in such a system and have constructed a multiplexing apparatus based on a two-dimensional acousto-optical deflector. Here, we show how this system could enable a multiplexed quantum repeater using collective excitations with non-trivial spatial phase profiles (spinwaves). Calculated entanglement generation rates over long distances reveal that such a multiplexed ensemble-cavity platform is a promising route towards long distance quantum entanglement and networking.
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.
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.
Single photons are the best carriers of quantum information for long-distance transmission. Nevertheless, maximal achievable distance is limited by the exponential decay of photons as a function of link length. The protocol of quantum repeater provides a promising solution by replacing direction transmission with segmented entanglement distribution and entanglement connection via swapping. The quantum repeater necessitates a key element of quantum memory for making efficient interconnections. An atomic ensemble is very suitable for this purpose due to the collective enhanced interaction. Single photons are stored as collective excitations in an atomic ensemble. Thus a comprehensive study of the physics relating to collective excitations is crucially important for improving the quantum memory performance and its reachable applications in quantum repeater and quantum network. In this article, we review our experimental work on cold atomic ensembles in recent years, focusing on the coherent manipulation of collective excitations. We first briefly introduce the general concept of collective excitations and the preparation process through spontaneous Raman scattering, and we review our experimental work on extending the coherence time, such as suppressing motional dephasing by increasing the spin-wave wavelength, by confining atoms with a three-dimensional optical lattice. Afterwards, we discuss about the retrieval process of collective excitations and review our experiments on using a ring-cavity enhanced setup to improve the retrieval efficiency. The coherent qubit operation in a quantum memory is very useful for enabling new functionalities for a quantum network, in a subsequent section, we thus review our work on developing Raman-based coherent operations for single excitations. Afterwards, we mention our experiments on creating a pair of atom-photon entanglement by interfering two modes of a collective excitation. Improving the entanglement preparation efficiency is crucially important, and Rydberg-based interaction provides a promising solution. Our experimental work in this direction is also reviewed. Additionally, as an application in coherent manipulation with collective excitations, we show several experiments on using excitations in remote atomic memories and demonstrating basic functionality of quantum repeater and quantum network. In short, significant progress has been made in the coherent manipulation of single collective excitations in cold atomic ensembles, and further improvement will be accelerated by the Rydberg-enabled interactions; practical applications in quantum repeater and quantum network is foreseeable in the near future.
A long-distance quantum network that could enable distributed quantum computing and unconditionally secure communication will require a quantum memory capable of supporting entanglement swapping over long distances. The memory qubit must preserve its entanglement to the communication qubit sent to a repeater for at least long enough to swap entanglement with an adjacent node. The most important figures-of-merit for a quantum memory are: fidelity, entanglement rate, and memory lifetime. Though many varieties of memories and repeaters are currently being researched (for a review see [1]) it remains an outstanding challenge to develop one that simultaneously meets all the necessary criteria to enable a practical long distance quantum network. We propose to use laser-cooled and trapped rubidium atoms, with Rydberg excitations, coupled to an optical cavity, to address these issues.
Abstract Quantum interfaces (QIs) that generate entanglement between photonic and spin-wave (atomic memory) qubits are basic building block for quantum repeaters. Realizing ensemble-based repeaters in practice requires quantum memory providing long lifetimes and multimode capacity. Significant progress has been achieved on these separate goals. The remaining challenge is to combine the two attributes into a single QI. Here, by establishing spatial multimode, magnetic-field-insensitive and long-wavelength spin-wave storage in laser-cooled atoms inside a phase-passively-stabilized polarization interferometer, we constructed a multiplexed QI that stores up to three long-lived spin-wave qubits. Using a feed-forward-controlled system, we demonstrated that a multiplexed QI gives rise to a 3-fold increase in the atom–photon (photon–photon) entanglement-generation probability compared with single-mode QIs. For our multiplexed QI, the measured Bell parameter is 2.51±0.01 combined with a memory lifetime of up to 1 ms. This work represents a key step forward in realizing fiber-based long-distance quantum communications.
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrate ES between different spatial modes for a cold-atom-ensemble quantum memory via the Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically correlated spin-wave-photon pairs in 12 spatial modes. We then prepare two entangled pairs of spin-wave modes through memory multiplexing. Via single-photon Bell-state measurement on retrieved fields from two spin-wave modes, we project the two remaining modes never entangled previously into an entangled state with a measured concurrence of C =0.0124±0.0030. The successful probability of ES in our scheme is increased by three times, compared with that in the scheme without memory multiplexing. Our presented work shows that the generation of entanglement ( C >0) between the remaining spin-wave modes requires the average cross-correlation function of the spin-wave-photon pairs to be ≥30. This presented work represents a significant step toward practical quantum repeaters.