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arXiv:1911.10746 · Physical Review A

High-dimensional entanglement between a photon and a multiplexed atomic quantum memory

Chang Li, Yukai Wu, W. Chang, Sanjun Zhang, Yunfei Pu, Nan Jiang, Luming Duan

Published Mar 10, 2020• 12 citations
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Abstract

Multiplexed quantum memories and high-dimensional entanglement can improve the performance of quantum repeaters by promoting the entanglement generation rate and the quantum communication channel capacity. Here, we experimentally generate a high-dimensional entangled state between a photon and a collective spin-wave excitation stored in the multiplexed atomic quantum memory. We verify the entanglement dimension by the quantum witness and the entanglement of formation. Then we use the high-dimensional entangled state to test the violation of the Bell-type inequality. Our paper provides an effective method to generate multidimensional entanglement between the flying photonic pulses and the atomic quantum interface.

DOI
10.1103/physreva.101.032312

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Multiplexed quantum memories and high-dimensional entanglement can improve the performance of quantum repeaters by promoting the entanglement generation rate and the quantum communication channel capacity. Here, we experimentally generate a high-dimensional entangled state between a photon and a collective spin-wave excitation stored in the multiplexed atomic quantum memory. We verify the entanglement dimension by the quantum witness and the entanglement of formation. Then we use the high-dimensional entangled state to test the violation of the Bell-type inequality. Our paper provides an effective method to generate multidimensional entanglement between the flying photonic pulses and the atomic quantum interface.
The generation and distribution of entanglement are key resources in quantum repeater schemes. Temporally multiplexed systems offer time-bin encoding of quantum information which provides robustness against decoherence in fibers, crucial in long distance communication. Here, we demonstrate the direct generation of entanglement in time between a photon and a collective spin excitation in a rare earth ion doped ensemble. We analyze the entanglement by mapping the atomic excitation onto a photonic qubit and by using time-bin qubit analyzers implemented with another doped crystal using the atomic frequency comb technique. Our results provide a solid-state source of entangled photons with embedded quantum memory. Moreover, the quality of the entanglement is high enough to enable a violation of a Bell inequality by more than two standard deviations.
The use of multiplexed atomic quantum memories (MAQM) can significantly enhance the efficiency to establish entanglement in a quantum network. In the previous experiments, individual elements of a quantum network, such as the generation, storage, and transmission of quantum entanglement have been demonstrated separately. Here we report an experiment to show the compatibility and integration of these basic operations. Specifically, we generate photon-atom entanglement from any chosen pair of memory cells in a 6×5 MAQM, convert the spin-wave to time-bin photonic excitation after a controllable storage time, and then store and retrieve the photon in a second MAQM for another controllable storage time. The preservation of quantum information in this process is verified by measuring the state fidelity. We also demonstrate that higher dimension quantum states can be transferred between the two distant MAQMs.
Promising access to high-speed quantum networks relies on the creation of high-dimensional entangled memories that provide quantum communication with higher capacity of noisy quantum channels, thereby reducing the transmission time of information. Yet, the distribution of multidimensional entanglement between remote memory nodes is still faintly investigated. We propose an experimentally feasible protocol of deterministic generation of high-dimensional entanglement between distant multilevel atoms confined in high-finesse optical cavities and driven by laser pulses. Three-dimensional entanglement is generated deterministically between remote atoms by triggering a two-photon wave packet, which mediates a superposition of states chosen by the laser pulse parameters. The efficient transfer of atomic states between remote nodes allows the construction of a three-dimensional quantum repeater, where the successful creation of entanglement can be verified by a simple method for reliable measurement of atomic ground Zeeman states.
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.
We demonstrate a novel way to efficiently create a robust entanglement between an atomic and a photonic qubit. A single laser beam is used to excite one atomic ensemble and two different modes of Raman fields are collected to generate the atom-photon entanglement. With the help of built-in quantum memory, the entanglement still exists after 20.5 micros storage time which is further proved by the violation of Clauser-Horne-Shimony-Holt type Bell's inequality. The entanglement procedure can serve as a building block for a novel robust quantum repeater architecture [Zhao, Phys. Rev. Lett. 98, 240502 (2007)10.1103/PhysRevLett.98.240502] and can be extended to generate high-dimensional atom-photon entanglements.
Realization of long-distance quantum communication through the quantum repeater network requires a combination of some key elements, including a multiplexed quantum memory for storage of entanglement and a telecom photon for propagation of information through the fiber.Although impressive experimental advances have demonstrated the individual elements, combining these key capabilities together and realizing them in a single experimental system remains a significant challenge.Here, we report an experimental realization of long-distance entanglement between a multiplexed quantum memory with 49 individually accessible memory cells and a telecom photon after transmission in a 10-km optical fiber.Excitation of an atomic ensemble generates narrow-band polarization entanglement between a telecom photon of 1530-nm wavelength and another photon of 780-nm wavelength, which is then stored into a memory cell of a multiplexed atomic quantum memory and read out after a controllable delay.The entanglement is verified through quantum-state tomography after quantum storage in the atomic memory and fiber transmission of the telecom photon.This experiment demonstrates an important step towards realization of long-distance quantum communication networks.