Multicell Atomic Quantum Memory as a Hardware-Efficient Quantum Repeater Node

For scalable quantum communication and networks, a key step is to realize a quantum repeater node that can efficiently connect different segments of atom-photon entanglement using quantum memories. We report a compact and hardware-efficient realization of a quantum repeater node using a single atomic ensemble for multicell quantum memories. A millisecond lifetime is achieved for individual memory cells after suppressing the magnetic-field-induced inhomogeneous broadening and the atomic-motion-induced spin-wave dephasing. Based on these long-lived memory cells, we achieve heralded asynchronous entanglement generation in two quantum repeater segments one after another and then an on-demand entanglement connection of these two repeater segments. As another application of the multicell atomic quantum memory, we further demonstrate storage and on-demand retrieval of heralded atomic spin-wave qubits by implementing a random access quantum memory with individual addressing capacity. This work provides a promising constituent for efficient realization of quantum repeaters for large-scale quantum networks.

Popular Summary

Photons are ideal carriers of information for long-distance quantum communication and large-scale quantum networks; however, they are still subject to exponential loss in optical fibers. To overcome this problem, quantum repeaters are required that divide the communication into multiple segments and use quantum memories at the nodes to connect segments of entanglement. A key step for scalable quantum networks is thus to realize quantum repeater nodes that can efficiently connect different segments of atom-photon entanglement using quantum memories with long enough coherence to boost the connection success probability.

Here, we report a compact and hardware-efficient realization of a quantum repeater node using a single atomic ensemble for multicell quantum memories. A millisecond lifetime is achieved for individual memory cells after suppressing the magnetic-field-induced inhomogeneous broadening and the atomic-motion-induced spin-wave dephasing. Based on these long-lived memory cells, we achieve heralded asynchronous entanglement generation in two quantum repeater segments one after another and then an on-demand entanglement connection of these two repeater segments. As another application of the multicell atomic quantum memory, we also demonstrate storage and on-demand retrieval of heralded atomic spin-wave qubits by implementing a random access quantum memory with individual addressing of multiple long-lived memory cells.

Compared with the previous work, this experimental implementation of the quantum repeater node has the advantages of being compact and hardware efficient, less vulnerable to environmental noise, and more convenient for manipulation of information stored in different memory cells. The experiment thus provides a promising constituent for efficient realization of quantum repeaters for future large-scale quantum networks.

Figure 1

The experimental setup for the long-lived MAQM. (a) A simplified setup: HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarization beam splitter; SPD, single-photon detector. The angles between the write or idler beams and the read or signal beams are the same ${1.5}^{\circ }$. The Gaussian sizes of the signal (idler), write (read), and pump 1 (pump 2) modes are $65\mu \mathrm{m}$, $125\mu \mathrm{m}$, and $275\mu \mathrm{m}$, respectively. For clarity, $3\times 4$ memory cells are sketched in the figure, which are the regions selected from the cold ensemble by the orthogonally placed AODs. The distance between two adjacent memory cells is $420\mu \mathrm{m}$. The Fabry-Perot cavities (etalons) are used to filter the leakage pulses from the signal and idler modes. The bias magnetic field $B$ is used to define the quantization axis. Two cells are addressed to manifest the entanglement generation between the signal photon and the spin-wave excitation in two spatial modes. More details about the setup can be found in Appendix pp1. (b) The energy diagram. The ensemble is initially prepared on the ground state $|g⟩$. The storage states, $|s⟩$ and $|s^{\mathrm{\prime }}⟩$, are used to store the spin-wave excitation. A write pulse (red solid arrow) is applied to generate a signal photon (orange dashed arrow) and a spin wave. The spin wave is transferred by two Raman pumping beams, $P1$ (green solid arrow) and $P2$ (gold solid arrow), and it is finally retrieved by a read pulse (blue solid arrow) into the idler photon (purple dashed arrow). The write beam is red detuned by $28\mathrm{MHz}$ and the detuning of $P1$ and $P2$ is $40\mathrm{MHz}$. (c) The experimental sequence. The ensemble in magneto-optical trap (MOT) is compressed, polarization gradient cooled, and further optical pumped to the ground state before the experiment. The circled part is one complete write-in and read-out cycle, with a storage time $t$ if a signal photon is detected. The pump 1 and pump 2 pulses are both in a temporal Gaussian profile (for details, see Appendix pp2).

Figure 2

The MAQM with a long lifetime. (a) The retrieval efficiency of the central cell (green dot) and the entanglement fidelity for a path qubit encoded in the two central cells with the corresponding signal photon (orange square) versus the storage time $t$. The error bars are calculated by Monte Carlo simulation with the assumption of a Poisson distribution for the photon counts. The lifetime of the cell is fitted from an exponential function. (b) The color map of the retrieval efficiency after $t=0.1\mathrm{ms}$ and the fitted storage lifetime (the number in each cell in units of milliseconds). The four circled cells are used to generate multiple spin waves in the following experiments.

Figure 3

Entanglement connection at a quantum repeater node. (a) The scheme of the entanglement connection. A successful Bell-state measurement (BSM) of two spin waves heralds the connection of two adjacent quantum repeater links. (b) The experimental sequence. After two entangled states are generated in cell pairs 1 and 2, we wait for $10\mu \mathrm{s}$ to project the $L$ modes of the spin waves onto the state $|{\varphi }_L⟩$ and then for another $5\mu \mathrm{s}$ to project the $R$ modes onto the state $|{\varphi }_R⟩$. (c) The reconstructed density matrix of the two signal photons. $L$ and $R$ represent the signal photons detected from the corresponding memory cells of the two pairs. The signal-photon detection counts are recorded only when the two idler photons are also detected. The measured state fidelity is $76.6\pm 3.9\mathrm{\%}$.

Figure 4

Random access quantum memory. (a) An illustration of a RAQM. Multiple memory cells are used to store the qubits and the qubits can be read out in an arbitrary order. (b) The time sequence for the experiment. First, cell pair 1 is addressed to generate the spin wave. Once a signal photon is detected, we wait for a time interval of $10\mu \mathrm{s}$ to switch all the AODs to address cell pair 2. Then, after the spin wave is generated in the second cell pair, we wait for $10\mu \mathrm{s}$ to read out the qubit $i(i=1,2)$ and for another $5\mu \mathrm{s}$ to read out the other qubit $j(j\ne i)$. We consider six complementary input states $|L⟩$, $|R⟩$, $|\pm ⟩=(|L⟩\pm |R⟩)/\sqrt{2}$, $|{\sigma }_{\pm }⟩=(|L⟩\pm i|R⟩)/\sqrt{2}$ for each qubit. In (c) and (d), we prepare the two qubits in the same states and read them out in the order of qubits 1–2 and qubits 2–1, respectively, to measure the fidelity. The qubits in different states are also stored as shown in (e) (fix qubit 1 to $|+⟩$ and vary qubit 2) and in (f) (fix qubit 2 to $|+⟩$ and vary qubit 1). The state fidelity is calculated by reconstructing the density matrix through quantum state tomography. The error bars denote one standard deviation.

Figure 5

The detailed configuration of the setup.

Figure 6

The crosstalk error at storage time $t=10\mu \mathrm{s}$.