Multiplexed ion-ion entanglement over $1.2$ kilometer fibers

Abstract

Quantum networks and quantum repeaters represent the promising avenues for building large-scale quantum information systems, serving as foundational infrastructure for distributed quantum computing, long-distance quantum communication, and networked quantum sensing. A critical step in realizing a functional quantum network is the efficient and high-fidelity establishment of heralded entanglement between remote quantum nodes. Multiplexing offers a powerful strategy to accelerate remote entanglement distribution, particularly over long optical fibers. Here, we demonstrate the first multiplexing-enhanced heralded entanglement between two trapped-ion quantum network nodes. By multiplexing $10$ temporal photonic modes, we achieve a 4.59-fold speedup in ion-ion entanglement generation and attain an entanglement fidelity of $95.9\pm1.5\%$ over $1.2$ km of fiber. Employing a dual-type architecture, our system is readily scalable to multiple nodes, thereby establishing a key building block for future large-scale quantum networks.

Figures (8)

• Figure 1: Multiplexed trapped-ion quantum network.a, The schematic of a multiplexed trapped-ion quantum network. Photonic interconnects are employed to link remote quantum network nodes, and multiplexing schemes are exploited to accelerate the remote entanglement generation. We use dual-type encoding to realize the trapped-ion quantum network node. Via the dual-type encoding, the lowest energy levels of a $^{40}$Ca$^+$ ion are splitted into two spectrally isolated subspaces, i.e., the communication qubit subspace and the memory qubit subspace, as shown in the lower inset ($|P_{1/2}\rangle$ is not shown here). The encoding conversions between the two subspaces and the qubit rotations are implemented by the Raman transitions and the $729\,$nm quadruple transition. The multiplexed ion-photon entanglement is generated by shining multiple $397\,$nm excitation pulses to the ion, as shown in the upper inset. We collect the $866\,$nm photons during the spontaneous emission of the excited state $|P_{1/2},m=-1/2\rangle$. All the cooling, pumping, ion-photon entanglement excitation, and state detection operations are performed in the communication subspace. b, The experimental setup of this work. The two quantum network nodes Alice and Bob each contain a $^{40}$Ca$^+$ inside, and a local controller is responsible for all the operations in each node. After the ion-photon entanglement generation, the emitted $866\,$nm photon from each node is transmitted through a $10\,$m or $600\,$m fiber to the measurement station in the center. The microprocessor 'QNetWorker' decides whether a successful heralding is achieved based on the pattern of the photon detector clicks, and sends back the heralding signal to each node, via a fiber of the same length for the flying qubit. The local controller in each node then determines the next step based on the returned heralding signal.
• Figure 2: Ion-ion entanglement over $20\,$m fibers.a, The histogram for the arrival times of $866\,$nm photons from both nodes. This histogram is measured by the detectors in the middle measurement station. The two wavepackets for photons from Alice and Bob have a small shift of $6\,$ps after careful adjustments. b, The reconstructed density matrices of the ion-photon entangled states for both Alice and Bob. c, The measurement results of correlations XX, YY, and ZZ for the heralded ion-ion entangled state. Totally $340$ entanglement events are recorded for the measurements, with an ion-ion entangling rate of $0.039\,$s$^{-1}$.
• Figure 3: Coherence time of each node. We measure the coherence time of Alice and Bob by preparing the ion in a superposition state $|+\rangle=\frac{|\uparrow_M\rangle+|\downarrow_M\rangle}{\sqrt{2}}$ at first. Then a spin echo is performed in the middle of the storage duration $\tau$. After $\tau$, the stored state is characterized, and the fidelity with respect to the initial state is obtained. The fidelity decay of the stored state versus the storage time $\tau$ is demonstrated in the figure. The fitted coherence times for Alice and Bob are $351\pm10\,$ms and $308\pm11\,$ms, respectively.
• Figure 4: Multiplexing-enhanced ion-ion entanglement over $1.2\,$km fibers.a, The time sequence for the multiplexed ion-ion entanglement generation. The combination of an EIT cooling of $200\,\mu$s and a block of remote entangling attempts is continuously running until an ion-ion entanglement is successfully heralded. Totally $10\times30=300$ photon excitations are performed in each block of attempts. b, The reconstructed density matrices for the ion-photon entangled states from Alice and Bob. The photon state is measured in the middle measurement station. c, The histogram for the arrival time of the $866\,$nm photons recorded by the detectors in the middle station. The $10$ narrower peaks correspond to the photons emitted in the ion-photon entanglement excitations, and the detection window for the ion-ion entanglement heralding is $45\,$ns. The $10$ wider peaks are the $866\,$nm photons emitted during the intermediate pumping. Successive photon excitations have a time interval of $500\,$ns in between. d, The correlations XX, YY, and ZZ for measuring the fidelity of heralded ion-ion entanglement. Totally $263$ entanglement events are recorded for the measurements, with an ion-ion entangling rate of $0.011$s$^{-1}$. e, The enhancement factor of the multiplexed ion-ion entanglement over single-mode case. The efficiency enhancement versus the mode number used in each excitation round is shown here. The efficiency of the remote ion-ion entangling is enhanced by $4.59$ times with $10$ time-bin modes in each round.
• Figure S1: Level scheme and transition diagram. a, A $\sigma_+$ polarized $397\,$nm continuous-wave laser is employed for state preparation and intermediate pumping, and a $397\,$nm picosecond pulses transfers the ion from $|S_{1/2},m=+1/2\rangle$ to $|P_{1/2},m=-1/2\rangle$. We collect the $866\,$nm photons in the spontaneous decay to $|D_{3/2}\rangle$ level for the ion-photon entanglement. b, $850/854\,$nm Raman transition connects the $|D_{3/2}\rangle$ manifold in the communication qubit subspace and $|D_{5/2}\rangle$ manifold in the memory qubit subspace. $866/866\,\mathrm{nm}$ Raman transition is used for rotating $\frac{1}{2}|1_C\rangle + \frac{\sqrt{3}}{2} |2_C\rangle$ into $|1_C\rangle$ coherently.