Pioneering Quantum Networking: Achieving Scalable Entanglement of Remote Distinguishable Qubits
Quantum networks – where entanglement is distributed across distant nodes – promise to revolutionize quantum computing, communication, and sensing. However, a major bottleneck has been scalability, as the entanglement rate in most existing systems is limited by a network design of a single qubit per node. A new study, led by Prof. A. Faraon at Caltech and conducted by A. Ruskuc et al., recently published in Nature [1-2], presents a groundbreaking solution: multiplexed entanglement using multiple emitters in quantum network nodes. By harnessing rare-earth ions coupled to nanophotonic cavities, researchers at Caltech and Stanford have demonstrated a scalable platform that significantly enhances entanglement rates and network efficiency. Let’s take a closer look at the two key challenges they tackled – multiplexing to boost entanglement rates and dynamic control strategies to ensure qubit indistinguishability – and how they overcame them.
Figure 1: A sketch depicting the experimental setup inspired by Ruskuc et al. [1-2], orchestrated by OPX+. The photonic emission entangled with 171Yb qubits in two remote nanophotonic cavities is interfered and measured at a central beamsplitter before collection via Single-Photon Detectors (SPDs), forming a quantum network link.
Breaking the Entanglement Bottleneck via Multiplexing
One of the biggest challenges in scaling quantum networks is the entanglement rate bottleneck, which arises due to the fundamental constraints of long-distance quantum communication. When two distant qubits are entangled via photon interference, the rate of entanglement distribution is typically limited by the speed of light and the node separation distance. In typical systems with a single qubit per node, this rate scales as c/L (where c is the speed of light and L is the distance between nodes), leading to long waiting times between successful entanglement events. This severely limits the scalability of quantum networks.
To overcome this limitation, the researchers in this study introduced a multiplexed quantum network architecture, where multiple qubits—each a spectrally distinct rare-earth ion—are housed within a single node. This allows multiple entanglement attempts to be made simultaneously, boosting the rate to Nc/L, where N is the number of qubits per node. By increasing the number of emitters per node, the team achieved a nearly twofold increase in the entanglement rate, demonstrating that multiplexing is a viable path toward high-throughput quantum communication.
Real-Time Feedforward for Rare-Earth Ion Entanglement
Optically addressable spin qubits have emerged as leading candidates for developing quantum repeater networks. However, the scalability of these networks beyond current few-node configurations necessitates radical improvements in quantum link efficiencies and fidelities. Solid-state emitters are particularly promising due to the extremely long coherence times of their host spins (in this work they demonstrate T2 times of the Bell state of more than 9 ms, with dynamical decoupling), and potential for nanophotonic integration.
On the other hand, inherent spatial and temporal variations in host crystals present formidable challenges, including static shifts and dynamic fluctuations in optical transition frequencies. In fact, if the emitters are not perfectly identical, then the emitted photons will not be indistinguishable. This leads to photons with slightly different frequencies that won’t interfere with each other. The research team at the California Institute of Technology introduces a scalable approach leveraging frequency-erasing photon detection alongside Adaptive Quantum Circuits. Here, Quantum Machines’ OPX control platform enabled the researchers to implement the necessary measurement-conditioned feed-forward operations. This protocol falls into the category of Quantum Real-Time (QRT) [3], i.e. a measurement-based conditional operation for which the classical feedback latency should be compared to (and significantly shorter than) the qubit coherence time – not an easy feat for classical control systems.
“The Quantum Machines OPX control system has been an enabling technology for our research. It provides unparalleled flexibility and ease of use for experiments requiring real-time quantum feedforward control. With the integrated high-resolution time tagging, this platform is a no-brainer for advanced quantum networking experiments.”
Prof. Andrei Faraon T.J. Watson Laboratory of Applied Physics, California Institute of Technology
Optimizing Entanglement: Real-Time Compensation for Quantum Nodes
A major challenge in using multiple qubits within a single node is that each qubit (ion) has a different optical transition frequency due to slight variations in its local environment. While this spectral distinguishability is what enables multiplexing, it also introduces frequency fluctuations that degrade entanglement fidelity. The quantum state of each ion is heralded by the detection of a single photon, but due to the stochastic nature of photon emission times, these frequency variations introduce random phase shifts between entangled qubits.
To maintain high-fidelity entanglement, the researchers employed real-time quantum feedforward control, a technique that dynamically corrects these phase shifts based on the measured photon emission time. This is where Quantum Machines’ OPX controller played a crucial role. The protocol was composed of the following steps:
- Detect and Timestamp Single Photons in Real Time
a. The controller detected a photon from one of the qubits and recorded its arrival time with very high precision.
b. The emission time was used to determine the phase shift induced by frequency fluctuations, calculated in real-time during the sequence. - Dynamically Rephase the Entangled State
a. Based on the detected photon time, the controller applied a dynamically adjusted Z-rotation to compensate for the accumulated phase error.
b. The required phase correction was computed and applied within a single experimental cycle, a task that would be impossible with traditional non-real-time controllers. - Implement Conditional Microwave and Optical Pulses
a. The entanglement sequence involved applying microwave and optical pulses to manipulate the rare-earth ion qubits.
b. The controller ensured these pulses were triggered in response to real-time photon detection events, allowing adaptive error correction strategies to be implemented with minimal delay. - Enable Multiplexed Entanglement Distribution
a. The controller orchestrated parallel entanglement attempts across multiple qubit pairs.
b.It tracked which qubits successfully generated entanglement and dynamically adjusted subsequent operations to focus on successful pairs.
By eliminating the stochastic phase, this protocol achieves two critical milestones: optical lifetime-limited entanglement rates with fidelities robust against spectral diffusion, and entanglement of optically distinguishable spin qubits without requiring frequency tuning. This efficient paradigm for quantum networking holds immense promise, enabling the realization of frequency-multiplexed multi-qubit nodes that can be robustly entangled.
Rare-Earth Ions: Catalysts for Quantum Communication Evolution
In essence, this research not only provides a practical solution to universal limitations imposed by non-uniformity and instability in solid-state emitters but also showcases the potential of single rare-earth ions as a scalable platform for the future quantum internet. As we venture further into the realm of quantum technologies, such pioneering advancements will undoubtedly shape the landscape of communication, computation, and more.
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References
[1] Ruskuc, Andrei, et al. “Scalable multipartite entanglement of remote rare-earth ion qubits.” arXiv preprint arXiv:2402.16224 (2024).
[2] Ruskuc, A., Wu, CJ., Green, E. et al. “Multiplexed entanglement of multi-emitter quantum network nodes”. Nature (2025).
[3] Ella, Lior, et al. “Quantum-classical processing and benchmarking at the pulse-level, 2023.” arXiv preprint arXiv:2303.03816 (2023)
