Insider Brief:
- Researchers at Caltech demonstrated multiplexed entanglement, using ytterbium ions in nanophotonic cavities to increase entanglement rates by leveraging multiple qubits per node.
- Their approach utilizes frequency-multiplexed photons and a quantum feed-forward correction system, which compensates for frequency mismatches and ensures stable entanglement distribution.
- Multiplexing significantly boosts quantum communication bandwidth and reliability, allowing entanglement to be established in parallel across multiple qubits, reducing delays and improving network resilience.
- This advancement supports the development of scalable quantum networks and a future quantum internet, though additional technologies, such as quantum repeaters, will be needed for long-distance implementation.
- Image Credit: Ruskuc et al. 2025, Figure 3, Nature, https://doi.org/10.1038/s41586-024-08537-z
At one point, entanglement featured mainly at the center of heated debates, most famously in the clash between Einstein and Bohr. Today, it serves as the foundation of quantum communication, threading its way through networks that could redefine information exchange. But as promising as quantum networks are, their practical limitations remain a challenge—especially when it comes to the bandwidth and stability of distributed entanglement.
A single optically addressed qubit per node has been a major constraint, limiting both communication speed and memory resources in quantum networks. Now, a new approach—multiplexed entanglement—provides a way to increase entanglement rates by using multiple qubits per node. Researchers at Caltech, as presented in a recent study published in Nature, have successfully demonstrated this method using ytterbium atoms embedded in nanophotonic cavities in pursuit of scalable quantum networking.
Entanglement in Quantum Networks
Entanglement is central to quantum networks, enabling two or more particles to share correlated states regardless of distance. When integrated into a communication system, entanglement allows for secure quantum key distribution and ultra-sensitive measurement techniques. However, distributing entanglement efficiently across distant nodes has been a persistent challenge. The fundamental bottleneck arises from the time it takes for quantum information to propagate—when entanglement is established between two remote nodes, photons carrying quantum states must travel from one to the other, limiting the rate of communication.
Traditional approaches rely on single qubits per node, meaning each entanglement event must complete before the next can begin. This restricts scalability, as networks must wait for each previous entangled state to be processed before establishing the next.
The Multiplexing Solution
In classical communication, multiplexing is a technique that allows multiple signals to be transmitted over the same channel simultaneously, increasing overall bandwidth. The concept translates into quantum networking through entanglement multiplexing, where multiple entanglement channels operate in parallel. Instead of using a single qubit at each node, multiple qubits, where each is spectrally distinguishable, may be entangled simultaneously.
The recent study demonstrates this by embedding ytterbium atoms inside yttrium orthovanadate crystals and coupling them to optical cavities. These cavities trap and guide light, allowing multiple qubits to emit and receive entangled photons concurrently. The main innovation, according to the study, is a quantum feed-forward protocol that compensates for frequency variations between different atoms, ensuring robust entanglement despite optical fluctuations.
“This is the first-ever demonstration of entanglement multiplexing in a quantum network of individual spin qubits,” said Andrei Faraon in a recent interview from Caltech news. “This method significantly boosts quantum communication rates between nodes, representing a major leap in the field.”
Building a Multi-Qubit Quantum Network
As described in the study, to test the potential of multiplexed entanglement, the researchers constructed a two-node quantum network using ytterbium ions embedded in nanophotonic cavities. Each node contained multiple qubits, each capable of emitting a photon that remained entangled with the ion. These photons traveled to a central station, where a detection event triggered a processing protocol that established entanglement between pairs of ions. Unlike previous approaches that relied on a single qubit per node, this setup allowed multiple qubits to participate in entanglement generation simultaneously, increasing efficiency and reducing delays.
One notable aspect of the experiment was embedding ytterbium atoms inside nanophotonic cavities, which enhanced their ability to act as optically active qubits. The researchers used frequency-multiplexed photons, allowing multiple qubits within a node to operate independently while still contributing to entanglement generation. However, a challenge arose from natural frequency mismatches between different qubits, which could lead to errors in entanglement distribution. To address this, the team developed a quantum feed-forward correction system, which actively adjusted for these mismatches in real time. This ensured that entanglement remained stable and usable across the network. By running entanglement attempts in parallel across multiple ion pairs, the researchers were able to increase the entanglement rate compared to conventional single-qubit methods.
Why This Matters for Quantum Communication
The ability to distribute entanglement efficiently is necessary for the development of functional quantum networks. This study demonstrates how multiplexing can help overcome long-standing bottlenecks by increasing entanglement rates and improving reliability. Using multiple qubits at each node allows networks to generate entanglement more efficiently, effectively increasing the bandwidth of quantum communication.
Reliability also improves with this approach. In traditional quantum networks, if a single qubit fails or experiences an error, entanglement generation is halted until the issue is resolved. In a multiplexed system, if one qubit encounters a problem, others can continue functioning, reducing the likelihood of communication breakdowns. This redundancy enhances the stability of quantum networks, making them more resilient to errors and disruptions.
Beyond immediate improvements in efficiency and reliability, this method may also be referenced for future efforts toward scalable quantum networking. As the number of interconnected nodes grows, a multiplexed approach may allow for more robust and flexible architectures, supporting the development of a global quantum internet.
A Step Toward Large-Scale Quantum Networking
Connecting quantum computers into a global network requires overcoming both entanglement distribution bottlenecks and transmission losses over long distances. The research at Caltech demonstrates how multiplexing may address the first issue by making better use of available qubits.
Further development of quantum repeaters, which can correct for photon loss, will be needed for practical long-distance quantum communication. But as this experiment shows, a new generation of quantum networks may be on the horizon—one in which entanglement is not only established but distributed efficiently, moving us closer to scalable and deployable quantum communication systems.
Contributing authors on the study include A. Ruskuc, C. J. Wu, E. Green, S. L. N. Hermans, W. Pajak, J. Choi, and A. Faraon.
