Chinese research team achieves genuine entanglement of 13 qubits without destroying photons

Quantum computers promise to solve problems beyond the reach of classical machines, but getting enough qubits to work together remains a major challenge. Recently, a Chinese research group demonstrated in a new study how to combine small entangled microwave photons into a larger, programmable quantum network without destroying the photons in the linking process. They successfully demonstrated genuine entanglement among 13 photonic qubits, paving the way for more scalable quantum computing and communication systems. The challenge lies in how to reliably establish these connections. Qubits can be linked through entanglement, a quantum phenomenon in which their shared state cannot be described independently.

Scientists can arrange these connections into structures known as graph states, which serve as resources for quantum computing and communication. "Large photonic graph states are an important resource for quantum communication, quantum networks, and measurement-based quantum computing," said Hongyi Zhang, a quantum researcher at Tsinghua University.

One way to link small entangled states is through a fusion process that combines them into larger states. However, the fusion methods used in traditional linear-optical systems are typically probabilistic, meaning some attempts fail. Repeated attempts and additional hardware make building larger states increasingly difficult. The research group introduced a strategy that uses superconducting circuits to combine small entangled microwave photons. Rather than creating a single massive entangled state in one go, the method builds a larger network by linking smaller prepared states. The key to this approach is a technique called quantum non-demolition measurement.

Quantum non-demolition measurement technique

Unlike measurements that absorb or destroy photons, this method extracts information while preserving the photons. In this experiment, the measurement also altered the paired photons' joint quantum state, enabling the researchers to link smaller graph states. "We explored whether deterministic fusion could be achieved by using quantum non-demolition measurement," Zhang said. "Our goal was to demonstrate a deterministic, programmable fusion operation and prove it provides a practical pathway for assembling larger, reconfigurable photonic graph states."

The researchers first used superconducting circuits to generate small entangled groups of microwave photons. They encoded quantum information in different time slots of the photons, allowing the information to be transmitted at different arrival times. They then used a quantum non-demolition detector to measure selected photon pairs, projecting them into a specific entangled state known as a Bell state. This allowed the smaller graph states to be linked without destroying the photons. Next, by adjusting the photons' frequency — also known as frequency tuning — the researchers could choose which pairs to measure, controlling how the smaller graph states connected to form larger graph states.

This made the fusion process programmable. The team also combined active reset and reuse of superconducting qubits with built-in error mitigation, helping limit the effects of decoherence due to environmental interactions. Through this method, the researchers demonstrated genuine multipartite entanglement among 13 photonic qubits. This means the entanglement extended across the entire ensemble rather than being confined to isolated subgroups. The achievement shows that the fusion process can produce larger, reconfigurable photonic graph states. "Our main contribution is a deterministic, programmable, and non-destructive fusion operation," Zhang said.

Applications of large photonic graph states

Large photonic graph states can support measurement-based quantum computing, in which computation is performed by carrying out carefully chosen measurements on the entangled state. They can also advance quantum networks and quantum error correction schemes designed to protect quantum information from errors. "This work provides an architecture for scaling photonic graph states by linking smaller, on-demand resource states," Zhang explained. "This is relevant for future measurement-based quantum computing, quantum networks, and potential quantum error correction schemes, because both the size and connectivity of the graph state are critical."

However, demonstrating entanglement among 13 qubits is not the same as building a practical large-scale quantum computer. The system still needs improvement in areas such as device fidelity, photon generation efficiency, and detector performance. The researchers now plan to develop a multi-detector architecture to perform more fusion operations and generate larger, more complex graph states. Whether this approach can scale efficiently while maintaining the required performance remains to be seen. The study was published in the journal Nature Physics.

Key breakthrough for achieving scalable quantum computing

The Chinese research team's success demonstrates an important advance in quantum computing, particularly in entanglement technology between qubits. Through quantum non-demolition measurement, researchers were able to extract and process information without damaging photons, which is crucial for future quantum computing and communication systems. The establishment of large photonic graph states not only advances quantum computing efficiency but also enhances the stability and reliability of quantum networks. As the technology develops further, this research may pave the way for realizing larger-scale quantum computers and solving the challenges facing current technology.