Researchers Entangle 13 Photons to Build Larger Quantum Graphs

Measurement-based quantum computing and memory-less quantum repeaters are key areas of research. Although some initial examples have been successfully prepared, creating large-scale graph states with programmable connections at scale remains an outstanding challenge. Fusion offers a promising “divide-and-conquer” approach by linking smaller resource states into larger, more complex graphs. However, its practical scalability has been limited by the probabilistic nature of conventional fusion protocols, requiring extensive hardware overhead and feed-forward resources to achieve deterministic outcomes. A deterministic fusion operation with built-in error correction is introduced and experimentally demonstrated.

Deterministic Fusion Enables Scalable Entanglement Across Thirteen Superconducting Qubits

Entanglement now extends to 13 qubits, surpassing prior demonstrations limited to 12, and opens possibilities for more complex quantum calculations; earlier methods struggled with topological complexity as entanglement scaled up. Researchers at Hefei National Laboratory and Tsinghua University achieved this milestone using a deterministic fusion operation that connects smaller entangled states into larger, reconfigurable structures via superconducting circuits. Actively resetting and reusing matter qubits during graph state generation overcomes limitations imposed by qubit coherence times which previously hindered scaling efforts.

This work establishes a scalable framework for creating photonic graph states essential for advanced measurement-based quantum computing and the development of distributed quantum networks capable of secure communication. Qubit coherence times previously capped demonstrations at 12 qubits, but this new method avoids those limits. Their system uses in-situ frequency tuning enabling control over individual qubit interactions; they also demonstrated programmable connections between building blocks allowing complex topologies not easily created before.

Scalable Photonic Graph State Synthesis via Nondestructive Microwave Qubit Fusion

The method mitigates limitations by connecting small time-bin-encoded cluster states, originally generated on-demand from a quantum emitter, into larger, reconfigurable photonic graph states of increased sizes. This fusion operation is realised by nondestructively projecting pairs of flying microwave photonic qubits into Bell states using superconducting circuits with in-situ programmability. Active reset and reuse of matter qubits during graph state generation circumvents coherence constraints of the quantum emitter, enabling genuine multipartite entanglement across 13 photons; this demonstrates scalability and high fidelity.

These results establish a scalable framework for photonic graph-state synthesis, paving the way for fault-tolerant measurement-based quantum computation and distributed quantum networks. Linear optical techniques utilising spontaneous parametric downconversion (SPDC) sources provide a powerful method for generating these photonic graph states, supporting several key proof-of-principle experiments.

However, SPDC sources face scalability challenges in large-scale quantum information processing due to their inherently probabilistic nature. Atom-like quantum emitters offer an alternative, deterministic and efficient, for generating single photons and multiphoton entanglement. Constructing large-scale graph states with the desired topology requires multiple interacting quantum emitters possessing sufficiently long coherence times; this remains a difficult technical challenge.

Genuine multipartite entanglement has been demonstrated in cluster states of up to 12 qubits, yet topological complexity was limited. Fusion offers a promising strategy for constructing large-scale photonic graph states by performing entangling measurements on qubits from multiple constant-size entangled resource states, enabling synthesis of larger, topologically complex many-body entangled structures. Recently, fusion of resource states from quantum emitters has occurred using two-photon interference and destructive projective measurements within linear optical setups.

These projective measurements are inherently probabilistic with a maximum success probability of 50%. Photon loss during detection further diminishes operation fidelity; therefore ancilla qubits and repeated operations become necessary, increasing the resource overhead for quantum information processing. Perfect overlap of temporal, spectral, and spatial modes is also required for successful two-photon interference. This imposes a stringent limitation on flexibility in the fusion operation and consequently limits entanglement scale and structure.

To overcome these challenges, they propose a deterministic and programmable fusion operation based on circuit/cavity quantum electrodynamics (cQED). Cavity-assisted interactions between flying photonic qubits have successfully enabled photon-photon gates, remote qubit entanglement, and non-demolition detection of flying states. In particular, performing QND parity measurement deterministically prepares two photons into Bell states, potentially enabling a deterministic and QND fusion operation. In this work, they demonstrate such an operation for microwave photonic qubits using superconducting quantum circuits.

A single-photon emitter generates itinerant timebin photonic qubits or multiphoton entangled cluster states; it is coupled to a detector that performs the QND parity measurement on incoming photons. They first show deterministic fusion of two initially uncorrelated time-bin qubits through selective parity measurements. Then, generating photonic cluster states allows them to fuse these into larger, higher-dimensional structures in a programmable manner.

Repetitive fusion operations are also presented by resetting matter qubits during graph state generation, creating an 11-vertex cluster state encoded with 13 photonic qubits, demonstrating scalability and feasibility for large-scale quantum information processing. The experimental setup includes circuit QED: a single-photon emitter and a QND detector cascadeconnected via microwave circulator; both consist of a transmon qubit dispersively coupled to a resonator. Frequency of emitted photons can be programmed on demand adjusting magnetic flux through the superconducting SQUID embedded at centre of the resonator.

Photonic states are analysed using homodyne detection then reconstructed with maximum likelihood method (details appear in Sections III & IV of Supplementary Information). This fusion operation relies on QND parity measurement, as illustrated. At this stage, projective measurement on transmon collapses two photons into Bell state; if initially in different graph states, fused Bell state serves as redundantly encoded vertex inheriting all bonds of both photons, merging them into a new one for deterministic fusion without heralding. Programmable fusion can be realised using frequency-tunable single photons with finite bandwidth of QND detector.

Superconducting Circuits Enable Scalable Entanglement via Active Qubit Reuse

This demonstration expands entangled systems beyond previous limitations, but achieving arbitrarily complex graph states isn’t guaranteed by simply scaling up qubit numbers, the abstract offers no detail regarding structural constraints within their approach. The team overcame probabilistic hurdles inherent in earlier techniques through superconducting circuits and active qubit reuse; this is an elegant solution to coherence problems that have long plagued similar efforts. Even if arbitrarily complex connections aren’t definitively proven possible (a point they acknowledge requires further investigation), this represents a leap forward for building larger quantum systems, overcoming longstanding issues by actively reusing qubits during graph creation. They successfully demonstrated deterministic method for creating complex entangled states using thirteen photonic qubits, surpassing previous limitations and establishing new benchmark in quantum information processing. By actively resetting and reusing components within superconducting circuits during graph state creation, they overcame challenges posed by qubit coherence, the tendency of quantum systems lose information over time, historically restricting scalability.

This research demonstrates a way to connect smaller entangled photon sources into larger, reconfigurable networks with up to thirteen qubits. The team achieved this through a deterministic fusion operation utilizing superconducting circuits that can reset and reuse matter qubits, addressing a key limitation in building scalable quantum technologies. This active reuse circumvents issues related to maintaining qubit stability for extended periods, enabling genuine multipartite entanglement. Their method establishes a framework for creating more complex photonic graph states without relying on probabilistic techniques requiring extensive hardware resources.

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