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Researchers at the University of Pennsylvania have developed a new method to entangle diamond qubits ten times faster than previous techniques while operating at room temperature. This breakthrough involves using a nitrogen-vacancy center, a tiny defect in diamond where a nitrogen atom sits next to a missing carbon atom. The team utilized an electron at this defect and three nearby carbon-13 nuclei to create four qubits. By applying a precisely timed sequence of controls, the electron interacted with all three nuclear qubits simultaneously in a single gate operation. This approach generated a four-qubit Greenberger–Horne–Zeilinger state, a specific type of entanglement where the qubits share a combined quantum arrangement.

The new parallel gate completed this task in 14.8 microseconds, which is ten times faster than the sequential two-qubit gates used in earlier experiments. Joseph D. Minnella and his colleagues reported that the parallel method also achieved higher fidelity, meaning the operation performed closer to its intended design. The sequential four-qubit gate had a fidelity of only 0.69(3), whereas the new parallel gate reached a fidelity of 0.92(4). To verify the entanglement, the researchers varied the quantum phases of the nuclear qubits and measured the light emitted by the diamond defect to determine how many qubits were linked together.

The authors state that multipartite entanglement is essential for executing quantum algorithms, implementing error correction, and achieving quantum-enhanced sensing. They note that solid-state quantum registers typically create entangled states using slow, sequential gates that suffer from crosstalk errors. The researchers found their parallel gate had higher fidelity than the sequential version and believe the approach is generalizable to other solid-state platforms. In the future, this method could contribute to scalable generation and control of entanglement in practical devices for quantum sensing and other technologies.