October 1, 2026 -- Quantum technologies rely on qubits, units of information that can exist in combinations of the states 0 and 1 instead of being limited to one or the other like conventional bits. Qubits can become entangled, which means their states become linked in ways that cannot be explained by considering each qubit separately.

A promising platform for realizing and studying entangled qubits is a nitrogen-vacancy center. This is a tiny defect in diamond in which a nitrogen atom sits beside a missing carbon atom.

In these diamond-based systems, researchers typically entangle groups of qubits through a series of gates (i.e., controlled operations), linking an electron at the defect to the nucleus of one carbon atom at a time. This process takes time and can cause crosstalk, a phenomenon in which an operation also affects qubits it was not supposed to target.

Researchers at the University of Pennsylvania recently introduced a new approach for implementing a gate that links multiple qubits in diamond at room temperature. Their method, outlined in a paper published in Nature Nanotechnology, allowed them to realize a four-qubit Greenberger–Horne–Zeilinger state, a type of entanglement in which the four qubits share a quantum combination of two collective arrangements, such as all 0s and all 1s.

"Multipartite entanglement is an essential aspect of quantum systems, needed to execute quantum algorithms, implement error correction and achieve quantum-enhanced sensing," Joseph D. Minnella, Mathieu Ouellet and their colleagues wrote in their paper. "In solid-state quantum registers such as nitrogen-vacancy centers in diamond, entangled states are typically created using sequential, pairwise gates between the central electron and individual nuclear qubits. This sequential approach is slow and suffers from crosstalk errors."

Realizing four entangled qubits in diamond

In their experiments, the researchers worked with a nitrogen-vacancy center in diamond at room temperature. They used an electron at the defect and three nearby carbon-13 nuclei as four qubits. The team implemented a precisely timed sequence of controls that prompted the electron to interact with the three nuclear qubits in parallel. This created a four-qubit entangled state in a single gate.

The researchers tested parallel gates that involved the electron and different pairs of nuclear qubits, then compared them with gates applied one pair at a time. To verify the entanglement, they varied the nuclear qubits' quantum phases and measured the light emitted by the diamond defect. The resulting pattern allowed them to determine how many qubits had become entangled.

Generation and verification of multipartite GHZ states. Credit: Nature Nanotechnology (2026). DOI: 10.1038/s41565-026-02254-6

"We demonstrate a parallelized multi-qubit entangling gate to generate a four-qubit Greenberger–Horne–Zeilinger state using a room-temperature nitrogen-vacancy center in only 14.8 μs—10 times faster than that using sequences of two-qubit gates and close to the fundamental limit set by the hyperfine coupling frequencies," wrote the authors. "Parallel three-qubit gates are also realized with all-nuclear-qubit subsets. The entangled states are verified by measuring multiple quantum coherences."

Promising results and future possibilities

The researchers found that the four-qubit parallel gate they implemented was 10 times faster than the sequential operations they compared it with. In addition, the parallel gate had a higher fidelity (i.e., a measure of how closely the operation performed as intended) than the sequential four-qubit gate.

"The four-qubit parallel gate has a fidelity of 0.92(4), whereas the sequential four-qubit gate fidelity is only 0.69(3)," wrote Minnella, Ouellet and their colleagues. "The approach is generalizable to other solid-state platforms, and it lays the foundation for scalable generation and control of entanglement in practical devices."

In the future, the researchers' method could be used to implement more accurate entangling gates in diamond-based quantum systems and other solid-state quantum platforms. Eventually, it could be scaled up to systems with more qubits and contribute to advances in quantum sensing or other quantum technologies.