Q-CTRL achieves largest entangled quantum state using new methods

Researchers from Sydney-based Q-CTRL have achieved significant computational performance gains on an IBM superconducting quantum processor by applying quantum error correction (QEC) primitives, according to a recently published peer-reviewed study.

The study reports two record-setting outcomes: the creation of the largest Greenberger–Horne–Zeilinger (GHZ) state to date, entangling 75 qubits, and enhanced high-fidelity long-range controlled-NOT (CNOT) gate operations.

Both advances were accomplished using error detection techniques with only modest resource requirements compared to previous logical encoding methods.

Quantum error correction is vital for protecting quantum information from noise and hardware imperfections, which are common obstacles in current quantum hardware. Complete, fully error-corrected calculations have been challenging on existing processors, typically requiring significant resource overhead with limited practical benefit.

Generating large-scale quantum entanglement, such as GHZ states, is important for the development of quantum computing and quantum communication but is technically demanding because of device-related noise and constraints.

Many prospective quantum algorithms will rely on entanglement to function effectively.

Q-CTRL researchers succeeded in bypassing traditional overheads associated with logical encoding by strategically deploying QEC primitives - fundamental parts of error correction protocols - at the physical level, without a full logical encoding framework. This approach led to noticeable advantages in operational superconducting processors.

The first key demonstration highlighted in the study involved a novel teleportation protocol using unitary preparation of a GHZ state, followed by a disentangling step. This process enabled the final quantum state to provide insights into errors occurring during the procedure.

According to the report, Q-CTRL achieved a long-range CNOT gate operation with over 85% fidelity across up to 40 lattice sites, surpassing comparable state-of-the-art results on superconducting hardware.

The second experimental highlight was the creation of large GHZ states using a protocol that integrates sparse error detection through ancillary stabiliser measurements. GHZ states are multipartite entangled states in which all qubits are correlated.

The routine was resource-efficient, requiring no more than nine flag qubits, and incorporated a deterministic error-suppression method.

The team successfully verified genuine multipartite entanglement in GHZ states of up to 75 qubits, measured in terms of multiple-quantum coherence fidelity. This achievement was described as a record in published academic literature.

Other existing methods in the field often discard the majority of experimental shots - in some cases nearly all - when dealing with large-scale entangled state creation. However, Q-CTRL reported a comparatively low discard rate, retaining more than 80% of shots when generating a 27-qubit GHZ state and over 21% of shots for the full 75-qubit state.

The results indicate that using QEC primitives at the physical hardware level can offer a net improvement in performance over previous techniques on near-term quantum computers.

Yuval Baum, Head of Quantum Computing Research at Q-CTRL, said, "This work demonstrates that QEC primitives, even without full logical encoding, can offer significant computational advantages with only modest resource overhead."

"By designing smart protocols, leveraging intrinsic symmetries and combining strategic error detection, we achieve high-fidelity long-range CNOT gates and generate a 75-qubit GHZ state with genuine multipartite entanglement - the largest reported to date."

"These results suggest that meaningful benefits from QEC are already accessible on current-generation hardware."

Doug Finke, Chief Content Officer at Global Quantum Intelligence, commented, "These demonstrations of Q-CTRL's innovative and efficient use of an approach that combines error suppression with error detection show how these techniques can create the largest GHZ state to date, as well as enable long-range, high-fidelity CNOT gates that can be useful in quantum networking."

"Such an approach may represent an interim step between the NISQ and the full Fault Tolerant Quantum Computing (FTQC) eras that could allow users to experience quantum advantage earlier than expected. We look forward to seeing how this approach will continue to develop and be put to use in real applications in the near future."

The findings suggest that practical benefits of quantum error correction are achievable on today's quantum computers, potentially accelerating the timeline for achieving quantum advantage relative to current supercomputer capabilities.

Q-CTRL's results contribute to broader efforts within the quantum research community to develop more robust and powerful quantum technologies by combining error suppression and detection without full-scale logical encoding.