IBM Quantum researchers have successfully prepared the largest Greenberger–Horne–Zeilinger (GHZ) state reported to date, consisting of 120 superconducting qubits. The experiment, detailed in a paper titled “Big cats: entanglement in 120 qubits and beyond,” achieved a measured fidelity of 0.56(3), surpassing the 0.5 threshold required to confirm genuine multipartite entanglement across all qubits.
The resource state was prepared on a fixed-frequency, tunable-coupler superconducting processor. The achievement was enabled by several specialized techniques within a novel compiler framework:
- Adaptive Compilation: The circuit was compiled adaptively to utilize the least noisy regions of the chip, maximizing error detection against non-uniform noise.
- Low-Overhead Error Detection: The protocol incorporated low-overhead parity checks and dynamical measurements, which acted as a single-shot form of error mitigation.
- Temporary Uncomputation: The researchers employed a process that temporarily disentangled the earliest qubits to allow them to relax to a stable ground state before re-entanglement, reducing noise accumulated in long idle regions.
The ability to faithfully entangle a large number of particles is a key benchmark for the quality of quantum hardware and control, demonstrating progress toward fault-tolerant quantum computing. IBM certified the fidelity using two statistical methods—Direct Fidelity Estimation (DFE) and parity oscillation tests—which are necessary because the full state cannot be classically simulated.
Beyond benchmarking, these large GHZ states are resource states that can be used to perform various quantum protocols and algorithms, including Quantum Secret Key Distribution (SKQD).
Read the full details of the research in the arXiv paper here.
November 3, 2025
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