State-backend-dependent fidelity of the CCZ gate on superconducting quantum computers

We present an experimental characterization of the three-qubit controlled-controlled-Z (CCZ) gate on IBM Quantum’s 127-qubit superconducting quantum computers, revealing state-backend-dependent performance variations. Using quantum state tomography (QST), we evaluate the gate’s output-state fidelity when applied to three algorithmically significant input states—the W state, the Greenberger–Horne–Zeilinger (GHZ) state, and the uniform superposition state. Each state probes distinct aspects of the reconstructed state fidelity, the GHZ state tests coherence in maximally entangled systems, the W state reveals sensitivity to asymmetric errors, and the uniform superposition assesses basis-state-independent behavior. Experiments were conducted across three computational environments, noise-free simulation, noisy simulation, and actual hardware executions on three IBM Quantum computers, ibm_osaka , ibm_brisbane , and ibm_sherbrooke . State fidelities for the qasm_simulator remained above 99.2% for all input states. Noisy simulations yielded fidelities around 80–82%, while hardware results showed more significant degradation: 67.6% (GHZ), 59.0% (uniform superposition), and 50.3% (W)—highlighting the combined importance of state selection and hardware characteristics for reliable quantum computation. These results quantify the current performance gap between simulated and physical implementations of multi-qubit gates and provide a benchmark for error profiles relevant to near-term quantum algorithms