Stress Testing Ibm_brisbane: Rapid Noise Saturation with Deep Random Circuits on 127 Qubits

The practical utility of Noisy Intermediate-Scale Quantum (NISQ) processors depends critically on their performance under demanding computational loads. We investigated the impact of circuit depth on the 127-qubit superconducting processor ibm_brisbane by executing stress-test circuits utilizing all available qubits. These circuits consisted of repeated layers combining random single-qubit rotations and linear chains of Controlled-Z entangling gates. We systematically increased the ideal circuit depth from 1 up to 30 layers, resulting in transpiled circuits ranging from approximately 550 to over 15,000 gates (optimization level 2). Using the qiskit-ibm-runtime Sampler primitive with 4096 shots and default error mitigation, we successfully obtained results for ideal depths up to 22 (transpiled depth ~11.7k gates), beyond which platform usage limits prevented execution. Our central finding is the immediate onset of noise saturation: the output distribution reached maximum sample entropy (12.0 bits) and exhibited near-zero average single-qubit Z magnetization even at the shallowest depth tested (ideal depth 1, transpiled depth ~550). These noise-dominated characteristics persisted despite significant increases in transpiled circuit depth. This demonstrates that for these specific full-width, complex random circuits, noise overwhelms the computation almost instantaneously on this device under default settings, highlighting the profound challenge of achieving computational fidelity for deep, wide algorithms in the NISQ era without advanced error handling.