Certifying large quantum systems without trusting the hardware

Self-testing lets you confirm what quantum state a device holds using only its measurement statistics, no trust in the hardware required. Until now, doing this for many-qubit states demanded an exponentially growing number of samples, making it useless at scale. This protocol achieves the same certification with polynomial sample complexity by efficiently evaluating multipartite Pauli measurements via a linear number of ancillary Bell pairs and standard operations already within reach of current hardware. The result extends beyond state verification to a broad class of learning and certification tasks in device-independent quantum networks.