Distributed quantum certification with Clifford encoders

Quantum state certification concerns a resource-efficient way to verify whether a quantum device produces a prescribed target state without reconstructing the state in full, making it a fundamental primitive for validating quantum systems. We consider the private-coin model with only quantum communication, where the parties have no shared randomness or preshared entanglement, and no additional classical communication is allowed. We establish the optimal sample complexity, thereby closing the logarithmic gap between the previously established upper and lower bounds in this model. Remarkably, our upper bound attains the log-free optimum using a fixed family of Clifford encoders. Thus, optimal distributed state certification under private coins and quantum-only communication does not require general quantum encoders.