Device-independent self-testing refers to the certification of quantum states based entirely on the correlations exhibited by measurements on separate subsystems. The fact that such a certification is possible at all is remarkable in its own right, and is intimately connected to the violation Bell’s inequalities by entangled quantum systems. In the bipartite case, it is known that, for every pure entangled bipartite state, there exists a device-independent self-testing protocol. Despite the growing interest in self-testing, an analogous universal result in the multipartite setting has remained elusive. In this work, we prove such a universal result for qubits: for every pure n-qubit entangled state, we give an explicit Bell-scenario correlations that self-tests that state, up to local isometries and, where relevant, complex conjugation. The authors prove that any pure entangled state of multiple quantum bits (qubits) can be uniquely identified using only measurement statistics, without trusting the devices, resolving a long-standing open problem in quantum certification.

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Nature

Researchers prove a universal result in quantum information theory: every pure entangled state of multiple qubits (n-qubit systems) can be uniquely certified using only measurement statistics, without trusting the devices involved. This device-independent self-testing result resolves a long-standing open problem in the multipartite setting, extending the previously known bipartite result. For each pure n-qubit entangled state, an explicit Bell-scenario correlation is constructed that self-tests the state up to local isometries and complex conjugation.

All pure multipartite entangled states of qubits can be self-tested