Researchers working in Singapore and the United States have discovered that all entangled states of two particles have a classical 'fingerprint'. This breakthrough could help engineers guard against errors and devices that don't do what they promise in quantum computing and quantum cryptography.

Goh Koon Tong and Valerio Scarani at the Centre for Quantum Technologies at the National University of Singapore, with Andrea Coladangelo at the California Institute of Technology, reported in Nature Communications on 26 May that a simple set of measurements can act as an identity check for any two-particle entangled state. The presence of this fingerprint could help certify quantum computers or quantum encryption devices purchased from third parties.

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