Entanglement verification and steering when Alice and Bob cannot be trusted

Understand

Various protocols exist by which a referee can be convinced that two observers share an entangled resource.

• Such protocols typically specify the types of communication allowed, and the degrees of trust required, between the referee and each observer.
• Here it is shown that the need for any degree of trust of the observers by the referee can be completely removed via the referee using classical and quantum communication channels appropriately.
• In particular, trust-free verification of Bell nonlocality, EPR-steering, and entanglement, respectively, requires two classical channels, one classical and one quantum channel, and two quantum channels.