Figure 2: Illustration of experimental apparatus.
From: Experimental measurement-device-independent verification of quantum steering
A pair of separate spontaneous parametric down-conversion (SPDC) sources create Alice’s, Bob’s and Charlie’s photons. One photon from Charlie’s source acts as a heralding signal, with the remaining photon prepared in the quantum state ωk and sent via optical fibre to the input of Bob’s partial BSM device, accompanied by a corresponding classical signal jε{1, 2, 3} sent to Alice. Using a 50:50 beam splitter, Bob combines Charlie’s photon (prepared in state ωk) with his own photon ρB (comprising half of the entangled state ρAB shared with Alice), and projects onto the singlet subspace |Ψ−BC›‹Ψ−BC|. Alice receives Charlie’s announcement j accompanied by the other half of the shared entangled state ρAB, and measures σj. To execute the entanglement verification, Charlie receives Alice’s and Bob’s output signals aε{±1} and bε{0, 1}, and computes a payoff function P, where P>0 witnesses quantum steering in a device-independent setting.
