Entanglement is a key resource for device-independent quantum key distribution and other secure quantum communication protocols. As such, verification of entanglement shared between distant parties is of highest importance to the quantum information community. Though the challenging task of loophole-free Bell test has been recently demonstrated, performing Bell-type verification over long distances is technologically intractable due to increased loss through the communication channel opening up the detection loophole. An alternative approach is quantum steering (EPR-steering), in which one of the parties, Bob, trusts quantum mechanics to describe his own measurement apparatus, but no assumptions are made on the other party, Alice. Trusting Bob’s measurement apparatus allows one to perform rigorous entanglement verification even in presence of loss in the untrusted quantum channel. However, the guaranteed success of such loss-tolerant protocol for very high channel loss relies on the use of perfect pure entangled states and infinite measurement settings from which Bob samples, and is not applicable to a realistic long-distance communication scenario.
In order to overcome this obstacle, we develop a new type of protocol, heralded quantum steering. We use entanglement swapping in order to teleport photons from the high-loss channel into another, high-transmission, quantum channel. With the additional heralding signal from successful teleportation events Bob can consider only those trials when the photon was transmitted through the loss channel. By using two high-efficiency spontaneous parametric downconversion sources of entangled photon pairs and superconducting nanowire photon detectors we were able to perform entanglement swapping with high fidelity, while maintaining effective heralding efficiency sufficient to violate the steering inequality in the presence of added loss. With our heralded steering protocol, we demonstrate detection-loophole-free quantum steering with n = 6 measurement settings over a quantum channel with at least 14.7 dB of added loss, equivalent to approximately 80 km of telecom optical fiber.