Within the hierarchy of quantum correlations, the Einstein-Podolsky-Rosen steering task (also called quantum steering) is distinguished from both entanglement verification and Bell nonlocality by the inherent asymmetry of the protocol. Apart from the ordinary two-way steering, it is predicted that Alice is able to steer Bob’s measurement outcomes, but not vice versa. This is known as one-way steering. One-way steering has been previously demonstrated for the restricted class of Gaussian measurements, but that situation is not fully general. There exist cases where states that are one-way steerable for Gaussian measurements are two-way steerable (i.e. not fundamentally asymmetric) for more complicated measurements.
Here, we proved and then observed one-way steerability of an experimentally accessible class of entangled polarisation-qubit states – the Werner states – in a general setting. Our high-heralding-efficiency photon source enabled us to violate the steering inequality by 6 standard deviations in one direction, whilst closing the detection loophole. In the other direction, tomographic reconstruction verified the creation of a state that was provably unsteerable for arbitrary quantum measurements .
Further, we tested our experimental results against a subsequently-derived necessary condition for steerability of arbitrary two-qubit states with loss . We tested our experimentally generated Werner-like states, having a 99% fidelity with a Werner state, against this practical necessary condition. We found that the assumption of Werner states opened a possible loophole in the demonstration.
We subsequently performed a conclusive demonstration of two-qubit one-way quantum steering  that unambiguously accounts for the condition in , without needing to assume a Werner state form.
In our experiments, it is photon loss between the entangled state source and the untrusted party that introduces the asymmetry that leads to one-way steering. However, this loss may be problematic if one wants to complete an entanglement verification task between two parties using quantum steering. We demonstrated a heralded quantum steering task that allowed steering to be completed, with the detection loophole closed, in the presence of 15dB of added loss (equivalent to the loss of 80km of telecom fiber) .
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Geoff J. Pryde is the leader of Quantum Optics and Information Laboratory at Centre for Quantum Dynamics. He has been elected a Fellow of the Optical Society (OSA). His research covers different aspects in quantum information, quantum computation and quantum communication, mainly on quantum measurement, quantum control and coherent control of semiclassical systems.