We experimentally demonstrate how adaptive measurement and readily-available entangled states can beat the standard limit in ab initio phase measurement, and how increasing measurement settings in EPR-steering inequality violations allows closure of the detection loophole.
Quantum measurement protocols make use of the quantum nature of the universe to gain more information about a system using fewer resources than would otherwise be possible. Achieving precision at the fundamental limits imposed by quantum mechanics nevertheless remains a challenge. Adaptive quantum measurement protocols may allow us to step closer to these ultimate limits. We present an overview of some interesting recent results of quantum measurement schemes as demonstrated in single-photon optical implementations, in the contexts of unambiguous phase estimation and multiple-copy non-orthogonal state discrimination.
It has been shown in earlier works that the vertices of Platonic solids are good measurement choices for tests of Einstein-Podolsky-Rosen (EPR)-steering using isotropically entangled pairs of qubits. Such measurements are regularly spaced, and measurement diversity is a good feature for making EPR-steering inequalities easier to violate in the presence of experimental imperfections. However, such measurements are provably suboptimal. Here, we develop a method for devising optimal strategies for tests of EPR-steering, in the sense of being most robust to mixture and inefficiency (while still closing the detection loophole, of course), for a given number n of measurement settings. We allow for arbitrary measurement directions, and arbitrary weightings of the outcomes in the EPR-steering inequality. This is a difficult optimization problem for large n, so we also consider more practical ways of constructing near-optimal EPR-steering inequalities in this limit.
Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date, all experimental tests with single-photon states have relied on post selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this 'detection loophole' by combining a highly efficient source of entangled photon pairs with superconducting transition-edge sensors. We achieve an unprecedented ∼62% conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 s.d.s. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test. Untrustworthy sources or detectors mean that quantum entanglement cannot always be ensured, but quantum steering inequalities can verify its presence. Using a highly efficient system, Smithet al. are able to close the detection loophole and clearly demonstrate steering between two parties.