Quantum steering without the detection loophole

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• February 2, 2012
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Our paper “Conclusive quantum steering with superconducting transition edge sensors” has now been published in Nature Communications.

Quantum steering was introduced by fellow Austrian Erwin Schroedinger in 1935 alongside the much better known quantum phenomenon of entanglement. It describes the ability of one party to steer the measurement outcome of a second party on a quantum state shared between them. The interesting feature of quantum steering is that it can demonstrate quantumness in a regime which is weaker than that needed to disprove local realistic theories.

Original demonstrations of quantum steering involved experimental tests of the Einstein Podolsky Rosen (EPR) paradox. Since then, Howard Wiseman (one of our co-authors) and others have given the concept of steering a facelift, reformulating it in the context of quantum information.

Our contribution was to experimentally demonstrate steering while closing the so-called detection loophole, which would allow one of the two involved parties to cheat the other into thinking their state was steered properly, even though it wasn’t. The detection loophole is an issue which also pops up in Bell inequalities performed with photons; it originates in our inability to detect entangled single photon pairs with 100% efficiency. In practice, a conclusive demonstration of quantum steering is performed by violating a steering inequality, which in it’s simplest form requires a conditional detection efficiency of at least 50%.

This number was considered out of bounds until only a few years ago, when superconducting transition edge sensors hit the scene. These detectors—pioneered by our collaborators at the National Institute for Standards and Technology (NIST) in Boulder, Colorado—are in principle capable of detecting incoming single photons with near-unity efficiency. The experimental challenge for us was to build a source for entangled photons which, combined with these detectors, would allow us to surpass the 50% efficiency limit. We were quite successful at that and achieved an unprecedented conditional detection efficiency of 62%.

Simultaneous to our efforts, two other groups reported steering experiments which successfully closed the detection loophole. The experiment performed in Vienna (Bernhard Wittmann et al.) simultaneously closed the locality loophole (similar to our previous experiment described here), which would allow cheating by information leakage between the measurement devices used by our two parties. The second experiment, performed at Griffith university also here in Brisbane (A. Bennet et al.) demonstrated steering over a 1 km fiber channel. Both of these experiments were performed with conventional photon detectors, exploiting clever theory which allows one to lower the efficiency requirements by using more measurement settings.