In 1935, Einstein, Podolsky and Rosen published their famous paper proposing a well-known paradox (the EPR paradox) that cast doubt on the completeness of quantum mechanics. To investigate the EPR paradox, Schrödinger introduced the concept of Steer, now known as the EPR steering. As an asymmetric concept, EPR steering, describing the ability of one system to non-locally affect another system’s states through local measurements, is logically intermediate between Bell non-locality and entanglement. EPR steerable states form a superset of the states that have Bell non-locality and form a subset of entangled states.

Professor LI Chuanfeng from Key Laboratory of Quantum Information, University of Science and Technology of China of Chinese Academy of Sciences, together with collaborators from Nankai University, have made breakthrough in the field of quantum non-locality. Based on the all-versus-nothing (AVN) proof proposed recently. They experimentally realize the Einstein-Podolsky-Rosen (EPR) steering game which presents a new form of quantum non-locality. The result is published online in Physical Review Letters on Sept. 30, 2014. 

Different from other experimental realization of EPR steering, Prof. LI’s group employed a new criterion based on the so-called all-versus-nothing proof which presents the strongest conflict between the predictions of quantum mechanics and the local-hidden-state model. Different from other general EPR steering inequalities, the AVN criterion requires only two measurement settings. It has been proved that, for a two-qubit entangled state, the normalized conditional states of one qubit can be regarded as two different pure states with a certain projective measurement performed on the other qubit in the AVN criterion. As the criterion is very sensitive to the experimental errors, the cooperative group further developed a theoretical method to deal with the noise and finite measurement statistics within the AVN framework and apply it to analyze the experimental data. 

In the experiment, the normalized conditional states of the qubit to be steered were verified to be two different pure states when the other qubit was performed by a projective measurement along with a certain direction. Then, a joint measurement was performed to check whether the state they shared is entangled or not. According to the practical meaning of EPR steering, the side to be steered should be able to store its state information. To meet this requirement, the group used a 50m optical fiber as the quantum memory. A free-space electro-optic modulator was employed on the side to be steered to respond to the signal from the other side. As a result, the EPR steering game is strictly demonstrated following the original logical idea. 

The results provide a particularly strong perspective for understanding the EPR steering and the quantum non-locality. Their method supplies a simple way to realize and verify EPR steering and shows the potential applications in the implementation of long-distance quantum state preparation and some other quantum information processings. 

The research was supported by Ministry of Science and Technology, Chinese Academy of Sciences, National Natural Science Foundation and Ministry of Education.

Figure:Illustration of the process of EPR steering game.

(School of Physical Sciences)