FIGURE 1. Testing our quantum picture of the world.

Rowe et al.¹ prepared two ions in an entangled state by using Raman laser beams. Next, they applied an arbitrary rotation to each ion by simultaneously controlling the phase of the Raman beams and the strength of the ion trap. Finally, they determined the quantum state of each ion by shining light (the probe laser) on the ion pair to identify whether each ion fluoresces. There are four possible outcomes: two in which the states are the same (both ions are bright or both are dark), and two in which they are different (only one ion or the other fluoresces). The measurements give the ratio of (N_same–N_different)/(N_same+N_different), where N is the number of detected ion pairs. The experimental result violates Bell's inequalities — thereby confirming the predictions of quantum mechanics — in a way that escapes the 'detection-efficiency' loophole.