Electron-positron collisions at SLAC produce a Υ(4s) resonance that results in an entangled pair of B mesons. When one meson decays at time t1 , the identity of the other is “tagged” but not measured specifically. In the top panel, the tagged meson is a “B̅ 0”. This surviving meson decays later at t2 , encapsulating a time-ordered event, which in this case corresponds to “B̅ 0” →B- . To study time reversal, the BaBar collaboration compared the rates of decay in one set of events to the rates in the time-reversed pair. In the present case, these would be the “B-” →B̅ 0 events, shown in the bottom panel.