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Quantum guessing game uses the future to predict the past

Scientists predict accurate energy levels nine times out of 10 in study

4:48pm, February 13, 2015

Predictions are much easier to make with the benefit of hindsight. Scientists successfully predicted an important quantum property midway through an experiment by analyzing measurements taken before and after. The research, scheduled to appear in the Feb. 13 Physical Review Letters, demonstrates that the present can help inform researchers about unknowns in the past, even in the uncertainty-dominated quantum realm.

Physicist Kater Murch of Washington University in St. Louis and colleagues prepared a specialized circuit that could have either of two energies. This superposition of quantum states enables calculations of the probability that the circuit will possess a particular energy when a measurement is made, but scientists usually can’t be confident of the outcome until they make the measurement. Murch and his team skirted the rules by gently probing the circuit with

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