Quantum guessing game uses the future to predict the past

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 microwaves, which relayed some information about the circuit without disturbing its delicate dual-energy state. About halfway through the experiment, the researchers definitively measured the circuit’s energy but didn’t immediately look at the result.

Playing soothsayer, Murch and his team used the premeasurement microwave readings to predict the circuit’s measured energy level; they came up with the correct energy level about half the time. In an attempt to improve their predictions, the scientists studied microwave data obtained after the measurement and projected backward in time from the end of the experiment. By combining information from both the forward and reverse projections, the researchers boosted the accuracy of their predictions to about 90 percent.