Predicting the Quantum Past

Hindsight in quantum physics isn’t exactly 20/20, but observing an object at a later time can allow a better guess about its earlier quantum state. In a new experiment, researchers used a weak probe to continuously monitor a single qubit over several microseconds, and with that data they tried to predict the qubit state at some intermediate time. Using only the “before” data, the prediction was right in just 50% of the trials. But adding “after” data boosted the success rate to 90%, suggesting that the quantum state reveals more of itself when past and future measurements are combined.