Predicting the Past in Quantum Mechanics
If you are not convinced that quantum mechanics is weird, keep reading and check out the source too. For the physics we live with every day, time runs in one direction, and the past influences the future, but not the other way around. As researchers at Washington University in St. Louis have discovered though, in the quantum world the future and the past can influence the state of a system, instead of just the past.
In our normal world, if you follow a system up to some event, you can predict what happens with the information you collected. In quantum mechanics though, the odds of guessing right are about 50-50, even if you know everything about a quantum particle leading up to the event. To confirm this, the researchers put a superconducting circuit into a superposition, creating a qubit, and placed it in a microwave box. When some microwave photons are put in the box, they will interact with the qubit and gain some information about it, without collapsing the superposition. The researchers then took a strong measurement of the qubit, which would cause it to collapse, but they hid the result and continued using the photons to make weak measurements.
When the researchers predicted what the strong measurement would be using just the information leading up to it, they were correct half of the time. However, when they ran the equations backwards with the weak measurements following the hard measurement, creating hindsight predictions or "retrodictions," they were right 90% of the time. This indicates that the state of the qubit incorporates information not only from what led up to the strong measurement, but also from what followed it. This results returns time symmetry to quantum mechanics and could have some interesting applications, such as making more robust chemical reactions and improving quantum computing.