This 250-year-old equation just got a quantum makeover

A team of international physicists has brought Bayes’ centuries-old probability rule into the quantum world. By applying the “principle of minimum change” — updating beliefs as little as possible while remaining consistent with new data — they derived a quantum version of Bayes’ rule from first principles. Their work connects quantum fidelity (a measure of similarity between quantum states) to classical probability reasoning, validating a mathematical concept known as the Petz map.