Probability Theorem Gets Quantum Makeover After 250 Years
August 28, 2025
Introduction:
(Eurekalert) How likely you think something is to happen depends on what you already believe about the circumstances. That is the simple concept behind Bayes’ rule, an approach to calculating probabilities, first proposed in 1763. Now, an international team of researchers has shown how Bayes’ rule operates in the quantum world.
“I would say it is a breakthrough in mathematical physics,” said Professor Valerio Scarani, Deputy Director and Principal Investigator at the Centre for Quantum Technologies, and member of the team. His co-authors on the work published on 28 August 2025 in Physical Review Letters are Assistant Professor Ge Bai at the Hong Kong University of Science and Technology in China, and Professor Francesco Buscemi at Nagoya University in Japan.
Additional extract:
Consider a case in which a person tests positive for flu. They may have suspected they were sick, but this new information would change how they think about their health. Bayes’ rule provides a method to calculate the probability of flu conditioned not only on the test result and the chances of the test giving a wrong answer, but also on the individual’s initial beliefs.
Bayes’ rule interprets probabilities as expressing degrees of belief in an event. This has been long debated, since some statisticians think that probabilities should be “objective” and not based on beliefs. However, in situations where beliefs are involved, Bayes’ rule is accepted as a guide for reasoning. This is why it has found widespread use from medical diagnosis and weather prediction to data science and machine learning.
Principle of minimum change
When calculating probabilities with Bayes’ rule, the principle of minimum change is obeyed. Mathematically, the principle of minimum change minimises the distance between the joint probability distributions of the initial and updated belief. Intuitively, this is the idea that for any new piece of information, beliefs are updated in the smallest possible way that is compatible with the new facts. In the case of the flu test, for example, a negative test would not imply that the person is healthy, but rather that they are less likely to have the flu.
Read more of the
Eurekalert articel here:
https://www.eurekalert.org/news-releases/1096395
Abstract
(Physical Review Letters)
Bayes’ rule, which is routinely used to update beliefs based on new evidence, can be derived from a principle of minimum change. This principle states that updated beliefs must be consistent with new data, while deviating minimally from the prior belief. Here, we introduce a quantum analog of the minimum change principle and use it to derive a quantum Bayes’ rule by minimizing the change between two quantum input-output processes, not just their marginals. This is analogous to the classical case, where Bayes’ rule is obtained by minimizing several distances between the joint input-output distributions. When the change maximizes the fidelity, the quantum minimum change principle has a unique solution, and the resulting quantum Bayes’ rule recovers the Petz transpose map in many cases.
Source of abstract:
https://journals.aps.org/prl/abstract/ ... 5n4p-bxhm
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