On the Conservation of Information and the Second Law of Thermodynamics How can these fundamental laws be consistent? The conservation of quantum information, quantified in term of von Neumann entropy, is a fundamental consequence of the linearity and unitarity of quantum mechanics. As the terms information and entropy are often used interchangeably in several branches of sciences, this may sound very strange to anyone familiar with the second law of thermodynamics, which says that entropy generally increases with time. How can these claims be…

https://aurelien-pelissier.medium.com/on-the-conservation-of-information-and-the-second-law-of-thermodynamics-f22c0645d8ec 

No-hiding theorem - Wikipedia