Irwin Oppenheim Award: Thermodynamics and quantum state discrimination

Bennett's exorcization of Maxwell's demon, building on the work of Szilárd and Landauer, taught us that the second law of thermodynamics forbids the erasure of information without a commensurate expenditure of energy. Following the same philosophy, in this talk we will explore how thermodynamics poses constraints on quantum information tasks. Specifically, I will show that the second law of thermodynamics restricts how well we can discriminate between quantum states. By examining an ideal gas with a quantum internal degree of freedom undergoing a cycle, we establish a nontrivial upper bound on the attainable accuracy of quantum state discrimination. This thermodynamic bound, which relies solely on the linearity of quantum mechanics and the constraint of no work extraction, matches Holevo's bound on accessible information, but is looser than the Holevo-Helstrom bound. This result places potential limitations on proposals beyond quantum mechanics and gives more evidence on the tension between thermodynamic entropy and von Neumann entropy. I will conclude by briefly discussing how this tension can be mitigated within quantum information theory using more operationally motivated notions of thermodynamic entropy.