Quantum fluctuation-response inequality and its application in quantum hypothesis testing.

We establish a general quantum fluctuation-response inequality that bounds the difference in expectation values of an observable between two quantum states via the quantum relative entropy. For observables with bounded spectra, we further strengthen this result by exploiting the sub-Gaussian property and explicitly relate our bound to the sub-Gaussian norm of the observable. This allows us to derive a novel nonasymptotic bound on the sum of statistical errors in quantum hypothesis testing, which complements and can be more informative than existing bounds. We also demonstrate the versatility of our results through problems such as thermodynamic hypothesis testing and quantum speed-limits in physics and generalizability of algorithms in machine learning.