How Many Tries Do You Need to Test an Unknown Quantum Battery? 

Our work shows that if you are given a tiny “quantum battery” whose internal condition is unknown, you usually cannot get any useful energy out of it in a single try. Earlier studies assumed that scientists already know everything about the microscopic quantum state of such a device, but real batteries in the lab are imperfect and only partly understood. In our paper, we instead treat the battery as a black box that keeps handing you identical, but unknown, quantum systems. We prove two new things. First, with only one copy, you are essentially stuck: almost no work can be reliably extracted. Second, if you can use the same device several times, we show how to test “how charged” it really is, using surprisingly few tries or measurements of the unknown state. Our work shows that such tests are within the reach of near-term quantum computers.

Publication: S. Chakraborty, S. Das, A. Ghorui, S. Hazra, and U. Singh, Sample Complexity of Black Box Work Extraction, Quantum Science and Technology 10, 045070 (2025). arXiv.