Using information from a measurement to adjust a system's parameters, enabling energy extraction in quantum protocols.Feedback in quantum physics is the process of using the outcome of a measurement to adjust the parameters of a quantum system in real time. Unlike classical feedback, which simply corrects errors, quantum feedback can be used to actively steer a system into desired states, extract energy from it, or protect it from decoherence. This is central to many quantum technologies, from error correction in quantum computers to the operation of quantum batteries and sensors.
The concept of feedback control in quantum systems was formalized in the 1980s–1990s, with key contributions by H. M. Wiseman and G. J. Milburn, who developed the theory of quantum feedback control.
A measurement on a quantum system yields a probabilistic outcome. The result is fed back to a controller, which applies a specific operation (e.g., a laser pulse or a magnetic field change) to the system. Because the measurement itself disturbs the quantum state, the feedback must be designed to account for this back-action. In energy extraction, for example, a measurement can reveal the system's state, and a subsequent operation can be tuned to extract work from it, effectively using information as a resource.
Used in quantum error correction, quantum metrology (e.g., adaptive measurements), and in proposed quantum battery charging protocols.
In the quantum Mpemba effect, feedback can be used to accelerate cooling by measuring and re-adjusting the system, making a hotter system cool faster than a colder one.














