Exact speed limit for measurement of quantum systems*

We present a fundamental lower limit on the time required for a measurement apparatus to extract the information of a quantum system. The "measurement speed limit" is exact, i.e., derived without any approximations. In particular, the bound is valid for any strength of coupling between system and device, and thus can account for arbitrarily strong non-Markovian correlation effects. The limit depends on the details of the coupling between the system and measurement device, and the correlation functions of the measurement device. In the context of quantum information processing, the result provides a fundamental lower limit (“best case scenario”) for the readout time of qubits for a given device, providing a possible benchmark and guiding principles for optimizing protocols and architectures. We compare the limit with actual readout times in circuit-based qubit architectures.

The bound also provides an upper limit ("worst case scenario") for the rate of uncontrolled information loss in quantum systems due to noise and fluctuations from the environment.