A measure of the phase space accessed by a probe in quantum-algebraic systems

The entropy of the response of a system to a probe is proposed as a quantitative measure for the fraction of the total phase space accessed by that probe. The measure has the required dynamic range and its value depends both on the Hamiltonian of the system and on the probe. Different probes can be considered. The role of quantal fluctuations is explicitly taken into account. The computational procedure can be implemented in general, but an algebraic approach offers a convenient framework for its interpretation and evaluation. An example is provided. The measure is suited for application to experimental data.