For one-dimensional continuous-variable quantum systems such as single-mode quantum optical systems, we give a quantification of the quantumness of such a system's state, { \rho}, by introducing the measure of quantumness, { \Xi}, which works for all states, pure or mixed. { \Xi} is a measure which is universal, sensitive, monotonic, and unbounded. { \Xi}[{ \rho}] yields a single positive value to quantify how nonclassical { \rho} is. { \Xi} employs phase space distributions to represent { \rho} and is a fixed function, { \Xi}[.], independent of the system, its environment or the type of state.
Categories: quant-ph