  The Keldysh-ordered full counting statistics is a quasi-probability
distribution describing the fluctuations of a time-integrated quantum
observable. While it is well known that this distribution can fail to be
positive, the interpretation and origin of this negativity has been somewhat
unclear. Here, we show how the full counting statistics can be tied to
trajectories through Hilbert space, and how this directly connects negative
quasi-probabilities to an unusual interference effect. Our findings are
illustrated with the example of energy fluctuations in a driven bosonic
resonator; we discuss how negative quasi-probability here could be detected
experimentally using superconducting microwave circuits.
