  Quantum physics was invented to account for two fundamental features of
measurement results -- their discreetness and randomness. Emblematic of these
features is Bohr's idea of quantum jumps between two discrete energy levels of
an atom. Experimentally, quantum jumps were first observed in an atomic ion
driven by a weak deterministic force while under strong continuous energy
measurement. The times at which the discontinuous jump transitions occur are
reputed to be fundamentally unpredictable. Can there be, despite the
indeterminism of quantum physics, a possibility to know if a quantum jump is
about to occur or not? Here, we answer this question affirmatively by
experimentally demonstrating that the jump from the ground to an excited state
of a superconducting artificial three-level atom can be tracked as it follows a
predictable "flight," by monitoring the population of an auxiliary energy level
coupled to the ground state. The experimental results demonstrate that the jump
evolution when completed is continuous, coherent, and deterministic.
Furthermore, exploiting these features and using real-time monitoring and
feedback, we catch and reverse a quantum jump mid-flight, thus
deterministically preventing its completion. Our results, which agree with
theoretical predictions essentially without adjustable parameters, support the
modern quantum trajectory theory and provide new ground for the exploration of
real-time intervention techniques in the control of quantum systems, such as
early detection of error syndromes.
