  A quantum system driven by a weak deterministic force while under strong
continuous energy measurement exhibits quantum jumps between its energy levels
(Nagourney et al., 1986, Sauter et al., 1986, Bergquist et al., 1986). This
celebrated phenomenon is emblematic of the special nature of randomness in
quantum physics. The times at which the jumps occur are reputed to be
fundamentally unpredictable. However, certain classical phenomena, like
tsunamis, while unpredictable in the long term, may possess a degree of
predictability in the short term, and in some cases it may be possible to
prevent a disaster by detecting an advance warning signal. Can there be,
despite the indeterminism of quantum physics, a possibility to know if a
quantum jump is about to occur or not? In this dissertation, we answer this
question affirmatively by experimentally demonstrating that the completed jump
from the ground to an excited state of a superconducting artificial atom can be
tracked, as it follows its predictable "flight," by monitoring the population
of an auxiliary level coupled to the ground state. Furthermore, the
experimental results demonstrate that the jump when completed is continuous,
coherent, and deterministic. Exploiting these features, we catch and reverse a
quantum jump mid-flight, thus deterministically preventing its completion. This
real-time intervention is based on a particular lull period in the population
of the auxiliary level, which serves as our advance warning signal. 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.
