  We add non-linear and state-dependent terms to quantum field theory. We show
that the resulting low-energy theory, non-linear quantum mechanics, is causal,
preserves probability and permits a consistent description of the process of
measurement. We explore the consequences of such terms and show that non-linear
quantum effects can be observed in macroscopic systems even in the presence of
de-coherence. We find that current experimental bounds on these non-linearities
are weak and propose several experimental methods to significantly probe these
effects. The locally exploitable effects of these non-linearities have enormous
technological implications. For example, they would allow large scale
parallelization of computing (in fact, any other effort) and enable quantum
sensing beyond the standard quantum limit. We also expose a fundamental
vulnerability of any non-linear modification of quantum mechanics - these
modifications are highly sensitive to cosmic history and their locally
exploitable effects can dynamically disappear if the observed universe has a
tiny overlap with the overall quantum state of the universe, as is predicted in
conventional inflationary cosmology. We identify observables that persist in
this case and discuss opportunities to detect them in cosmic ray experiments,
tests of strong field general relativity and current probes of the equation of
state of the universe. Non-linear quantum mechanics also enables novel
gravitational phenomena and may open new directions to solve the black hole
information problem and uncover the theory underlying quantum field theory and
gravitation.
