  Macroscopic realism is the name for a class of modifications to quantum
theory that allow macroscopic objects to be described in a
measurement-independent manner, while largely preserving a fully quantum
mechanical description of the microscopic world. Objective collapse theories
are examples which aim to solve the quantum measurement problem through
modified dynamical laws. Whether such theories describe nature, however, is not
known. Here we describe and implement an experimental protocol capable of
constraining theories of this class, that is more noise tolerant and
conceptually transparent than the original Leggett-Garg test. We implement the
protocol in a superconducting flux qubit, and rule out (by ~84 s.d.) those
theories which would deny coherent superpositions of 170 nA currents over a ~10
ns timescale. Further, we address the 'clumsiness loophole' by determining
classical disturbance with control experiments. Our results constitute strong
evidence for the superposition of states of nontrivial macroscopic
distinctness.
