  The principle of detailed balance (DB) states that every kinetic transition
in a system with many micro-states, $\mu$, is balanced, on average, with the
opposite transition, $\mu_i\leftrightharpoons\mu_j$. Since its introduction by
Boltzmann, this principle has been used by luminaries, such as Einstein,
Eddington, Kramers, Pauli, Ehrenfest, Dirac, Onsager, and many others to derive
significant results that underpin much of our scientific understanding. The
current belief is that DB is satisfied only in equilibrium systems, while
non-equilibrium steady states can only be balanced by cycles, such as $A\to
B\to C\to A$. We show here experimentally that DB can exist and is commonly and
robustly satisfied in a family of quasi-statically cyclically sheared granular
systems. We further study the approach to DB as a function of system size and
time. Given the significant impact that this principle has had on equilibrium
systems, we believe that this discovery paves the way for better models of the
dynamics of non-equilibrium systems.
