  Scale-invariant fluxes are the defining property of turbulent cascades, but
their direct measurement is a notorious problem. Here we perform such a
measurement for a direct energy cascade in a turbulent quantum gas. Using a
time-periodic force, we inject energy at a large lengthscale and generate a
cascade in a uniformly-trapped Bose gas. The adjustable trap depth provides a
high-momentum cutoff $k_{\textrm{D}}$, which realises a synthetic dissipation
scale. This gives us direct access to the particle flux across a momentum shell
of radius $k_{\textrm{D}}$, and the tunability of $k_{\textrm{D}}$ allows for a
clear demonstration of the zeroth law of turbulence: we observe that for fixed
forcing the particle flux vanishes as $k_{\textrm{D}}^{-2}$ in the
dissipationless limit $k_{\textrm{D}}\rightarrow \infty$, while the energy flux
is independent of $k_{\textrm{D}}$. Moreover, our time-resolved measurements
give unique access to the pre-steady-state dynamics, when the cascade front
propagates in momentum space.
