  Transition frequencies of atoms and ions are among the most accurately
accessible quantities in nature, playing important roles in pushing the
frontiers of science by testing fundamental laws of physics, in addition to a
wide range of applications such as satellite navigation systems. Atomic clocks
based on optical transitions approach uncertainties of $10^{-18}$, where full
frequency descriptions are far beyond the reach of the SI second. Frequency
ratios of such super clocks, on the other hand, are not subject to this
limitation. They can therefore verify consistency and overall accuracy for an
ensemble of super clocks, an essential step towards a redefinition of the
second. However, with the measurement stabilities so far reported for such
frequency ratios, a confirmation to $1 \times 10^{-18}$ uncertainty would
require an averaging time $\tau$ of multiple months. Here we report a
measurement of the frequency ratio of neutral ytterbium and strontium clocks
with a much improved stability of $4 \times 10^{-16} ({\tau}/s)^{-1/2}$.
Enabled by the high stability of optical lattice clocks interrogating hundreds
of atoms, this marks a 90-fold reduction in the required averaging time over a
previous record-setting experiment that determined the ratio of Al+ and Hg+
single-ion clocks to an uncertainty of $5.2 \times 10^{-17}$. For the Yb/Sr
ratio, we find R = 1.207 507 039 343 337 749(55), with a fractional uncertainty
of $4.6 \times 10^{-17}$.
