  Many-particle optical lattice clocks have the potential for unprecedented
measurement precision and stability due to their low quantum projection noise.
However, this potential has so far never been realized because clock stability
has been limited by frequency noise of optical local oscillators. By
synchronously probing two 87Sr lattice systems using a laser with a thermal
noise floor of 1e-15, we remove classically correlated laser noise from the
intercomparison, but this does not demonstrate independent clock performance.
With an improved optical oscillator that has a 1e-16 thermal noise floor, we
demonstrate an order of magnitude improvement over the best reported stability
of any independent clock, achieving a fractional instability of 1e-17 in 1000 s
of averaging time for synchronous or asynchronous comparisons. This result is
within a factor of 2 of the combined quantum projection noise limit for a 160
ms probe time with ~10^3 atoms in each clock. We further demonstrate that even
at this high precision, the overall systematic uncertainty of our clock is not
limited by atomic interactions. For the second Sr clock, which has a
cavity-enhanced lattice, the atomic-density-dependent frequency shift is
evaluated to be -3.11e-17 with an uncertainty of 8.2e-19.
