  We propose a precision measurement of time variations of the proton-electron
mass ratio using ultracold molecules in an optical lattice. Vibrational energy
intervals are sensitive to changes of the mass ratio. In contrast to
measurements that use hyperfine-interval-based atomic clocks, the scheme
discussed here is model-independent and does not require separation of time
variations of different physical constants. The possibility of applying the
zero-differential-Stark-shift optical lattice technique is explored to measure
vibrational transitions at high accuracy.
