We demonstrate spin gradient thermometry, a new general method of measuring
the temperature of ultracold atoms in optical lattices. We realize a mixture of
spins separated by a magnetic field gradient. Measurement of the width of the
transition layer between the two spin domains serves as a new method of
thermometry which is observed to work over a broad range of lattice depths and
temperatures, including in the Mott insulator regime. We demonstrate the
thermometry in a system of ultracold rubidium atoms, and suggest that
interesting spin physics can be realized in this system. The lowest measured
temperature is 1 nK, indicating that the system has reached the quantum regime,
where insulating shells are separated by superfluid layers.Comment: 5 pages, 3 figures, minor edits for clarit