Resolving the gravitational redshift across a millimetre-scale atomic sample.

Einstein's theory of general relativity states that clocks at different gravitational potentials tick at different rates relative to lab coordinates-an effect known as the gravitational redshift 1 . As fundamental probes of space and time, atomic clocks have long served to test this prediction at distance scales from 30 centimetres to thousands of kilometres 2-4 . Ultimately, clocks will enable the study of the union of general relativity and quantum mechanics once they become sensitive to the finite wavefunction of quantum objects oscillating in curved space-time. Towards this regime, we measure a linear frequency gradient consistent with the gravitational redshift within a single millimetre-scale sample of ultracold strontium. Our result is enabled by improving the fractional frequency measurement uncertainty by more than a factor of 10, now reaching 7.6 × 10 -21 . This heralds a new regime of clock operation necessitating intra-sample corrections for gravitational perturbations.