Cold-atom clocks as part of a timing ensemble

Four rubidium fountains have been in continuous operation at USNO for more than 3 years. They contribute to the local timescale and are reported to the BIPM as continuously running clocks — as opposed to secondary standards — the first and only cold-atom clocks so reported. Measurements among individual fountains demonstrate fractional-frequency stability that routinely reaches the low 10∼
¹⁶
s. Intervals during which there is little user intervention show fountain stabilities reaching the mid 10∼
¹⁷
s. As an ensemble, the four fountains generate a timescale with short-term stability below 1×10
⁻¹³
at 1 second, an order of magnitude better than the ensemble of ∼70 commercial cesiums in the USNO timescale. Rare frequency changes (on order of 1 per year, possibly correlated with operator intervention) have been observed. Multiple fountains allow the effect of this type of behavior on timing applications to be mitigated. The first 1.5 years of fountain data were used to put stricter limits on violations of Local Position Invariance and on the coupling of certain fundamental constants to gravitational potential. Some of these latter constraints require microwave clocks, and we expect to be able to improve upon these measurements using additional fountain data.