Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10 18 enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10 –18 after only 7 hours of averaging.

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An atomic clock with $10^{-18}$ instability

Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 x 10(-16) fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.

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Sr lattice clock at 1 x 10(-16) fractional uncertainty by remote optical evaluation with a Ca clock.

Scientists demonstrate a cavity-stabilized laser system with a reduced thermal noise floor, exhibiting a fractional frequency instability of 2 × 10−16. They use this system as a stable optical source in an ytterbium optical lattice clock to resolve an ultranarrow 1 Hz linewidth for the 518 THz clock transition. Consistent measurements with a clock instability of 5 × 10−16/√τ are reported.

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Making optical atomic clocks more stable with 10-16-level laser stabilization

Optical clocks with a record low zero-dead-time instability of 6 × 10–17 at 1 second are demonstrated in two cold-ytterbium systems. The two systems are interrogated by a shared optical local oscillator to nearly eliminate the Dick effect. Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with the ‘dead’ time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise from the laser interrogating the atomic transition1,2. Despite recent advances in optical clock stability that have been achieved by improving laser coherence, the Dick effect has continually limited the performance of optical clocks. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock that is based on the interleaved interrogation of two cold-atom ensembles3. This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability assessed to be for an averaging time τ in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce the standard quantum limit of clock stability, achieving a spectroscopy line quality factor of Q > 4 × 1015.

Ultrastable optical clock with two cold-atom ensembles

We describe and review the current state of the art in atomic fountain clocks. These clocks provide the best realization of the SI second possible today, with relative uncertainties of a few parts in 1016.

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Atomic fountain clocks

Atomic frequency standards using trapped ions or cold atoms work intrinsically in a pulsed mode. Theoretically and experimentally, this mode of operation has been shown to lead to a degradation of the frequency stability due to the frequency noise of the interrogation oscillator. In this paper a physical analysis of this effect has been made by evaluating the response of a two-level atom to the interrogation oscillator phase noise in Ramsey and multi-Rabi interrogation schemes using a standard quantum mechanical approach. This response is then used to calculate the degradation of the frequency stability of a pulsed atomic frequency standard such as an atomic fountain or an ion trap standard. Comparison is made to an experimental evaluation of this effect in the LPTF Cs fountain frequency standard, showing excellent agreement.

Frequency stability degradation of an oscillator slaved to a periodically interrogated atomic resonator

In this paper we evaluate the effect of the interrogation oscillator frequency noise on the stability of a pulsed atomic frequency standard such as an atomic fountain or ion trap frequency standard. The atomic response to a phase perturbation in Ramsey and multi-Rabi interrogation schemes has been calculated using the density matrix formalism. The results of these calculations are used to obtain a simple model for the limitation of the frequency standard stability. An experimental evaluation of this effect has been performed by using the Cs atomic fountain frequency standard. Possible means to reduce this effect are considered.

Theoretical description and experimental evaluation of the effect of the interrogation oscillator frequency noise on the stability of a pulsed atomic frequency standard

We point out key points of a microgravity clock using cold Cs atoms. The main characteristics of the experimental set-up (resonator, optical set-up, ...) are presented. Preliminary tests of a prototype are performed in the reduced gravity of aircraft parabolic flights.

C. Audoin

Papers

Frequency stability degradation of an oscillator slaved to a periodically interrogated atomic resonator

Theoretical description and experimental evaluation of the effect of the interrogation oscillator frequency noise on the stability of a pulsed atomic frequency standard

Toward a space clock using cold cesium atoms: the Pharao project