Jeff Sherman

An Atomic Clock with 10–18 Instability

Tick, Tick, Tick… Many aspects of everyday life from communication to navigation rely on the precise ticking of the microwave transitions of the atoms in atomic clocks. Optical transitions occur at much higher frequency and so offer the opportunity to reduce the scale of the ticks even more. Hinkley et al. (p. 1215, published online 22 August; see the Perspective by Margolis) compare the ticking of two optical clocks and report an instability near the 10−18 level. Such performance will improve tests of general relativity and pave the way for a redefinition of the second. An ytterbium-based optical clock exhibits a precision of nearly one part per quintillion. [Also see Perspective by Margolis] 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 1018 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.

Ultrastable optical clock with two cold-atom ensembles

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.

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

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.

Search for domain wall dark matter with atomic clocks on board global positioning system satellites

Cosmological observations indicate that dark matter makes up 85% of all matter in the universe yet its microscopic composition remains a mystery. Dark matter could arise from ultralight quantum fields that form macroscopic objects. Here we use the global positioning system as a ~ 50,000 km aperture dark matter detector to search for such objects in the form of domain walls. Global positioning system navigation relies on precision timing signals furnished by atomic clocks. As the Earth moves through the galactic dark matter halo, interactions with domain walls could cause a sequence of atomic clock perturbations that propagate through the satellite constellation at galactic velocities ~ 300 km s−1. Mining 16 years of archival data, we find no evidence for domain walls at our current sensitivity level. This improves the limits on certain quadratic scalar couplings of domain wall dark matter to standard model particles by several orders of magnitude.The composition of dark matter in the universe remains a mystery, with one hypothetical form being topological defects. Here the authors determine a stronger constraint on the coupling of this dark matter to atomic clocks on board global positioning satellites through the analysis of 16 years of archival data.

A high stability optical frequency reference based on thermal calcium atoms

Here we report an imprecision below 10^-14 with a simple, compact optical frequency standard based upon thermal calcium atoms. Using a Ramsey-Bordé spectrometer we excite features with linewidths <; 5 kHz for the ¹S₀-³P₁ intercombination line at 657 nm. We have measured a fractional frequency instability below 6 × 10^-15 at 1 s, with good prospects for still quieter performance. The key remaining issue for this standard is how well first- and second-order Doppler drifts can be suppressed. Due to its experimental simplicity such a system could find application as a less accurate, but high stability reference for commercial applications.

Coherent excitation of the6S1/2to5D3/2electric-quadrupole transition in138Ba+

The electric dipole-forbidden, quadrupole 6S1/2 5D3/2 transition in Ba+ near 2051 nm, with a natural linewidth of 13 mHz, is attractive for potential observation of parity non-conservation, and also as a clock transition for a barium ion optical frequency standard. This transition also offers a direct means of populating the metastable 5D3/2 state to measure the nuclear magnetic octupole moment in the odd barium isotopes. Light from a diode-pumped, solid state Tm,Ho:YLF laser operating at 2051 nm is used to coherently drive this transition between resolved Zeeman levels in a single trapped 138Ba+ ion. The frequency of the laser is stabilized to a high finesse Fabry Perot cavity at 1025 nm after being frequency doubled. Rabi oscillations on this transition indicate a laser-ion coherence time of 3 ms, most likely limited by ambient magnetic field fluctuations.

Optical atomic clock measurements at the mHz level

Time (or its inverse, frequency) is the most precisely measured physical quantity and therefore is exploited in many fundamental investigations of nature. The advent of atomic clocks based on optical transitions has led to 10-100 times improvement in our timekeeping capabilities, with some measurements of optical frequencies reaching the mHz level [1,2]. Timekeeping precision at 1 part in 1018 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 of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium, and we discuss their operation at 10-18 instability as well as measurements toward the goal of 10-18 uncertainty.

Single-branch Er:fiber frequency comb for precision optical metrology with 10 −18 fractional instability

The comparison of optical atomic clocks with frequency instabilities reaching 1 part in 1016 at 1 s will enable more stringent tests of fundamental physics. These comparisons, mediated by optical frequency combs, require optical synthesis and measurement with a performance better than, or comparable to, the best optical clocks. Fiber-based mode-locked lasers have shown great potential for compact, robust, and efficient optical clockwork but typically require multiple amplifier and fiber optic paths that limit the achievable fractional frequency stability near 1 part in 1016 at 1 s. Here we describe an erbium-fiber laser frequency comb that overcomes these conventional challenges by ensuring that all critical fiber paths are common mode and within the servo-controlled feedback loop. Using this architecture, we demonstrate a fractional optical measurement uncertainty below 1×10−19 and fractional frequency instabilities less than 3×10−18 at 1 s and 1×10−19 at 1000 s.

Atomic Clock with1×10−18Room-Temperature Blackbody Stark Uncertainty

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

An atomic clock with

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.