Stephan Falke

A strontium lattice clock with 3 × 10 −17 inaccuracy and its frequency

We have measured the absolute frequency of the optical lattice clock based on 87Sr at PTB with an uncertainty of 3.9 × 10−16 using two caesium fountain clocks. This is close to the accuracy of todayʼs best realizations of the SI second. The absolute frequency of the 5 s2 1S0 – 5s5p 3P0 transition in 87Sr is 429 228 004 229 873.13(17) Hz. Our result is in excellent agreement with recent measurements performed in different laboratories worldwide. We improved the total systematic uncertainty of our Sr frequency standard by a factor of five and reach 3 × 10−17, opening new prospects for frequency ratio measurements between optical clocks for fundamental research, geodesy or optical clock evaluation.

8 × 10⁻¹⁷ fractional laser frequency instability with a long room-temperature cavity.

We present a laser system based on a 48 cm long optical glass resonator. The large size requires a sophisticated thermal control and optimized mounting design. A self-balancing mounting was essential to reliably reach sensitivities to acceleration of below Δν/ν<2×10(-10)/g in all directions. Furthermore, fiber noise cancellations from a common reference point near the laser diode to the cavity mirror and to additional user points (Sr clock and frequency comb) are implemented. Through comparison with other cavity-stabilized lasers and with a strontium lattice clock, instability of below 1×10(-16) at averaging times from 1 to 1000 s is revealed.

High accuracy correction of blackbody radiation shift in an optical lattice clock.

We have determined the frequency shift that blackbody radiation is inducing on the 5s2 (1)S0-5s5p (3)P0 clock transition in strontium. Previously its uncertainty limited the uncertainty of strontium lattice clocks to 1×10(-16). Now the uncertainty associated with the blackbody radiation shift correction translates to a 5×10(-18) relative frequency uncertainty at room temperature. Our evaluation is based on a measurement of the differential dc polarizability of the two clock states and on a modeling of the dynamic contribution using this value and experimental data for other atomic properties.

Tackling the Blackbody Shift in a Strontium Optical Lattice Clock

A major obstacle for optical clocks is the frequency shift due to blackbody radiation (BBR). We discuss how one can tackle this problem in an optical lattice clock, in our case, 87Sr: first, by a measurement of the dc-Stark shift of the clock transition and, second, by interrogating the atoms in a cryogenic environment. Both approaches rely on transporting ultracold atoms over several centimeters within a probe cycle. We evaluate the mechanical movement of the optical lattice and conclude that it is feasible to transport the atoms over 50 mm within 300 ms. With this transport, a dc-Stark shift measurement will allow reducing the contribution of the BBR to fractional uncertainty below 2 × 10-17 at room temperature by improving the shift coefficient, known only from atomic-structure calculations up to now. We propose a cryogenic environment at 77 K that will reduce this contribution to a few times 10-18.

Ultrastable laser with average fractional frequency drift rate below 5 × 10−19_s

Cryogenic single-crystal optical cavities have the potential to provide high dimensional stability. We have investigated the long-term performance of an ultrastable laser system that is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a primary caesium fountain standard and to the 87Sr optical lattice clock at Physikalisch-Technische Bundesanstalt (PTB). With fractional frequency instabilities of σ(y)(τ)≤2×10(-16) for averaging times of τ=60  s to 1000 s and σ(y)(1  d)≤2×10(-15) the stability of this laser, without any aid from an atomic reference, surpasses the best known microwave standards for short averaging times and is competitive with the best known hydrogen masers for longer times of 1 day. The comparison of modeled thermal response of the cavity with measured data indicates an average fractional frequency drift below 5×10(-19)/s, which we do not expect to be a fundamental limit.

Delivering pulsed and phase stable light to atoms of an optical clock

In optical clocks, transitions of ions or neutral atoms are interrogated using pulsed ultra-narrow laser fields. Systematic phase chirps of the laser or changes of the optical path length during the measurement cause a shift of the frequency seen by the interrogated atoms. While the stabilization of cw-optical links is now a well-established technique even on long distances, phase stable links for pulsed light pose additional challenges and have not been demonstrated so far. In addition to possible temperature or pressure drift of the laboratory, which may lead to a Doppler shift by steadily changing the optical path length, the pulsing of the clock laser light calls for short settling times of stabilization locks. Our optical path length stabilization uses retro-reflected light from a mirror that is fixed with respect to the interrogated atoms and synthetic signals during the dark time. Length changes and frequency chirps are compensated for by the switching AOM. For our strontium optical lattice clock, we have ensured that the shift introduced by the fiber link including the pulsing acoustooptic modulator is below 2×10-17.

Potassium ground-state scattering parameters and Born-Oppenheimer potentials from molecular spectroscopy

We present precision measurements with MHz uncertainty of the energy gap between asymptotic and well bound levels in the electronic ground state

A compact and efficient strontium oven for laser-cooling experiments

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Long-range transport of ultracold atoms in a far-detuned one-dimensional optical lattice

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Born-Oppenheimer approximation for mass scaling of cold-collision properties

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