Optical lattice clocks with uncertainty and instability in the 10 −17 -range and below have so far been demonstrated exclusively using fermions. Here, we demonstrate a bosonic optical lattice clock with 3 × 10 −18 instability and 2.0 × 10 −17 accuracy, both values improving on previous work by a factor 30. This was enabled by probing the clock transition with an ultra-long interrogation time of 4 s, using the long coherence time provided by a cryogenic silicon resonator, by careful stabilization of relevant operating parameters, and by operating at low atom density. This work demonstrates that bosonic clocks, in combination with highly coherent interrogation lasers, are suitable for highaccuracy applications with particular requirements, such as high reliability, transportability, operation in space, or suitability for particular fundamental physics topics. As an example, we determine the 88 Sr -87 Sr isotope shift with 12 mHz uncertainty.

We present an interrogation laser system for a transportable strontium lattice clock operating at 698 nm, which is based on an ultra-low-expansion glass reference cavity. Transportability is achieved by implementing a rigid, compact, and vibration insensitive mounting of the 12 cm-long reference cavity, sustaining shocks of up to 50 g. The cavity is mounted at optimized support points that independently constrain all degrees of freedom. This mounting concept is especially beneficial for cavities with a ratio of length L over diameter D L/D > 1. Generally, large L helps to reduce thermal noise-induced laser frequency instability while small D leads to small cavity volume. The frequency instability was evaluated, reaching its thermal noise floor of mod σy ≈ 3 × 10−16 for averaging times between 0.5 s and 10 s. The laser system was successfully operated during several field studies.

We present a transportable ultra-stable clock laser system based on a Fabry–Perot cavity with crystalline Al0.92Ga0.08As/GaAs mirror coatings, fused silica (FS) mirror substrates, and a 20 cm-long ultra-low expansion (ULE) glass spacer with a predicted thermal noise floor of mod σy = 7 × 10−17 in modified Allan deviation at one second averaging time. The cavity has a cylindrical shape and is mounted at 10 points. Its measured sensitivity of the fractional frequency to acceleration for the three Cartesian directions are 2(1) × 10−12 /(ms−2), 3(3) × 10−12 /(ms−2), and 3(1) × 10−12 /(ms−2), which belong to the lowest acceleration sensitivities published for transportable systems. The laser system’s instability reaches down to mod σy = 1.6 × 10−16

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