The pursuit of better atomic clocks has advanced many research areas, providing better quantum state control, new insights in quantum science, tighter limits on fundamental constant variation and improved tests of relativity. The record for the best stability and accuracy is currently held by optical lattice clocks. Here we take an important step towards realizing the full potential of a many-particle clock with a state-of-the-art stable laser. Our 87Sr optical lattice clock now achieves fractional stability of 2.2 × 10−16 at 1 s. With this improved stability, we perform a new accuracy evaluation of our clock, reducing many systematic uncertainties that limited our previous measurements, such as those in the lattice ac Stark shift, the atoms' thermal environment and the atomic response to room-temperature blackbody radiation. Our combined measurements have reduced the total uncertainty of the JILA Sr clock to 2.1 × 10−18 in fractional frequency units.

Strontium optical lattice clocks have the potential to simultaneously interrogate millions of atoms with a high spectroscopic quality factor of 4 × 10 Previously, atomic interactions have forced a compromise between clock stability, which benefits from a large number of atoms, and accuracy, which suffers from density-dependent frequency shifts. Here we demonstrate a scalable solution that takes advantage of the high, correlated density of a degenerate Fermi gas in a three-dimensional (3D) optical lattice to guard against on-site interaction shifts. We show that contact interactions are resolved so that their contribution to clock shifts is orders of magnitude lower than in previous experiments. A synchronous clock comparison between two regions of the 3D lattice yields a measurement precision of 5 × 10 in 1 hour of averaging time.

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Engineered spin-orbit coupling (SOC) in cold-atom systems can enable the study of new synthetic materials and complex condensed matter phenomena. However, spontaneous emission in alkali-atom spin-orbit-coupled systems is hindered by heating, limiting the observation of many-body effects and motivating research into potential alternatives. Here we demonstrate that spin-orbit-coupled fermions can be engineered to occur naturally in a one-dimensional optical lattice clock. In contrast to previous SOC experiments, here the SOC is both generated and probed using a direct ultra-narrow optical clock transition between two electronic orbital states in Sr atoms. We use clock spectroscopy to prepare lattice band populations, internal electronic states and quasi-momenta, and to produce spin-orbit-coupled dynamics. The exceptionally long lifetime of the excited clock state (160 seconds) eliminates decoherence and atom loss from spontaneous emission at all relevant experimental timescales, allowing subsequent momentum- and spin-resolved in situ probing of the SOC band structure and eigenstates. We use these capabilities to study Bloch oscillations, spin-momentum locking and Van Hove singularities in the transition density of states. Our results lay the groundwork for using fermionic optical lattice clocks to probe new phases of matter.

We conduct frequency comparisons between a state-of-the-art strontium optical lattice clock, a cryogenic crystalline silicon cavity, and a hydrogen maser to set new bounds on the coupling of ultralight dark matter to Standard Model particles and fields in the mass range of 10 −16 − 10 −21 eV. The key advantage of this two-part ratio comparison is the differential sensitivities to time variation of both the fine-structure constant and the electron mass, achieving a substantially improved limit on the moduli of ultralight dark matter, particularly at higher masses than typical atomic spectroscopic results. Furthermore, we demonstrate an extension of the search range to even higher masses by use of dynamical decoupling techniques. These results highlight the importance of using the best performing atomic clocks for fundamental physics applications as all-optical timescales are increasingly integrated with, and will eventually supplant, existing microwave timescales.

We implement imaging spectroscopy of the optical clock transition of lattice-trapped degenerate fermionic Sr in the Mott-insulating regime, combining micron spatial resolution with submillihertz spectral precision. We use these tools to demonstrate atomic coherence for up to 15 s on the clock transition and reach a record frequency precision of 2.5×10^{-19}. We perform the most rapid evaluation of trapping light shifts and record a 150 mHz linewidth, the narrowest Rabi line shape observed on a coherent optical transition. The important emerging capability of combining high-resolution imaging and spectroscopy will improve the clock precision, and provide a path towards measuring many-body interactions and testing fundamental physics.

We present the first characterization of the spectral properties of superradiant light emitted from the ultranarrow, 1-mHz-linewidth optical clock transition in an ensemble of cold 87 Sr atoms. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. By comparing the frequency of our superradiant source to that of a state-of-the-art cavity-stabilized laser and optical lattice clock, we observe a fractional Allan deviation of 6.7ð1Þ × 10 −16 at 1 s of averaging, establish absolute accuracy at the 2-Hz (4 × 10 −15 fractional frequency) level, and demonstrate insensitivity to key environmental perturbations.

We measure atom number statistics after splitting a gas of ultracold 87 Rb atoms in a purely magnetic double-well potential created on an atom chip. Well below the critical temperature for Bose-Einstein condensation Tc, we observe reduced fluctuations down to −4.9 dB below the atom shot noise level. Fluctuations rise to more than +3.8 dB close to Tc, before reaching the shot noise level for higher temperatures. We use two-mode and classical field simulations to model these results. This allows us to confirm that the super-shot noise fluctuations directly originate from quantum statistics. Since the achievement of Bose-Einstein condensation (BEC) in dilute atomic gases, different experimental techniques have been developed in order to coherently split a BEC into two spatially separate parts [1][2][3][4], with atom interferometry as one of the motivations. Even though BECs are usually in the weakly interacting regime, the interactions between the particles dramatically affect the physics of the splitting. In particular, repulsive interactions limit the phase coherence between the two split parts [5], but also reduce atom number difference fluctuations, giving rise to non-classical squeezed states [6][7][8][9][10].In this Letter, we use a purely static magnetic potential created on an atom chip to realize a nonlinear spatial "beam" splitter for a BEC. We investigate the physics of the splitting and focus on atom number fluctuations and the role of temperature. At low temperatures, where the interaction energy dominates, we directly observe number squeezed states with relative population fluctuations −4.9 dB below shot noise, as first shown in [10] and indirectly observed in [8]. The two separated but weakly linked parts of the BEC constitute a bosonic Josephson junction, usually described by a two mode model (TMM) [11]. Our results are in agreement with the TMM, which also predicts that the observed squeezing is accompanied by high phase coherence.The magnetic trap configuration allows barrier heights up to several µK and straightforward evaporative cooling, so that we can separate clouds with increasing temperature all the way to the non-degenerate regime. In the intermediate temperature regime, where both a significant condensate and thermal fraction are present, we observe large super-binomial fluctuations in the number difference between the two parts. This excess of fluctuations is a direct signature of the Bose statistics, in close analogy to the bunching effect in quantum optics [16].Close to the BEC transition, the condensates show significant depletion and the TMM breaks down. We complement our experiments by a theoretical investigation of this regime using a classical field approach and show that large super-binomial fluctuations are a general feature at thermal equilibrium. Although the experiments are not performed at equilibrium, our observations are still in qualitative agreement with these theoretical results.Our experiment uses a two-layer atom chip to prepare a 87 Rb BEC in the |F = 2, m F = 2 hyp...