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Optical atomic clocks require local oscillators with exceptional optical coherence owing to the challenge of performing spectroscopy on their ultranarrow-linewidth clock transitions. Advances in laser stabilization have thus enabled rapid progress in clock precision. A new class of ultrastable lasers based on cryogenic silicon reference cavities has recently demonstrated the longest optical coherence times to date. Here we utilize such a local oscillator with two strontium (Sr) optical lattice clocks to achieve an advance in clock stability. Through an anti-synchronous comparison, the fractional instability of both clocks is assessed to be 4.8 × 1 0 - 17 ∕ τ for an averaging time τ (in seconds). Synchronous interrogation enables each clock to average at a rate of 3.5 × 1 0 - 17 ∕ τ , dominated by quantum projection noise, and reach an instability of 6.6 × 10−19 over an hour-long measurement. The ability to resolve sub-10−18-level frequency shifts in such short timescales will affect a wide range of applications for clocks in quantum sensing and fundamental physics. By using an ultrastable oscillator based on a cryogenic Si cavity, the fractional instability of two Sr optical lattice clocks at 1 s reaches 4.8 × 10−17 and 3.5 × 10−17 through anti-synchronous and synchronous comparisons, respectively. [ABSTRACT FROM AUTHOR]

Overtone vibration–laser double resonance studies of DF(v=1–3) energy transfer yield self-relaxation rate constants for v=1, 2 and 3 of k1=(0.37±0.06)×10-12 cm3 mol-1 s-1, k2=(22.0±2.0)×10-12 cm3 mol-1 s-1, and k3=(17.0±1.8)×10-12 cm3 mol-1 s-1, respectively. The approach also directly measures the relative importance of vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V–T,R) energy transfer. The fraction of DF(v) molecules relaxing by V–V energy transfer is 1.1±0.1 and 0.72±0.10 for v=2 and v=3, respectively. Essentially all of the vibrational energy transfer in v=2 occurs via the V–V mechanism. The slower relaxation of DF(v=3) compared to DF(v=2), in contrast to simple scaling law predictions, reflects the decreasing influence of the V–V mechanism, even though it is still the primary relaxation pathway for DF(v=3). Comparison with HF self-relaxation qualitatively indicates that V–R energy transfer is important in V–T,R relaxation of DF(v=1). [ABSTRACT FROM AUTHOR]

Overtone vibration-laser double resonance measurements determine the vibrational relaxation rates of DF(v=1) and HF(v=1) by the DF dimer. Both monomers are efficiently relaxed by the dimer at a rate that is 20% of the gas kinetic rate. The similarity of the rate constants for the two systems, which have very different energy defects, indicates that the relaxation occurs by collision complex formation and energy redistribution, rather than direct vibration-to-vibration energy transfer from the monomer to the dimer. [ABSTRACT FROM AUTHOR]

Overtone vibration-laser double resonance directly measures the relative importance of vibration-to-vibration and vibration-to-translation-and-rotation energy transfer for HF(v=3 and v=4) at room temperature. The fraction of HF(v) molecules relaxing by V–V energy transfer is 0.44±0.05 and 0.16±0.05 for v=3 and v=4, respectively, compared to 0.59±0.10 for v=2. These measurements show that V–T,R energy transfer is the dominant relaxation mechanism for HF(v≥3) and the observed decreased amount of V–V energy transfer for higher initially excited vibrational levels is in good agreement with a chemiluminescence measurement and several theoretical calculations. The data demonstrate that the magnitude of the energy defects for the component pathways primarily determines the energy transfer mechanism for HF(v=2–4). [ABSTRACT FROM AUTHOR]

The temperature dependencies of the total self-relaxation rate constants for the vibrational deactivation of HF(v=2) and HF(v=1) and the state-to-state vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V-T,R) energy transfer components of the HF(v=2) self-relaxation process are measured using the overtone vibration excitation-laser double resonance technique. The total self-relaxation rate constants vary inversely with temperature. The much weaker temperature dependence of HF(v=2) self-relaxation compared to that of HF(v=1) arises from the significant role of the V–V energy transfer route. Competition between energetics and collision duration results in a weaker inverse variation with temperature for the slightly endothermic V–V route than for the exothermic V-T,R route for HF(v=2). The branching ratio for V–V energy transfer increases slightly with temperature and the data suggest that two quantum relaxation processes constitute no more than 10% of the total self-relaxation of HF(v=2). The available temperature dependence data on self-relaxation of HF(v=1–5) form a consistent picture in which the energetics of the V–V and V-T,R relaxation pathways control their relative contributions to the total energy transfer. [ABSTRACT FROM AUTHOR]

Overtone vibration–laser double resonance measurements determine the vibrational relaxation rate of HF(v=1) by HF dimers. Vibration-to-vibration energy transfer from the excited monomer to the dimer followed by vibrational predissociation of the dimer provides an efficient pathway for vibration-to-translation energy transfer that deexcites the monomer at 40% of the gas kinetic collision rate. Analysis of the pressure dependence of the observed decay constants using a simple kinetic model establishes a rough upper limit of 10 ns on the predissociation lifetime of the collisionally excited dimer. [ABSTRACT FROM AUTHOR]