In this paper the method for reducing the shift of the "clock" resonance by the example of 87 Sr has been described. This shift arises due to the presence in the optical spectrum of the lattice laser of the incoherent component. This component is a traveling wave. It is shown that the effective decrease of the impact on the shift of the "clock" resonance by introducing the interference filter is achieved. This filter was inserted directly into the optical scheme after the laser source. Examples of the optical lattice spectra with and without filter are considered. The estimates of the efficiency ratio of the use of the interference filters to improve the final uncertainty of the optical reference are made. It is shown that the filter reduces the relative uncertainty from the impact of the incoherent component of the optical radiation of the optical lattice by two orders of magnitude. It should help to reduce the lower bound for the total uncertainty limit of the optical frequency clocks down to 10 −18 .
In this paper the method for reducing the shift of the "clock" resonance by the example of 87 Sr has been described. This shift arises due to the presence in the optical spectrum of the lattice laser of the incoherent component. This component is a traveling wave. It is shown that the effective decrease of the impact on the shift of the "clock" resonance by introducing the interference filter is achieved. This filter was inserted directly into the optical scheme after the laser source. Examples of the optical lattice spectra with and without filter are considered. The estimates of the efficiency ratio of the use of the interference filters to improve the final uncertainty of the optical reference are made. It is shown that the filter reduces the relative uncertainty from the impact of the incoherent component of the optical radiation of the optical lattice by two orders of magnitude. It should help to reduce the lower bound for the total uncertainty limit of the optical frequency clocks down to 10 −18 .
Recent assessments have enhanced the evaluation of the lattice ac Stark shift in optical clocks to a fractional frequency level below ${10}^{\ensuremath{-}18}$. With this level of accuracy, meticulous attention is required to address shifts from the background spectrum of the lattice laser. Therefore, it is essential for the lattice laser to exhibit a minimal background spectrum. In this investigation, we employ a system based on sum-frequency generation (SFG) of the fiber-amplified lasers as the source for the strontium optical-clock lattice laser. The spectrum purity of this laser system has been confirmed through a heterodyne measurement involving a titanium:sapphire (Ti:Sa) laser. Consequently, we have characterized the background light shift resulting from its background spectrum, which is more than one order of magnitude lower than that of a tapered amplifier (TA). Through the implementation of carefully selected spectral filtering, the background light shift can be suppressed to a level of mid-${10}^{\ensuremath{-}20}$. Additionally, the suitability of the SFG laser is also experimentally verified in a strontium clock.
In this paper, we present recent experiments conducted with two87Sr optical lattice clocks operated at LNE-SYRTE. We report on the first calibrations of TAI with optical clocks, a necessary step towards the redefinition of the SI second. Additionally, we report on the experimental realization of a cavity-assisted non-destructive detection whose improved signal-to-noise ratio can improve the stability of optical clocks by reducing the Dick effect instability contribution, possibly beyond the quantum projection noise. Finally, we discuss the possibility to operate lattice clocks with semi-conductor sources as a lattice trap laser, by evaluating the frequency shift induced by their incoherent background light.
Experiments involving optical traps often require careful control of the ac Stark shifts induced by strong confining light fields. By carefully balancing light shifts between two atomic states of interest, optical traps at the magic wavelength have been especially effective at suppressing deleterious effects stemming from such shifts. Highlighting the power of this technique, optical clocks today exploit Lamb-Dicke confinement in magic-wavelength optical traps, in some cases realizing shift cancelation at the ten parts per billion level. Theory and empirical measurements can be used at varying levels of precision to determine the magic wavelength where shift cancelation occurs. However, lasers exhibit background spectra from amplified spontaneous emission or other lasing modes that can easily contaminate measurement of the magic wavelength and its reproducibility in other experiments or conditions. Indeed, residual light shifts from laser background have plagued optical lattice clock measurements for years. In this work, we develop a simple theoretical model allowing prediction of light shifts from measured background spectra. We demonstrate good agreement between this model and measurements of the background light shift from an amplified diode laser in a Yb optical lattice clock. Additionally, we model and experimentally characterize the filtering effect of a volume Bragg grating bandpass filter, demonstrating that application of the filter can reduce background light shifts from amplified spontaneous emission well below the ${10}^{\ensuremath{-}18}$ fractional clock frequency level. This demonstration is corroborated by direct clock comparisons between a filtered amplified diode laser and a filtered titanium:sapphire laser.
We discuss the extension of a recently proposed method based on continuous spectroscopy of 88 Sr atoms trapped in a moving optical lattice [Appl. Phys. Express, 14 , 072 006 (2021)] to the operation of bicolor thulium optical clocks. Simultaneous interrogation of two clock transitions at 1.14 μ m and build-up of a synthetic frequency provides a straightforward way to significantly suppress the first-order Doppler shift and cancel out the second-order Zeeman shift, the latter being one of the major sources of uncertainty in the originally proposed 88 Sr setup. In our configuration, the total systematic frequency shift is 4 × 10 −16 and instability is at or lower than 10 −18 after 1 h of integration.
Atoms confined in the optical lattice can be interrogated with Doppler- and recoil-free operation. However, if not properly controlled, the optical lattice may limit clock accuracy. To improve the lattice-light-shift uncertainty, the cooling and trapping lasers' frequency stability is optimized, and the atom's signal stability is enhanced. A ring-cavity Ti:sapphire laser is locked to the optical frequency comb, which is referenced to a 578 nm ultra-stable laser, and the beat note's stability is on the order of 10−16. Using a 10 cm Fabry–Pérot cavity referenced to the Ti:sapphire laser, the optical frequency stability is transferred to the 399 nm cooling laser, creating favorable conditions for evaluating the lattice-light-shift accurately. We reevaluate lattice-light-shift in our 171Yb optical lattice clock with an uncertainty of 8.1 × 10−18, which is an order lower than our previous result, and the magic frequency is determined to be 394 798 266.6(1.3) MHz.
Frequency metrology underpins precise measurements. At the forefront of this field are optical clocks which can perform measurements that are accurate to more digits than those in the number representing seconds elapsed from the beginning of the universe. Frequency estimation errors in the operation of optical clocks lead to frequency instability, prohibiting them from reaching high accuracies on short to medium time scales, and limiting access to interesting measurements of the atomic environment and exotic external perturbations such as gravity fields or fields of fundamentally new physical origins. Optical lattice traps are typically employed in state-of-the-art optical clocks to suppress Doppler effects. By carefully tuning the optical lattice to the magic wavelength, unwanted Stark shifts can be suppressed significantly, allowing operation with thousands of atoms and minimising the fundamental source of frequency estimation errors, namely the quantum projection noise, to around 3 × 10−17 at 1 s. In this thesis, the status of the two Strontium optical lattice clocks at NPL, Sr1 and Sr2, is presented going over a few interesting measurement campaigns from the past years involving Sr1 and the latest experimental efforts to re-enable operation of Sr2. A model of clock stability is developed by incorporating known and new frameworks to characterise the relation between the operating parameters of optical clocks and their performance, with relevance to the NPL lattice clocks and beyond. In particular, a method is identified for addressing the main technical noise for optical lattice clocks, the Dick effect, which typically has a magnitude of 10−16 at 1 s and prevents optical lattice clocks from reaching the quantum projection noise level. By taking advantage of dynamical decoupling we can engineer an optimised duty cycle with benefits in general for systems limited by the optical coherence of the clock laser. Moreover, a study of other noise sources in the NPL lattice clocks is conducted using synchronous measurements between Sr1 and Sr2, revealing a large contribution from linear Zeeman shifts, also at the 10−16 level, and a plan to tackle them is detailed. A hybrid clock architecture is discussed showing how optical lattice clocks can improve the optical coherence of ion clocks thanks to their higher servo rate, and a preliminary experimental demonstration is reported. Finally, a new type of spectroscopy is presented which completely avoids aliasing of linear Zeeman shifts by simultaneously driving transitions with opposite Zeeman sensitivities. Such a technique also enables the continuous interrogation of atoms, a crucial missing component in the Dick-free conveyor belt lattice clock concept heralded as the next standard of stable optical clocks.