Abstract We describe a method of duty cycle engineering in optical lattice clocks to reduce clock instability through tailoring of the Dick effect noise contribution. We apply dynamically decoupled interrogation sequences to realise an estimated improvement in single clock instability approaching a factor of two, as well as providing a route to zero dead-time operation for two interleaved clocks which are otherwise unable to reach the required duty cycle. We show through simulations and synchronous measurements of clocks NPL-Sr1 and NPL-Sr2 that a quantum projection noise limited instability < 3 − 10−17 τ −1/2 can be realised in a near ideal zero dead-time configuration.
Abstract We describe a method of duty cycle engineering in optical lattice clocks to reduce clock instability through tailoring of the Dick effect noise contribution. We apply dynamically decoupled interrogation sequences to realise an estimated improvement in single clock instability approaching a factor of two, as well as providing a route to zero dead-time operation for two interleaved clocks which are otherwise unable to reach the required duty cycle. We show through simulations and synchronous measurements of clocks NPL-Sr1 and NPL-Sr2 that a quantum projection noise limited instability < 3 − 10−17 τ −1/2 can be realised in a near ideal zero dead-time configuration.
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.
Optical atomic clocks play a crucial role in fundamental physics, relativistic geodesy, and the future redefinition of the Systeme International second. Standard operation relies on cyclic interrogation sequences, which alternate between atomic interrogation and dead time used for state preparation and readout. This approach introduces the Dick effect, where laser frequency noise aliases onto the atomic transition frequency. Although reducing laser noise improves clock stability, the Dick effect remains a key limitation. In this Letter, we demonstrate a zero-dead-time optical clock based on two interleaved ensembles of cold ^{87}Sr atoms. Our system significantly suppresses this noise and achieves a fractional frequency instability at the 10^{-19} level between 10 000 and 20 000 s over repeated measurements, with a best value of 2.9×10^{-19} at τ=20000 s. The estimated long-term stability based on the combined data of these measurements reaches 2.5×10^{-19} at 1 day. These results represent a more than ninefold improvement over a conventional single-ensemble clock, highlighting its potential for next-generation timekeeping applications.
Abstract The frequency stability of many optical atomic clocks is limited by the coherence of their local oscillator. Here, we present a measurement protocol that overcomes the laser coherence limit. It relies on engineered dynamical decoupling of laser phase noise and near-synchronous interrogation of two clocks. One clock coarsely tracks the laser phase using dynamical decoupling; the other refines this estimate using a high-resolution phase measurement. While the former needs to have a high signal-to-noise ratio, the latter clock may operate with any number of particles. The protocol effectively enables minute-long Ramsey interrogation for coherence times of few seconds as provided by the current best ultrastable laser systems. We demonstrate implementation of the protocol in a realistic proof-of-principle experiment, where we interrogate for 0.5 s at a laser coherence time of 77 ms. Here, a single lattice clock is used to emulate synchronous interrogation of two separate clocks in the presence of artificial laser frequency noise. We discuss the frequency instability of a single-ion clock that would result from using the protocol for stabilisation, under these conditions and for minute-long interrogation, and find expected instabilities of σy(τ) = 8 × 10−16(τ/s)−1/2 and σy(τ) = 5 × 10−17(τ/s)−1/2, respectively.
We describe the Sr optical lattice clock apparatus at NPL with particular emphasis on techniques used to increase reliability and minimise the human requirement in its operation. Central to this is a clock-referenced transfer cavity scheme for the stabilisation of cooling and trapping lasers. We highlight several measures to increase the reliability of the clock with a view towards the realisation of an optical time-scale. The clock contributed 502 hours of data over a 25 day period (84% uptime) in a recent measurement campaign with several uninterrupted periods of more than 48 hours. An instability of 2 x 10 -17 was reached after 10 5 s of averaging in an interleaved self-comparison of the clock.
Optical lattice clocks demonstrate advantages in metrology and frontier physics because of their high stability.Here, we present approaches to enhancing the stability by decoupling the noise related to the short-term and long-term stability.For the short-term stability, we optimize the clock laser by decoupling the frequency noise, and optimize each noise contribution individually until it is below the thermal noise limit.For the long-term stability, we introduce a method to decouple the instability caused by systematic effects.Having identified that the collision frequency shift was the main limiting factor in our systems, we thus optimized the atom number fluctuations in optical lattices.Through targeted optimization, we achieve a synchronous comparison of two clocks with an average stability of 3.2 × 10 -16 = τ p and a long-term stability of 2.4 × 10 -18 at 8000 s.This work provides an analytical framework for enhancing optical clock stability.
Laser noise usually limits the stability of optical frequency ratio measurements, limiting the speed and precision one can compare two atomic frequency standards. In this talk I will describe two methods, correlation and differential spectroscopy, which utilize correlations in laser noise to increase the achievable interrogation time and thus increase the frequency comparison stability. Correlation spectroscopy is a technique which uses a parity measurement following a synchronized Ramsey interrogation to measure the relative frequency of two similar frequency atomic clocks. With this technique we achieve a measurement instability of (4×10^(-16))⁄√(τ⁄s) for a comparison of two single 27Al+ ion clocks. Differential spectroscopy uses an atomic clock with low projection noise, here a 171Yb lattice clock, to correct the phase noise of a second, higher frequency clock’s local oscillator thereby reducing the measurement instability to the level of the first. This can be further extended using two lattice clocks in a zero dead time configuration to correct the phase noise beyond the interrogation time reachable for a single Yb lattice clock. With these techniques we achieve measurement stabilities of (2.5×10^(-16))⁄√(τ⁄s) and (2×10^(-16))⁄√(τ⁄s) for a comparison between a single 27Al+ ion clock and a 171Yb lattice clock running as single clock and in a zero dead time configuration respectively. In addition to these techniques, I will also discuss recent progress towards characterizing the systematics of the NIST 40Ca+/27Al+ optical atomic clock.