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.
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.
This thesis presents the latest achievements regarding the Sr optical lattice clock experiment at LNE-SYRTE, Observatoire de Paris. After having described the general principles for optical lattice clocks and the operation of the clock in question, the emphasis is put on the features that have been added to the experiment since 2007. The most important new elements are an ultra-stable reference cavity for the clock laser, the development of a non-destructive detection technique, and the construction of a second Sr lattice clock. The ultra-stable cavity is constructed from a ULE spacer and fused silica mirrors and has shown a thermal noise floor at 6.5E-16, placing it among the best in the world. The non-destructive detection is effectuated by a phase measurement of a weak probe beam that traverses the atoms placed in one arm of a Mach-Zender interferometer. The non-destructive aspect enables a recycling of the atoms from cycle to cycle which consequently increases the duty cycle, allowing for an increase of the stability of the clock. With these new tools the frequency stability is expected to be 2.2E-16/tau^(1/2) for an optimized sequence. The most recent comparisons between the two Sr clocks reach an accuracy level of 1E-16 after about 1000 s, and this way we have been able to characterize lattice related frequency shifts with an unprecedented accuracy. The measurements ensure a control of lattice related effects at the 1E-18 level even for trap depths as large as 50 Er.
This thesis presents the latest achievements regarding the Sr optical lattice clock experiment at LNE-SYRTE, Observatoire de Paris. After having described the general principles for optical lattice clocks and the operation of the clock in question, the emphasis is put on the features that have been added to the experiment since 2007. The most important new elements are an ultra-stable reference cavity for the clock laser, the development of a non-destructive detection technique, and the construction of a second Sr lattice clock. The ultra-stable cavity is constructed from a ULE spacer and fused silica mirrors and has shown a thermal noise floor at 6.5E-16, placing it among the best in the world. The non-destructive detection is effectuated by a phase measurement of a weak probe beam that traverses the atoms placed in one arm of a Mach-Zender interferometer. The non-destructive aspect enables a recycling of the atoms from cycle to cycle which consequently increases the duty cycle, allowing for an increase of the stability of the clock. With these new tools the frequency stability is expected to be 2.2E-16/tau^(1/2) for an optimized sequence. The most recent comparisons between the two Sr clocks reach an accuracy level of 1E-16 after about 1000 s, and this way we have been able to characterize lattice related frequency shifts with an unprecedented accuracy. The measurements ensure a control of lattice related effects at the 1E-18 level even for trap depths as large as 50 Er.
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.
Abstract We present several developments aimed at improving the stability of optical lattice clocks. First, we present our developments of an optical lattice clock using neutral mercury. We mention recent advances made with the fermionic isotope 199Hg. We also mention our work aimed at using bosonic isotopes, which offer the possibility to circumvent the relatively short lifetime of the upper clock state in 199Hg. Second, we present our work on a non-destructive detection in a Sr optical lattice clock. We describe developments that brought the detection scheme from the classical non-destructive regime to the quantum non-destructive regime. Our detection scheme is practical and has the capability to the used beyond proving the principle. Finally, we present our work on laser stabilization using spectral hole burning in a Eu3+:Y2SiO5 crystal at cryogenic temperatures. We describe our development of low noise interrogation based on digital IQ modulation and detection that can probe multiple spectral features simultaneously, as well as several investigations towards fluctuating environmental factors. These advances, individually or combined for example with spectral purity transfer with combs and composite clock approaches, shall bring significant progress in clock stability and accuracy.
High-bandwidth, high-stability clocks offer unique chances to study fundamental physics beyond simple metrology. Today, the most stable clocks are optical lattice clocks and they can reach a fractional stability of 3.1 × 10^−18 at 1 s averaging time. Their applications are ranging from the redefinition of the SI time unit, the Second, to the possible detection of gravitational waves in space. The stability of optical lattice clocks is primarily limited by quantum projection noise and Dick noise. In this thesis, I present our effort in creating a platform for studying quantum enhanced metrology and its application towards optical lattice clocks. We demonstrated the laser cooling in an optical cavity with Raman-sideband cooling and achieved average vibration number of n = 0.2. We utilized cavity feedback squeezing to demonstrate unitary spin squeezing on magnetic sub-levels of 171Yb, achieving 6.5 dB quantum projection noise reduction over the standard quantum limit (SQL). With coherent optical state transfer by ultra stable laser pulses, we successfully transferred the spin squeezed state from magnetic sub-levels to the optical transition, and demonstrated quantum enhanced metrology in the optical lattice clock for the first time. We achieved 4.4 dB of metrological gain over the SQL, which corresponds to a reduction of the averaging time by a factor of 2.8 in clock operation. Furthermore, we studied quantum enhanced metrology with time-reversal-based protocols. We performed a signal amplification through time-reversed interaction protocol between the ground state magnetic sublevels of 171Yb, achieving the largest sensitivity improvement beyond SQL in any interferometers to date at 11.8dB. Besides that, we experimentally studied the connection between quantum information scrambling, by measuring out-of-time-order correlators (OTOCs), and quantum metrology, by measuring metrological gain. We found these values to agree with each other, and concluded that the time-reversal quantum metrology can saturate the attainable gain from the quantum information scrambling.