Absolute frequency measurement of an 171Yb lattice clock and optical clock comparisons

The measurement of time and frequency is at the heart of many technological applications and scientific measurements alike. In fact, the SI-unit the second is by quite a margin the SI-unit with the best relative uncertainty (ca. 10^{-16}), given by the accuracies of Cs fountain clocks probing the F = 3 - F = 4 ground-state transition in 133Cs. Still, demands for even higher accuracy and especially stability (a Cs fountain needs up to two weeks for the statistics to reach its declared uncertainty) are uttered in support of technological advancements (e.g. geodesy and GNSS systems) as well as fundamental science (physics beyond the standard model, tests of relativity). Nowadays optical lattice clocks confining a large number of neutral atoms in Stark shift free optical traps (the Stark shift free condition is characterised by a so-called magic wavelength of the trap) propose good candidates for a future redefinition of the SI-second in terms of an optical transition. Their accuracy and stability already surpass the Cs-fountains by two and three orders of magnitude, respectively. With further improvements to be expected in the near future, the application of optical lattice clocks to relativistic gravimetry, quantum computing, quantum simulation and fundamental physics keeps evolving. This thesis describes the development and characterisation of an 171Yb lattice clock at INRIM as well as its first frequency measurement campaigns and technolo- gies towards improved optical frequency measurements. The lattice clock confines cold atoms in a 1D optical dipole trap at the magic wavelength, which also cancels any Doppler- and recoil-related effects on the ultra-narrow clock transition. The first chapter offers a general overview of the physics behind lattice clocks and opti- cal frequency measurements. In the second chapter the 171Yb lattice clock developed during this work is expounded, including the trapping, state-preparation and state-probing of ultracold atoms inside the optical lattice. An exhaustive uncertainty budget for the clock transition is given and discussed showing already a performance beyond state-of- the-art Cs fountain clocks. An absolute frequency measurement obtained during this work is laid out. The result represents the lowest uncertainty achieved in a measurement of this transition against a primary frequency standard so far and is in agreement with previous values obtained by other groups around the world. A proof-of-principle experiment demonstrating for the first time the feasibility of transportable optical lattice clocks for geodesy and metrology applications outside of laboratory environments is described in chapter three. This experiment was conducted in collaboration with PTB and NPL and included a geodetic measurement with a transportable optical lattice clock that agreed with conventional methods as well as an optical 171Yb-87Sr frequency ratio measurement, enlarging the database on this particular ratio and thereby contributing to a possible redefinition of the SI-unit the second in terms of an optical transition or frequency-ratio matrix in the future. The fourth chapter discusses improvements added to the Yb lattice clock after the aforementioned measurements, in particular the stabilisation of the cooling and trapping lasers on a single stable low-drift cavity using mirrors coated for three disparate wavelengths across the optical spectrum. The simultaneous offset sideband locking and a throughout characterisation of the cavity are discussed. The last chapter is about the characterisation and optimisation of the NPL universal oscillator, which was conducted during my secondment at the NPL research facilities in the UK. The universal oscillator consists out of a femtosecond frequency comb, an ultra stable master laser and six slave oscillators. The femtosecond comb is transferring the stability of the superior master oscillator cavity to all six slave oscillators, which includes five lasers ranging from the infrared to the visible region. The principle of operation is explained and the obtained high performance of the spectral purity transfer set forth and discussed. This experiment demonstrated an unprecedented spectral purity transfer performance in a multi-branch configuration, opening the way for the interrogation of whole clock ensembles by just one master oscillator.