Master thesis in physics at University of Torino. ABSTRACT :Optical lattice clocks occupy a very important position in metrological research. Diffused worldwide, they are a strong candidate for a future redefinition of the second of the International System of Units (SI) and thanks to their extremely small fractional uncertainty about 10−18 they are involved in fundamental physics experiments, for example in the field of general relativity. In a optical clock, the lattice is used to trap ultracold atoms and it is realized with a retro-reflected laser tuned on magic wavelength to reduce perturbations on the clock transition. An optical lattice is generated usually starting by a titanium:sapphire laser: its main qualities are high output power and narrow linewidth, but it is delicate and difficult to use so its application in new kinds of atomic clocks, such as transportable or spacial, is not strongly recommended. Replacing it with an amplified laser diode would be preferable because these instruments are better for cost, reliability, size and weight, so suitable for applications out of laboratory, but at the same time present the problem of spectral purity as they have a not negligible amplified spontaneous emission (ASE): this phenomenon produces a broad spectrum detuned from magic wavelength inducing a perturbation on the clock transition. In my thesis I investigated experimentally the possibility to generate an optical lattice starting from an amplified laser diode emitting at 759.35 nm, ytterbium magic wavelength, and I applied the laser system that I developed on the atomic clock IT-Yb1 at Istituto Nazionale di Ricerca Metrologica (INRiM) located in Torino. The target of this work is to achieve fractional uncertainty due to ASE below 10−18, making the performances of the two different lasers sources equivalent. First, I calculated the theoretical model of the light shift induced by the lattice on the clock transition between ytterbium atomic states 1S0 →3 P0, comparing it with experimental results and I applied it to estimate ASE contribution to lattice shift. In the experimental part of this work I realized an optical system to study amplified laser diode emission and manipulate laser beam to obtain a good lattice from this laser source, filtering ASE with a volume Bragg grating and controlling power with an acousto optic modulator. Moreover I studied ASE spectral distribution in different conditions and its interaction with optical elements. Next studies foresee to measure experimentally with IT-Yb1 the shift induced by the laser system that I realized to calculate its uncertainty contribution and compare this result with the one obtained with a titanium:sapphire laser. This work will be useful to the development of compact or transportable atomic clocks based exclusively on semiconductor lasers with applications in the generation of optical timescales, travelling optical standards and the measurements of Earth gravitational redshift.
Master thesis in physics at University of Torino. ABSTRACT :Optical lattice clocks occupy a very important position in metrological research. Diffused worldwide, they are a strong candidate for a future redefinition of the second of the International System of Units (SI) and thanks to their extremely small fractional uncertainty about 10−18 they are involved in fundamental physics experiments, for example in the field of general relativity. In a optical clock, the lattice is used to trap ultracold atoms and it is realized with a retro-reflected laser tuned on magic wavelength to reduce perturbations on the clock transition. An optical lattice is generated usually starting by a titanium:sapphire laser: its main qualities are high output power and narrow linewidth, but it is delicate and difficult to use so its application in new kinds of atomic clocks, such as transportable or spacial, is not strongly recommended. Replacing it with an amplified laser diode would be preferable because these instruments are better for cost, reliability, size and weight, so suitable for applications out of laboratory, but at the same time present the problem of spectral purity as they have a not negligible amplified spontaneous emission (ASE): this phenomenon produces a broad spectrum detuned from magic wavelength inducing a perturbation on the clock transition. In my thesis I investigated experimentally the possibility to generate an optical lattice starting from an amplified laser diode emitting at 759.35 nm, ytterbium magic wavelength, and I applied the laser system that I developed on the atomic clock IT-Yb1 at Istituto Nazionale di Ricerca Metrologica (INRiM) located in Torino. The target of this work is to achieve fractional uncertainty due to ASE below 10−18, making the performances of the two different lasers sources equivalent. First, I calculated the theoretical model of the light shift induced by the lattice on the clock transition between ytterbium atomic states 1S0 →3 P0, comparing it with experimental results and I applied it to estimate ASE contribution to lattice shift. In the experimental part of this work I realized an optical system to study amplified laser diode emission and manipulate laser beam to obtain a good lattice from this laser source, filtering ASE with a volume Bragg grating and controlling power with an acousto optic modulator. Moreover I studied ASE spectral distribution in different conditions and its interaction with optical elements. Next studies foresee to measure experimentally with IT-Yb1 the shift induced by the laser system that I realized to calculate its uncertainty contribution and compare this result with the one obtained with a titanium:sapphire laser. This work will be useful to the development of compact or transportable atomic clocks based exclusively on semiconductor lasers with applications in the generation of optical timescales, travelling optical standards and the measurements of Earth gravitational redshift.
Atomic clocks constitute a fundamental tool for time and frequency metrology and their application is widespread in many technological fields. In particular, the International System of units (SI) defines the second on a microwave transition of Caesium atoms. The realization is made by clocks reaching uncertainties of few parts in 10^−16, making the second to be the quantity realized with the smallest uncertainty in the SI. However, a new generation of atomic clocks, called optical clocks, have already demonstrated to surpass Caesium standards both in accuracy and stability. The research performed during my PhD activity has been focused on the development and characterization of Ytterbium (Yb) optical lattice clocks. These systems operate with a large number of ultra-cold neutral atoms having a clock transition in the visible region of the electromagnetic spectrum. The atomic sample is trapped in an periodical optical potential called optical lattice that gives the advantage to interrogate many quantum absorbers for an extended time, with small perturbations, allowing to achieve an unprecedented stability and accuracy. The main experimental work has been carried out in the laboratories of the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, where several atomic clocks are present including the Italian primary frequency standard, the Caesium fountain ITCsF2, and where an Yb optical lattice clock is currently being developed. This thesis describes the functioning of the system along with the full characterization of systematic effects, the complete uncertainty budget and its first absolute frequency measurement against the primary frequency standard. The clock exhibited an accuracy of 1.6 × 10^−16 and the comparison with the Cs fountain resulted in a frequency of f171Yb = 518 295 836 590 863.59(31) Hz, limited by the fountain uncertainty. This measurement is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the SI and constitutes the first measurement of a Yb clock in Europe and the second one in the world against a primary frequency standard. Several upgrades have been applied after the absolute measurement. In particular, the design and realization a system capable to frequency stabilize several lasers on a single optical cavity is illustrated. This cavity has been implemented to lock the lasers used to cool and trap the atomic sample at 399, 556 and 759nm using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method that gives an extended frequency tunability. The system proved to be an easy-to-use and reliable tool for the experimental activity showing a linewidth below 300 Hz at 556nm, which is the wavelength that requires the most stringent performance, and a long term drift below 20 kHz per day at 759nm. That is suitable for operating the lattice laser with a light shift uncertainty below 1 × 10−18. During my PhD I have been guest researcher at the National Institute of Standards and Technology (NIST) of Boulder, Colorado, for nine months in 2016. In these laboratories two Yb optical lattice clocks are operative. I worked on the instability measurement of a composite system exploiting the two clocks to suppress the Dick effect, called zero-dead-time (ZDT) clock, which demonstrated a fractional instability of 3 × 10−17 at 1s. The two clocks can also be operated to extend the interrogation time obtaining a spectroscopic feature after 4 s of 120(20)mHz corresponding to a quality factor Q > 4 × 10^15. I also worked on the characterization of several systematic shifts that allowed to complete the uncertainty budget of the clocks at 1.6 × 10^−18. In particular, I contributed to the characterization of lattice light shifts considering the effect of atomic sample temperature and the identification of a metrological regime called operational magic frequency where frequency shifts are insensitive to changes in trap depth.
The work described in this thesis was done at the Istituto Nazionale di Ricerca Metrologica (I.N.Ri.M.), in the optic division, within the Ph.D. of Metrology of the Politecnico di Torino. My work consisted in developing a new generation optical frequency standard based on Ytterbium atoms: the goal of this project is to realize a clock whose performance overcomes the ones of the standard which actually realizes the Second, the atomic Cesium fountain. This means realizes an apparatus which shows stability and accuracy better then the fountain ones: a possible way adopted in several new generation clock is using optical transitions, because they offer an higher quality factor of the clock transition itself, which means a better stability and the possibility to reach the 10^-17 level in the global accuracy of the clock. The fundamental idea of a clock is to realize as close as possible the basic idea under the actual definition of the Second, that is to fix the unit if time with a particular atomic transition, when the atom is completely unperturbed. Accomplishing this prescription means a lot of work in cooling down the atoms until the 10^-6 K regime using laser cooling techniques, developing a trap where atoms can be excited by the clock laser (the optical lattice) and of course a very careful frequency and amplitude stabilization of the clock laser itself is needed to reach our goal. The work described in this thesis regards the development of the two cooling stages of the Yb atoms and it is presented as follows. Chapter 1 explains with more details why an optical clock is needed to improve the accuracy of the actual realization of the Second, then describing the theoretical framework in which the clock will be developed. Chapter 2 contains a description of the experimental apparatus used, with more attention on the instrumentation related to the laser cooling of the atoms. Chapter 3 is about the characterization of the first cooling stage, both in a static and in a dynamic way, with some attention on the theoretical model developed to perform both the characterizations. Chapter 4 focuses on the development of the second cooling stage and it describes the actual state of the art of this stage
We present an optical lattice clock based on ytterbium 171Yb atoms developed in the laboratories of INRIM. In the experiment, we cool and trap ytterbium atoms in a two stage magneto-optical trap (MOT) (at 399nm and 556nm for the first and second stage, respectively). Atoms are then transferred in a horizontal, one-dimensional optical lattice at the magic wavelength (759 nm). Here the clock transition at 578nm is probed by a laser stabilized on an ultra-stable cavity. We describe the generation of all the laser sources, the physic package and the operation of the clock. Lasers at 399 nm, 556nm and 578nm are obtained, with different techniques, using non-linear crystals starting from infrared sources. The clock laser is stabilized using a high finesse notched ULE cavity. The lattice is made with a titanium-sapphire laser. The aluminum vacuum chamber is designed for wide optical access and its temperature is measured by 8 thermistors for blackbody shift evaluation. Our system allows for fast loading of the lattice with 1 × 104atoms trapped in the lattice in 250 ms. We obtained preliminary spectroscopy results and we locked the clock laser to the atomic line. Future perspectives are discussed.
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.
Optical clocks are being developed worldwide and are at the forefront of frequency metrology. Today they outperform microwave clocks instability and inaccuracy by two orders of magnitude, raising discussion about the redefinition of the SI second. Their outstanding performances are paving the way to applications in different fields apart from metrology. Optical clocks can be used for geodetic measurements, such as chronometric levelling, and fundamental physics experiments. These applications require transportable, and even space-born optical clocks that are in development. Space optical clocks will serve as a high accurate references for intercontinental clock comparisons, and will also provide a unique test bench for fundamental physics theories, such as Einstein’s prediction of time dilation and equivalence principle.
This work presents the development and characterization of a transportable strontium optical lattice clock, realised in the framework of the ESA I-SOC (Space Optical Clock on the ISS) mission. The aim of the project is to investigate new technologies and techniques, in order to develop a robust apparatus as demonstrator for space applications. Special effort has been made to reduce the size and weight of the clock, so as to meet the stringent requirements of a space mission. The system reported here is one of the most compact optical clock to date, with a total volume of only 1 m3, excluding electronics.
The clock is operated with the strontium bosonic isotope 88Sr: this is preferable to the fermionic counterpart 87Sr, commonly used in optical lattice clocks, if simplicity and reliability are required. However, in terms of accuracy, bosonic clocks present some disadvantages compared with the fermionic ones, which have significantly slowed down their development for metrological applications.
We characterised the relevant operating parameters and their effect on the clock accuracy and instability. Exploiting a clock laser offering an outstanding stability, we were able to demonstrate a clock fractional instability of 3×10-18 , and an inaccuracy of 2×10-17: both values represent a factor of 30 improvement compared with previous works on bosonic optical clocks. These results suggest that, in the near future, bosonic clocks may become competitive to their fermionic counterparts. This is of interest for applications such as time scale realization, transportable clocks, or for test of fundamental physics. As an example, we measured the 88Sr-87Sr isotope shift, with an uncertainty of 12 mHz.
The optical lattice clock with neutral atoms occupies an outstanding position in the research field of atomic clocks, demonstrating the great potential of its performance (like the uncertainty and the stability). At present, the optical lattice clock has realized a 10-18 level of its uncertainty. In this paper, we present the realization of loading bosonic atoms 88Sr (strontium, alkaline-earth metals) into a one-dimensional (1D) optical lattice in our laboratory. The optical lattice where the atoms are trapped can make the energy level shift, called Stark shift. But there is the special optical lattice operating at the “magic” wavelength for clock transitions (5s2) 1S0-(5s5p) 3P0, which can make the same Stark light-shift for both of them, indicating a zero light-shift relative to the clock. In our experiment, Sr atoms are cooled in a two-stage cooling and its temperature can be as low as 2 μK. Then these cold atoms are confined in the Lamb-Dicke region by the lattice laser output from an amplified diode laser operating at the “magic” wavelength, 813 nm. Experimentally, it is straightforward to provide 850 mW of lattice power focused to a 38 μm beam radius. After the cold atoms have trapped in the optical lattice, the lifetime of atoms in 1D optical lattice is measured to be 270 ms. The temperature and the number are about 3.5 μK and 1.2×105 respectively. Besides, effects of the power of the lattice laser on both the number and temperature are analyzed. The number changes linearly with the laser power, while there is no obvious influence on the temperature by the power. This original and special approach for atoms trapped in the optical lattice can provide a long interrogation time for probing the clock transition. Furthermore, it may be the foundation for developing our optical lattice clock of strontium atoms.