Atomic interferometry is a very sensitive technique in precision measurements. In this PhD work we explore ways to improve certain aspects of atomic interfer- ometry. We show that there are some advantages on using magnetic sensitive states. We introduce the required infrastructures to implement Ramsey type interferometry on particular transitions. Using a Magneto Optical Trap (MOT) which is the main prerequisite, the atoms are trapped and cooled. The trapping procedure with the MOT requires ingredients such as lasers, the homogeneous and inhomogeneous magnetic fields, the chamber containing the sample, the vacuum system and the detection part (see Chapter 2). At the beginning of my PhD, this facility for trapping the rubidium atoms was available. Later on we upgraded the vacuum system. During this PhD work we could further cool down the trapped sample thanks to the optical molasses technique. We maintain the atoms at temperatures typically below some tenths of micro Kelvin. The coherent manipulation of the atoms is the significant part of this thesis since we aim to do atomic interferometry. Using radiations in microwave domain, it is feasible to couple any two states and do the Rabi oscillations between the hyperfine and Zeeman levels.\nMy first work as a PhD student was to contribute in the Dual Isotope MOT (DIMOT) experiment (see Chapter 3). The idea was to trap both isotopes of rubidium simultaneously by using only single laser and an Electro-Optical Modulator (EOM). This work introduces huge simplifications on the optical part and is extendable to trap more isotopes at the same time. The experiment has the optical and RF parts and my contribution was to complete the RF part.\nStarting all the atoms at the particular initial point for the interferometry takes us some considerable time to be sure that we correctly transfer the atomic population into the desired state (see the discussion in Chapter 4). From that point we focused on two transitions to use the Ramsey type interferometer. Each of these two transitions have an optimum operating point in order to be far from the magnetic field fluctuations since the interferometers are quite sensitive to the field perturbations sensed from the environment. At the end we aim to combine these two transitions of interest to make a dual interferometer (see Chapter 6). The idea is to obtain the minimum magnetic sensitivity at a tunable magnetic field by changing the fraction of atoms in each interferometer. The dual interferometer may be useful in applications where low magnetic sensitivity is required at a particular magnetic field.
Atomic interferometry is a very sensitive technique in precision measurements. In this PhD work we explore ways to improve certain aspects of atomic interfer- ometry. We show that there are some advantages on using magnetic sensitive states. We introduce the required infrastructures to implement Ramsey type interferometry on particular transitions. Using a Magneto Optical Trap (MOT) which is the main prerequisite, the atoms are trapped and cooled. The trapping procedure with the MOT requires ingredients such as lasers, the homogeneous and inhomogeneous magnetic fields, the chamber containing the sample, the vacuum system and the detection part (see Chapter 2). At the beginning of my PhD, this facility for trapping the rubidium atoms was available. Later on we upgraded the vacuum system. During this PhD work we could further cool down the trapped sample thanks to the optical molasses technique. We maintain the atoms at temperatures typically below some tenths of micro Kelvin. The coherent manipulation of the atoms is the significant part of this thesis since we aim to do atomic interferometry. Using radiations in microwave domain, it is feasible to couple any two states and do the Rabi oscillations between the hyperfine and Zeeman levels.\nMy first work as a PhD student was to contribute in the Dual Isotope MOT (DIMOT) experiment (see Chapter 3). The idea was to trap both isotopes of rubidium simultaneously by using only single laser and an Electro-Optical Modulator (EOM). This work introduces huge simplifications on the optical part and is extendable to trap more isotopes at the same time. The experiment has the optical and RF parts and my contribution was to complete the RF part.\nStarting all the atoms at the particular initial point for the interferometry takes us some considerable time to be sure that we correctly transfer the atomic population into the desired state (see the discussion in Chapter 4). From that point we focused on two transitions to use the Ramsey type interferometer. Each of these two transitions have an optimum operating point in order to be far from the magnetic field fluctuations since the interferometers are quite sensitive to the field perturbations sensed from the environment. At the end we aim to combine these two transitions of interest to make a dual interferometer (see Chapter 6). The idea is to obtain the minimum magnetic sensitivity at a tunable magnetic field by changing the fraction of atoms in each interferometer. The dual interferometer may be useful in applications where low magnetic sensitivity is required at a particular magnetic field.
The utilization of light-pulse atom interferometry presents a valuable technique that has found application in both fundamental scientific inquiries, such as investigating constants such as measurements of fine structure constant α and gravitational constant G, and practical endeavors including acceleration sensing and rotation measurements. This thesis delves into our implementation of a dual atom interferometer design involving the clock transition 1S0 → 3P0 in strontium, concurrently conducting the interferometric sequence on atoms in both ground and excited states. To facilitate the dual atom interferometer’s operation, we have firstly established a compact experimental setup with a transversely loaded bi-dimensional magneto-optical-trap (2D-MOT) to create a high flux source of cold strontium atoms. Introducing a novel cross-polarized bi-color atomic beam slower, we simultaneously addresses two excited Zeeman substates of the transition 1S0 → 1P1 in strontium-88, significantly enhancing the number of atoms prepared in the 461 nm MOT by around 10-fold. Following the 461 nm MOT, the atomic ensemble undergoes cooling in a 689 nm MOT, achieving atomic temperatures below 1 µK. We also install a network of eight magnetic field probes arranged around the cold atomic sample, enabling precise three-axis active control of magnetic fields down to the milligauss level. With the atoms confined in a 2D lattice at the magic-wavelength of 813.427 nm after the initial preparation phase, we conduct the magnetically-induced clock transition to create a statistical mixture state of ground and excited states. After a velocity selection stage, implementing a Mach-Zehnder interferometer with Bragg pulses operating at the magic-wavelength along the vertical axis, we conduct tests of the weak equivalence principle (WEP). The Eötvös ratio we found in our system stood at (1.8 ± 3.1) × 10−4, mainly limited by the finite temperature of the atomic cloud constraining the interrogation time. Furthermore, the dual atom interferometer scheme enables the measurement of state-dependent force and eventually polarizability difference via the phase shift between the atoms in different states, and we experimentally demonstrate this by introducing an additional light field gradient.
Interferometry with atoms is a young discipline of physics. The ¯rst interfer- ometers were realised in the early 1990s, using light pulses as beamsplitters in momentum space. Recent developments in atom optics have led to new proposals for interferometers, where the splitting takes place spatially. Mag- netic and optical traps are both suited for this kind of interferometer if a well de¯ned and highly controllable trap is realised. In this thesis results from two experiments that in principle allow the creation of such traps are presented. After reviewing the principles and techniques of atom optics necessary for the understanding of this thesis, a theoretical discussion about the spatial single atom interferometer in a double well potential follows. It is shown that within reasonable limits the system can be reduced to two levels and solved by the Bloch equations. Using the realistic case of a not perfectly symmetric double well potential allows an understanding of the physics behind such an interferometer: how phase is accumulated and how localisation of the atoms leads to the loss of the interferometric signal. Then two experiments are presented. One experiment was newly built up as part of this thesis: it uses a novel hybrid atom chip' with a combination of a magneto-optical ¯lm and current-carrying structures to produce the magnetic trapping potentials. This experiment allowed 5¢108 87Rb atoms to be captured in a magneto-optical trap, where the atom-chip's surface acts as a mirror. The atoms were then transferred into a purely magnetic trap which was created by current carrying structures on the chip. From there on, RF radiation enforced evaporative cooling, so that a quantum degenerate Bose-Einstein condensate of up to 105 atoms was created. It was also shown that the magneto-optical ¯lm is able to hold and trap the atoms. In the second experiment atoms are trapped in the spatially varying inten- sity of light of 1.03 ¹m wavelength. This experiment used an existing set-up as a basis and was modi¯ed and improved for this new project. This experiment starts with 109 87Rb atoms in a 6-beam magneto-optical trap. After examining and optimising the loading process, 1:5 ¢ 105 atoms could be loaded directly from the Magneto-optical trap into the optical dipole potential of two crossed laser beams at 1.03 ¹m. Evaporative cooling was demonstrated though the phase transition to quantum degeneracy was not reached.
Atomic interferometers are often affected by magnetic field fluctuations. Using the clock transition at zero magnetic field minimizes the effect of these fluctuations. There is another transition in rubidium that minimizes the magnetic sensitivity at 3.2 G. We combine the previous two transitions to obtain minimum magnetic sensitivity at a tunable magnetic field between 2.2 and 3.2 G. The two interferometers evolve independently from each other and we control the magnetic sensitivity by changing the population in both transitions with a microwave pulse.
The use of strontium in an atom interferometer presents the opportunity to perform detailed measurements of gravitational fields, rotation rates, and clock transitions as well as other natural phenomena. Strontium can be particularly effective in atom interferometry experiments for two reasons. The first is that the intrinsic magnetic properties of strontium result in less sensitivity of the transition frequencies to magnetic noise, allowing for better measurements. Second, strontium atoms can be cooled to a lower temperature just through ordinary Doppler cooling than other atoms typically used in interferometry (e.g., rubidium). A lower temperature means less initial kinetic energy and hence lower spatial dispersion over the course of an experiment. A magneto-optical trap consisting of two primary cooling lasers and two repump lasers cools and confines strontium atoms for use in the interferometer. This thesis focuses on the repump lasers and includes the design and performance of the optical system that frequency stabilizes each laser. The system demonstrated the ability to simultaneously frequency lock the two repump lasers to the degree necessary to form a strontium magneto-optical trap.
Atoms in magnetically sensitive sublevels constitute a natural sensor for absolute magnetic field measurements, which are explored here to form an optical Ramsey interferometer for determining the magnetic field distribution. An optical Ramsey atom interferometer provides a convenient way to map a magnetic field in vacuum. In comparison with the often used Raman spectroscopy method, a Ramsey interferometer can not only improve the sensitivity, but also dramatically suppress the contribution of the ac Stark effect. Furthermore, in this work, the magnetically sensitive atoms are velocity-selected, which improves the spatial resolution for the magnetometer as well as the contrast of the Ramsey fringes. A short-term sensitivity of 1.4nT/Hz1/2 for magnetic field measurement is finally achieved even with an interrogation time as short as $500\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$. The presented approach for absolute magnetic field measurement is expected to find wide applications in atom-interferometry-based precision measurements.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.