This thesis manuscript presents experiments realized with ultracold atomic gases of dysprosium. This element has specific properties such as a large spin J=8 and tunable light-spin couplings originating from significant tensor light-shift contributions close to resonance. An overview of the experimental setup is given, from magneto-optical trapping to evaporative cooling. In particular we present the role of the tensor light-shift in the in-trap Doppler cooling process. Details on the use of spatial modulation to tune the shape of an optical dipole trap are also given. In the second part we describe the preparation of squeezed and ‘oversqueezed’ spin states as well as Schrödinger cat states using light-induced spin couplings. The magnetic sensitivity of these states is characterized using Ramsey spectroscopy. The Husimi function is reconstructed from population measurements along various directions, also allowing us to compute the purity of the prepared states. While squeezed states show an increased metrological gain, oversqueezed states exhibit a strong Ramsey oscillations contrast reduction. The Hellinger distance is used to extend the notion of metrological gain and the sensitivity measured in this case saturates the Cramér-Rao bound. Finally a system of 16 indistinguishable interacting spins 1/2 is realized. In the third part we present the measurement of pairwise entanglement from photon absorption measurements, which was not realized experimentally. Finally we present two projects where spatial degrees of freedom are added to the dynamics of internal levels. The first one consists in the realization of a system analogous to the Landau Hamiltonian of a charged particle in a magnetic field, with one spatial dimension and one synthetic dimension. The second project uses two spatial dimensions and exhibits an artificial magnetic field originating from positiondependent light-shifts.
This thesis manuscript presents experiments realized with ultracold atomic gases of dysprosium. This element has specific properties such as a large spin J=8 and tunable light-spin couplings originating from significant tensor light-shift contributions close to resonance. An overview of the experimental setup is given, from magneto-optical trapping to evaporative cooling. In particular we present the role of the tensor light-shift in the in-trap Doppler cooling process. Details on the use of spatial modulation to tune the shape of an optical dipole trap are also given. In the second part we describe the preparation of squeezed and ‘oversqueezed’ spin states as well as Schrödinger cat states using light-induced spin couplings. The magnetic sensitivity of these states is characterized using Ramsey spectroscopy. The Husimi function is reconstructed from population measurements along various directions, also allowing us to compute the purity of the prepared states. While squeezed states show an increased metrological gain, oversqueezed states exhibit a strong Ramsey oscillations contrast reduction. The Hellinger distance is used to extend the notion of metrological gain and the sensitivity measured in this case saturates the Cramér-Rao bound. Finally a system of 16 indistinguishable interacting spins 1/2 is realized. In the third part we present the measurement of pairwise entanglement from photon absorption measurements, which was not realized experimentally. Finally we present two projects where spatial degrees of freedom are added to the dynamics of internal levels. The first one consists in the realization of a system analogous to the Landau Hamiltonian of a charged particle in a magnetic field, with one spatial dimension and one synthetic dimension. The second project uses two spatial dimensions and exhibits an artificial magnetic field originating from positiondependent light-shifts.
In this thesis, I present the study of the laser trapping and cooling of a Dysprosium atomic gas. This latter belong to the lanthanide family, it exhibits a large angular momentum in its electronic ground state, making it a suitable candidate for investigating dipolar quantum gases. These systems present a major interest as they can lead to the observation of novel quantum phenomena thanks to the anisotropic and long-range character of the interaction between magnetic dipoles. Moreover, Dysprosium has a rich electronic structure offering the possibility to implement strong light-spin coupling with a reduced heating with respect to alkali species, which paves the way toward the realization of synthetic gauge fields.In this work, I present the experimental investigation of different interaction mechanisms occurring in an ultracold gas of Dysprosium, ranging from light-assisted collisions to dipolar relaxation and evaporative cooling. I expose also the experimental realization of an effective magnetic field, using spin-dependent light-shift, allowing optical control over atomic interactions by means of Feshbach resonances.
This thesis describes three studies using ultracold gases of bosonic dysprosium atoms. We encode quantum states of interest in the large electronic ground state spin J=8 of each atom. Narrow-line optical transitions are used to manipulate these states by creating strong spin-dependent tensor light shifts with negligible heating. Our detection scheme uses arbitrary spin rotations followed by measurement of projection probabilities with single magnetic sublevel resolution. Firstly, we study a transverse-field Ising model of quantum spin-1/2’s with infinite-range interactions. This model shows a paramagnetic to ferromagnetic quantum phase transition in the thermodynamic limit. Our experiment relies on the formal equivalence between the states of a spin J=8 and exchange-symmetric states of 2J=16 qubits in the sector of maximum total spin. We probe the thermodynamic and dynamical properties, revealing finite-size quantum critical behaviour around the transition point. We also directly test the fundamental link between symmetry breaking and the appearance of a finite order parameter. This is enabled by access to the collective observable defining the underlying Z2 symmetry. Secondly, we partition the electronic spin to reveal entanglement in nonclassical states. By optically coupling the ground level to an excited state J' = J-1, we remove a pair of encoded qubits in a state defined by the light polarisation. We probe the concurrence of qubit pairs extracted from W and squeezed states to quantify their nonclassical character. We directly demonstrate entanglement between the 14- and 2-qubit subsystems via an increase in entropy upon partition. In a complementary set of experiments, we probe the decoherence of a state prepared in the excited level J' = J + 1 and interpret spontaneous emission as a loss of a qubit pair in a random state. This allows us to contrast the robustness of nonclassical pairwise correlations of the W state with the fragility of the coherence of a Schrödinger cat state. Thirdly, we simulate a quantum Hall system by exploiting the electronic ground state spin as a discrete synthetic dimension, and optically coupling it to atomic motion along a second real dimension. This creates an artificial magnetic field, with the two coupled dimensions forming a Hall ribbon with sharp edges along the synthetic axis. We demonstrate that the large number of magnetic sublevels leads to distinct bulk and edge behaviours, and a Hall response characteristic of a non-trivial topology. We perform a preliminary investigation of the entanglement spectrum associated with a partition at the centre of the synthetic dimension, and we explain how future studies could directly implement the corresponding entanglement Hamiltonian. We conclude with a roadmap for extending the techniques presented in this thesis in order to realise many-body topological states.
The subject of this thesis is the construction and characterization of an experimental setup for studying the interaction of an ensemble of 10^5 - 10^6 cold 87 Rb atoms held in a far off-resonance optical dipole trap with a probe detuned by about 100 natural line widths, a regime in which refractive effects are resonantly enhanced but absorption is small. An important feature of this type of atom-light interface is that it facilitates quantum non-demolition measurements which in turn have numerous applications in quantum information technology, as well as in the field of ultra-precise metrology where they allow the measurements to overcome the so-called standard quantum limit.
The use of a dipole trap has several advantages over more conventional approaches in which vapor cells or magneto-optical traps (MOT) have been used. Most importantly interaction strength of comparable magnitude can be achieved with a far lower number of atoms and with longer coherence times. The elongated geometry that can be realized in a dipole trap maximizes the interaction strength. One of the complications that arise is that the achievable interaction strength is highly sensitive to the mode-matching between the trapped ensemble and the probe beam.
The theoretical description of the interaction between a trapped ensemble and a freely-propagating probe beam is provided in the framework of continuous variables. The relevant quantities are a pseudo-spin, defined for an ensemble of atoms in their ground state, and the Stokes vector that characterizes the polarization state of light. In practice the experiment consists of four main blocks: the atom traps, the optical pumping setup, the probe laser setup, and the photodetection.
Before loading into the dipole trap, atoms are cooled in a MOT. An existing MOT setup has been improved by implementing a new, more powerful laser system. A dedicated dipole trap, which had been constructed in collaboration with another project, has been thoroughly characterized and the results are presented in this thesis. Setups for optical pumping and for the probe have been built, characterized and incorporated into the apparatus.
Several experimental techniques have been developed and tested in order to optimize the alignment of the probe with the trapped ensemble. The effect of the geometrical overlap on the achievable signal to noise ratio is discussed and illustrated using simple, intuitive models.
Photo-detection is a critical part of the setup. In order to facilitate detection of non-classical (squeezed) states of the atomic (pseudo-) spin, the detection system must be shot-noise limited within the range of photon numbers used in the experiments. Such a system has been constructed and tested, and is described in detail. It employs a charge-sensitive amplifier (CSA) but, unlike earlier implementations, it does not use a pulse-shaping stage. Instead, an algorithm has been devised and implemented that derives the quantities of interest directly from the output signal of the CSA. This novel approach avoids the technical problems that arise in systems using hardware-based pulse shaping. It also makes the detector more flexible in terms of the shape and timing of the probe pulses that can be used.
The atomic states produced in the setup are intrinsically sensitive to stray magnetic fields. Therefore an efficient cancellation of stray fields is essential. On the other hand having the possibility to apply a fully controlled, external magnetic field makes the experimental setup more versatile. Both goals can be achieved with a magnetic field control unit that has been built during this project. It facilitates active cancellation of stray magnetic fields at the frequency of the mains and in addition allows to apply an arbitrary waveform magnetic fields along all three axes. One of the main results reported in this thesis is the observation of polarization rotation caused by population imbalance created in the process of optical pumping and, in a separate series of measurements, due to an applied magnetic field. These measurements have allowed us to assess the strength of the atom-light interaction available in the current setup. They indicate that with the necessary modifications to the optical pumping scheme the system should prove successful in implementing quantum information protocols, in particular quantum memory for light.
An interesting off-shoot has been the spectroscopic characterization of the atomic state in the dipole trap. In these measurements Doppler free spectra are recorded in a very simple setup without the need to use nonlinear techniques. Apart from providing a method to characterize the efficiency of the optical pumping into the hyperfine level of interest, the acquired spectra are a very direct illustration of the power of the laser cooling techniques developed in the recent years and without which this work would not have been possible.
Laser cooling and trapping of the most magnetic fermionic atom, dysprosium (Dy), may provide a framework to explore quantum liquid crystal (QLC) theory (Chapter 1). This thesis presents details of the Dy laser cooling and trapping apparatus including the laser systems at 421, 741, and 1064 nm, the ultra-high vacuum (UHV) chamber, and the computer control that has produced a magneto-optically (MOT) and magneto-statically (MT) trapped Dy gas (Chapters 3, 4, 5). Despite the fact that Dy has a complex energy level structure with nearly 140 metastable states (Chapter 2), Dy MOT at 421-nm transition with 32-MHz linewidth was realized without any rempumper, exploiting its large magnetic moment, which brought a strong magnetic confinement of metastable states of Dy. This unique MOT/MT dynamics is discussed and its quantitative measurements are shown in Chapter 6. When the Dy atoms dropped from the MOT were adsorptively imaged, it was observed that Dy MOT had a bimodal temperature distribution in contrast to the usual MOT described by a single temperature (Chapter 7). Such novel anisotropic sub-Doppler laser cooling of Dy, which breaks the symmetry in cooling, is due to Dy's large magnetic spin aligned along a strong axis of the quadrupole field of the MOT, and we further support this plausible conjecture with the velocity selective resonance (VSR) theory. The MOT at ~ 1 mK was cooled to ~ 10 uK by narrow-line cooling at 741 nm with a linewidth of 2 kHz, and we were able to load the optical dipole trap (ODT) at 1064 nm. By loading two isotopes of 164Dy and 163Dy in sequence to the MOT and narrow-line cooling them simultaneously, ultracold Bose-Fermi mixtures of Dy in the ODT were realized (Chapter 8). This thesis is concluded with a discussion of prospect on the Bose-Fermi mixtures of Dy.
This thesis presents novel techniques for the experimental study of ultracold quantum gases of fermionic lithium and potassium atoms.In the first part of this thesis, we describe the design and characterization of the new components of our experimental apparatus capable of trapping and cooling simultaneously 6Li and 40K atoms to ultracold temperatures.We report on a novel sub-Doppler cooling mechanism, operating on the D1 line transition of alkali atoms, for laser cooling of lithium and potassium. The measured phase space densities after this molasses phase are on the order of 10−4 for both 6Li and 40K. We present the forced evaporative cooling of 40K atoms, starting in an optically plugged magnetic quadrupole trap and continuing in an optical dipole trap. In this context, we report on the production of a quantum degenerate Fermi gas of 1.5×105 atoms 40K in a crossed dipole trap with T/TF=0.17, paving the way for the study of strongly interacting superfluids of 40K.In the second part of this thesis, we present a narrow-linewidth, all-solid-state laser source, emitting 5.2 W in the vicinity of the lithium D-line transitions at 671 nm. The source is based on a diode-end-pumped unidirectional ring laser operating on the 1342 nm transition of Nd:YVO4, capable of producing 6.5 W of single-mode light delivered in a diffraction-limited beam. We report on three different approaches for second-haromonic generation of its output beam, namely by employing an enhancement cavity containing a ppKTP crystal, intracavity frequency doubling and a ppZnO:LN waveguide structure.
This thesis reports on the experimental investigation of controlled spin dependent interactions in a sample of ultracold Rubidium atoms trapped in a periodic optical potential. In such a situation, the most basic interaction between only two atoms at one common potential well, forming a micro laboratory for this atom pair, can be investigated. Spin dependent interactions between the atoms can lead to an intriguing time evolution of the system. In this work, we present two examples of such spin interaction induced dynamics. First, we have been able to observe and control a coherent spin changing interaction. Second, we have \nachieved to examine and manipulate an interaction\ninduced time evolution of the relative phase of a spin 1/2-system, both in the case of particle pairs and in the more general case of N interacting particles. The first part of this thesis elucidates the spin-changing interaction mechanism underlying many fascinating effects resulting from interacting spins at ultracold temperatures. This process changes the spin states of two colliding particles, while preserving total magnetization. If initial and final states have almost equal energy, this process is resonant and leads to large amplitude\noscillations between different spin states. The measured coupling parameters of such a process allow to precisely infer atomic scattering length differences, that e.g. determine the nature of the magnetic ground state of the hyperfine states in Rubidium.\nMoreover, a method to tune the spin oscillations at will based on the AC-Zeeman effect has been implemented. This allowed us to use resonant spin changing \ncollisions as a quantitative and non-destructive particle pair probe in the optical lattice. This led to a series of experiments shedding light on the Bosonic superfluid to Mott insulator transition.\nIn a second series of experiments we have been able to coherently manipulate the interaction induced time evolution of the relative phase in an ensemble of spin 1/2-systems. For two particles, interactions can lead to an entanglement oscillation of the particle pair. For the general case of N interacting particles, the ideal time evolution leads to the creation of spin squeezed states and even Schrödinger cat states. In the experiment we have been able to control the underlying\ninteractions by a Feshbach resonance. For particle pairs we could directly observe the entanglement oscillations. For the many particle case we have been able to observe and reverse the interaction induced dispersion of the relative phase.\nThe presented results demonstrate how correlated spin states can be engineered through \ncontrol of atomic interactions. Moreover, the results point towards the possibility to simulate quantum\nmagnetism phenomena with ultracold atoms in optical traps, and to realize and analyze many novel quantum spin states which have not been experimentally realized so far.