BoseEinstein condensates (BECs) of ultracold atoms have been subjects of a large research effort, that started a century ago as a purely theoretical subject and is now, since the invention of evaporative cooling thirty years ago, a rich research topic with many experimental apparatuses around the world. A deep knowledge of its underlying physics has been now acquired, for example on the thermodynamics of the gas, superfluidity, topological excitations and manybody physics. However, many topics are still open for investigation, thanks to the flexibility and the high degree of control of these systems. During the course of my PhD, I developed and realized a new experimental apparatus for the realization of coherentlycoupled mixtures of sodium BECs. The highly stable and lownoise magnetic environment of this apparatus enables the experimental investigation of a previously inaccessible regime, where the energy of the coupling becomes comparable to the energy of spin excitations of the mixture. With this apparatus, I concluded two experimental investigations: I produced and investigated nondispersive spinwaves in an twocomponent BEC and I experimentally observed the quantum spintorque effect on a elongated bosonic Josephson junction.The research activity in multicomponent BECs of alkali atoms begun shortly after the first realization of a condensate, thanks to the low energy splitting between the internal substates of the electronic ground state. These internal states can be coherently coupled with an external electromagnetic field and can interact via mutual meanfield interaction, generating interestinc effects such as ground states with different magnetic ordering depending on their interaction constants, density as well as spin dynamics and internal Josephson effects. The research interest on mixtures of sodium atoms sparks from the peculiar characteristic of the system: in the $ket{F = 1, m_F = pm 1}$ states, the interaction constants are such that the ground state has antiferromagnetic ordering and the system is perfectly symmetric for exchanges of the two species. In these peculiar system, density and spinexcitations have very different energetic cost, with the latter being much less energetic, and can be completely decoupled. Moreover, spinexcitations, that are connected to excitations in the relativephase between the components, change drastically in nature when a coupling of comparable energy is added between the states. The presence of the coupling effectively locks the relativephase in the bulk and spin excitations become localized. While extensive theoretical predictions on the spin dynamics of this system has been already performed, experimental confirmation was still lacking because of the high sensitivity to external forces (due to the very low energy of the spin excitations) and the impossibility of realizing a lowenergy coupling between these states in the presence of environmental magnetic noise. During my PhD, I realized an experimental apparatus where magnetic noises are suppressed by five orders of magnitude using a multilayer magnetic shield made of an highpermeability metal alloy (μmetal), that encases the science chamber. In this apparatus, I developed a protocol, compatible with the technical limitations of the magnetic shield, to produce BECs in a spininsensitive optical trapping potential. I then characterized the residual magnetic noise and found it compatible with the requirements for observing spindynamics effects. Finally, I realized a system and a set of protocols for the manipulation of the internal state of the sample allowing arbitrary preparation of the sample while maintaining the long coherence times necessary to observe the spin dynamics, that have been used in the subsequent experimental observations. The first experimental result discussed in this thesis, is the production of magnetic solitons and the observation of their dynamic in a trapped sample. Waves in general spread during their propagation in a medium, however this tendency can be counterbalanced by a selffocusing effect if dispersion of the wave is nonlinear, generating nondispersive and longlived wavepackets commonly named solitons. These have been found in many fields of physics, such as fluid dynamics, plasma physics, nonlinear optics and coldatoms BECs, attracting interest because of their ability to transport information or energy unaltered over long distances, as they are robust against the interaction with inhomogeneities in the medium. Of these systems, coldatoms can be widely manipulated to generated different kinds of solitons, both in single and in multicomponents systems. A new kind of them, named magnetic solitons, has been predicted in a balanced mixture of BECs of sodium in $ket{F = 1, m_F = pm 1}$, however experimental observation was still lacking. I deterministically produced magnetic solitons via phase engineering of the condensate using a spinsensitive optical potential. I then developed a tomographic imaging technique to semiconcurrently measure the densities of both components and the discontinuities in their relative phase, allowing for the reconstruction of all the relevant quantities of the spinor wavefunction. This allowed to observe the dispersionless dynamics of the solitons as they perform multiple oscillation in the trapped sample in a timescale of the order of the second. Moreover, I engineered collisions between different kinds of magnetic solitons and observed their robustness to mutual interaction. The second experimental results presented in this thesis is the observation of the breaking of magnetic heterostructures in BECs due to the quantum spin torque effect, an effect with strong analogies with electronic spins traveling through magnetic devices. Spins in magnetic material precess around the axis of the effective magnetic field, and their dynamics must take into account the external field as well as nonlinear magnetization and the inhomogeneity of the material. These effects are commonly described by the LandauLifshitz equation and have been mainly studied for electronic spins in magnetic heterostructures, where the inhomogeneity in the material at the interfaces enhances the exchange effects between spins. For homogeneous materials, this description reduces to the Josephson system, a closely related effect that is more known in coldatoms systems. The Josephson effect arises when a macroscopic number of interacting bosonic particles are distributed in two possible states, weakly tunnelcoupled together, with the average energy of particles occupying each of the states depending on the occupation number itself. In these conditions, the dynamics of the system depends on the difference in occupation numbers, the relative phase between the states and the selfinteraction to tunneling ratio, giving raise to macroscopic quantum effects such as oscillating AC and DC Josephson currents and selftrapping. While these phenomena has been historically studied in junctions between superconducting systems, they can be also realized with coldatoms systems, allowing the study of Josephson junctions with finite dimensions and in regimes that are hard to reach for superconducting systems. In this thesis, I realized a magnetic heterostructure in a twocomponent elongated BECs thanks to the simultaneous presence of selftrapped (ferromagnetic) and oscillating (paramagnetic) regions in the sample. While the dynamics at short times is correctly described by the Josephson effects, at the interface between the regions the particle nature of the gas creates a strong exchange effect, named the quantum spin torque, that produces magnetic excitations that spread trough the sample and break the local Josephson behaviour. I experimentally studied the spread and nature of these magnetic excitations, while numerical simulations confirmed the dominant role played by the quantum spin torque effect. The structure of this thesis is the following: in the first chapter is given a review of theoretical concepts and existing literature. In the second chapter is described the experimental apparatus and the protocols developed to prepare the ultracold atoms sample. In the third chapter is presented the experimental observation of magnetic solitons. In the fourth chapter is presented the experimental investigation of the quantum spin torque effect in magnetic heterostructures. The last chapter is devoted to conclusions and outlook of this work.
Spin dynamics in twocomponent BoseEinstein condensates / Farolfi, Arturo.  (2021 Apr 14), pp. 1124. [10.15168/11572_299835]
Spin dynamics in twocomponent BoseEinstein condensates
Farolfi, Arturo
20210414
Abstract
BoseEinstein condensates (BECs) of ultracold atoms have been subjects of a large research effort, that started a century ago as a purely theoretical subject and is now, since the invention of evaporative cooling thirty years ago, a rich research topic with many experimental apparatuses around the world. A deep knowledge of its underlying physics has been now acquired, for example on the thermodynamics of the gas, superfluidity, topological excitations and manybody physics. However, many topics are still open for investigation, thanks to the flexibility and the high degree of control of these systems. During the course of my PhD, I developed and realized a new experimental apparatus for the realization of coherentlycoupled mixtures of sodium BECs. The highly stable and lownoise magnetic environment of this apparatus enables the experimental investigation of a previously inaccessible regime, where the energy of the coupling becomes comparable to the energy of spin excitations of the mixture. With this apparatus, I concluded two experimental investigations: I produced and investigated nondispersive spinwaves in an twocomponent BEC and I experimentally observed the quantum spintorque effect on a elongated bosonic Josephson junction.The research activity in multicomponent BECs of alkali atoms begun shortly after the first realization of a condensate, thanks to the low energy splitting between the internal substates of the electronic ground state. These internal states can be coherently coupled with an external electromagnetic field and can interact via mutual meanfield interaction, generating interestinc effects such as ground states with different magnetic ordering depending on their interaction constants, density as well as spin dynamics and internal Josephson effects. The research interest on mixtures of sodium atoms sparks from the peculiar characteristic of the system: in the $ket{F = 1, m_F = pm 1}$ states, the interaction constants are such that the ground state has antiferromagnetic ordering and the system is perfectly symmetric for exchanges of the two species. In these peculiar system, density and spinexcitations have very different energetic cost, with the latter being much less energetic, and can be completely decoupled. Moreover, spinexcitations, that are connected to excitations in the relativephase between the components, change drastically in nature when a coupling of comparable energy is added between the states. The presence of the coupling effectively locks the relativephase in the bulk and spin excitations become localized. While extensive theoretical predictions on the spin dynamics of this system has been already performed, experimental confirmation was still lacking because of the high sensitivity to external forces (due to the very low energy of the spin excitations) and the impossibility of realizing a lowenergy coupling between these states in the presence of environmental magnetic noise. During my PhD, I realized an experimental apparatus where magnetic noises are suppressed by five orders of magnitude using a multilayer magnetic shield made of an highpermeability metal alloy (μmetal), that encases the science chamber. In this apparatus, I developed a protocol, compatible with the technical limitations of the magnetic shield, to produce BECs in a spininsensitive optical trapping potential. I then characterized the residual magnetic noise and found it compatible with the requirements for observing spindynamics effects. Finally, I realized a system and a set of protocols for the manipulation of the internal state of the sample allowing arbitrary preparation of the sample while maintaining the long coherence times necessary to observe the spin dynamics, that have been used in the subsequent experimental observations. The first experimental result discussed in this thesis, is the production of magnetic solitons and the observation of their dynamic in a trapped sample. Waves in general spread during their propagation in a medium, however this tendency can be counterbalanced by a selffocusing effect if dispersion of the wave is nonlinear, generating nondispersive and longlived wavepackets commonly named solitons. These have been found in many fields of physics, such as fluid dynamics, plasma physics, nonlinear optics and coldatoms BECs, attracting interest because of their ability to transport information or energy unaltered over long distances, as they are robust against the interaction with inhomogeneities in the medium. Of these systems, coldatoms can be widely manipulated to generated different kinds of solitons, both in single and in multicomponents systems. A new kind of them, named magnetic solitons, has been predicted in a balanced mixture of BECs of sodium in $ket{F = 1, m_F = pm 1}$, however experimental observation was still lacking. I deterministically produced magnetic solitons via phase engineering of the condensate using a spinsensitive optical potential. I then developed a tomographic imaging technique to semiconcurrently measure the densities of both components and the discontinuities in their relative phase, allowing for the reconstruction of all the relevant quantities of the spinor wavefunction. This allowed to observe the dispersionless dynamics of the solitons as they perform multiple oscillation in the trapped sample in a timescale of the order of the second. Moreover, I engineered collisions between different kinds of magnetic solitons and observed their robustness to mutual interaction. The second experimental results presented in this thesis is the observation of the breaking of magnetic heterostructures in BECs due to the quantum spin torque effect, an effect with strong analogies with electronic spins traveling through magnetic devices. Spins in magnetic material precess around the axis of the effective magnetic field, and their dynamics must take into account the external field as well as nonlinear magnetization and the inhomogeneity of the material. These effects are commonly described by the LandauLifshitz equation and have been mainly studied for electronic spins in magnetic heterostructures, where the inhomogeneity in the material at the interfaces enhances the exchange effects between spins. For homogeneous materials, this description reduces to the Josephson system, a closely related effect that is more known in coldatoms systems. The Josephson effect arises when a macroscopic number of interacting bosonic particles are distributed in two possible states, weakly tunnelcoupled together, with the average energy of particles occupying each of the states depending on the occupation number itself. In these conditions, the dynamics of the system depends on the difference in occupation numbers, the relative phase between the states and the selfinteraction to tunneling ratio, giving raise to macroscopic quantum effects such as oscillating AC and DC Josephson currents and selftrapping. While these phenomena has been historically studied in junctions between superconducting systems, they can be also realized with coldatoms systems, allowing the study of Josephson junctions with finite dimensions and in regimes that are hard to reach for superconducting systems. In this thesis, I realized a magnetic heterostructure in a twocomponent elongated BECs thanks to the simultaneous presence of selftrapped (ferromagnetic) and oscillating (paramagnetic) regions in the sample. While the dynamics at short times is correctly described by the Josephson effects, at the interface between the regions the particle nature of the gas creates a strong exchange effect, named the quantum spin torque, that produces magnetic excitations that spread trough the sample and break the local Josephson behaviour. I experimentally studied the spread and nature of these magnetic excitations, while numerical simulations confirmed the dominant role played by the quantum spin torque effect. The structure of this thesis is the following: in the first chapter is given a review of theoretical concepts and existing literature. In the second chapter is described the experimental apparatus and the protocols developed to prepare the ultracold atoms sample. In the third chapter is presented the experimental observation of magnetic solitons. In the fourth chapter is presented the experimental investigation of the quantum spin torque effect in magnetic heterostructures. The last chapter is devoted to conclusions and outlook of this work.File  Dimensione  Formato  

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