Details
Original language  English 

Qualification  Doctor rerum naturalium 
Awarding Institution  
Supervised by 

Date of Award  13 Oct 2023 
Place of Publication  Hannover 
Publication status  Published  2023 
Abstract
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Hannover, 2023. 122 p.
Research output: Thesis › Doctoral thesis
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TY  BOOK
T1  Quantum Dynamics in a Ferromagnetic Atomic Gas
AU  MeyerHoppe, Bernd
PY  2023
Y1  2023
N2  BoseEinstein condensates (BECs) provide an extraordinary system to study manybody quantum effects with a high degree of control. Using such ultracold gases, microscopic quantum effects become visible on a macroscopic scale as thermal ﬂuctuations are negligible. In particular, quantum phase transitions can be observed. These phase transitions can be indicated by an order parameter that abruptly changes at the critical value of a certain control parameter. Throughout this work, a spin1 BEC with ferromagnetic interactions and zero magnetization is considered. This system exhibits three groundstate quantum phases that can be controlled by an effective magnetic ﬁeld. These phases have been explored both theoretically and experimentally in the last two decades. Quantum phase transitions are by deﬁnition only applicable to the ground state of a system. However, this powerful concept can be extended to states with nonzero energy. Such excitedstate quantum phase transitions (ESQPTs) can be driven by a conventional control parameter, but, interestingly, also by a variation of the excitation energy only. ESQPTs have been studied theoretically and their existence itself has been revealed, e.g., in molecular spectra. However, a thorough investigation by an order parameter and in particular the experimental mapping of the corresponding phase diagram remain an open challenge in any physical system. In this thesis, an interferometric order parameter is employed to experimentally map out an excitedstate quantum phase diagram. This order parameter is based on dynamical behavior of coherent states that resemble the meanﬁeld phasespace trajectories of excitedstate phases. While a ferromagnetic spin1 BEC with zero magnetization serves as an exemplary platform, the ﬁndings can be applied to other quantum systems with similar Hamiltonians. Importantly, the distinction of excitedstate quantum phases utilizes the excitation energy as a second control parameter, which presents a key feature of ESQPTs. Our experiments therefore extend the powerful concept of quantum phases and quantum phase transitions to the entire Hilbert space of the spin1 BEC.
AB  BoseEinstein condensates (BECs) provide an extraordinary system to study manybody quantum effects with a high degree of control. Using such ultracold gases, microscopic quantum effects become visible on a macroscopic scale as thermal ﬂuctuations are negligible. In particular, quantum phase transitions can be observed. These phase transitions can be indicated by an order parameter that abruptly changes at the critical value of a certain control parameter. Throughout this work, a spin1 BEC with ferromagnetic interactions and zero magnetization is considered. This system exhibits three groundstate quantum phases that can be controlled by an effective magnetic ﬁeld. These phases have been explored both theoretically and experimentally in the last two decades. Quantum phase transitions are by deﬁnition only applicable to the ground state of a system. However, this powerful concept can be extended to states with nonzero energy. Such excitedstate quantum phase transitions (ESQPTs) can be driven by a conventional control parameter, but, interestingly, also by a variation of the excitation energy only. ESQPTs have been studied theoretically and their existence itself has been revealed, e.g., in molecular spectra. However, a thorough investigation by an order parameter and in particular the experimental mapping of the corresponding phase diagram remain an open challenge in any physical system. In this thesis, an interferometric order parameter is employed to experimentally map out an excitedstate quantum phase diagram. This order parameter is based on dynamical behavior of coherent states that resemble the meanﬁeld phasespace trajectories of excitedstate phases. While a ferromagnetic spin1 BEC with zero magnetization serves as an exemplary platform, the ﬁndings can be applied to other quantum systems with similar Hamiltonians. Importantly, the distinction of excitedstate quantum phases utilizes the excitation energy as a second control parameter, which presents a key feature of ESQPTs. Our experiments therefore extend the powerful concept of quantum phases and quantum phase transitions to the entire Hilbert space of the spin1 BEC.
U2  10.15488/15167
DO  10.15488/15167
M3  Doctoral thesis
CY  Hannover
ER 