Generalized Quantum Phase Transitions for QuantumState Engineering in Spinor BoseEinstein Condensates
 verfasst von
 Polina Feldmann
 betreut von
 Luis Sanchez Santos
 Abstract
Entanglement lies at the core of emergent quantum technologies such as quantumenhanced metrology, quantum communication and cryptography, and quantum simulation and computing. Spinor BoseEinstein condensates (BECs) offer a promising platform for the generation and application of entangled states. For example, a spin1 BEC has served for the proofofprinciple demonstration of a quantumenhanced atomic clock. Ferromagnetic spin1 BECs with zero magnetization exhibit three groundstate quantum phases with different entanglement properties. The control parameter can be tuned by a magnetic field or by microwave dressing. As already experimentally demonstrated, an entangled ground state can be reached from a well accessible, nonentangled one by driving the control parameter across quantum phase transitions (QPTs). We investigate which of the entangled ground states afford quantumenhanced interferometry. The interferometric usefulness is quantified by the quantum Fisher information (QFI), which we analyze throughout all groundstate phases. A large QFI at about half the Heisenberg limit, and thus far above the standard quantum limit, is attained by the wellknown TwinFock state and by the central brokenaxisymmetry (CBA) state. We detail how the CBA state can be used as a probe for quantumenhanced interferometry. Furthermore, we observe that the large QFI of the CBA state can be traced back to enclosed macroscopic superposition states (MSSs). Measuring the atom number in one out of three modes generates, with high probability and heralded by the measurement outcome, a MSS similar to a NOON state. Our proposal promises NOONlike MSSs of unprecedentedly many atoms. Both proposed applications of the adiabatically prepared CBA state depend only on existent technology. Our numerical results show that they tolerate a reasonably swift quasiadiabatic passage in the presence of atom loss as well as uncertainties of atom counting. Excitedstate quantum phase transitions (ESQPTs) extend the concept of QPTs beyond the ground state. While they have been extensively investigated theoretically, there are only few experimental results. From the perspective of quantumstate engineering, it is furthermore surprising how rarely order parameters of ESQPTs are discussed in the literature. Meanfield models for spinor BECs imply ESQPTs, to which some experimental observations on the meanfield dynamics can be attributed. However, so far, neither theoretical nor experimental studies have specifically addressed ESQPTs in spinor BECs. We extend the groundstate phase diagram of ferromagnetic spin1 BECs with zero magnetization across the spectrum. There are three excitedstate phases, corresponding to one groundstate phase each. The ESQPTs are signaled by a diverging density of states. The meanfield phasespace trajectories can be characterized by a winding number that is in onetoone correspondence to the excitedstate phases. We derive a closely related order parameter encoded in the dynamics of coherent states and discuss how this order parameter can be interferometrically measured in current experiments. Remarkably, the meanfield model governing the ESQPTs in spin1 BECs with zero magnetization is encountered also, e. g., in molecular and nuclear physics. Because of the superior experimental control, spinor BECs can be considered as simulators of the ESQPTs in those systems. Our results contribute to quantumstate engineering and quantumenhanced interferometry in spinor BECs and to the characterization of excitedstate quantum phases. The latter may, in turn, lead on to applications in quantumstate engineering.
 Organisationseinheit(en)

Institut für Theoretische Physik
QUEST Leibniz Forschungsschule
 Typ
 Dissertation
 Anzahl der Seiten
 131
 Publikationsdatum
 2021
 Publikationsstatus
 Veröffentlicht
 Elektronische Version(en)

https://doi.org/10.15488/10772 (Zugang:
Offen)