Here we have studied the dynamical response of an interacting, three-species Fermi-Hubbard model to an rfπ-pulse. Taking final state interactions into account, the near-perfect population transfer realised a quantum quench setup. To make connection to experiments, we included the harmonic trapping of the fermionic gas in our description of our model and worked in the low density regime. We found different dynamics ensuing as a result of the population trans-fer. While the pair correlation in the final state initially increases due to the population of the upper level, it exhibits a surprising decrease even before theπ-pulse is completed, which we attribute to fast decoherence processes determined by the final state. Additionally, all discussed observables, most notable the particle density show another, slow dynamical time scale in their evolution. We find that the rf-quench activates the collective monopole mode of the trap and discuss its non-trivial effect on the pairing dynamics. Our simulations are in good, qualitative agreement with rf-quench experiments performed in the BCS-BEC crossover in the group of Michael Köhl.
Chapter 8
Conclusion
In this thesis we have presented a comprehensive investigation of the non-equilibrium dy-namics of interacting Fermi gases. We focussed on two different fermionic models, a three-dimensional Fermi gas in the BCS-BEC crossover, and a one-three-dimensional Fermi-Hubbard model confined to an optical lattice geometry. Both models have been studied in the past and in this thesis we build upon these results, by extending the Hilbert space to include an initially un-populated state corresponding to a third fermionic species. By coupling this final level to an initially occupied state using radiofrequency (rf) modulation to redistribute the population be-tween the three fermionic species, we drove the system away from equilibrium and probed its excitation spectrum. In this thesis we have explored the rf-modulation as a tool with three-fold purpose.
Firstly, we developed a novel excitation technique based upon the rf-transfer. We studied a three-dimensional, homogeneous Fermi gas in the BCS-BEC crossover. By transferring a small fraction of particles between the initial, correlated ground state and the empty final state, we excited the Higgs mode of the system, a collective excitation of the superconducting state. This mode is usually short-lived, but in the BCS regime it is stabilised by an effective Lorentz invari-ance of the equations of motion provided by the particle-hole symmetry of the Hamiltonian near the Fermi momentum. As it is a scalar excitation it does not couple directly to gauge fields and is therefore difficult to excite and observe experimentally. In this chapter, we have devised a novel excitation mechanism based on far red-detuned rf-modulation of the Fermi gas and theoretically showed thedirectcoupling to the superconducting order parameter. We revealed the collective nature of the Higgs mode from Fourier spectra of the momentum-resolved su-perconducting order parameter, which served as an unambiguous final proof of its excitation, and confirmation of the proposed excitation scheme. Finally, we have compared our simula-tions in the BCS regime to an experiment conducted in the group of Michael Köhl and found excellent agreement between the two. Our investigations provide a route to use rf-techniques beyond spectroscopic tools to dynamically excite and stabilise complex quantum many-body states away from equilibrium.
In a closely related study, we explored the influence of the duration of quantum quenches upon the ensuing quantum dynamics. Here we tuned thes-wave scattering length of an inter-acting two-species Fermi gas described by the mean-field BCS Hamiltonian. As we lowered the
interaction strength over time, we uncovered three distinct dynamical regimes in the subse-quent dynamics of the superconducting order parameter. For fast quenches, the order param-eter vanished, while the pair amplitude remained finite. This illustrated the delicate nature of the superconducting state, and the importance of long-range phase coherence between Cooper pairs. By rapidly changing the interaction, the Cooper pairs of the initial state were projected onto the new basis of the final Hamiltonian. We showed that this rapid quench predominantly generated excited Cooper pairs in the new basis, which quickly dephased. The observed final state was therefore characterised by incoherent, pre-formed pairs. Increasing the quench dura-tion we found that intermediate ramps gave rise to a reduced, but finite order parameter with long-lived oscillations in the final state. The competition between the free evolution driven by the system’s chemical potential and the dephasing mechanism of the excited Cooper pairs was the origin of this dynamical regime, where the dephasing has not been sufficient to fully erode the superconducting state. For slow quenches in the (near) adiabatic regime, the system followed the instantaneous interaction strength and the final state was shown to be given by a finite temperature thermal state with a static, reduced superconducting order parameter. Our results thus show how the coherence of Cooper pairs can be dynamically tuned by the quench duration, and demonstrate a new avenue to engineer non-trivial quantum states away from equilibrium.
Secondly, we used the rf-transfer as a spectroscopic tool to uncover the intricate excita-tion spectrum of the attractive, one-dimensional Fermi-Hubbard model. Our quasi-exact time-dependent matrix product state simulations, revealed two distinct dynamical regimes in the evolution of the system. One was characterised by (off)resonant oscillations of the upper level’s population whilst the other, through coupling to a continuous band of final states, was domi-nated by a net linear rise. By monitoring the population of the final state in time, we gained detailed information of the excitation spectrum, in particular about the coupling strength to different excitations in the initial state. Using linear response calculations in the weak coupling regime, we related the transfer rate to the spectral function of the initial system. Surprisingly, despite not always being in the linear response regime, we were able to extract the underlying excitation spectrum with reasonable accuracy. We compared our numerical findings to exact results from Bethe ansatz, and find overall good agreement between the different methods.
However, the t-MPS simulations allowed us to go beyond the spectral response and study in detail the full out-of-equilibrium evolution, enabling us to characterise the two distinct dynami-cal regimes. In this way we presented a comprehensive overview of the nature of the rf-transfer and how it affects a quantum many-body system at the microscopic level.
Thirdly, we explored the possibility of employing rf-modulation to perform quench experi-ments on time scales difficult to attain using more traditionally used magnetic ramp techniques.
We proposed a scheme which exploits the differences in thes-wave scattering length near a Fes-hbach resonance. By completely transferring particles from a strongly interacting initial state to a weakly interacting final state, we realised apopulation quenchof the system on a time-scale of a few hopping periods. We performed t-MPS simulations of a large system in the low den-sity limit, explicitly taking the final state interactions and time-dependence of the rf-drive into account. Our simulations revealed the excitation of a collective trap mode and we observed its