We propose a technique for engineering momentum-dependent dissipation in Bose-Einstein condensates with nonlocal interactions. The scheme relies on the use of momentum-dependent dark states in close analogy to velocity-selective coherent population trapping. During the short-time dissipative dynamics, the system is driven into a particular finite-momentum phonon mode, which in real space corresponds to an ordered structure with nonlocal density-density correlations. Dissipation-induced ordering can be observed and studied in present-day experiments using cold atoms with dipole-dipole or off-resonant Rydberg interactions. Because of its dissipative nature, the ordering does not require artificial breaking of translational symmetry by an optical lattice or harmonic trap. This opens up a perspective of direct cooling of quantum gases into strongly interacting phases.
We propose a technique for engineering momentum-dependent dissipation in Bose-Einstein condensates with nonlocal interactions. The scheme relies on the use of momentum-dependent dark states in close analogy to velocity-selective coherent population trapping. During the short-time dissipative dynamics, the system is driven into a particular finite-momentum phonon mode, which in real space corresponds to an ordered structure with nonlocal density-density correlations. Dissipation-induced ordering can be observed and studied in present-day experiments using cold atoms with dipole-dipole or off-resonant Rydberg interactions. Because of its dissipative nature, the ordering does not require artificial breaking of translational symmetry by an optical lattice or harmonic trap. This opens up a perspective of direct cooling of quantum gases into strongly interacting phases.
We propose a technique for engineering momentum-dependent dissipation in Bose-Einstein condensates with non-local interactions. The scheme relies on the use of momentum-dependent dark-states in close analogy to velocity-selective coherent population trapping. During the short-time dissipative dynamics, the system is driven into a particular finite-momentum phonon mode, which in real space corresponds to an ordered structure with non-local density-density correlations. Dissipation-induced ordering can be observed and studied in present-day experiments using cold atoms with dipole-dipole or off-resonant Rydberg interactions. Due to its dissipative nature, the ordering does not require artificial breaking of translational symmetry by an opticallattice or harmonic trap. This opens up a perspective of direct cooling of quantum gases into strongly-interacting phases.
In a driven dense ensemble of two-level atoms the decay acts in a correlated way on several atoms simultaneously. The competition between the coherent laser excitation and the dipole-dipole interaction induced by the dissipation gives rise to a bimodal distribution of the excitation density. More interestingly, the presence of nonlocal dissipation lifts the direct connection between the photon emission rate and the density of excited atoms present in typical open atomic systems formed by, for example, Rydberg gases or trapped ions. In contrast, here the bimodality present in the excitation density distribution, which indicates the existence of a dynamical first-order transition, is not accompanied by an intermittent behavior of the photon emission. Moreover, due to the nonlocal character of the dissipation the spatial coherence of the steady state can be measured experimentally by analyzing solely global quantities---the mean values of the photon emission rate and the density of excited atoms. We show that these effects can be experimentally probed in a system of trapped Sr atoms.
Interacting ultracold atomic quantum gases provide an ideal test bed to study strongly correlated quantum matter. The interactions between atoms in such an ultracold quantum gas are typically short-ranged and well described by an effective contact potential. Introducing longer-range interactions promises the realization of novel quantum phases which are absent in systems with only short-range interactions. Trapped atoms, resonantly laser-coupled to highly excited, strongly interacting Rydberg states have been proposed as a versatile platform to realize such long-range interacting quantum matter. In a first experiment presented in this thesis, we combined the single-atom-sensitive local preparation and detection enabled by a quantum gas microscope to prepare and microscopically characterize an ensemble of rubidium-87 atoms trapped in an optical lattice in the regime of strong Rydberg blockade. There, we observe collectively enhanced optical coupling in an effective two-level system, the so-called “superatom”, and infer the presence of entanglement. Detuned optical coupling to Rydberg states, termed “Rydberg dressing”, has been proposed as an alternative approach to induce long-range interactions. Rather than directly exciting an atom to a Rydberg state, the properties of the latter are admixed to a ground state, which consequently acquires long-range interactions. In the context of this thesis, we have designed a laser system in the ultraviolet spectral range to admix Rydberg P-state character to the ground state of atoms in an optical lattice. In an emerging Ising quantum magnet, the presence of Rydberg-dressed interactions was demonstrated by an interferometric technique combined with spatially resolved spin correlation measurements. The theoretically predicted tunability of the isotropy and range of the interaction were confirmed experimentally. While these initial measurements exhibited unexpected dissipation, in a subsequent experiment in a one-dimensional spin chain this dissipation was overcome. We substantiated the improved coherence times by tracking the time evolution of the magnetization upon suddenly switching on interactions and observing coherent collapse and revival dynamics, one of the hallmarks of coherent quantum evolution. Our results establish Rydberg-dressed interactions in ultracold gases and pave the way to realize and study novel exotic quantum phases.
Spontaneous symmetry breaking and self-organized pattern formation in nonlinear systems are fundamental phenomena with broad implications. Studying these effects in cold atomic gases, particularly through the optical control of laser-dressed Rydberg atoms, bridges atomic physics and modern optics. Here, we investigate the ground-state matter-wave pattern formation in cold Rydberg-dressed Bose-Einstein condensates (BECs) with considerations of the Lee-Huang-Yang (LHY) correction and Raman-induced spin-orbit coupling (SOC). A surprising discovery is that the combination of these effects can induce spontaneous symmetry breaking in a plane-wave state of matter waves, giving rise to a variety of exotic self-organized structures, including stripes, positive and negative hexagons, squares, black-eye patterns, and quasi-lattices, among others. Furthermore, we uncover intriguing structural behaviors by considering inter- and intra-specific nonlocal interactions and the hard-core (contact, short-range) and soft-core (long-range) interactions. Our findings advance optical control of quantum matter, with potential applications in atomic-based information processing.
The remarkable progress in control over cold atomic gases has led to a point where people are no longer satisfied with merely studying these systems, but rather put them to use to understand complex quantum many-body systems. The basis of this development is a deep understanding of the interaction between atoms, and how to exploit those in order to engineer interesting and novel quantum-systems. The aim of this particular thesis is to contribute to this third quantum revolution [1] and hence help to understand the inner workings of complex many-body systems. We present a method to control the shape and character of the interaction between cold atoms based on dressing the atomic ground-state with a Rydberg-state. The latter induces a van der Waals interaction between all the atoms in the ensemble, and allows for control via the coupling light-field. We find that with increasing atom densities the ensemble shows a direct transition into a collective regime that preempts the onset of three-body interactions associated with a break-down of the first Born-approximation. The reason for this intriguing behavior is the strong interaction between Rydberg atoms that gives rise to the blockade-mechanism, and prevents the simultaneous excitation to the Rydberg-state for spatially close atoms. The non-trivial behavior of the interaction-potential within the collective regime yields a novel tool for shaping the interaction between ground-state atoms beyond s-wave scattering. We study this collective regime and the resulting interaction-potential between the atoms within a variational/mean-field approach, and discuss its effects on a trapped Bose-Einstein condensate. Artificial atoms show remarkable properties, that are often superior to real atoms. In particular, since they are built out of many constituents, such systems often exhibit an enhanced coupling to the light-field as well as strong optical non-linearities even for small light-fields. On the other hand, noise in quantum-mechanical systems can not only destroy coherence, but rather can be used in order to robustly drive a system into an interesting state. We study the effect of a controlled dephasing onto an artificial atom in the context of an ensemble of atoms coherently coupled to a Rydberg state and demonstrate that such an enhanced artificial atom allows for the deterministic absorption of a single photon from an arbitrary incoming probe field. Such behavior yields a unique tool in light-matter interaction, and opens the path to realise quantum-networks or to fabricate novel quantum-devices. Here, we discuss the applicability of this single-photon absorber as a single-photon transistor, a high fidelity n-photon counter, and a device that allows for the deterministic creation of non-classical states of light via photon-subtraction. A non-trivial topological order of quantum-states leads to conservation of certain properties and, hence, increases their robustness against external perturbations. This can even stabilize quantum-states against local fluctuations. The latter usually corrupts the coherence within a macroscopic object and thereby prevents quantum-phenomena to occur in our macroscopic world. As an example of such a topological state, we study the behavior of vortex-excitations in a two-dimensional superfluid confined to a periodic potential, as can be realised within a cold atomic gas in an optical lattice. For large superfluid filling factors and strong interactions, the healing-length and, accordingly, the vortex core is much smaller than the lattice spacing. As a result, vortices are confined to the plaquettes of the lattice, and can be described in the framework of an effective tight-binding Hamiltonian. Via a first-principle calculation based on coherent-state path-integrals we derive the microscopic parameters of this model and provide an analytic expression for the vortex mass. Moreover, we show that such a quantum vortex is not obliged to follow the superfluid flow, but rather exhibits Bloch-oscillations perpendicular to it, which is a telltale sign for quantum interference of this macroscopic many-body excitation. Recently, Jonathan Simon et al. [2] performed a major step towards simulating quantum many-body systems in cold atomic gases by simulating the paramagnet-antiferro-magnet transition of a one-dimensional Ising-model. Fundamental excitations in the phase with broken translational symmetry are domain-walls carrying fractional statistics. The question is, whether experimentally accessible single-particle excitations, which correspond to two closely-bound domain-walls, decay into fractional excitations or remain closely-bound. By use of perturbation theory, we derive an analytic model for the time-evolution of these fractional excitations in the framework of a tilted Bose-Hubbard model, and demonstrate the existence of a repulsively bound state above a critical center-of-mass momentum. The validity of the perturbative approach is confirmed by the use of t-DMRG simulations. Together with the recent demonstration of single-site addressing and readout in optical lattices, these findings open the path for experimental observation of fractional excitations within cold atomic gases.
We theoretically investigate optically controlled pattern formation in a Rydberg-dressed Bose-Einstein condensate by employing two-frequency modulation of the control laser field. This approach enables direct and periodic tuning of the long-range nonlocal interactions in time, thereby intrinsically modulating the system's nonlinearity. A Floquet analysis reveals parametric resonances that simultaneously excite roton and Faraday instabilities (RI and FI), identifying distinct regimes of pure RI, pure FI, and a joint RI-FI phase. Direct numerical simulations confirm the emergence of novel Faraday patterns, which are selected by the relative amplitude and frequency ratio of the modulation. Our results establish a theoretical framework for studying nonlinear dynamics in driven nonlocal quantum fluids, providing a phase diagram that may serve as a benchmark for future experiments in Rydberg-dressed systems.