This Research Letter reports on the observation of superradiance decoherence caused by long-range dipole-dipole interactions between Rydberg atoms. A cold atom cloud is prepared in the mode volume of a weakly driven optical cavity and excited to a Rydberg state. The cavity transmission monitors the Rydberg dynamics in real time and detects superradiant enhancement of the transition rates between neighboring Rydberg states. The observed enhancement is reduced for increasing Rydberg atom density: a signature of atom pair interactions. The observations are explained within a model based on the idea of Rydberg blockade reducing the atom number participating in the Dicke state, responsible for superradiance.
This Research Letter reports on the observation of superradiance decoherence caused by long-range dipole-dipole interactions between Rydberg atoms. A cold atom cloud is prepared in the mode volume of a weakly driven optical cavity and excited to a Rydberg state. The cavity transmission monitors the Rydberg dynamics in real time and detects superradiant enhancement of the transition rates between neighboring Rydberg states. The observed enhancement is reduced for increasing Rydberg atom density: a signature of atom pair interactions. The observations are explained within a model based on the idea of Rydberg blockade reducing the atom number participating in the Dicke state, responsible for superradiance.
We study the superradiant phase transition of an array of Rydberg atoms in a dissipative microwave cavity. Under the interplay of the cavity field and the long-range Rydberg interaction, the steady state of the system exhibits an interaction-enhanced superradiance, with vanishing critical atom-cavity coupling rates at a discrete set of interaction strengths. We find that, while the phenomenon can be analytically understood in the case of a constant all-to-all interaction, the enhanced superradiance persists under typical experimental parameters with spatially dependent interactions, but at modified critical interaction strengths. The diverging susceptibility at these critical points is captured by emergent quantum Rabi models, each of which comprises a pair of collective atomic states with different numbers of atomic excitations. These collective states become degenerate at the critical interaction strengths, resulting in a superradiant phase for an arbitrarily small atom-cavity coupling.
Electron dynamics in dipole-dipole (DD) coupled Rydberg atoms show some unique properties that do not exist in individual atoms.The goal of the research in this dissertation is to reveal and control the electron dynamics influenced by DD interaction in cold Rydberg gases.We have studied the decay of Rydberg excitations in a cold Rb gas and find no evidence for superradiance.The decay rates and population redistribution we observe are consistent with a model that considers only spontaneous emission from, and blackbody redistribution within, isolated atoms.Suppression of superradiant emission is likely due to variations in transition energies across the cold Rydberg atom sample.For initial s states, these variations are dominated by inhomogeneities in DD exchange interactions within the random ensemble.Such inhomogeneities will necessarily be present in any measurement involving a large number of atoms where the separation between atoms is not well defined.For initial p states, the suppression is likely due to a combination of DD exchange and electric field inhomogeneities.We have also explored the evolution of Rydberg wavepackets in the presence of strong dipole-dipole interactions in a frozen gas.The distribution of atom separations results in an inhomogeneity in the strength of the exchange coupling between neighboring atoms, causing a dephasing of the macroscopic coherence.We have also explored the mechanism of wavepacket coherence transfer from one Rydberg atom to a neighboring Rydberg atom via DD interactions utilizing the resonant DD coupling transition 25s33s↔24p34p.The phase-shift of the observed interference modulations in the 34p signal, relative to that in 25s, is a signature of wavepacket coherence transfer between atoms driven by electron correlations resulting from the controlled DD coupling between them.I would like to thank my advisor Professor Bob Jones for his guidance and help, not only in the detailed research performance but also in scientific ethics and methodology.He taught me how to operate most of the instruments as well as helped me fix failing apparatus.His rigorous, yet optimistic, working attitude always inspires me in my research.I don't remember how many times I have felt frustrated after getting nothing from a several days' experiment.At those times, I was always motivated and energetic from his guidance, constructive criticism and sincere encouragement, after even a short talk with him.The accomplishment of this dissertation involved a lot of his help, ranging from grammar checking, to language polishing, to error correction.I really feel grateful and fortunate to have worked with him for more than 5 years.Thanks also goes to my colleagues who have helped me in the research.Brian Richards worked with me a lot to fix failing instruments.He also programmed the data collection software.Sha Li taught me how to build setups to generate the THz radiation and build dye lasers.Kisra Nayomal Egodapitiya helped me adjust lasers and amplifiers many times.I also got invaluable inspiration from daily talks with them.
Integrating Rydberg-state control of atom-atom interactions with cavity control of atom-light interactions enables the combination of local entangling operations with photon-mediated long-range coupling, measurement, and communication. Such integration is challenging because electric fields generated by charges on cavity optics and by cavity-stabilizing actuators may disturb Rydberg-state coherence. Here, we couple a Rydberg tweezer array to a high-cooperativity optical cavity, suppressing the electric fields produced by cavity length actuation so that the residual field variation of about 3mV/cm rms does not limit Rydberg coherence. We demonstrate two capabilities that neither system provides alone. First, we use Purcell-enhanced fluorescence to read out atoms directly in the ground-Rydberg basis within 7.5μs, well within the Rydberg-state lifetime, with ground- and Rydberg-state detection infidelities of 3% and 5%, respectively, and with low atom loss. Second, we use Rydberg-Rydberg interactions to control collective light-atom scattering into the cavity mode. Specifically, we drive blockaded Rabi oscillations between a state with two ground-state atoms, which scatter collectively with a superradiant or subradiant rate controlled by the atomic positions, and an entangled state with a single Rydberg excitation, which scatters like a single atom. These capabilities open routes toward networked Rydberg processors, mid-circuit measurements in analog quantum simulators, and many-body quantum optics with entangled emitters coupled collectively to light.
Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes. Here, we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model.
Resonant laser excitation of multiple Rydberg atoms are prohibited, leading to Rydberg blockade, when the long-range van der Waals interactions are stronger than the laser-atom coupling. Rydberg blockade can be violated, i.e., simultaneous excitation of more than one Rydberg atom, by off-resonant laser excitation, causing an excitation antiblockade. Rydberg antiblockade gives rise to strongly correlated many-body dynamics and spin-orbit coupling and also finds quantum computation applications. Instead of commonly used van der Waals interactions, we investigate antiblockade dynamics of two Rydberg atoms interacting via dipole-dipole exchange interactions. We study typical situations in current Rydberg atom experiments, where different types of dipole-dipole interactions can be achieved by varying Rydberg state couplings. An effective Hamiltonian governing underlying antiblockade dynamics is derived. We illustrate that geometric gates can be realized with the Rydberg antiblockade which is robust against the decay of Rydberg states. Our study may stimulate new experimental and theoretical exploration of quantum optics and strongly interacting many-body dynamics with Rydberg antiblockade driven by dipole-dipole interactions.
High optical nonlinearities can be achieved at the single photon level by coupling the photon states to strongly interacting Rydberg excitations under the conditions of electromagnetically induced transparency. The nonlinear response in Rydberg quantum optics comes as a direct result of the long range dipole-dipole interactions via Rydberg blockade and interaction induced dephasing. In this thesis, Rydberg quantum optics experiments are performed with two spatially separated mediums with a nonlinear response at the single photon level. Two ultracold rubidium atomic clouds are tightly confined by a pair of in-vacuo aspheric lenses such that only a few Rydberg excitations can exist in each one of the clouds simultaneously. The long range character of the dipole-dipole interactions leads to the generation of quantum states of light. The effective photon-photon interactions are directly observed as an anti-correlation in the simultaneous photon retrieval as a result of the non resonant van der Waals interactions. The collective Rydberg excitations, stored in the non-overlapping mediums, experience an additional spatial non-uniform phase. In addition, we experimentally characterize the cooperative optical response of a cold atomic medium at a single photon level. This aims to exploit the enhancement of the spontaneous emissions decay rate (superradiance) as a dependence of the number of atoms involved in the ensemble. The observed atomic response in two different regimes gives a further understanding of the system dynamics and unlocks a promising potential route towards the implementation in scalable, multichannel quantum optical devices.