Van der Waals explosion of cold Rydberg clusters

We report on the direct measurement in real space of the effect of the van der Waals forces between individual Rydberg atoms on their external degrees of freedom. Clusters of Rydberg atoms with interparticle distances of around 5μm are created by first generating a small number of seed excitations in a magneto-optical trap, followed by off-resonant excitation that leads to a chain of facilitated excitation events. After a variable expansion time the Rydberg atoms are field ionized, and from the arrival time distributions the size of the Rydberg cluster after expansion is calculated. Our experimental results agree well with a numerical simulation of the van der Waals explosion

Experimental signatures of an absorbing-state phase transition in an open driven many-body quantum system

Understanding and probing phase transitions in non-equilibrium systems is an ongoing challenge in physics. A particular instance are phase transitions that occur between a non-fluctuating absorbing phase, e.g., an extinct population, and one in which the relevant order parameter, such as the population density, assumes a finite value. Here we report the observation of signatures of such a non-equilibrium phase transition in an open driven quantum system. In our experiment rubidium atoms in a quasi one-dimensional cold disordered gas are laser-excited to Rydberg states under so-called facilitation conditions. This conditional excitation process competes with spontaneous decay and leads to a crossover between a stationary state with no excitations and one with a finite number of excitations. We relate the underlying physics to that of an absorbing state phase transition in the presence of a field (i.e. off-resonant excitation processes) which slightly offsets the system from criticality. We observe a characteristic power-law scaling of the Rydberg excitation density as well as increased fluctuations close to the transition point. Furthermore, we argue that the observed transition relies on the presence of atomic motion which introduces annealed disorder into the system and enables the formation of long-ranged correlations. Our study paves the road for future investigations into the largely unexplored physics of non-equilibrium phase transitions in open many-body quantum systems

Real-space measurement of the mechanical effect of the van der Waals-force on Rydberg atoms.

The van der Waals force is the sum of all attractive or repulsive forces between atoms or molecules due to the interactions between permanent or induced dipole moments; it is a topic of widespread interest in many fields of science, such as atomic and condensed matter. physics, chemistry and biology as it is at the basis of very different phenomena, from the complex nature of interatomic and intermolecular interactions, to the characteristic three-dimensional shape of biological macromolecules, to effects in the macroscopic domain such as the capability of geckos to stick on walls without falling. Even if van der Waals force has been known for over a century, it is only in the last few years that experiments targeted to measure the force between two isolated atoms have been carried out: these experiments focus mainly on measuring the effect of the force on internal degrees of freedom of the system, that is the displacement of atomic energy levels due to the interaction. A more direct approach to studying the effect of van der Waals force is to perform measurements on the external degrees of freedom instead, observing the spatial dynamics of the atoms involved in the interaction. However, traditional techniques that have been employed in a lot of experiments in atomic physics through the years, as such those based on fluorescence, cannot be applied in this kind of experiment: in fact, we must deal with small atom numbers, distributed within a large volume of around a cubic millimeter, which the above imaging techniques are not able to detect. Our experiment is performed using an ultracold gas of rubidium atoms trapped in a magneto-optical trap, and we observe the van der Waals force between atoms excited to the 70S Rydberg state, for which the interaction is repulsive. We use Rydberg atoms because, thanks to their larger electrical dipole moments with respect to their ground state counterparts, they lead to larger values of the van der Waals interaction coefficient and thus facilitate the observation of the resulting force. In order to measure the effect of the van der Waals force, we excite about ten atoms in the atomic cloud to the Rydberg state and study their expansion over time by field ionizing them at different moments and gathering information about the arrival times of the corresponding ions to the detector. Our detection technique is based on the analysis of the arrival times of the ions, in a similar way to the study of Coulomb explosions; in order to use this tool we need to make a detailed characterization of our detection apparatus and a calibration of the arrival times. Through this technique we can monitor the spatial expansion of the cloud, as each time we ionize the atoms we take a snapshot of the cloud as it is right before ionization. We also compare the results of the expansions with a numerical simulation without using any free parameter, and a good agreement with the experimental data is achieved. Our experiment represents an innovative approach to the measurement of the van der Waals force using real-space measurements of its mechanical effects, that lead to what may be called a "van der Waals explosion"

Experimental studies of the blackbody induced population migration in dissipative Rydberg systems

My work concerns the experimental study of many-body physics using ultracold atoms. Cold atoms experiments represent ideal quantum simulators and allow us to study the complete quantum dynamics of a system under investigation, once its Hamiltonian is known. Among the many implementations, Rydberg atoms, i.e., atoms excited to highly excited states, represent a suitable framework for simulating certain types of physics, such as absorbing state phase transitions and other non-equilibrium phenomena. In fact, Rydberg atoms can naturally implement dissipation through two radiative processes, which are the spontaneous decay and the blackbody induced transitions to neighboring Rydberg states, and its characterization is important for simulations. In my thesis I have developed an experimental method for measuring the lifetimes of high-lying Rydberg states, where the application of traditional techniques results impractical. For this purpose, a detailed characterization of the apparatus, of the detection system and of the electric fields acting on the atoms has been necessary. This measurement allows to distinguish between an initially populated Rydberg state, a target state, from all the other states which are populated through blackbody radiation, the support states. The measurement also allows to obtain the lifetime of the total ensemble of Rydberg atoms in the system. Through this measurement, it is possible to characterize the blackbody induced migration between Rydberg states

Driven dissipative dynamics in an open many-body quantum system

In a large variety of systems in nature, a small change of an external parameter around a critical value could affect the microscopic dynamics in such a deep way that the entire system changes sharply its macroscopic state and properties, experiencing what is known as a phase transition. Non equilibrium phase transitions are a particular class of phase transitions which occur in systems far from the thermal equilibrium. An example is that of absorbing state phase transitions, in which the behaviour of the system is determined by the competition of two processes, where one increases the order parameter of the transition, while the other lowers it. The system thus ends in one of two different states, either an oscillating state, or an absorbing state in which the order parameter equals zero and from which the system cannot escape. An example is represented by the infectious spreading of a disease, in which the two processes are the infection and the spontaneous healing, while the two final states are a partially infected or a completely healed population

Optical gain switching by thermo-responsive light-emitting nanofibers through moisture sorption swelling

The development of intelligent photonic systems made of stimuli-responsive materials, i.e., with features tunable and switchable by environmental signals, is gaining increasing attention. Here, the study reports on switchable optical gain based on complex arrays of nanofibers made of thermo-responsive poly(2-n-propyl-2-oxazoline), incorporating a blue-emitting chromophore. The fluorescent component endows the nanofibers with optical gain in addition to the moisture absorption capability of the polymer. Light amplification is found with temperature- and humidity-dependent excitation threshold. The threshold value is halved close to the polymer cloud point temperature, enabling reversible switching of the emission intensity upon temperature change. Waveguiding analysis by back-focal plane imaging on individual fibers allows the switching mechanisms to be rationalized, in terms of moisture sorption swelling-induced morphological changes. These responsive light-emitting nanofibers may find application in a novel class of lasers with dynamically-controlled properties, environmentally-switchable optoelectronics, and smart sensors

Van-der-Waals explosion of cold Rydberg aggregates

No abstract available