18 research outputs found
Driven dissipative dynamics in an open many-body quantum system
No full textIn 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
No full textThe 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
Experimental signatures of an absorbing-state phase transition in an open driven many-body quantum system
No full textUnderstanding 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
Photonic devices with transient functionality
No full textTransient and smart functionalities can be obtained by devices that can physically disappear in a controlled way. Water-soluble polymers and materials that can dissolve/disintegrate offer self-degradable opportunities for use in various domains. Here we report on our recent results on combinatory, transient photonics based on water-soluble compounds and sublimating materials for application in full-field imaging and labelling. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 682157, “xPRINT”) and from the Italian Minister of University and Research PRIN 20173L7W8K project
Van der Waals explosion of cold Rydberg clusters
No full textWe 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
Dissipative and non-dissipative many-body dynamics in a cold Rydberg gas
No full textDissipative and non-dissipative many-body dynamics in a cold Rydberg ga
Strongly correlated excitation dynamics in an ultracold Rydberg gas
No full textThe van-der-Waals interaction between ultracold atoms excited to highlying Rydberg states can lead to strongly correlated excitation dynamics [1, 2]. We study these correlations for resonant and off-resonant driving of cold atoms in a magneto-optical trap. In the resonant case we observe dynamical constraints similar to those found in glassy systems [3]. By contrast, for off-resonant driving facilitated excitations mediated by Rydberg-Rydberg interactions lead to avalanche-like events in which a chain reaction of successive excitations occurs. We show that this effect can be induced by a seed excitation and that the resulting full counting statistics of excitation events can be reproduced by a simple analytical model
Experimental studies of the blackbody induced population migration in dissipative Rydberg systems
No full textMy 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
