We demonstrate a non-equilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α = −0.53 ± 0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.Non-equilibrium systems displaying phase transitions are found throughout nature and society, for example in ecosystems, financial markets and climate [1]. The steady state of a non-equilibrium system is a dynamical equilibrium between driving and dissipative processes. In atomic physics, one of the most studied non-equilibrium phase transitions is optical bistability where the driving is provided by a resonant laser field and the dissipation is inherent in the atom-light interaction. In most examples of optical bistability feedback is provided by an optical cavity, as in the pioneering work of Gibbs [2,3]. However, bistability can also arise in systems where many dipoles are located within a volume which is much smaller than the optical wavelength; in this case the feedback is due to resonant dipole-dipole interactions [4,5]. This latter case is known as intrinsic optical bistability [6] and has, so far, only been observed in an up-conversion process between densely packed Yb 3+ ions in a solid-state crystal host cooled to cryogenic temperatures [7]. Intrinsic optical bistability generally cannot be observed for simple two-level systems such as atomic gases, because the resonance broadening, which is larger than the line shift [8], suppresses the bistable response [9,10].A solution to this problem is provided by highlyexcited Rydberg states, where the dipole-dipole induced level shifts between neighbouring states can be much larger than the excitation linewidth. This property of optical excitation of Rydberg atoms, known as dipole blockade [11], enables a diverse range of applications in quantum many-body physics, quantum information processing [12], non-linear optics [13] and quantum optics [14][15][16][17]. An interesting feature of Rydberg systems is that the range of the interaction can be much larger than the optical excitation wavelength, giving rise to non-local interactions [18]. This also creates the possibility of observing intrinsic optical bistability, and hence non-equilibrium phase transitions [19] over macroscopic, optically-resolvable length scales.In this letter, we demonstrate a non-equilibrium phase transition in a thermal Rydberg ensemble. In contrast to previous experiments, we directly observe...

We study the interaction of thermal rubidium atoms with the guided mode of slot waveguides integrated in a vapor cell. Slot waveguides provide strong confinement of the light field in an area that overlaps with the atomic vapor. We investigate the transmission of the atomic cladding waveguides depending on the slot width, which determines the fraction of transmitted light power interacting with the atomic vapor. An elaborate simulation method has been developed to understand the behavior of the measured spectra. This model is based on individual trajectories of the atoms and includes both line shifts and decay rates due to atom-surface interactions that we have calculated for our specific geometries using the discrete dipole approximation. Furthermore, we investigate density-dependent effects on the line widths and line shifts of the rubidium atoms in the subwavelength interaction region of a slot waveguide.

We investigate an integrated optical chip immersed in atomic vapor providing several waveguide geometries for spectroscopy applications. The narrow-band transmission through a silicon nitride waveguide and interferometer is altered when the guided light is coupled to a vapor of rubidium atoms via the evanescent tail of the waveguide mode. We use grating couplers to couple between the waveguide mode and the radiating wave, which allow for addressing arbitrary coupling positions on the chip surface. The evanescent atom-light interaction can be numerically simulated and shows excellent agreement with our experimental data. This work demonstrates a next step towards miniaturization and integration of alkali atom spectroscopy and provides a platform for further fundamental studies of complex waveguide structures.

We investigate experimentally and theoretically the coherent and incoherent processes in open 3-level ladder systems in room temperature gases and identify in which regime electromagnetically induced transparency (EIT) occurs. The peculiarity of this work lies in the unusual situation where the wavelength of the probe field is shorter than that of the coupling field. The nature of the observed spectral features depends considerably on the total response of different velocity classes, the varying Doppler shifts for bichromatic excitation fields, on optical pumping to additional electronic states and transit time effects. All these ingredients can be absorbed in a model based on optical Bloch equations with only five electronic states.

We demonstrate the use of electrically contacted vapor cells to switch the transmission of a probe laser. The excitation scheme makes use of electromagnetically induced transparency involving a Rydberg state. The cell fabrication technique involves thin-film-based electric feedthroughs, which are well suited for scaling this concept to many addressable pixels like in flat panel displays.

Strongly interacting atom-cavity systems within a network with many nodes constitute a possible realization for a quantum internet which allows for quantum communication and computation on the same platform. To implement such large-scale quantum networks, nanophotonic resonators are promising candidates because they can be scalably fabricated and interconnected with waveguides and optical fibers. By integrating arrays of ring resonators into a vapor cell we show that thermal rubidium atoms above room temperature can be coupled to photonic cavities as building blocks for chip-scale hybrid circuits. Although strong coupling is not yet achieved in this first realization, our approach provides a key step towards miniaturization and scalability of atom-cavity systems.

The paradigm of cavity QED is a two-level emitter interacting with a high quality factor single mode optical resonator. The hybridization of the emitter and photon wave functions mandates large vacuum Rabi frequencies and long coherence times; features that so far have been successfully realized with trapped cold atoms and ions and localized solid state quantum emitters such as superconducting circuits, quantum dots, and color centers 1,2 . Thermal atoms on the other hand, provide us with a dense emitter ensemble and in comparison to the cold systems are more compatible with integration, hence enabling large-scale quantum systems. However, their thermal motion and large transit time broadening is a major challenge that has to be circumvented. A promising remedy could benefit from the highly controllable and tunable electromagnetic fields of a nano-photonic cavity with strong local electric-field enhancements. Utilizing this feature, here we calculate the interaction between fast moving, thermal atoms and a nano-beam photonic crystal cavity (PCC) with large quality factor and small mode volume. Through fully quantum mechanical calculations, including Casimir-Polder potential (i.e. the effect of the surface on radiation properties of an atom) we show, when designed properly, the achievable coupling between the flying atom and the cavity photon would be strong enough to lead to Rabi flopping in spite of short interaction times. In addition, the time-resolved detection of different trajectories can be used to identify single and multiple atom counts. This probabilistic approach will find applications in cavity QED studies in dense atomic media and paves the way towards realizing coherent quantum control schemes in large-scale macroscopic systems aimed at out of the lab quantum devices.

Surface active N-oxyl radicals containing aliphatic chains were prepared from palmitinic, stearic and oleic acid chlorides by reaction with 4-hydroxy-(or-4-amino-) 2,2,6,6-tetramethylpiperidine or its N-oxyl. In the former case the resulting esters or amides were subsequently oxidized. A dioleyl ester (3 b) was prepared analogously from 2,2,6,6-tetramethyl-4-bis(2-hydroxyethy1)aminopiperidine. Surface tension data of aqueous solutions were determined and micelle formation was confirmed by light transmission measurements. The size of the water droplets in hexane solution of the dioleoyl derivative was estimated by the fluorescence spectroscopic method to 4,62 nm.