Session U9: Rydberg Gases

Trapped Circular Rydberg Atoms for Quantum Simulation

Condensed-matter systems are interesting and important to understand but they are difficult to study, even numerically, given the significant sizes of their Hilbert space. Quantum simulation proposes to mimic those out-of-reach quantum systems with more controllable and accessible ones. The high polarizability of Rydberg atoms allows for strong and tunable short-range interactions, making them nice candidates for a quantum simulation platform. However, low angular momentum Rydberg atoms cannot be efficiently laser-trapped and their lifetimes would limit the scope of such a quantum simulator. We propose instead to use circular Rydberg atoms (of maximum angular momentum) which can be laser-trapped and whose lifetimes can be extended to the one minute range by placing them in a spontaneous emission-inhibiting capacitor. We aim at the deterministic preparation of a 1D-chain of 40 atoms, trapped in a Laguerre-Gauss hollow laser beam, with a collective lifetime of $2$ seconds. With exchange rates in the $10-100$ kHz range, this would provide a platform able to simulate quantum many-body physics for more than $10^4$ exchange times. In this talk I will present this novel quantum simulation platform and our latest experimental results in the laser-trapping of circular Rydberg atoms.   

Control of Rydberg atom blockade by dc electric field orientation in a quasi-one-dimensional sample

Rydberg atoms posse a strong atom-atom interaction, which limits its density in an atomic sample. Such effect is known as Rydberg atom blockade. Here, we present a novel way to control such effect by direct orienting the induced atomic dipole moment using a dc external electrical field. To demonstrate it, we excite the $50S_{1/2}$ Rb atomic state in a quasi-one-dimensional sample held in a quasi-electrostatic trap. A pure $nS$ state holds only van der Waals interaction at long range, but in the presence of an external electric field the state mixing leads to strong dipole-dipole interactions. We have measured the Rydberg atom population as a function of ground state atoms density for several angles between the electric field and the main axis of the unidimensional sample. The results indicate that the limit on the final Rydberg density can be controlled by electric field orientation. Besides, we have characterized the sample by using direct spatial ion imaging, demonstrating that it does behave as an unidimensional sample.   

Excitation dynamics in a lattice of Rydberg superatoms and steady-state bistability

Due to the strong and long-range interactions between Rydberg atoms, a mesoscopic atomic ensemble within a certain blockade volume suppresses more than one optical excitation. This so called superatom was realized in recent experiments. In the limit of strong dephasing during laser excitation, the superatom excitation probability reaches unity allowing for a much stronger driving strength beyond a two-level system. We study the many-body excitation dynamics of superatom lattices and observe interesting phases and phase transitions of open many body systems. In particular, we investigate the possibility of bistability in the steady state of the open many body system and explore signatures thereof. We clarify the role of long range correlations and a thermodynamic limit on the phase transition to a bistable regime. In a 2D lattice with nearest neighbor blockade, we observe an antiferromagnetically ordered phase with broken sublattice symmetry.   

Time-domain Ramsey interferometry with interacting Rydberg atoms

Many-body effects govern a variety of important quantum phenomena such as the emergence of superconductivity and magnetism in condensed matter physics. Here, we present a theoretical investigation of a many-body system formed by interacting Rydberg atoms. We follow the evolution of the electronic coherence of the atoms in Rydberg states by a time-domain Ramsey-interferometry protocol [1]. An Ising-type Hamiltonian with long range interactions is employed to describe the many-body dynamics. We show that fully analytic expressions for the coherence and the Ramsey-interferometry signal can be obtained in an ultracold gas under a continuous limit assumption and that this treatment can be further extended to correlation functions of the system. From the Ramsey signal a characteristic contrast degradation and phase accumulation signal is obtained which is showing corresponding scaling laws for different ensemble densities and dimensionalities. Good agreement is found between the theoretical analysis and recent experimental results [2]. References [1] C. Sommer et al. Phys. Rev. A. 94, 053607 (2016) [2] N. Takei et al. Nat. Commun. 7, 13449 (2016)   

Many-body dynamics of driven-dissipative Rydberg cavity polaritons

The usage of photons as long-range information carriers has greatly increased the interest in systems with nonlinear optical properties in recent years. The nonlinearity is easily achievable in Rydberg mediums through the strong van der Waals interaction which makes them one of the best candidates for such a system. Here, we propose a way to analyze the steady state solutions of a Rydberg medium in a cavity through the combination of the variational principle\footnote{H.~Weimer,Variational Principle for Steady States of Dissipative Quantum Many-Body Systems,Phys. Rev. Lett. {\bf 114}, 040402 (2015).} for open quantum systems and the P-distribution of the density matrix. To get a better understanding of the many-body-dynamics a transformation into the polariton picture is performed and investigated.   

Lifetimes of Ultralong-range Strontium Rydberg Molecules in a Dense BEC

Ultralong-range Rydberg molecules created in a dense BEC can be used to explore collective many-body phenomena such as the creation of polarons. The atom densities in a BEC, however, are such that even for moderate values of $n$, $n>50$, the electron orbit can enclose tens to hundreds of ground-state atoms. Collisional destruction therefore becomes important and can limit the molecular lifetimes. Measurements of the loss of Rydberg molecules with $n=49$, 60, and 72 excited in a BEC of $^{84}$Sr with a peak density of $4\times10^{14}$ cm$^{-3}$ reveal large loss rates of $1-3\times10^5$ s$^{-1}$. This loss is attributed to two mechanisms: the formation of Sr$^{2+}$ molecules through associative ionization, and $\ell$-changing reactions involving the Rydberg electron, with associative ionization being dominant. Collisional loss limits the time available to explore collective effects and possible techniques to increase this time are being examined.   

Two-Photon Excitation of Launched Cold Atoms in Flight

We demonstrate two-photon bi-chromatic excitation of cold rubidium atoms in flight, using the pathway $5S_{1/2}\rightarrow5P_{3/2}\rightarrow5D_{5/2}$ with two resonant photons. In our experiment, atoms are laser-cooled in a magneto-optical trap and launched upward in discrete clouds with a controllable vertical speed of 7.1$\pm$0.6 m/s and a velocity spread that is less than 10\% of the launch speed. Outside the cooling beams, as high as 14 mm above the original center of the trap, the launched cold atoms are illuminated simultaneously by spatially-localized horizontal excitation beams at 780 nm ($5S_{1/2}\rightarrow5P_{3/2}$) and 776 nm ($5P_{3/2}\rightarrow5D_{5/2}$). We monitor transmission of the 780-nm beam over a range of intensities of 780-nm and 776-nm light. As the center of the moving cloud passes the excitation beams, we observe as much as 97.9$\pm$1.2\% transmission when the rate of two-photon absorption is high and the $5S_{1/2}$ and $5P_{3/2}$ states are depopulated, compared to 87.6$\pm$0.9\% transmission if only the 780-nm beam is present. This demonstrates two-photon excitation of a launched cold-atom source with controllable launch velocity and narrow velocity spread, as a foundation for three-photon excitation to Rydberg states.   

Nondestructive Detection of Polar Molecules via Rydberg Atoms

Research on cold and ultracold molecules is impeded by the difficulty in many cases to efficiently detect molecules, with the choice of molecule species often influenced by the need for a suitable detection scheme. We demonstrate the possibility to efficiently and nondestructively detect basically any polar molecule species using Rydberg atoms [1]. A Rydberg atom senses the presence of a molecule based on F\"orster resonance energy transfer. We show that huge interaction cross sections of more than $10^{-6}\,$cm$^2$ exist for low collision energies, allowing for efficient detection [1]. First experimental results on detection of room temperature ammonia molecules with Rubidium Rydberg atoms will be presented. [1] M. Zeppenfeld, arXiv:1611.08893 [physics.atom-ph] (2016).   

$n\ell \rightarrow n'\ell'$ transition rates in electron and proton - Rydberg atom collision

Electrons and protons drive the recombination dynamics of highly excited Rydberg atoms in cold rarefied plasmas found in astrophysical conditions such as primordial recombination or star formation in H-II clouds. It has been recognized that collisions induce both energy and angular momentum transitions in Rydberg atoms, although in different proportions, depending on the initial state, temperature and the given species considered in the collision (electron or proton). Most studies focused on one collision type at a time, under the assumption that collision types are independent or their effects are not competing. The classical Monte-Carlo trajectory simulations presented in this work calculate the rates for both energy and angular momentum transfers and show their interdependence. For example, energy transfer with small angular momentum change are more efficient for target states with initial large angular momentum.