Session H05: Cold Rydberg gases and plasmas

Magnetic Confinement of an Ultracold Neutral Plasma

Ultracold neutral plasmas (UNPs), created by photoionization of a cold gas, are an excellent tool for studying strongly coupled plasmas, in which the ratio of the nearest neighbor Coulomb energy to the average thermal energy, $\Gamma_{i}$, is greater than one. Magnetized UNPs are of current interest because of the interplay of magnetization and strong coupling, connection to plasma confinement, and modification of recombination dynamics in strong fields. Here, we demonstrate magnetic confinement of an UNP within the same quadrupole magnetic fields used in the magneto-optical trapping of the initial atom cloud. Preliminary results show that the plasma expansion is initially unaffected by the presence of the magnetic field, but after a time that scales with $\sigma(0)/\sqrt{T_e}$, where $\sigma(0)$ is the initial plasma size and $T_{e}$ is the electron temperature, the expansion essentially ceases and the density stabilizes. We will also discuss complications for the laser-induced-fluorescence ion imaging arising from the presence of magnetic fields.   

Matter-wave interferometry with atoms in high Rydberg states

Rydberg states of atoms and molecules can possess large static electric dipole moments which allow forces to be exerted on them using inhomogeneous electric fields [1]. For samples prepared in coherent superpositions of Rydberg states with different electric dipole moments, these forces can be exploited to generate superpositions of momentum states for atom interferometry. Here, we report the experimental realization of such a Rydberg-atom interferometer - an electric analogue of the longitudinal Stern-Gerlach interferometer [2]. The experiments were performed with helium atoms in pulsed supersonic beams. Sequences of microwave and inhomogeneous electric field pulses were applied to implement the interferometry scheme. The results presented open new possibilities for measurements of the acceleration of Rydberg positronium or antihydrogen atoms in the Earth's gravitational field. The Rydberg states used in the experiments had sizes of ~320 nm. Matter-wave interferometry with such giant atoms is also of interest for the study of spatial decoherence in large quantum systems. [1] S. D. Hogan, EPJ Techniques and Instrumentation 3, 1 (2016) [2] S Nic Chormaic et al., J. Phys. B 26, 1271 (1993)   

Observation of a large, resonant, cross-Kerr nonlinearity in a free-space Rydberg medium

Rydberg-Rydberg interactions combined with electromagnetically induced transparency (EIT) are an extremely promising platform for nonlinear optics at the level of single photons. In light of this, there have been concerted theoretical and experimental efforts endeavoring to harness the powerful interactions of Rydberg atoms to implement a Kerr nonlinearity. We report the experimental observation of an enhanced cross-Kerr nonlinearity in a free-space medium based on resonantly-excited, interacting Rydberg atoms and electromagnetically induced transparency. The nonlinearity is used to implement cross-phase modulation between two optical pulses. The nonlinear phase written onto the ``probe" pulse is measured to be as large as 8mrad per nW of ``signal" power, corresponding to a $\chi^{\mathrm{(3)}}$ of 10$^{\mathrm{-8\thinspace }}$m$^{\mathrm{2}}$V$^{\mathrm{2}}$. The cross-Kerr nonlinearity scales with the adjusted principal quantum number like n$^{\mathrm{5.6+/-0.4}}$, consistent with our expectations for a van der Waals based nonlinearity. Our results have applications ranging from optical quantum information processing to quantum non-demolition measurement of photon number.   

Quantum gas microscopy of Rydberg macrodimers

Rydberg macrodimers - molecules consisting of two bound highly excited Rydberg atoms - provide huge bond lengths even resolvable with optical wavelengths. Here we report on the microscopic observation, characterization and control over the formation of such Rydberg macrodimers in a gas of ultracold atoms in an optical lattice. The size of about 0.7 micrometers matches the diagonal distance of two atoms in the lattice. Starting from a two-dimensional array of one atom per site, the discrete density provided by atoms in their motional ground state combined with a narrow-linewidth ultraviolet laser enables the resolved two-photon photoassociation of more than 50 theoretically predicted vibrational states. Using our spatially resolved detection, we observe the macrodimers by correlated atom loss and demonstrate control of the molecular alignment by the vibrational state and the polarization of the excitation light. Our results allow for a detailed test of Rydberg interactions and establish quantum gas microscopy as a new tool for quantum chemistry.   

Quantum Control Methodology for Creation of GHZ and W states of Rydberg atoms

Rydberg atoms with very high principal quantum number trapped in a optical trap are used to study the collective spin properties of ultracold atomic systems. The interaction Hamiltonian of the collective spin states were explored in the blockade regime for diatomic and triatomic chains. The diagonal of the Hamiltonian is taken to find all locations where energy level states are in resonance. Control parameters including two-photon detuning $\delta$, Rabi interaction $\vee$, and chirp rates $\alpha,\beta$ can be manipulated to get resonance between ground and excited energy levels and suppress all other interactions. Rydberg atoms have two well defined entangled states known as the Greenberger-Horne-Zeilinger (GHZ) state and the W state. By suppressing unwanted interactions both entangled states can be found by choosing $\vee$ and $\Omega$ and then sweeping over values of one-photon detuning $\Delta$.   

Optical circularization of Rydberg atoms

We present theoretical calculations concerning three new methods of circularizing F-state Rydberg atoms in ponderomotive laser traps via two-photon interactions arising from the A*A term of the minimal coupling Hamiltonian. For the first method, Rydberg atoms in the m=3 magnetic suborbital are promoted to the m=n-1 circular state from two Laguerre-Gaussian modes that radially trap them. The second method involves an RF-modulated optical lattice providing quadrupole-like couplings between hydrogenic states for adiabatic rapid passage into the circular state. In the third method, an RF-modulated, two-dimensional optical lattice harmonically shifts the trap center such that the atoms see the phase of a circularly-polarized photon, which excites them directly into the circular state without population loss from opposite-handed transitions.   

Dynamics and electric fields of photo-excited plasma

The development of atom trapping and cooling processes has made it possible for researchers to photo-excite cold plasmas in the laboratory. This enables models of hard-to-access plasmas that occur in nature, such as the insides of stars and gas planets, and artificial plasmas such as inertial-confinement fusion plasmas. We have created cold, cylindrically shaped, and positively charged plasmas from laser-cooled rubidium atoms. We have done so by establishing a cylindrical excitation region and using ultraviolet pulses to strip away the valence electrons. We report our findings on time-of-flight plasma expansion, obtained via spatially- and time-resolved imaging on a micro-channel-plate detector. We discuss our ongoing efforts of using Rydberg-electromagnetically induced transparency (Rydberg-EIT) to measure the internal electric fields.   

Ion friction at small values of the Coulomb logarithm

We report relaxation measurements in a Ca$^{\mathrm{+}}$/Yb$^{\mathrm{+}}$ dual species ultracold neutral plasma. The nearly 4:1 mass ratio of the ion species in our plasma is similar to the alpha:proton mass ratio important for fusion-class systems. Our system provides a platform for using Ca$^{\mathrm{+}}$ and Yb$^{\mathrm{+}}$ ions to find the Coulomb logarithm for momentum transfer collisions in a strongly coupled plasma environment. The velocity distributions are determined using laser-induced fluorescence. Measurements are compared to a two-fluid code calculation that include convection, adiabatic expansion, pressure acceleration, ion friction, ambipolar field acceleration, and Joule heating to describe dual species plasma expansion. We compare our measurements with a range of expressions for the Coulomb logarithm from the literature.   

Dynamical shell structure from Rydberg impurities in a many-body environment

We consider a Rydberg impurity in a single-component Fermi gas. The Rydberg electron of the impurity induces a giant potential for the bath particles. We develop a theoretical approach to predict the full absorption line shape of Rydberg molecules composed of an impurity and bath particles, including dressing effects by low-energy excitations of the fermionic environment. We find striking differences in the dynamics of fermions and bosons arising from the interplay of molecule formation, scattering physics and quantum statistics. Analyzing the full spectral absorption line shape we demonstrate that the dynamics is governed by states that exhibit a shell structure similar to that found in nuclear physics. We study the evolution with increasing density from the few-body limit with resolved molecular spectral lines to the many-body limit with a profile representing a giant Fermi superpolaronic state in which a large number of fermions bind to the Rydberg impurity. A compression of the superpolaronic spectrum allows Rydberg impurities to serve as an in-situ sensor of local density fluctuations. We further discuss the orthogonality catastrophe in the new setting of giant impurity excitations far from equilibrium.