Rydberg atoms with principal quantum number $n⪢1$ have exaggerated atomic properties including dipole-dipole interactions that scale as ${n}^{4}$ and radiative lifetimes that scale as ${n}^{3}$. It was proposed a decade ago to take advantage of these properties to implement quantum gates between neutral atom qubits. The availability of a strong long-range interaction that can be coherently turned on and off is an enabling resource for a wide range of quantum information tasks stretching far beyond the original gate proposal. Rydberg enabled capabilities include long-range two-qubit gates, collective encoding of multiqubit registers, implementation of robust light-atom quantum interfaces, and the potential for simulating quantum many-body physics. The advances of the last decade are reviewed, covering both theoretical and experimental aspects of Rydberg-mediated quantum information processing.

Quantum information with Rydberg atoms

It is shown that under the usual conditions of zero‐electron‐kinetic‐energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high‐lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

Factors affecting lifetimes and resolution of Rydberg states observed in zero-electron-kinetic-energy spectroscopy

Atomic polarization phenomena impinge upon a number of areas and processes in physics. The dielectric constant and refractive index of any gas are examples of macroscopic properties that are largely determined by the dipole polarizability. When it comes to microscopic phenomena, the existence of alkaline-earth anions and the recently discovered ability of positrons to bind to many atoms are predominantly due to the polarization interaction. An imperfect knowledge of atomic polarizabilities is presently looming as the largest source of uncertainty in the new generation of optical frequency standards. Accurate polarizabilities for the group I and II atoms and ions of the periodic table have recently become available by a variety of techniques. These include refined many-body perturbation theory and coupled-cluster calculations sometimes combined with precise experimental data for selected transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index measurements in microwave cavities, ab initio calculations of atomic structures using explicitly correlated wavefunctions, interferometry with atom beams and velocity changes of laser cooled atoms induced by an electric field. This review examines existing theoretical methods of determining atomic and ionic polarizabilities, and discusses their relevance to various applications with particular emphasis on cold-atom physics and the metrology of atomic frequency standards.

https://iopscience.iop.org/article/10.1088/0953-4075/43/20/202001/pdf

Theory and applications of atomic and ionic polarizabilities

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2 O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

/pdf/photodissociation-and-photoionisation-of-atoms-and-molecules-1bnk2a3c2i.pdf

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

Publisher Summary This chapter discusses resonance and threshold phenomena in low-energy electron collisions with molecules and clusters. Low-energy collisions of electrons with atoms and molecules are among the most important elementary processes in gaseous environments such as discharges, arcs, gas lasers, gaseous dielectrics and the earth's atmosphere. The dynamics behavior of low-energy electron-molecule collisions is discussed. The dynamical behavior of slow electrons traversing gases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances). Some aspects of resonance and threshold phenomena are discussed. The theoretical description of electron-molecule collisions generally requires an adequate description of electronic, vibrational and rotational degrees of freedom. However, if the typical collision time is short compared to the rotational period, the molecule can be treated as having a fixed orientation during the collision process, and the result for the cross-section can be averaged over orientations. Treatment of vibrational dynamics is usually more important and more challenging to the theory. In the electron energy region important for applications, many inelastic processes such as vibrational excitation and dissociative electron attachment are driven by negative-ion resonances. The theoretical description of vibrational dynamics in these cases is usually based on the nonlocal complex potential describing the nuclear motion in the intermediate negative-ion state.

/pdf/resonance-and-threshold-phenomena-in-low-energy-electron-42hkyjmsxl.pdf

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

We have observed the sharply resonant enhancement of the cross section of the collision process $ns+ns\ensuremath{\rightarrow}np+(n\ensuremath{-}1)p$ between two $\mathrm{Na}ns$ ($16l~nl~27$) atoms which occurs when the levels are shifted by an electric field so that the $ns$ level lies midway between the two $p$ states. The widths of the collisional resonances scale as ${n}^{\ensuremath{-}2}$ and the magnitudes of the cross sections as ${n}^{4}$, with values of 0.5 GHz and 8\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{\AA{}}}^{2}$ for the $20s$ state. These results are in good agreement with calculations based on the long-range dipole-dipole interaction between the two atoms.

Resonant Rydberg-Atom-Rydberg-Atom Collisions,

Using a three-step laser excitation approach, we have studied the autoionizing ${(6{p}_{j}nd)}_{J=3}$ states of neutral barium. The $6{p}_{\frac{3}{2}}\mathrm{nd}$ series is essentially unperturbed and exhibits a constant quantum defect and autoionization widths which scale as ${\ensuremath{\nu}}^{\ensuremath{-}3}$, where $\ensuremath{\nu}$ is the effective quantum number. The $6{p}_{\frac{1}{2}}\mathrm{nd}$ states are narrower than the $6{p}_{\frac{3}{2}}\mathrm{nd}$ states and are perturbed by their interaction with the $6{p}_{\frac{3}{2}}\mathrm{nd}$ states. The perturbation leads to variations in the quantum defects of the $6{p}_{\frac{1}{2}}\mathrm{nd}$ states and departures from the ${\ensuremath{\nu}}^{\ensuremath{-}3}$ scaling of the widths. The interaction is most apparent in the excitation of the $6{p}_{\frac{3}{2}}10d$ state which is degenerate with the $6{p}_{\frac{1}{2}}\mathrm{nd}$ states for $n\ensuremath{\sim}20$. The observed excitation spectrum clearly shows the influence of the degenerate $6{p}_{\frac{1}{2}}\mathrm{nd}$ states and is shown to represent the spectral density of the $6{p}_{\frac{3}{2}}10d$ state. This spectrum as well as all the above-mentioned observations may be connected by a quantum-defect treatment.

Interaction between two Rydberg series of autoionizing levels in barium

The experimental investigation of the resonant collision process Nans+Nans..-->..Nanp+Na(n-1)p which occurs when the levels are tuned with an electric field so that the ns level lies midway between the two p levels is described in detail. The observations are shown to be in good agreement with a long-range resonant dipole-dipole interaction.

W. Sandner

Papers

Resonant Rydberg-Atom-Rydberg-Atom Collisions,

Interaction between two Rydberg series of autoionizing levels in barium

Resonant Rydberg-atom — Rydbert-atom collisions

Photoionization of potassium in the vicinity of the minimum in the cross section

Probing configuration interaction of the Ba 5d7d 1 D 2 state using radio-frequency spectroscopy and lifetime measurements