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

This paper reviews the recent theoretical and experimental advances in the study of ultra-cold gases made of bosonic particles interacting via the long-range, anisotropic dipole–dipole interaction, in addition to the short-range and isotropic contact interaction usually at work in ultra-cold gases. The specific properties emerging from the dipolar interaction are emphasized, from the mean-field regime valid for dilute Bose–Einstein condensates, to the strongly correlated regimes reached for dipolar bosons in optical lattices. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/72/12/126401/pdf

The physics of dipolar bosonic quantum gases

This paper presents a review of the current state of the art in the research field of cold and ultracold molecules. It serves as an introduction to the focus issue of New Journal of Physics on Cold and Ultracold Molecules and describes new prospects for fundamental research and technological development. Cold and ultracold molecules may revolutionize physical chemistry and few-body physics, provide techniques for probing new states of quantum matter, allow for precision measurements of both fundamental and applied interest, and enable quantum simulations of condensed-matter phenomena. Ultracold molecules offer promising applications such as new platforms for quantum computing, precise control of molecular dynamics, nanolithography and Bose-enhanced chemistry. The discussion is based on recent experimental and theoretical work and concludes with a summary of anticipated future directions and open questions in this rapidly expanding research field.

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Cold and ultracold molecules: science, technology and applications

A "Schrodinger cat"-like state of matter was generated at the single atom level. A trapped 9Be+ ion was laser-cooled to the zero-point energy and then prepared in a superposition of spatially separated coherent harmonic oscillator states. This state was created by application of a sequence of laser pulses, which entangles internal (electronic) and external (motional) states of the ion. The Schrodinger cat superposition was verified by detection of the quantum mechanical interference between the localized wave packets. This mesoscopic system may provide insight into the fuzzy boundary between the classical and quantum worlds by allowing controlled studies of quantum measurement and quantum decoherence.

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A "Schrodinger Cat" Superposition State of an Atom

A Schrödinger cat superposition state of an atom

We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to 100 cm(-1). When the atoms are initially bound by 70 cm(-1), this evolution occurs when most of the atoms are translationally cold, <1 mK, but a small fraction, approximately 1%, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma.

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Spontaneous evolution of rydberg atoms into an ultracold plasma

By using a magneto-optical trap we have measured the Rb $ns\ensuremath{-}(n+1)s$ and ${\mathrm{nd}}_{j}{\ensuremath{-}(n+1)d}_{j}$ two-photon millimeter-wave transitions for $32l~nl~37,$ observing 100-kHz-wide resonances, in spite of the trap's 10 G/cm magnetic-field gradient, in which one might expect to observe resonances 5 MHz wide. This resolution is possible because of the similarity of the ${g}_{j}$ factors in the initial and final states. Under the same conditions, the single-photon ns-np resonances are \ensuremath{\sim}5 MHz wide. To make useful measurements of these intervals, we turned off the trap field and used the 300-K atoms of the background Rb vapor. Together these measurements improve the accuracy of the s, p, and d quantum defects by an order of magnitude.

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Millimeter-wave spectroscopy of cold Rb Rydberg atoms in a magneto-optical trap: Quantum defects of the ns , np , and nd series

An atomic electron is prepared in a state closely analogous to Schr{umlt o}dinger{close_quote}s coherent superposition of {open_quote}{open_quote}live cat{close_quote}{close_quote} and {open_quote}{open_quote}dead cat.{close_quote}{close_quote} The electronic state is a coherent superposition of two spatially localized wave packets separated by approximately 0.4 {mu}m at the opposite extremes of a Kepler orbit. State-selective ionization is used to verify that only every other atomic level is populated in the {open_quote}{open_quote}cat state,{close_quote}{close_quote} and a Ramsey fringe measurement is used to verify the coherence of the superposition. {copyright} {ital 1996 The American Physical Society.}

/pdf/excitation-of-an-atomic-electron-to-a-coherent-superposition-32nf53gyke.pdf

Excitation of an Atomic Electron to a Coherent Superposition of Macroscopically Distinct States

Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as $100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high $n$ states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.

/pdf/evolution-dynamics-of-a-dense-frozen-rydberg-gas-to-plasma-53c1edp6gy.pdf

Evolution dynamics of a dense frozen Rydberg gas to plasma

Atoms in an ultracold highly excited sample are strongly coupled through the dipole-dipole interaction. In an effort to understand and manipulate the complicated interactions in this system we are investigating their dependence on the relative orientation of the dipoles. By focusing a 480 nm beam from a tunable dye laser into a magneto-optical trap, we produce a nearly one-dimensional sample of Rydberg atoms. The trap lies at the center of four conducting rods with which we can vary the magnitude and direction of the electric field at the trap, thus controlling the orientation of the dipoles with respect to the sample axis. We have measured the strength of the interaction for a variety of relative orientations.

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Michael W. Noel

Papers

Spontaneous evolution of rydberg atoms into an ultracold plasma

Millimeter-wave spectroscopy of cold Rb Rydberg atoms in a magneto-optical trap: Quantum defects of the ns , np , and nd series

Excitation of an Atomic Electron to a Coherent Superposition of Macroscopically Distinct States

Evolution dynamics of a dense frozen Rydberg gas to plasma

Angular Dependence of the Dipole-Dipole Interaction in a Nearly One-Dimensional Sample of Rydberg Atoms