Rydberg physics is the land of gentle giants—highly-excited electronic states where an electron is, on average, far from the nucleus. They are created with a lot of energy, have unusually large size and live relatively long. They are useful because they are extremely sensitive to their environment; so much so that a single photon of light can change the behaviour of not only one, but dozens or hundreds of them, over distances we can easily see under an optical microscope. Wherever Rydberg states are created—in atoms, molecules or solids—they all share common features that are exemplified by the physical description of highly-excited hydrogen atoms, with energy levels described by the Rydberg formula. In this ebook, we outline the universal properties of Rydberg systems and then present a selection of ideas that demonstrate their increasing relevance to applications, including sensing, the manipulation of light on the single photon level (quantum optics) and in engineering the interactions between individual quanta (quantum simulation).
Rydberg physics is the land of gentle giants—highly-excited electronic states where an electron is, on average, far from the nucleus. They are created with a lot of energy, have unusually large size and live relatively long. They are useful because they are extremely sensitive to their environment; so much so that a single photon of light can change the behaviour of not only one, but dozens or hundreds of them, over distances we can easily see under an optical microscope. Wherever Rydberg states are created—in atoms, molecules or solids—they all share common features that are exemplified by the physical description of highly-excited hydrogen atoms, with energy levels described by the Rydberg formula. In this ebook, we outline the universal properties of Rydberg systems and then present a selection of ideas that demonstrate their increasing relevance to applications, including sensing, the manipulation of light on the single photon level (quantum optics) and in engineering the interactions between individual quanta (quantum simulation).
Rydberg physics is the land of gentle giants—highly-excited electronic states where an electron is, on average, far from the nucleus. They are created with a lot of energy, have unusually large size and live relatively long. They are useful because they are extremely sensitive to their environment; so much so that a single photon of light can change the behaviour of not only one, but dozens or hundreds of them, over distances we can easily see under an optical microscope. Wherever Rydberg states are created—in atoms, molecules or solids—they all share common features that are exemplified by the physical description of highly-excited hydrogen atoms, with energy levels described by the Rydberg formula. In this ebook, we outline the universal properties of Rydberg systems and then present a selection of ideas that demonstrate their increasing relevance to applications, including sensing, the manipulation of light on the single photon level (quantum optics) and in engineering the interactions between individual quanta (quantum simulation).
This introductory chapter presents the scope of this book: namely that the book is devoted to Rydberg (i.e., highly-excited) atoms and molecules They have striking distinctions to atoms and molecules in the ground or low-excited states. For Rydberg atoms, with the increase of the principal quantum number n, their characteristic size and the electric dipole moment scales as n 2 (which is why they are extremely sensitive to even weak electric fields and electromagnetic radiation) and their radiative lifetime scales as n 3. They strongly interact not only with an electric field, but also with each other: their dipole–dipole interaction scales as n 4/R 3 and their van der Waals interaction scales as n 11/R 6, where R is their separation from each other. In laboratory conditions experimentalists deal with Rydberg atoms having the quantum number of several tens. In the Universe, in H II regions, the principal quantum number of Rydberg atoms can reach n ~ 1000. In the latter case, the Rydberg atoms cannot be considered anymore as microscopic objects: the diameter of their electron orbit is ~ 0.1 mm, i.e., twice the diameter of human hair. Studies of Rydberg atoms and molecules have numerous applications. Examples are highly-responsive sensing, quantum optics (where the light is manipulated at the level of a single photon), quantum computing, quantum chaos, ultracold Rydberg gases and plasmas, anti-hydrogen formation, and plasma diagnostics.
The creation of giant atoms whose size is comparable to that of a grain of sand might sound like the stuff of science fiction, but in fact such species exist in nature and can now be created in the laboratory using advanced laser systems. Such exotic atoms, in which one electron is placed in a highly-energetic state, are termed Rydberg atoms, after the Swedish spectroscopist J. R. Rydberg who first characterised their properties. As might be expected, such extreme atoms possess very unusual physical and chemical properties. Their study has provided many new insights into the properties of Rydberg atoms themselves, their interactions with other atoms and molecules, and phenomena that arise from their collective interactions. The extreme properties of Rydberg atoms now enable emerging technological applications in sensing and quantum computation.
Introduction Series of atomic spectral lines had been studied by emission from excited vapors even before Balmer described the regularity of the visible lines of hydrogen in 1884. However, direct excitation had been restricted to the few lowest-lying levels, particularly those accessible by electric dipole transitions from the ground state using classical light sources. The advent of tunable lasers in the 1970s changed such spectroscopic limitations enormonsly, allowing excitation to a very much larger number of states. Atoms in highly excited states, typically with principal quantum numbers n ≥ 10, are called Rydberg atoms. They are oversize, fragile states of significant intrinsic interest. They are very rare in nature because of their size and fragility, but they are found in interstellar space (H I regions). One of the best sources for comprehensive descriptions of Rydberg atoms and references to the literature describing their exploration is Ref. [120]. It describes their characteristic properties and behavior both classically and quantum mechanically, and especially their strongly enhanced interactions with external fields. The first thing to note is that Rydberg atoms are usually characterized by a single, highly excited electron while all the other electrons remain in their original states. Thus their properties are very nearly classical, and the atoms are much closer to the ideal hydrogen-like model of Chap. 7, along with appropriate quantum defects (see Chap. 10). The spectroscopy of such states is especially interesting because the active electron is much less perturbed by the other electrons in the atom than when it is in lower states. Moreover, there are some special circumstances in which the energy of a Rydberg state is degenerate with an atomic state having two electrons excited to lower levels, and such configurations yield further new information. Second, Rydberg atoms do not usually decay back to the ground state by emission of characteristic radiation, so their study is mediated by other techniques that reveal new and interesting properties. Such properties arise because Rydberg atoms have a huge dipole moment and very low binding energy resulting from the large average distance between the excited electron and the nucleus-plus-core of the rest of the atom with net charge e . Third, transitions between Rydberg states are quite different from those connecting to the ground state for at least three reasons.
A whole community of physicists is under the spell of the oversized atoms called Rydberg atoms. Created when one or sometimes two of their electrons is excited to very high energy levels, far from the nuclear core, Rydberg atoms can detect subtle electric fields and probe chemical reactions that ordinarily proceed too fast for study. And because their outermost electrons sometimes orbit the nucleus like planets orbiting the sun—a behavior reminiscent of classical physics, not the quantum physics of the atom—physicists are using Rydberg atoms to study the border between the quantum and classical worlds.
Controlling molecular binding at the level of single atoms is one of the holy grails of quantum chemistry. Rydberg macrodimers─bound states between highly excited Rydberg atoms─provide a novel perspective in this direction. Resulting from binding potentials formed by the strong, long-range interactions of Rydberg states, Rydberg macrodimers feature bond lengths in the micrometer regime, exceeding those of conventional molecules by orders of magnitude. Using single-atom control in quantum gas microscopes, the unique properties of these exotic states can be studied with unprecedented control, including the response to magnetic fields or the polarization of light in their photoassociation. The high accuracy achieved in spectroscopic studies of macrodimers makes them an ideal testbed to benchmark Rydberg interactions, with direct relevance to quantum computing and information protocols where these are employed. This review provides a historic overview and summarizes the recent findings in the field of Rydberg macrodimers. Furthermore, it presents new data on interactions between macrodimers, leading to a phenomenon analogous to Rydberg blockade at the level of molecules, opening the path toward studying many-body systems of ultralong-range Rydberg molecules.