A Rydberg atom is an atom in an excited state. This occurs when a single valence electron of any atom is promoted to a state of high principal quantum number n, the electron experiences an essentially Coulombic potential and behaves in many ways like an excited electron in hydrogen. The atom in a Rydberg state have new properties, including an exaggerated response to electric and magnetic fields, long decay periods and electron wavefunctions that approximate, under some conditions, the classical orbits of electrons around the nuclei.
The existence of the Rydberg series was first demonstrated in 1885 by Johann Balmer, who discovered an empirical formula for the wavelengths of light associated with transitions in atomic hydrogen. Swedish physicist, Johannes Rydberg, followed this with a generalized version of Balmer's formula which came to be known as the Rydberg formula, which indicated the existence of an infinite series of more closely spaced discrete energy levels converging on a finite limit.
The existence of atoms in a Rydberg state also was predicted in Niels Bohr's original paper on hydrogen, which had been discovered with high quantum numbers. These were observed in nature but could not be created in the laboratory until the invention of the laser.
The field of Rydberg physics and the study of the Rydberg atom has since generated more interest, with investigations of pressure shifts and broadening Rydberg levels by rare gases. The detection of these gases in dilute interstellar regions have helped grow the interest. In these dilute environments, the atoms in a Rydberg state can live long enough to decay radiatively, and can be sensitive to weak magnetic fields, and act as a magnetic probe of interstellar media.
Rydberg atoms have been considered as a building block for quantum technologies. The Rydberg atom offers a strong and controllable atomic interaction that can be tuned by selecting states with different quantum number or orbital angular momentum. As well, Rydberg atoms are comparatively long-lived, and offers a number of available energy levels and separations which allow coupling to electromagnetic fields spanning over 6 orders of magnitude in frequency.
In order to use Rydberg atoms in quantum technology, the atoms need to be trapped. A number of studies have demonstrated trapping of the Rydberg atoms using magnetic, electric, or laser technology, but the trapping times of these methods have been relatively short. A research group at the Laboratoire Kastler Brossel (LKB) have achieved a longer 2D laser trapping time of circular Rydberg atoms (or atoms in a Rydberg circular state). This was done by producing a donut-shaped laser beam and using the Rydberg atoms repulsion by light to trap them in the donut-shaped beam. The time of trapping was up to 10 milliseconds, up from 100 microseconds previous methods had achieved.
Researchers at the California Institute of Technology and the National University of Singapore have used arrays of strontium Rydberg atoms in quantum computers. Their research arranged quantum entangled alkaline-earth Rydberg atoms arranged in arrays. Most quantum computers are based on superconducting qubits, or trapped ion platforms, and this research used Rydberg atoms serving as a qubit. This requires a lot of atoms arranged in an array. These arrays use Rydberg qubits which realize photon coupling between different levels of Rydberg ground-state qubits, to avoid scattering. These qubits also allow for efficient detection of Rydberg states. The research at both institutes did not resolve a computing system, but rather investigated the possibility of other neural-network style quantum computing systems.
Rydberg atoms offer polarizability and strong dipole transitions between energetically nearby states that are sensitive to electric fields. This offers it as a good choice for quantum sensors. It has been used for sensing weak radio frequency electric fields, and offers applications in antenna calibration, signal detection, terahertz sensing, and the characterization of electronics and materials in the radio frequency spectrum.
Researchers at the University of Durham have developed a terahertz imaging system using atoms in a Rydberg state. These imaging systems have a range of applications including security screening, medical imaging, and industrial quality control.
The system fills a cell with caesium vapor and focus three infrared lasers on it. Each laser is tuned to one of three successive atomic transitions in caesium. When excited by these lasers, the caesium atoms end in an excited Rydberg state. The atom can then absorb a 0.55 THz photon, which decays after around a microsecond. The decay emits a green photon which in turn can be detected by a standard optical camera. This technique is about 100 times more sensitive than other techniques.
The United States Army Research Laboratory have developed quantum sensor systems for radio-frequency communications. These systems have provided much higher data rates than comparably sized traditional antenna. The atoms, in the system, are held in a glass cell at room temperature, and they use two colors of laser light to excited the Rydberg states and probe their reaction to the external electric fields. The system has been described as a compass needle for electric fields.
The atoms in this state are able to operate over a band of frequencies, from kilohertz to terahertz, while the atoms on the receiver can achieve maximum performance theoretically allowed, such that it was limited by the fundamental collapse of the quantum wave-function. The atoms convert the communication signal into modulated optical light which offers sensitive and high speed detection. And, when they detect signals, the system is nondestructive (and, thereby, covert) where traditional antenna systems absorb and damage the energy.
Arrays of strontium Rydberg atoms show promise for use in quantum computers
June 4, 2020
First successful laser trapping of circular Rydberg atoms
April 8, 2020
L23.3 Rydberg atoms.
July 5, 2017
Localized Excitation of Single Atom to a Rydberg State with Structured Laser Beam for Quantum Information | IntechOpen
Leila Mashhadi, Gholamreza Shayeganrad
April 3, 2019
Microwave Sensing with Rydberg Atoms
September 23, 2020
- Quantum sensorQuantum sensors and quantum sensing describes the use of a quantum system, quantum properties, or quantum phenomena to perform a measurement of a physical quality.
- Quantum computingQuantum computing is a type of computation that harnesses the collective properties of quantum states, such as superposition, interference, and entanglement, to perform calculations. The devices that perform quantum computations are known as quantum computers.