the best laboratory vacuums attainable on Earth, Rydberg states could persist for long periods of time without being destroyed by collisions. Strongly interacting systems Due to their large size, Rydberg atoms can exhibit very large electric dipole moments. Calculations using perturbation theory show that this results in strong interactions between two close Rydberg atoms. Coherent control of these interactions combined with their relatively long lifetime makes them a suitable candidate to realize a quantum computer. In 2010 two-qubit gates were achieved experimentally. Strongly interacting Rydberg atoms also feature quantum critical behavior, which makes them interesting to study on their own. Current research directions Since 2000's Rydberg atoms research encompasses broadly five directions: sensing, quantum optics, quantum computation, quantum simulation and quantum matters. High electric dipole moments between Rydberg atomic states are used for radio frequency and terahertz sensing and imaging, including non-demolition measurements of individual microwave photons. Electromagnetically induced transparency was used in combination with strong interactions between two atoms excited in Rydberg state to provide medium that exhibits strongly nonlinear behaviour at the level of individual optical photons. The tuneable interaction between Rydberg states, enabled also first quantum simulation experiments. In October 2018, the United States Army Research Laboratory publicly discussed efforts to develop a super wideband radio receiver using Rydberg atoms. In March 2020, the laboratory announced that its scientists analysed the Rydberg sensor's sensitivity to oscillating electric fields over an enormous range of frequencies—from 0 to 1012 Hertz (the spectrum to 0.3mm wavelength). The Rydberg sensor can reliably detect signals over the entire spectrum and compare favourably with other established electric field sensor technologies, such as electro-optic crystals and dipole antenna-coupled passive electronics. Classical simulation A simple 1/r potential results in a closed Keplerian elliptical orbit. In the presence of an external electric field Rydberg atoms can obtain very large electric dipole moments making them extremely susceptible to perturbation by the field. Figure 7 shows how application of an external electric field (known in atomic physics as a Stark field) changes the geometry of the potential, dramatically changing the behaviour of the electron. A Coulombic potential does not apply any torque as the force is always antiparallel to the position vector (always pointing along a line running between the electron and the nucleus): , . With the application of a static electric field, the electron feels a continuously changing torque. The resulting trajectory becomes progressively more distorted