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Rydberg Atoms: Quantum Electric Field Sensors

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Figure 1 Artistic representation of excited Rydberg atoms in a tightly focused laser beam interacting with incoming electromagnetic radiation. (CREDIT – US Army)

Professor Barry Dunning discusses new research into quantum electric field sensors using Rydberg atoms of higher sensitivity

Electromagnetic (EM) radiation is critically important to the functioning of modern technological society in areas such as telecommunications, radar and GPS navigation. While electromagnetic waves are now typically detected using antennas and electronic receivers, recent work using atoms in highly excited states, called Rydberg atoms, has demonstrated that, following in the footsteps of quantum time sensors (atomic clocks) and magnetic fields (magnetometers), controllable quantum systems can also provide a powerful electric field sensor (electrometer) that has the potential to achieve unprecedented levels of sensitivity and selectivity. Quantum atomic sensors have the advantage over more traditional technologies in that all atoms of a given species possess identical characteristics, allowing repeatable, high-precision measurements that can be directly related to known absolute standards. .

As a recent e-book points out, atoms in which an electron is excited to a Rydberg state possess many new characteristics. In particular, as their level of excitement increases, they become very large and can approach the size of a grain of sand. The excited electron orbits so far from the nucleus and the remaining electrons that the electric field it experiences is very weak. Consequently, its movement can be strongly disturbed by even very weak external electric fields and it is this sensitivity which is the basis of the use of Rydberg atoms in electrometry. Indeed, at room temperature, even a perfect vacuum represents a hostile environment for a Rydberg atom due to its interactions with the background thermal radiation of the black body emitted by the walls of its enclosure.

Rydberg-based EM field sensing exploits a phenomenon called electromagnetically induced transparency (EIT). A typical experimental setup includes a small glass gas cell filled with atomic vapor through which two counter-propagating laser beams are directed. The ‘probe’ laser is tuned to excite atoms in the cell to a low excited state, the ‘control’ laser is tuned to excite atoms from this low state to a selected Rydberg state. In the absence of the control beam, the laser beam from the probe is strongly absorbed due to the excitation of the atoms in the cell. The addition of the control beam leads to the creation of a quantum “dark” state which involves a superposition of the ground and excited states which opens a narrow window in the transmission of the probe beam. The introduction of an additional external EM field leads to so-called Autler-Townes separation and AC Stark shifts which perturb the dark state and cause changes in the transmission window. These changes are sensitive to the intensity and frequency of the field and can be observed by measuring the transmission of the probe beam. Techniques commonly used in radio receivers, such as using a local oscillator in a heterodyne configuration, can further improve sensitivity

Figure 2 Schematic illustration of a Rydberg receiver and quantum analyzer used to detect <a class=electromagnetic radiation over a wide spectral range. (CREDIT – US Army)” width=”800″ height=”533″ srcset=”https://www.openaccessgovernment.org/wp-content/uploads/2022/08/2.jpg 800w, https://www.openaccessgovernment.org/wp-content/uploads/2022/08/2-768×512.jpg 768w, https://www.openaccessgovernment.org/wp-content/uploads/2022/08/2-696×464.jpg 696w, https://www.openaccessgovernment.org/wp-content/uploads/2022/08/2-630×420.jpg 630w, https://www.openaccessgovernment.org/wp-content/uploads/2022/08/2-100×67.jpg 100w” sizes=”(max-width: 800px) 100vw, 800px”/>
Figure 2 Schematic illustration of a Rydberg receiver and quantum analyzer used to detect electromagnetic radiation over a wide spectral range. (CREDIT – US Army)

Rydberg atomic sensors have been shown to detect electromagnetic radiation over a wide spectral range from kilohertz to terahertz frequencies that extend from AM and FM radio bands to Bluetooth and WiFi signals and beyond. Additionally, the use of small vapor cells allows spatially localized observations of electric field strengths such as, for example, at points in a microwave waveguide without disturbing the local field as would the introduction of an antenna. Measurements of Rydberg atoms can also be directly related to known physical constants and can therefore provide accurate absolute electric field values ​​which can be used to calibrate more conventional antenna-receiver combinations. While current experiments have already demonstrated the remarkable sensitivity of Rydberg’s electrometers, creating even more sensitive sensors seems feasible whose performance is ultimately limited by quantum noise itself and the extent to which this is true is an area of ​​debate. active research.

Since the response of Rydberg sensors to EM radiation can be very frequency specific, they promise higher transmission capabilities in cluttered electromagnetic environments, especially when high bandwidth is not required, such as, for example, for the transmission of speech. This capability also enables precise measurement of Doppler shifts that result from the reflection of EM waves from a moving target allowing measurement of velocities from a few micrometers to a few kilometers per second, a range that covers cell migration to hyper -speed. Additionally, laboratory studies have demonstrated that Rydberg’s atomic sensors can be used to determine the direction of incoming radiation, and therefore the location of its source.

Although the use of Rydberg atoms as quantum electric field sensors is still in its infancy, they have been shown to provide superior performance to current technology with respect to sensitivity, resistance to interference, wide tunability, accuracy and small physical size. These capabilities speak of future applications not only in defense and communications, but also in more specialized fields such as radio astronomy and Doppler velocimetry.

FB Dunning

Physics teacher Sam and Helen Worden

Rice University, Houston, TX 77005

Giants on the atomic landscape, Open Access Government Research and Development online e-book, February 2022, available at openaccessgovernment.org.

NSFBAA - Air Force Scientific Research Office 2014