Inside DARPA's Use Of Rydberg Atoms To Create New RF Sensors

By John Oncea, Editor

DARPA leverages Rydberg atoms' extreme electromagnetic sensitivity to create compact quantum RF sensors, replacing large antenna arrays for defense applications.

Back in 1885, Swiss mathematician Johann Balmer – encouraged by colleague Eduard Hagenbach-Bischoff* – discovered a simple empirical formula for the wavelengths of light associated with transitions in atomic hydrogen.

Three years later, Swedish physicist Johannes Rydberg presented a generalized and more intuitive version of Balmer’s formula. This formula – known today as the Rydberg formula – indicated the existence of an infinite series of ever more closely spaced discrete energy levels converging on a finite limit.

In 1913, Danish theoretical physicist Niels Bohr provided a semi-classical explanation for the formula, showing how quantized angular momentum leads to the discrete energy levels. Thirteen years later, Austrian-Swiss theoretical physicist and quantum mechanics pioneer Wolfgang Pauli provides a full quantum mechanical derivation of the hydrogen spectrum, fully explaining the Rydberg series and – 41 years after Balmer’s discovery – the Rydberg atom was fully introduced to the world of physics.

Finally, in 1997, the advent of laser trapping and cooling techniques became a key enabler for modern Rydberg physics and put into motion a series of uses, applications, and discoveries, including:

• Late 1990s - 2000s: Researchers begin to explore the strong interactions between Rydberg atoms, predicting and later observing new phenomena such as large molecules (macrodimers and macrotrimers).
• 2000s - 2010s: Rydberg atoms are recognized for their potential in various applications due to their strong interactions and sensitivity.
• 2011: The ability to use neutral Rydberg atoms for building quantum computers is explored, overcoming limitations of charged ions.
• 2015: A “trilobite” Rydberg molecule is experimentally observed.
• 2016: A “butterfly” Rydberg molecule is observed by a collaboration of international researchers.
• 2022: Research highlights the promise of Rydberg atoms for use in highly sensitive detectors of RF electric fields.

The DARPA Quantum Apertures (QA) program is actively developing technology based on Rydberg atoms, using them to create new kinds of RF sensors. This research leverages the unique properties of Rydberg atoms to create highly sensitive, directional, and portable receivers that overcome the limitations of classical antenna systems for future Department of Defense missions.

* Hagenbach-Bischoff is best known for developing an electoral quota system – a variant of the D'Hondt method – for allocating seats in party-list proportional representation that is used by Luxembourg to allocate seats in its European Parliament elections to this day.

What Is A Rydberg Atom?

Rydberg atoms are a special type of atom in which one or more electrons are excited to extremely high energy levels, known as high principal quantum numbers, symbolized by n. These highly excited electrons orbit far from the nucleus, sometimes making the atom up to 1,000 times larger than a ground-state atom, according to NIST. Because of this vast size and loose binding, Rydberg atoms exhibit exaggerated atomic behaviors that are crucial in modern quantum technologies.

At a fundamental level, Rydberg atoms behave similarly to hydrogen atoms but with modified energy levels due to the presence of inner electrons that shield the nucleus. Their energy levels follow the Rydberg formula, which explains why their energy spacing decreases rapidly as n increases, according to the University of Connecticut. This results in a series of closely packed states called the Rydberg series, which converge toward the ionization limit of the atom.

These atoms are remarkable because many of their physical properties scale dramatically with the quantum number n. Their radius and electric dipole moment scale as n², while their polarizability and radiative lifetime scale even more steeply – as n⁷ and n³, respectively.

This makes Rydberg atoms extremely sensitive to external electric and magnetic fields, as well as to interactions with other atoms. The large dipole moments lead to strong long-range interactions, often used to study quantum many-body physics and implement quantum logic gates in atomic computing platforms, according to the University of Wisconsin.

Rydberg physics plays a central role in quantum information science. In “Rydberg blockade” systems, only one atom in a group can be excited to the Rydberg state at a time due to strong interatomic interactions. This phenomenon allows quantum bits, or qubits, to interact controllably, forming the basis for quantum gates used in neutral atom quantum computers, according to Sandia National Laboratories.

Extensive research at the University of Wisconsin and other institutions has demonstrated that Rydberg-based systems enable coherent control of large arrays of neutral atoms, opening new frontiers for fault-tolerant quantum processing.

The exaggerated sensitivity of Rydberg atoms also allows them to serve as precise measurement tools. Scientists at NIST developed a new thermometer using Rydberg atoms that can measure temperature from 0 to 100 °C without touching the sample, relying purely on how the atoms respond to blackbody radiation. The technique’s high precision stems from tracking transitions between energy levels that respond subtly to heat, giving it applications in quantum metrology and atomic clock calibration.

In astrophysics, naturally occurring Rydberg atoms have been detected in interstellar space, where their long lifetimes and sensitivity to weak magnetic fields allow them to serve as probes of cosmic environments. Their presence in these dilute regions has provided valuable insights into the effects of atomic collisions and radiation in the interstellar medium.

Rydberg atoms represent a frontier in atomic physics where classical and quantum behavior overlap. Their enormous size, sensitivity, and controllable interactions make them indispensable tools across quantum computing, precision measurement, and astrophysical research. The ongoing exploration of Rydberg atoms continues to reveal how extreme atomic states can be harnessed to advance both fundamental science and emerging technologies.

How DARPA Is Using Rydberg Atoms To Create New RF Sensors

The Defense Advanced Research Projects Agency (DARPA) has emerged as a key player in advancing quantum-enabled RF detection and communications technologies using Rydberg atoms. These highly excited atomic states serve as highly sensitive resonators and transducers for electromagnetic fields, enabling a fundamentally new class of quantum RF sensors that can outperform traditional antenna-based receivers in terms of bandwidth, size, and spectral awareness.

The agency’s most prominent efforts come under the Science of Atomic Vapors for New Technologies (SAVaNT) and Quantum Apertures (QA) programs, both of which aim to demonstrate practical, scalable quantum RF sensing systems for defense and communication applications.

Rydberg Atoms And Quantum Electrometry

Rydberg atoms – atoms whose outermost electrons are excited to extremely high energy levels – are prized for their extraordinary sensitivity to electric fields. Their large dipole moments enable them to detect minute fluctuations in an electromagnetic environment, making them well-suited for RF detection.

In these systems, an atomic vapor cell, typically containing alkali atoms such as rubidium or cesium, is illuminated by lasers tuned to excite the atoms into Rydberg states. Variations in the RF field cause measurable shifts in the optical response of the vapor, enabling an all-optical readout without traditional mixers, oscillators, or RF amplifiers, according to NASA.

This technique effectively replaces the antenna’s physical capture area with quantum coherence, allowing the sensor to operate across extremely wide frequency ranges – from MHz radar bands up to the terahertz regime – without changing hardware configurations, adds the University of Colorado.

DARPA’s SAVaNT Program

DARPA’s SAVaNT initiative specifically focuses on advancing vapor-based quantum devices such as Rydberg atomic sensors, optical magnetometers, and miniature atomic clocks. One key research direction aims to develop compact, wafer-scale laser systems and integrated vapor cells that can enable deployable Rydberg sensors. These sensors exploit the quantum interference effects of atomic states to detect electromagnetic fields with high precision.

DARPA’s approach under SAVaNT seeks to combine the physics of atomic vapors with photonic integration, aiming to shrink laboratory-grade quantum sensors into rugged systems suitable for battlefield use.

A related DARPA Spark Tank presentation in 2025 emphasized the potential of Rydberg atoms in high-frequency (HF) bands for over-the-horizon radar and compact communications systems. The report described feasibility demonstrations of Rydberg atoms in alkali vapors operating at microwave frequencies, showing that quantum-sensing-based receivers could achieve centimeter-scale detection apparatuses independent of signal wavelength, a significant advantage over conventional hundred-meter-scale antennas, according to DARPA.

Quantum Apertures: A Next-Generation Receiver Paradigm

The QA program represents DARPA’s broader vision for quantum-enabled RF reception. QA aims to engineer “field programmable quantum receivers” based on atomic vapor technologies that can dynamically tune to various frequencies, detect signals buried below the thermal noise floor, and achieve directionality without mechanically moving components.

The DARPA QA framework supports collaborations with the U.S. Army Research Laboratory and university programs such as the University of Maryland Quantum Technology Center, which recently demonstrated multiband Rydberg atomic sensors capable of demodulating five RF signals simultaneously over a 17–116 GHz range, according to the University of Maryland.

By leveraging quantum coherence rather than classical resonance, these devices effectively transcend traditional bandwidth and noise limitations. According to the National Center for Biotechnology Information, Rydberg systems achieve baseband optical readouts, where the RF waveform is translated directly into an optical signal measurable by a photodetector, eliminating multiple intermediate RF stages and reducing susceptibility to thermal and electronic noise.

The Broader Research Ecosystem

Beyond direct defense applications, DARPA’s sponsorship has fueled extensive academic and industrial research in atomic electrometry. At the University of Colorado Boulder, researchers funded by DARPA and Lockheed Martin have developed theoretical frameworks linking Rydberg sensor physics to radio engineering metrics such as noise figure, dynamic range, and instantaneous bandwidth.

Meanwhile, national laboratories and agencies such as NASA and NIST are collaborating with DARPA to explore manufacturable pathways for quantum sensor integration at scale, addressing challenges in cell fabrication, laser stability, and miniaturization.

DARPA’s investment in Rydberg atom physics represents a transformative approach to RF sensing and communication. By exploiting quantum coherence in atomic vapors, the agency is fostering the development of compact, self-calibrating, broadband sensors capable of detecting faint or stealthy signals across the electromagnetic spectrum, ushering in a new era of quantum-enhanced electromagnetics for defense, communications, and beyond.