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May 21, 2021

NIST: How America's Authority on Measurements Relies on Ansys HFSS to Calibrate Their Measurements

Established in 1901, National Institute of Standards and Technology (NIST) is one of the oldest physical science laboratories in the U.S. In 2015, I held a post-doctoral position there. I worked on numerous projects associated with wireless technology for more than three years at NIST, including some of the earliest work on measurement tools to support next-generation 5G networks.

I’d like to share some interesting work I recently engaged in with my former colleagues at NIST and other researchers from the University of Colorado in Boulder. This work has demonstrated an atom-based sensor that can determine the direction of incoming radio signals, which is the key to more reliable communications than conventional technology has achieved.

Fig. 1. Researchers determined the angle of arrival of an incoming radio frequency signal based on laser measurements at two locations inside a cesium vapor cell. Fig. 1. Researchers determined the angle of arrival of an incoming radio frequency signal based on laser measurements at two locations inside a cesium vapor cell.


Atom-based sensors have attracted a lot of attention lately. Measurement standards have adopted a plethora of atom-based measurements over the last couple of decades, such as length (m), frequency (Hz), time (s) and mass (kg) standards. There is a strong interest in extending this adoption to magnetic (H) and electric (E) field sensors.

A Rydberg atom is an atom excited into a high energy state whose outermost electrons are in very high orbits around the nucleus. Rydberg atoms have many interesting properties including a sensitive response to external radio frequency (RF) fields. The rich resonant response of these atoms to the external RF fields occurs across wide frequency range from MHz to THz.

Sponsored by the Defense Advanced Research Projects Agency (DARPA) Quantum-Assisted Sensing and Readout (QuASAR) program, NIST has made significant progress in developing Rydberg atom-based RF e-field sensors. These sensors offer advantages over current technology, including the ability to measure amplitude, polarization and phase of the RF fields. Numerous applications are emerging from this technology such as detection of fields and modulated signals, detection of an angle of arrival (AoA), vector fields, different waveform characterization, DC to THz detection, and detection of very weak and very strong fields.

Collaboration Calculates Angle of Arrival

NIST collaborated with Ansys to investigate the capability of Rydberg atom-based sensors to determine the AoA of an incident RF field measurement, which is of great interest to the radar and communications industries. NIST has developed a heterodyne technique that uses a Rydberg atom-based mixer for measuring E-field phase. An atom-based mixer takes input signals and converts them into different frequencies. While one signal is used as a reference, the second one is detuned to a lower frequency. The mixer essentially determines the phase of the detuned signal at two different locations inside an atomic vapor cell. Scientists can calculate a signal’s AoA based on the phase difference at those two locations.

To confirm the validity of the proposed method, NIST measured phase differences at two locations inside the vapor cell for different AoAs and compared those with Ansys HFSS simulations and a theoretical model. HFSS proved to be a very accurate simulation tool capable of efficiently matching measured and theoretical data, and addressing the errors caused by measurement setup. Because the atomic vapor cell was made of dielectric, the RF field can exhibit multi-reflections inside the cell and generate standing waves perturbing the RF field that was measured. NIST scientists relied on Ansys HFSS simulations to correct the atom-based sensor response due to field perturbation issues in order to get improved agreement with the theoretical model.

Agreement of experimental, Ansys HFSS and theoretical data
Agreement of experimental, Ansys HFSS and theoretical data

Fig. 2. Agreement of experimental, Ansys HFSS and theoretical data


At Ansys, we constantly encounter researchers who are surprised after seeing excellent agreement between simulated and measured results. They truly shouldn’t be surprised. We are actually doing exactly the same thing; the only difference is that metrologists are doing their research in labs and simulation gurus on computers. Both of us are solving Maxwell’s equations, which proved to be universally correct over 150 years ago and – as formulated in Ansys HFSS with automatic adaptive meshing – are deterministically accurate.

This outstanding R&D effort is helping to ensure the performance of modern radar and wireless communications. In this and many other studies, Ansys once again demonstrates the accuracy and the value of our simulation tools. Simulation helps researchers achieve high fidelity results in less time than ever before, thereby boosting development of cutting-edge technologies.


1. NIST Demo Adds Key Capability to Atom-Based Radio Communications

2. NIST Quantum Probe Enhances Electric Field Measurements

3. A.K. Robinson, N. Prajapati, D. Senic, M.T. Simons and C.L. Holloway. Determining the Angle-of-Arrival of a Radio-Frequency Source with a Rydberg Atom-Based Sensor. Applied Physics Letters. Published online March 15, 2021.

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