Rydberg atom-based radio frequency sensors benefit from the unique features of atoms, such as their stability and uniformity. These features contribute to making Rydberg atom-based radio frequency sensors calibration-free. A low density of atoms, such as that found in a room temperature atomic cesium vapour contained in a vapour cell, is sufficient to generate a strong detectable signal.

For applications where self-calibration and accuracy are important, it is crucial to perform close to distortion-free radio frequency field measurements. The main challenge is the vapour cell, since it is a dielectric object placed in the target radio frequency field. It becomes imperative to engineer the vapour cell so that it minimally perturbs the target radio frequency field.

Rather than simply being an obstacle, engineering of the vapour cell can offer new opportunities. Taking an engineering approach to vapour cell design, we created vapour cells with reduced scattering cross-section, better target radio frequency field uniformity, and even amplification. Novel bonding schemes have also been developed.

Vapour Cell Engineering

2 different vapour cell models are shown to compare their normalized electric fields.
Fig. 1: Calculated field profiles for two vapour cell designs. In the first design (a), which is a plain square design, there is a large field gradient in the cell, resulting in a smeared-out field distribution. In the second design (b), metamaterial principles are used in the design to reduce the field gradient, resulting in a very narrow field distribution and a more accurate measurement of the incident field.

We developed a vapour cell based on metamaterial principles in order to reduce the effective refractive index and hence lower the radio frequency scattering. These principles can also be used to improve the field uniformity in the vapour cell so that the target radio frequency field measured inside the vapour cell is almost the same as that which is incident on the sensor. The scattering of the target radio frequency field can be characterized through the radar scattering cross-section. These types of vapour cells are advantageous for metrological applications.

Higher radio frequency field sensitivity is desired for noise-free detection of short pulses which is needed for radar and communications applications. Using photonic crystal concepts, waveguiding and a cavity, we built a receiver that effectively amplifies the atomic response.  These vapour cells are entirely dielectric and create comparatively little scattered radiation when compared to conventional technology.

Point graph showing scattering cross section in millimeters squared on the Y axis, and frequency in gigahertz on the X axis.
Fig. 2: Measured scattering cross sections of our vapour cells as a function of the frequency of the incident RF field.

Vapour Cell Testing and Fabrication

We developed a number of methods to characterize our vapour cells. Homogeneous RF field distribution inside a vapour cell is a key requirement to measure the true RF field. For example, the metamaterial-based vapour cells minimize the radio frequency field variation inside the cell. One way to investigate the vapour cell is to illuminate it with the radio frequency test field while the sensor is operational and map out the field inhomogeneities, the Stark shifts, and the variation in the target radio frequency field inside the vapour cell. We have also measured radio frequency scattering cross-sections for our vapour cells in our radio frequency test facilities.

All our vapour cells are fabricated in our MEMs fabrication facility. We have coating equipment, bonding equipment, diagnostic equipment, and the apparatus to fill our vapour cells with cesium in a number of different ways. The MEMs fabrication facilities are complemented by a surface science lab where the surface chemistry of the vapour cells can be investigated.

Testing is done in our optics labs and in our radio frequency test chamber. Novel bonding schemes have been developed and are currently under development in our laboratories, as well as methods to control the environment where the atoms sense the radio frequency field.

Lasers interacting with Cs atoms in a photonic crystal vapour cell.
Fig. 3: The photonic crystal vapour cell used as an RF receiver. The atoms inside the cell see an enhanced RF field due to the photonic crystal, resulting in higher sensitivity to the incident RF field.
Graph with vertical position on the Y axis and horizontal position on the X axis, both in millimetres. A coloured scale also depicts the RF field strength, in percent.
Fig. 4: Calculated RF field strength inside one of our cell designs at an RF frequency of 4.01 GHz.

Representative Publications:

  1. “Vapor cell characterization and optimization for applications in Rydberg atom-based radio frequency sensing,” M. Noaman, H. Amarloo, R. Pandiyan, S. Bobbara, S. Mirzaee, K. Nickerson, C. Liu, D. Booth and J. P. Shaffer, Proceedings Volume 12447, Quantum Sensing, Imaging, and Precision Metrology; 124470V (2023).

Funding agencies:

DARPA logo
DRDC logo
NRC logo

Interested in Collaborating or Joining Our Team?

If you are interested in collaborating with us or becoming a technical staff member, including student internships and postdoctoral training, please contact James Shaffer at [email protected].