Physicists Develop a New Type of Antenna
A team of University of Otago researchers and physicists have demonstrated a new form of antenna, developed with a small glass bulb containing an atomic vapor. The bulb was wired with laser beams and could therefore be placed far from any receiver electronics.
Dr. Susi Otto, from the Dodd-Walls Centre for Photonic and Quantum Technologies, led the field testing of the portable atomic radio frequency sensor. Such sensors, that are enabled by atoms in a so-called Rydberg state, can provide superior performance over current antenna technologies as they are highly sensitive, have broad tunability, and small physical size, making them attractive for use in defense and communications.
For example, they could simplify communications for soldiers on the battlefield as they cover the full spectrum of radio frequencies, rather than needing multiple antennas to cover different frequency bands, and are super sensitive and accurate to detect a wide range of critical signals. The ability to eliminate the need for multiple sensors also makes them useful in satellite technology.
Importantly, compared to more traditional sensors, Rydberg sensors can function without any metal parts, which can scatter the radio frequency field of interest and the atomic sensor is accessed via laser light, replacing the need for electric cables. The new design developed by the team of Otago physicists is portable and can be taken outside the laboratory. In a first out-of-lab demonstration, the sensor was able to efficiently measure fields in a distance of 30 meters using a freespace laser link, adding important flexibility to Rydberg-atom based sensing technologies.
The team detailed the development of the new sensor and its demonstration in a paper that was recently published in Applied Physics Letters.
Moving Beyond the Lab and Physical Constraints
In 2012, a seminal work by Sedlacek et al. demonstrated the use of Rydberg-excited 87Rb atoms in a glass vapor cell as a sensitive detector for microwave fields. In their scheme, the presence and strength of microwave radiation resonant with the transition between two Rydberg states were measured by sending counter-propagating blue and infrared laser beams through the vapor cell. In the absence of microwave radiation, the blue coupling light would establish electromagnetically induced transparency (EIT) for the infrared probe light, while an incoming microwave field of constant amplitude would split the EIT transmission peak into two, and from the peak separation of these two Autler–Townes (AT) peaks, the microwave field strength could be inferred. Because atomic transitions lie at the heart of this ingeniously simple experimental scheme, it offers SI traceable and calibration free electrometry. 2
The decade following the original work of Sedlacek1 et al. witnessed a number of developments. For example, the sensitivity of the on-resonant EIT/AT technique can be greatly improved using a superheterodyne receiver architecture and an optical ground-state repumping technique, making it possible to detect fields down to 5 μV m–1. In the other extreme, at large RF field strengths, the AT splitting is no longer linear with the strength of the applied field due to the AC Stark shift, which scales as the field strength squared. Instead, this shift can be measured to infer the field strengths of sufficiently large fields.
Measurements of the AC Stark shift also allow detection of frequencies outside the discrete set of atomic Rydberg transitions extending the technique to a continuous RF spectrum. Rydberg-atomic systems for communication constitute a particular application that leverages the technique of Sedlacek et al., and fundamental working principles of analog and digital communication have been demonstrated in a range of atom-based systems.
Although the systems mentioned above allow for a large variety of RF measurements and applications, they are almost invariably confined to an optical table in a laboratory due to the requirement to counter-align the two (or more) laser beams within the atomic vapor cell. The Otago team created a proof-of-concept setup capable of increased mobility, replacing the fiber access to the vapor cell with two freespace laser beams and a corner-cube prism reflector, which reflects the probe beam back to a photodetector. Without any significant effort, the portable atomic RF probe can be deployed to sense fields at a distance exceeding 30 m from the active components — the lasers and photodetector.
Figure 1 shows a schematic of the experimental setup, highlighting the base station and the separate portable transducer, in the following referred to as “sensing unit.” Also shown are the relevant atomic levels of 87Rb involved in the RF-to-optical transduction. The base station contains all the required elements to prepare two light fields for a two-photon Rydberg EIT system, i.e., a coupling and probe laser, their driver electronics, and a setup for the frequency stabilization of the two lasers. Specifically, the team employs a coupling laser with a wavelength of 480 nm and a maximum power of 10 mW. After passing through a beam expander, the coupling beam has a 1/ e2 beam diameter of ~ 6 mm corresponding to a Rayleigh length of about 60 m. For parts of the demonstration, the coupling laser is stabilized to a high-finesse cavity using the Pound–Drever–Hall technique, which results in a root-mean square (RMS) linewidth below 100 kHz.
The stand-alone sensing unit is optically linked to the base station via the two free space laser beams and only incorporates two elements: a 150 mm-long and 27 mm-wide cylindrical vapor cell containing a rubidium vapor at room temperature and a corner-cube prism reflector (Thorlabs PS975-A). The prism reflector allows us to accommodate two pairs of counter-propagating probe and coupling beams, passing through the vapor cell with minimal alignment effort.
When coupled to a Rydberg state via the optical two-photon transition, the rubidium atoms become sensitive to RF radiation. In particular, this happens for RF frequencies that couple the Rydberg level resonantly to other nearby Rydberg levels. For sufficiently large fields, the atoms are receptive to a broad range of RF signals via the AC Stark shift. In the demonstration, a home-built helical antenna broadcasts an amplitude modulated (AM) microwave signal with carrier frequencies between 16 and 20 GHz. The antenna radiates circularly polarized microwave fields along the axis of the helix. The AM signal is imprinted onto the probe light field via the atoms and is retrieved from the light field at the base station using a photo detector and a spectrum analyzer.
This research and demonstration presents a simple technique for a remote Rydberg-atom-based RF sensor, which was verified for distances of up to 30 m between the passive sensing unit and the base station — currently limited by the length of the hallway used in the demonstration. The researchers remark that their work shares challenges with remotely interrogated atomic magnetometers, which operate in a similar fashion to a Rydberg microwave detector. A free-space optical setup, as discussed in this work, is applicable to these systems, though a polarization-preserving retroreflector might be needed.
The researchers envision these developments eventually making quantum sensors more robust and cost-effective, enabling them to move out of labs and into the real world.
- J. Sedlacek, A. Schwettmann, H. Kübler, R. Löw, T. Pfau, and J. Shaffer, Nat. Phys. 8, 819 (2012). https://doi.org/10.1038/nphys2423
- C. L. Holloway, M. T. Simons, J. A. Gordon, A. Dienstfrey, D. A. Anderson, and G. Raithel, J. Appl. Phys. 121, 233106 (2017). https://doi.org/10.1063/1.4984201
Material for this article was provided by the University of Otago and part of the paper published by the team of physicists, in Applied Physics Letters. For more information, visit here .