‘Unique’ Quantum Sensor Technology Shows Promise in Boosting Battlefield EMS Awareness

A groundbreaking lab-based quantum sensor can offer warfighters unprecedented insight into the electromagnetic spectrum (EMS) space on the battlefield; however, the tool has several limitations that researchers and engineers at the U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL) must address before soldiers can use the technology in real-life operations. 

To make the Rydberg quantum sensor practical for the field, scientists must improve the fundamental performance and sensitivity of the framework systems and consistently achieve the standard quantum limit; create resilient and adaptable foundational lasers; and increase the sensor’s mobility, decrease the system’s power requirements and make the necessary advancements to unlock its ability to function outside of the lab without surrendering its potential and ability to give soldiers a warfighting advantage, per David Meyer, lead research scientist in the quantum sciences and technology branch at the ARL. The latter challenge is one the team at the ARL is actively confronting head-on and attempting to resolve. 

ARL officials are collaborating with the private sector to explore new ways of developing the Rydberg quantum sensor, specifically by making it smaller and more agile, reducing its power requirements and making it self-sufficient and not reliant on the lab. Crews could then feasibly transport the Rydberg quantum sensor, which could then operate in the unpredictable and extreme conditions of the real world without the assistance of the lab. Right now, despite the potential the sensor possesses, warfighters in battle cannot take advantage of the sensor’s abilities because it is too large and impractical for soldiers to carry. The task of cutting down its size, which would also include limiting its power needs, is a large component toward opening the door for the capability to leave the lab—a development that needs to happen for personnel to thoroughly put the sensor to the test and show the tool’s abilities to commanders and investors, Meyer said.

“[Getting platforms that can leave the lab can] allow us to sort of do demonstrations that are more true to life, which there are a lot of lessons learned when you do those kinds of demonstrations, and it also makes it more tangible for stakeholders to really understand what it’s capable of,” Meyer explained during an interview with SIGNAL Media. “It’s one thing to write the paper; it’s another thing to actually go out in the field and do it, so that’s a big part of what we’re working on.”

He added that they are diving into the physics surrounding the Rydberg quantum sensor to gain a deeper understanding of the limitations of the platform and the improvements they can realistically achieve and apply. 

The goal of making the Rydberg quantum sensor more mobile and compatible with the outside world goes hand in hand with another hurdle the ARL team has to clear: creating resilient and adaptable foundational lasers, or as Meyer called them, “robust lasers.” Researchers and developers must engineer a way to enhance these lasers so they can withstand heat, humidity, temperature changes, unstable environments, etc. Right now, scientific-grade lasers typically do not operate effectively in uncontrolled climates because they are designed to be in a lab. Meyer assured that crafting robust lasers is possible and has been done before; however, previously, crews have done it only for specific wavelengths, which are not applicable to the Rydberg quantum sensor. Additionally, building robust lasers requires a significant amount of power and precision, so only a few commercial manufacturers can produce them, which also means they are expensive. This detail leads to funding challenges in this space.

Meyer has noted that procuring and managing the funding for this project has been tricky. He said that if they receive enough funding, then performance will improve, but for that to happen, they need to conduct a fair amount of experimentation to get the exact laser they desire. Meyer said there are a plethora of options when it comes to lasers, so they have to endure a lot of trial and error before achieving their goal, a process that will take a substantial amount of money.

“You don’t necessarily want to overinvest in a particular wavelength too early, and realizing actually we invested all this money in this laser, this laser color, but actually we need it in this laser color for this particular application, and therefore, half of our investment is essentially null and void, and we have to start over,” Meyer said. “So that’s a big problem with Rydberg sensors is that our ability to develop performance and capabilities is still somewhat limited by the tools that we have, especially the laser systems.” 

The research scientist at the ARL added that the need for additional funding presents a catch-22-type obstacle for the project. “There is this chicken-and-egg issue where we need to invest money into the engineering of the tooling in order to be able to make the system performance better, but people are hesitant to invest a lot in the tooling without knowing that the performance will work. You need to demonstrate it. I won’t say it’s an insurmountable challenge or anything, but it makes the process slow.”

He added that this whole process is a feedback cycle, and it takes time. 

The third main gap the ARL team needs to fill regarding the Rydberg quantum sensor is improving the fundamental performance and sensitivity of the framework systems and consistently achieving the standard quantum limit. Meyer noted that the demonstrated sensitivity of the systems is still behind purpose-built single-frequency systems, but he also acknowledged that this element does not necessarily have to be a limitation if they could elevate these systems to operate at the standard quantum limit. This level requires a lot of precision and care. Another fundamental principle of quantum is that the system of quantum information tends to be susceptible to error sources, and once these threats contaminate the data, it’s hard to reach the standard quantum limit, according to Meyer. He warned that they are likely still far from achieving the standard quantum limit, but considering how relatively new the field is, Meyer is giving himself and his team grace.

“I think it’s the technology has only been around for like 15 years, so I don’t think it’s totally unreasonable that we haven’t quite gotten there yet, but it is a challenge, and it’s a very difficult challenge,” Meyer said. 

He further explained that for the Rydberg quantum sensor system specifically to achieve the standard quantum limit, it requires a more effective measuring instrument. “For the system that we demonstrated here, the limitation for us hitting the standard quantum limit is actually the spectroscopy mechanism itself. The electromagnetically induced transparency is not a very performant way to measure something at the standard quantum limit. It has inherent inefficiencies in it that you can’t really surmount by the way it’s constructed, and exploring alternative probing schemes in order to get closer to the standard quantum limit is a very high-priority research area that the whole field is working on.”

The quantum sensor is based on Rydberg atoms, hence the “Rydberg quantum sensor” name, and the platform can offer soldiers a never-before-seen view of the EMS. During a demonstration organized by ARL personnel, the Rydberg quantum sensor measured the full 3D direction of radio-frequency electromagnetic fields, per the ARL public affairs team. This proved a theory the researchers published in Physical Review Applied, claiming that the sensor can discover the 3D polarization orientation and propagation direction of an electromagnetic wave, otherwise known as the k-vector, in addition to calculating the electromagnetic field strength. Typical quantum sensors can solely measure the strength of an electromagnetic field in one direction at a time. The Rydberg quantum sensor immensely propels this space forward because for the first time, operators can have a complete 3D picture, including the direction and motion, of the electromagnetic field.

ARL scientists made this theory into reality through the advancements of two major lasers: the 780-nanometer laser and the 480-nanometer laser. The former is common, low power and easy to create, but the latter uses a good chunk of power. The 480-nanometer laser, which emits a bright blue color, did not exist before 2010. This feature, in addition to the integration of the rubidium vapor cell and delving deeper into atomic physics and other topics, enabled engineers to create the Rydberg quantum sensor. 

This capability could pave the way for the Army to acquire tools that could bolster communications, accelerate decision-making and boost situational awareness, especially in the EMS. Meyer implied that furthering these three areas is essential for the Army to outperform their adversaries given the state of the battlefield.

“Spectrum awareness is definitely a concern for the Army,” Meyer said. “The current battlefield is a very contested electromagnetic environment, and being able to get angle of arrival information over a large span EMS quickly is something that’s very important for the soldier.”

He added that, “A lot of our communications, they’re done over wireless radio links of various frequencies, and that includes radars or comms or anything, and a lot of these technologies rely on these free space communications, and that means that you as a soldier, you’re in an environment where lots of people are transmitting lots of things, and oftentimes you want to know where they are, and it can be very difficult to mask those signatures because when you’re transmitting power, you are transmitting power, so if you can detect that and you can detect it quickly, you can know where things are on the battlefield, and that’s obviously very important.”

Meyer and his team see the Rydberg quantum sensor as a tool that offers Army officials a one-of-a-kind role in the modern-day fight, which in and of itself he considered a benefit, but he also admitted that filling this void has a major drawback. 

“We don’t really envision [the capability] to be a drop-in replacement for some existing technology,” Meyer noted. “Right now, we definitely envision it as something that provides a unique capability that’s not capable from other places, which on the one hand is really cool and that’s actually very exciting and very impactful. On the downside though, it does make finding the applications trickier, because there isn’t some lookup table of, you’re going to replace this, and let’s just put it wherever that thing exists, and that does make it a little bit more of a challenge.”