Big Atoms Make Small, Super-Sensitive Quantum Receivers

UNITED STATES ARMY

Classical physics describes how big things behave and interact in our physical world. Throw a ball into the air, and you know it will go up and come down in a predictable path every time. In the quantum realm, if you could throw something up, it might come down, or it might not, or it might come down in another city. That is because quantum mechanics governs the ultra-small world of atoms, electrons, and photons. It is an invisible world where particles mysteriously disappear then reappear, where particles exert an influence on other distant and unconnected particles,  and where particles can assume many quantum values at the same time. To add to the strangeness, some scientists believe that quantum particles can time travel. In contrast, others believe particles behave strangely because they move in and out of alternate universes.

Despite quantum weirdness, scientists have used these bizarre properties to create powerful computers and ultra-sensitive quantum devices for communications, measurements, and sensing.

Quantum on the battlefield

Quantum sensing is an area of interest to our military because it is useful on and off the battlefield. For that reason, Army scientists are doing leading-edge research, and exploring the use of quantum sensing for such applications as submarine detection, underwater communications, geolocation, navigation, and communications.  

Left, Bohr model of the hydrogen atom with a proton orbited by an electron. Right, Bohr model of an atom with inner electrons and valence electron excited into a Rydberg state. 
 JACOB T. PAUL, DEPARTMENT OF PHYSICS, URSINUS COLLEGE

The Army is building experimental quantum radios using Rydberg atoms created from Rubidium, an alkali metal. Alkali metal atoms are used because they have a single valence electron in the outer shell. The valence electron is weakly bound to the atom because it is the only electron in its energy level and is shielded from the nucleus by the inner core electrons. To create a Rydberg atom, lasers excite the outer electron to move it up many energy levels. Because of the extreme location of its outer electron, a Rydberg atom is typically very large.

Army Research Laboratory

U.S. Army Research Laboratory
 U.S. ARMY

I recently spoke with David Meyer, Kevin Cox, and Paul Kunz, a team of research scientists at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. The team is perfecting a quantum sensor that they  previously announced as the world’s first quantum radio receiver using Rydberg atoms. However, you won’t be listening to a personal quantum radio any time soon. According to Kunz, it might take years for their research to find its way into commercial products.

Cox further clarified the lab’s mission, saying, “Our main research focus is finding ways in general, and in theory, that quantum sensors can be useful for Department of Defense applications.  We’re interested in how quantum sensors can be useful for military communications, electronic warfare, or other  military sensing applications.”  

Although the researchers have a long-term goal of perfecting quantum radio receivers for military purposes, Kuntz noted that the sensors also have the potential to be used as  a calibration standard.  

Although Rydberg sensors are quantum in nature, it will be many years before sensors incorporate one of the most useful quantum properties called entanglement, a state when particles act similarly, and they are dependent on each other. With entanglement, the received signal will be greatly enhanced relative to the noise and lead to even more advanced receiving capabilities.

 How quantum sensors catch radio waves

Electric and magnetic fields in a radio signal. 
WEATHER.GOV

Radio communications are  essential for many applications, such as battlefield communications, disaster management, and even international communications with our embassies.

A transmitted radio signal consists of a magnetic field and an electric field. Information, such as voice or music or data,  creates changes in those fields. Rydberg atoms are ultra-sensitive to variations in the transmitted electrical fields which allows them to detect weak signals over an extensive range of frequencies.

Radio spectrum from 0 to 100 GHz 
TRANSPORTATION.GOV

According to the Army researchers, a quantum radio will be portable enough for soldiers to carry it undetected. It will also be able to receive communication signals over the entire radio spectrum, from zero to 100 GHz, said the researchers. A big plus is that the device can also act as its own antenna.  Such broad frequency coverage by a single antenna isn’t possible with traditional receivers. To receive that much of the RF spectrum would require multiple receivers using multiple antennas, amplifiers, and other components.

“I think it is interesting that the fundamental physical limits of quantum sensors are different from the fundamental limitations of classical receiver systems. That has led to new conversations between physicists and antenna engineers and cross-pollination of ideas. Getting that kind of integration with a new community and two different ideas coming together is one of the really neat aspects of this research.”  – Kevin Cox, Army quantum research scientist

Why quantum receivers are needed

For a modern receiver to receive weak and distant RF signals it must use large and directional high-gain antennas tuned to the transmitted frequency.  Different segments of the radio spectrum require multiple antennas designed to match the wavelength of the transmitted signal. Companies that depend on  RF signals to communicate go to great lengths to improve reception and capture very weak signals. 

The author stands in front of large magnetic loop antennas tuned for receiving signals in the 3 to 30 MHz portion of the electromagnetic spectrum. To receive signals from every direction requires a fully equipped array containing 32 of these large loops occupying over an acre of ground. 
 HYSKY TECHNOLOGIES

Charlie Maynard, CEO of HySky, is an expert on the technology needed for weak signal reception. He said, “Our system filters weak and otherwise unusable signals from the noise and makes them usable. Our  specialized hardware, software, and antennas allows us to transmit signals equivalent to the sub-watt power of a cheap walkie-talkie, then retrieve signals mixed with noise up to two thousand miles away.”

We are not alone

There is no doubt that a functioning quantum radio would provide our military with a significant advantage. However, our scientists are not the only ones doing research on Rydberg atoms. According to Cox, over the past few years, there has been growing interest in studying Rydberg atoms for electric field sensing. The other research has primarily been in the United States, Europe, and China.

In addition to its military usefulness, a quantum radio would significantly reduce the complexity and size of large receiving systems. Maynard said he’s been following the Army’s research on quantum receivers. He said if Rydberg receiving technology ever became a commercial product, it would definitely be a “game-changer.”