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Quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits. Quantum sensors are measuring device that takes advantage of quantum correlations, such as states in a quantum superposition or entanglement, for better sensitivity and resolution than can be obtained by classical systems.
When measurements are based on quantum phenomena, such as the energy difference between two well-defined quantum states, sensors have the ability to reach unprecedented precision and accuracy that doesn’t drift over time. High sensitivity and stability in combination with a small form factor provide transformational capabilities with applications spanning from GPS-free global navigation and cryogen-free high-precision magnetometry to sensing within mesoscale structure and measurements of individual nuclear or electron spins. Quantum sensors are just becoming commercially available.
Researchers at the National Institute of Standards and Technology (NIST) have demonstrated that quantum physics might enable communications and mapping in locations where GPS and ordinary cellphones and radios don’t work reliably or even at all, such as indoors, in urban canyons, underwater and underground.
The NIST team is experimenting with low-frequency magnetic radio—very low frequency (VLF) digitally modulated magnetic signals—which can travel farther through building materials, water and soil than conventional electromagnetic communications signals at higher frequencies. The lower a signal’s frequency, the farther it travels, but at the expense of the precision afforded with higher frequencies.
The researchers built a direct-current magnetometer that detects the “spin” of certain atoms using polarized light. Because the atoms are highly sensitive and respond quickly, the resulting quantum sensors would be able to enhance very low-frequency radio with the best of both worlds—precise signals at an ideal bandwidth. A paper detailing the work was published in the Review of Scientific Instruments.
VLF electromagnetic fields are already used underwater in submarine communications. But there’s not enough data-carrying capacity for audio or video, just one-way texts. Submarines also must tow cumbersome antenna cables, slow down and rise to periscope depth (18 meters, or about 60 feet, below the surface) to communicate.
“The big issues with very low-frequency communications, including magnetic radio, is poor receiver sensitivity and extremely limited bandwidth of existing transmitters and receivers. This means the data rate is zilch,” NIST project leader Dave Howe said.
“The best magnetic field sensitivity is obtained using quantum sensors. The increased sensitivity leads in principle to longer communications range. The quantum approach also offers the possibility to get high bandwidth communications like a cellphone has. We need bandwidth to communicate with audio underwater and in other forbidding environments,” he said.
Listening To Quantum Radio
Researchers at Delft University of Technology have created a quantum circuit that enables them to listen to the weakest radio signal allowed by quantum mechanics. This new quantum circuit opens the door to possible future applications in areas such as radio astronomy and medicine (MRI). It also enables researchers to do experiments that can shed light on the interplay between quantum mechanics and gravity.
We have all been annoyed by weak radio signals at some point in our lives: our favourite song in the car turning to noise, being too far away from our wifi router to check our email. Our usual solution is to make the signal bigger, for instance by picking a different radio station or by moving to the other side of the living room. What if, however, we could just listen more carefully?
Weak radio signals are not just a challenge for people trying to find their favourite radio station, but also for magnetic resonance imaging (MRI) scanners at hospitals, as well as for the telescopes scientists use to peer into space.In a quantum ‘leap’ in radio frequency detection, researchers in the group of Prof. Gary Steele in Delft demonstrated the detection of photons or quanta of energy, the weakest signals allowed by the theory of quantum mechanics.
Quantum chunks
One of the strange predictions of quantum mechanics is that energy comes in tiny little chunks called ‘quanta’. What does this mean? “Say I am pushing a kid on a swing”, lead researcher Mario Gely said. “In the classical theory of physics, if I want the kid to go a little bit faster I can give them a small push, giving them more speed and more energy. Quantum mechanics says something different: I can only increase the kid’s energy one ‘quantum step’ at a time. Pushing by half of that amount is not possible.”
For a kid on a swing these ‘quantum steps’ are so tiny that they are too small to notice. Until recently, the same was true for radio waves. However, the research team in Delft developed a circuit that can actually detect these chunks of energy in radio frequency signals, opening up the potential for sensing radio waves at the quantum level.
From quantum radio to quantum gravity?
Beyond applications in quantum sensing, the group in Delft is interested in taking quantum mechanics to the next level: mass. While the theory of quantum electromagnetism was developed nearly 100 years ago, physicists are still puzzled today on how to fit gravity into quantum mechanics.
“Using our quantum radio, we want to try to listen to and control the quantum vibrations of heavy objects, and explore experimentally what happens when you mix quantum mechanics and gravity”, Gely said. “Such experiments are hard, but if successful we would be able to test if we can make a quantum superposition of space-time itself, a new concept that would test our understanding of both quantum mechanics and general relativity.”
