Howe aligns a laser beam to pass through a tiny glass cell of rubidium atoms

‘Quantum radio’ could improve underwater and underground communications

Image credit: Burrus/NIST

Researchers at the US National Institute of Standards and Technology (NIST) have demonstrated that it could be possible to use ‘quantum radio’ to enable communications and mapping in areas where GPS and ordinary radios and mobile phones do not work, such as in mines, indoors and underwater.

Building ‘quantum radio’ technology requires the researchers to merge the fields of quantum physics with low-frequency magnetic radio communications.

Electromagnetic communications systems such as radio systems use transmitters to produce electromagnetic waves. These are propagated through the air and detected by a receiver, which converts the waves into readable electronic signals.

These conventional signals often struggle to penetrate water, buildings and the ground. In many locations, such as on submarines or in military zones with many interfering radio devices, they cannot be relied on for communications.

The NIST team has been working to use very low-frequency (VLF) magnetic radio signals. These digitally modulated (comprising of strings of 0s and 1s) signals can travel far further through water and other materials than conventional electromagnetic signals, and are already used for basic submarine communications.

However, these VLF magnetic signals have insufficient data-carrying capacity for the transmission of types of media other than basic text, such as video footage or voice calls, and in order to gain signals, submarines are required to tow antenna cables or rise to just below the surface of the water.

“The big issues with [VLF] communications, including magnetic radio, is poor receiver sensitivity and extremely limited bandwidth of existing transmitters and receivers,” said Dave Howe, project leader at NIST. “This means the data rate is zilch.”

“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,” Howe continued. “We need bandwidth to communicate with audio underwater and in other forbidding environments.”

In a step towards achieving communication underwater and in other challenging locations, Howe’s team at NIST demonstrated that it was possible to control the horizontal and vertical positions of a magnetic signal and detect these digital signals with a magnetic field sensor (magnetometer).

The team developed a new direct current magnetometer which uses polarised light to measure the “spin” (a quantum property) of rubidium atoms, which corresponds to oscillations in the magnetic field and creates electronic signals. The NIST sensor was capable of detecting signals weaker than the ambient magnetic fields normally measured with these magnetometers.

The researchers were also able to demonstrate a new technique for signal processing, which reduces magnetic noise from the environment, allowing for fainter signals to be detected, or signals to be detected across a greater range.

“Atoms offer very fast response plus very high sensitivity,” said Howe. “Classical communications involves a tradeoff between bandwidth and sensitivity; we can now get both with quantum sensors.”

The researchers estimated this approach could allow for a range of tens of metres in noisy environments, or hundreds of metres if levels of noise are reduced. Such a technique could prove highly valuable to surveyors, soldiers and sailors.

In the future, Howe and his colleagues hope to develop a custom quantum magnetometer which detects signals by switching between the internal energy levels of atoms, as well as working on improving their transmitters and the ability of their technique to pinpoint location.

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