Photo:NIST physicist Dave Howe aligns a laser beam to pass through a tiny glass cell of rubidium atoms inside the cylindrical magnetic shield. The atoms are the heart of an atomic magnetometer demonstrated as a receiver for magnetic radio. These very low frequency (VLF) digitally modulated magnetic signals can travel farther through building materials, water, and soil than conventional communications signals at higher frequencies and, with further advances in receivers and transmitters, could improve communications and mapping indoors at long range, in urban canyons, underwater and underground.
Researchers at the National Institute of Standards and Technology (NIST) have demonstrated that quantum physics might enable communication and mapping in locations where GPS, cell phones, and radio is not reliable or don’t work at all, such as indoors, in urban canyons, underwater, and underground. NIST announced the technology advance on January 2. The technology may have marine, military, and surveying applications. The NIST team is experimenting with very low frequency (VLF) digitally modulated magnetic signals, which propagate farther through buildings, water, and soil than conventional electromagnetic signals at higher frequencies.
“The big issues with very low-frequency communications, including magnetic radio, are poor receiver sensitivity and extremely limited bandwidth of existing transmitters and receivers. This means the data rate is zilch,” said NIST project leader Dave Howe, AD0MR.
“The best magnetic field sensitivity is obtained using quantum sensors. The increased sensitivity leads in principle to better 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.
NIST researchers have demonstrated detection of digitally modulated magnetic signals by a magnetic-field sensor that relies on the quantum properties of rubidium atoms. The NIST technique varies magnetic fields to modulate or control the frequency — specifically, the horizontal and vertical positions of the signal’s waveform — produced by the atoms.
NIST developed a direct current magnetometer that uses polarized light as a detector to measure the “spin” of rubidium atoms in a tiny glass cell induced by magnetic fields. Changes in the atoms’ spin rate correspond to an oscillation in the dc magnetic fields, creating alternating current voltages at the light detector that are more useful for communications.
“Atoms offer very fast response plus very high sensitivity,” Howe said. “Classical communications involves a tradeoff between bandwidth and sensitivity. We can now get both with quantum sensors.” Howe speculated on an Amateur Radio application.
“The quantum radio is great fun, far better sensitivity than any other receiver, at room temperature, anyway,” Howe told ARRL. “The atoms in the gas cell replace the ‘antenna’ and detection in the classical sense. It would be nice to try modulation in the 2200-meter band using the quantum receiver for detection.” In the future, the NIST team plans to develop improved transmitters.
In the NIST tests, the sensor detected digitally modulated magnetic field signals with strengths of 1 picotesla — one millionth of Earth’s magnetic field strength — and at frequencies below 1 kHz.
To further improve performance, the NIST team is building and testing a custom quantum magnetometer. Like an atomic clock, the device will detect signals by switching between atoms’ internal energy levels as well as other properties, Howe said. The researchers hope to extend the range of low-frequency magnetic field signals by boosting the sensor sensitivity, suppressing noise more effectively, and increasing and efficiently using the sensor’s bandwidth.
The NIST strategy requires inventing an entirely new field, which combines quantum physics and low-frequency magnetic radio, Howe said.
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