Quantum radio may outperform other types of communications within harsh environments
Quantum physics makes nearly everything better, and communications is no exception. Researchers at the National Institute of Standards and Technology (NIST – Gaithersburg, Maryland) are experimenting with a low-frequency magnetic radio, using very low-frequency (VLF) digitally modulated magnetic signals. Its signals are able to travel through building materials, in water, and underground farther than conventional electromagnetic communications signals that operate at higher frequencies.
This technology may be an intriguing, potentially more secure, alternative for military use, because GPS signals can’t be sent very deeply or at all into water, underground, or through the walls of buildings.
What’s so special about VLF electromagnetic fields? They’re used for submarine communications, which is impressive, but only have enough data-carrying capacity for one-way texts, not audio or video. And to use this technology, submarines need to transport antenna cables, slow down, and rise to periscope depth (18 meters below the surface) to communicate.
The biggest hurdles for VLF communications, including magnetic radio, are “poor receiver sensitivity and the extremely limited bandwidth of existing transmitters and receivers,” says Dave Howe, NIST project leader. “Data rate is zilch. The best magnetic-field sensitivity is obtained using quantum sensors.”
Why quantum sensors? Because they offer “increased sensitivity, which leads in principle to a longer communications range,” Howe asserts. “The quantum approach also offers the possibility to get high-bandwidth communications like cellphones. We need bandwidth to communicate with audio underwater and within other forbidding environments.”
To this end, Howe and colleagues at NIST recently demonstrated the detection of digitally modified magnetic signals – messages consisting of digital bits – by a magnetic-field sensor that relies on the quantum properties of rubidium atoms. This 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.
“Classical communications involve a trade-off between bandwidth and sensitivity. We can get both with quantum sensors,” Howe says.
These types of atomic magnetometers traditionally were used for measuring magnetic fields that occur naturally, but the NIST researchers are using them to receive coded communications signals. They want to take it a step further to develop better transmitters because the quantum method is “more sensitive than conventional magnetic sensor technology and could be used to communicate,” Howe says.
The researchers developed a signal processing technique to reduce environmental magnetic noise – such as generated by electrical power grids – that limits the communications range. Now, receivers can detect weaker signals and the signal range can be increased, Howe adds.
For this work, the researchers developed a direct-current (DC) magnetometer that uses polarized light as a detector to measure the spin of rubidium atoms produced by magnetic fields. Atoms are housed in a tiny glass jar, and changes in their spin rate correspond to an oscillation in the DC magnetic fields, creating alternating current (AC) electronic signals, or voltages at the light detector, which are more useful for communications.
Beyond sensitivity, these so-called optically pumped magnetometers offer advantages such as room-temperature operation, small size, low power and cost, and reduced interference. These types of sensors also won’t drift or require calibration.
During testing, their sensor detected signals significantly weaker than typical ambient magnetic-field noise. It picked up digitally modulated magnetic-field signals with strengths of 1 picotesla (one millionth of Earth’s magnetic-field strength) and at very low frequencies (below 1 kilohertz). This is below the frequencies of VLF radio, which spans from 3 to 30 kHz. But modulation techniques can suppress the ambient noise and its harmonics, or multiples of these, which effectively increases the channel capacity.
To estimate its communication and location-ranging limits, they performed calculations and found that the spatial range corresponding to a “good” signal-to-noise ratio was tens of meters in the indoor noise environment. This is “better than what’s now possible indoors” and can be extended to hundreds of meters if the noise were reduced to the sensitivity levels of the sensor, Howe says.
Pinpointing location was trickier: The uncertainty in location capability was 16 meters, which is well above the target of 3 meters, but it can be improved through future noise suppression techniques, increased sensor bandwidth, and improved digital algorithms that can accurately extract distance measurements.
The NIST researchers are now building a custom quantum magnetometer, which requires inventing an entirely new field that combines quantum physics and low-frequency radio, says Howe.