GPS signals do not penetrate very deeply or at all in water, soil, or building walls, and therefore can’t be used by submarines or in underground activities such as surveying mines. GPS also may not work well indoors or even outdoors among city skyscrapers. For soldiers, radio signals may be blocked in environments cluttered by rubble or many interfering electromagnetic devices during military or disaster recovery missions.
Low-frequency magnetic radio — very low frequency (VLF) digitally modulated magnetic signals — can travel farther through building materials, water, and soil than conventional electromagnetic communications signals at higher frequencies. VLF electromagnetic fields are already used underwater in submarine communications. But there’s not enough data-carrying capacity for audio or video — just oneway 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 best magnetic field sensitivity is obtained using quantum sensors.
Researchers demonstrated detection of digitally modulated magnetic signals — messages consisting of digital bits 0 and 1 — by a magnetic-field sensor that relies on the quantum properties of rubidium atoms. The 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. Classic communications involve a tradeoff between bandwidth and sensitivity, both of which are obtained with quantum sensors.
The quantum method is more sensitive than conventional magnetic sensor technology and could be used to communicate. A signal processing technique was demonstrated to reduce environmental magnetic noise, such as from the electrical power grid, that otherwise limits the communications range. This means receivers can detect weaker signals or the signal range can be increased.
For this work, a direct-current (DC) magnetometer was developed in which polarized light is used as a detector to measure the “spin” of rubidium atoms induced by magnetic fields. The atoms are in a tiny glass container. Changes in the atoms’ spin rate correspond to an oscillation in the DC magnetic fields, creating alternating current (AC) electronic signals, or voltages, at the light detector that are more useful for communications. Such “optically pumped” magnetometers, in addition to high sensitivity, offer advantages such as room temperature operation, small size, low power and cost, and reduced interference. A sensor of this type would not drift or require calibration.
In tests, the sensor detected signals significantly weaker than typical ambient magnetic-field noise. The sensor detected digitally modulated magnetic-field signals with strengths of 1 picotesla (one millionth of the Earth’s magnetic field strength), and at very low frequencies — below 1 kilohertz (kHz). (This is below the frequencies of VLF radio, which spans 3-30 kHz, and is used for some government and military services.) The modulation techniques suppressed the ambient noise and its harmonics, or multiples, effectively increasing the channel capacity.
Calculations were performed to estimate communication and location-ranging limits. The spatial range corresponding to a good signal-to-noise ratio was tens of meters in the indoor noise environment of the tests, but could be extended to hundreds of meters if the noise were reduced to the sensitivity levels of the sensor.
Pinpointing location is more challenging. The measured uncertainty in location capability was 16 meters, much higher than the target of 3 meters, but this metric can be improved through future noise suppression techniques, increased sensor bandwidth, and improved digital algorithms that can accurately extract distance measurements.