Researchers at the National Institute of Standards and Technology (NIST) have succeeded in demonstrating that quantum physics could enable communications and mapping in locations where GPS, ordinary radios and cellphones do not work consistently or even at all - e.g. indoors, in urban canyons, underground and underwater.
NIST physicist Dave Howe aligns a laser beam to pass through a tiny glass cell of rubidium atoms inside the cylindrical magnetic shield. Photo credit: Burrus/NIST
The technology could be helpful to soldiers, mariners and surveyors, among others. GPS signals do not penetrate very deeply or at all in water, building walls or soil. Therefore they cannot be used by submarines or for activities such as surveying mines. GPS may not also be able to function well indoors or even outdoors in the midst of city skyscrapers. For soldiers, there are chances for radio signals to be blocked in environments cluttered by rubble or several other interfering electromagnetic devices during disaster recovery or military missions.
The NIST team is presently experimenting with low-frequency magnetic radio — extremely low frequency (VLF) digitally modulated magnetic signals — capable of traveling farther via building materials, soil and water than standard electromagnetic communications signals at greater frequencies.
VLF electromagnetic fields are already being used underwater in submarine communications. However, there is not enough data-carrying capacity for video or audio, just one-way texts. It is also necessary for submarines to tow cumbersome antenna cables, slow down and then rise to periscope depth (18 m, or about 60′, below the surface) in order 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... 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.
Dave Howe, NIST Project Leader
The NIST researchers established detection of digitally modulated magnetic signals, messages containing digital bits 0 and 1, by a magnetic-field sensor that depends on the quantum properties of rubidium atoms. The NIST method varies magnetic fields in order to control or modulate the frequency — specially, the vertical and horizontal positions of the signal’s waveform — generated by the atoms.
“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.”
Conventionally, such atomic magnetometers are employed for measuring naturally occurring magnetic fields, but in this NIST project, they have been used for receiving coded communications signals. Going forward, the NIST team plans to produce enhanced transmitters. The results obtained by these researchers have been published in the Review of Scientific Instruments.
The quantum method is considered to be more sensitive than standard magnetic sensor technology and could be employed for communicating, Howe stated. The researchers also established a signal processing technique that helps decrease environmental magnetic noise, such as that from the electrical power grid, which otherwise restricts the communications range. This explains the fact that receivers are capable of detecting weaker signals or it is possible to increase the signal range, Howe said.
A direct-current (DC) magnetometer was developed by NIST for these studies. In this magnetometer, polarized light is employed as a detector for measuring the “spin” of rubidium atoms induced by magnetic fields. The atoms are in a small glass container. Variations in the atoms’ spin rate match with an oscillation in the DC magnetic fields, producing alternating current (AC) electronic signals, or voltages at the light detector, which are considered to be more useful for communications.
Such “optically pumped” magnetometers, besides high sensitivity, offer benefits such as small size, room-temperature operation, reduced interference, and low power and cost. A sensor of this kind would not require calibration or drift.
In the NIST tests, the sensor detected signals majorly weaker than usual ambient magnetic-field noise. The sensor was able to detect digitally modulated magnetic field signals with strengths of 1 picotesla (one millionth of the Earth’s magnetic field strength) and at extremely low frequencies, below 1 kilohertz (kHz). This is considered to be below the frequencies of VLF radio, which spans 3-30 kHz and is employed for some military and government services. The modulation techniques blocked the ambient noise and along with its harmonics, or multiples, efficiently increasing the channel capacity.
The researchers also carried out calculations for estimating communication and location-ranging limits. The spatial range equivalent to a good signal-to-noise ratio was observed to be tens of meters in the indoor noise environment of the NIST tests, but could also be extended to hundreds of meters if the noise were decreased to the sensitivity levels of the sensor.
“That’s better than what’s possible now indoors,” Howe said.
Pinpointing location is considered to be more challenging. The measured uncertainty in location capability was 16 m, much greater than the target of 3 m, but this metric can be enhanced via future noise suppression methods, increased sensor bandwidth, and enhanced digital algorithms that can precisely extract distance measurements.
To enhance performance further, the NIST team is presently building and also testing a custom quantum magnetometer. Like an atomic clock, the device will be able to identify signals by switching between internal energy levels of atoms and also various other properties, Howe said. The researchers expect to extend the range of low-frequency magnetic field signals by suppressing noise more effectively, enhancing the sensor sensitivity, and increasing and efficiently employing the sensor's bandwidth.
The NIST strategy necessitates inventing a wholly new field, which incorporates quantum physics and low-frequency magnetic radio, Howe stated. The team plans to boost the sensitivity by producing low-noise oscillators in order to enhance the timing between receiver and transmitter and examining how to employ quantum physics for surpassing the available bandwidth limits.