We take easy communication for granted in our globally-connected world, but there are plenty of places where the environment hinders the straightforward use of radio signals: under water, for example, or underground, or in areas of high electromagnetic interference. This also poses challenges for mapping the oceans or inside mines, where GPS cannot penetrate.
The issue is that the higher the radio frequency, the less good the signal is at penetrating matter. This is why your phone cuts out while you’re driving in tunnels, while the lower frequency FM radio reception gets patchy but the even lower frequency AM radio continues to be reasonably audible.
A solution, however, may be at hand. Researchers at the National Institutes of Standards and Technology (NIST) in Boulder, Colorado, US, have managed a proof-of-concept for “quantum radio”, manipulating the magnetic field of rubidium atoms to send digital signals. The work is described in the journal Review of Scientific Instruments.
“The big issues with very low-frequency communications, including magnetic radio, is poor receiver sensitivity and extremely limited bandwidth of existing transmitters and receivers. This means the data rate is zilch,” NIST project leader Dave Howe explains.
“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 cell-phone has.”
Modulating the spin of the atoms vertically and horizontally via a magnetic field sensor allowed the researchers to create “zero” and “one” positions which could be transmitted to, and read by, a direct-current magnetometer.
The atoms themselves are in a tiny glass container, and the changes in their spin creates alternating current electric signals. The voltages are then read by a light detector. Since these are (mostly) well-understood technologies, such detectors should be able to be made at a low cost, at small size, and work in a normal temperature range, making them a practical system for field use.
The researchers also developed a signal processing technique to reduce environmental noise – for example, from the electrical grid – and thereby increase the range of communications. The plan from here is to develop ways in which the signal can be boosted and ways to improve bandwidth further.
There are also some challenges in pinpointing the location of a receiver, which they are hopeful will be addressed by better digital algorithms and noise suppression.
“Atoms offer very fast response plus very high sensitivity [while] classical communications involves a trade-off between bandwidth and sensitivity,” says Howe. “We can now get both with quantum sensors.”