At the smallest scales, our universe becomes bizarre. The particles act like billiard balls or waves on the water, depending on how you probe them. Properties cannot be measured simultaneously or tend to spread uncertainly over a range of values. Human intuition fails us.
For much of the last century, all of this weirdness was primarily the domain of physicists. But more recently, the theoretical and the experimental have come closer to the practical. This trend is most visible in the growing menagerie of early quantum computers, but bizarre quantum behavior is useful for more than computation. Some scientists and engineers are building quantum communication networks that are impossible to hack; others have their eyes glued to the sensors.
In a recent pre-print article published on the arXiv, a team from the French National Center for Scientific Research describes a quantum accelerometer that uses lasers and ultra-cold rubidium atoms to measure motion in three dimensions with extreme precision.
The work extends quantum accelerometers into the third dimension and could bring precise GPS-free navigation and reliable detection of valuable mineral deposits underfoot.
atomic waves
We already rely on accelerometers daily. Pick up a phone and the screen lights up. Turn it sideways and the page you’re reading changes orientation. A tiny mechanical accelerometer – essentially a mass attached to a spring-like mechanism – makes these actions possible (alongside other sensors, like gyroscopes). Every time a phone moves through space, its accelerometer tracks that movement. This includes short periods of time when GPS fails, such as in tunnels or cellular signal dead spots.
As useful as they are, mechanical accelerometers tend to drift. Left long enough, they will accumulate errors on a mile scale. This isn’t critical for phones that are briefly out of GPS contact, but it is an issue when devices travel out of range for long periods of time. And for industrial and military applications, accurate position tracking would be useful on submarines – which cannot access GPS underwater – or as backup navigation on ships in the event of GPS loss.
Researchers have long been developing quantum accelerometers to improve the accuracy of position tracking. Instead of measuring a mass compressing a spring, quantum accelerometers measure the wave properties of matter. The devices use lasers to slow down and cool the clouds of atoms. In this state, the atoms behave like waves of light, creating interference patterns as they move. More lasers induce and measure how these patterns change to track the device’s location in space.
At first, these devices, called atomic interferometers, were a jumble of wires and instruments strewn across lab benches and could only measure one dimension. But as lasers and expertise have advanced, they’ve gotten smaller and tougher, and now they’ve gone 3D.
A quantum upgrade
The new 3D quantum accelerometer, developed by the team in France, looks like a metal box the length of a laptop computer. It uses lasers along all three spatial axes to manipulate and measure a cloud of rubidium atoms trapped in a small glass box and cooled to near absolute zero. Like earlier quantum accelerometers, these lasers induce ripples in the cloud of atoms and interpret the resulting interference patterns to measure motion.
To improve the stability and bandwidth required for use outside the lab, the new device combines readings from classical and quantum accelerometers in a feedback loop that exploits the strengths of both technologies.
Because the team can control atoms with extreme precision, they can make equally precise measurements. To test the accelerometer, they attached it to a table designed to shake and spin and found that the system was 50 times more accurate than conventional navigation sensors. Over a period of hours, the position of the device measured by a conventional accelerometer was off by one kilometer; the quantum accelerometer nailed it to less than 20 meters.
Reduce radius
The accelerometer, which is still relatively large and heavy, won’t be ready for your iPhone anytime soon. But made a bit smaller and sturdier, the team says it could be fitted to ships or submarines for precise navigation. Or it could end up in the hands of field geologists searching for mineral deposits by measuring subtle changes in gravity.
Other groups are also working to miniaturize and harden quantum sensors for the field. A team at Sandia National Laboratory recently built a cold atom interferometer, like the one used here, in a sturdy case the size of a shoebox. In a paper describing the work, Sandia researchers say further miniaturization will likely be driven by advances in photonic chips. In the future, they say, the optical components needed for a cold atom interferometer like theirs could fit on a chip just eight millimeters across.
Other quantum sensors, like gyroscopes, could join the fun. Although they also need a few cycles of shrinking and hardening before escaping from the lab.
For now, moving to 3D is a step forward.
“Three-dimensional measurement is a big deal, a necessary and excellent technical step towards any practical use of quantum accelerometers,” said John Close of the Australian National University recently. new scientist.
Image Credit: Interference patterns appear in a cloud of cold rubidium atoms trapped in a quantum gyroscope / National Institute of Standards and Technology (NIST)
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