Scientists Tracked Earth's Rotation With Frankly Unbelievable Precision
We know Earth’s rotation on a cosmic scale, but seeing it close up requires quantum mechanics.
Interferometry is the use of light waves, sound, etc., to identify changes in matter or motion.
New research uses entangled photons and fiber optics to measure Earth’s nanorotations.
In a paper recently published in the journal Science Advances, scientists from the Vienna Center for Quantum Science and Technology described how they used a “path-entangled quantum interferometer” to measure how fast the Earth rotates. It’s a step forward in securing a measurement that’s been surprisingly hard to pin down, and shows the potential of quantum-enabled technologies to help us better understand the world around us.
Interferometry is a technique involving displaced and refracted light—the patterns created reveal what ‘interfered’ in the first place. It was originally conceived of to help prove or disprove the idea that all the air around us was filled with a wild proposed material called luminiferous ether, but now it’s used in a lot of different scientific disciplines. Most interferometers use mirrors, curved lenses, and more to bend and recapture light waves, but some may be acoustic, using a crystal to resonate through a cloud of liquid or gas.
These setups predate the study of quantum mechanics, but the researchers from the Vienna Center explain in their paper how quantum interferometry improves on previous forms. “[T]he enhanced sensitivity of quantum interferometers,” they wrote, “opens up opportunities for precision measurements that can explore new frontiers in physics.”
Focusing on quantum phenomena like superposition instead of qualities based in the world of classical physics means we need even tinier readings than usual. That’s where quantum interferometry comes in, and it’s definitely more precise, but it’s also a work in progress. Over time, scientists are working to develop better and better ways to reduce “noise” (a quantum side effect) and other downsides to these methods.
In this research, scientists use an established paradigm called N00N—short for a complex mathematical expression—where a cloud of n photons are all in the same state of superposition, and no non-superpositioned photons are allowed. A number of N00N states with two photons each are put into a prepared interferometer that’s dozens of meters long on one side, using long fiber optic strands. Then, Earth’s rotation itself causes a measurable change within the photons.
It may seem silly to spend so much time and so many resources on clocking the speed of Earth’s rotation—something we surely must know all about by now. A day is a certain length, and we know because we end up in the same place in about 24 hours... right?
Mostly. But those facts reinforce what the problem is underneath. Earth is so enormous and steady that detecting tiny changes in its rotational state is actually very difficult.
To help, these researchers implemented helpful tools like a toggle that switches off how Earth’s rotation affected their interferometric setup, and they built their model to involve mapping the idea of rotation into a measurement that does not rotate like a calculus-based magic trick.
The results demonstrate not only the precision of their best-yet quantum interferometer, the researchers conclude, but also a “milestone” in our collective quest to marry general relativity and quantum mechanics into one unified theory. Indeed, being able to check the general, classical motion of the Earth against quantum phenomena could help move physics forward—24 hours at a time.
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