German scientists have demonstrated quantum entanglement between two atoms separated by 33 km (20.5 miles) of fiber optics. This is a record distance for this type of communication and represents a significant step toward a fast and secure quantum internet.

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Researchers successfully quantum entangled two atoms across 30 kilometers of fiber optic cables. Depositphotos

The strange phenomenon of quantum entanglement occurs when two particles become so inextricably linked that examining one can reveal the state of the other. Even stranger, changing something about one particle instantly changes its partner, regardless of how far apart they are. This has the unsettling implication that information is being “teleported” faster than the speed of light, an idea that even Einstein, who famously described it as “spooky action at a distance,” found unsettling.

Despite its apparent impossibility, quantum entanglement has been consistently demonstrated in experiments for decades, with scientists exploiting its peculiar nature to rapidly transmit data over long distances. Ludwig-Maximilians-Universität München (LMU) and Saarland University have now broken a distance record for quantum entanglement between two atoms over fiber optics in the new study.

The team used optical traps in two different buildings on the LMU campus to entangle two rubidium atoms. They were separated by 700 m (2,297 ft) of fiber optics, which was extended to 33 km with additional cable spools. A laser pulse was used to excite each atom, causing it to emit a quantum photon entangled with the atom.

The photons are then sent down the fiber optic cables, where they will eventually collide at a receiving station in the middle. The photons undergo a joint measurement, which entangles them – and because they’re already involved with their own atom, the two atoms become entangled as well.

While photons have previously been entangled over long distances, this study establishes a new distance record for entangling two atoms over fiber optics, which could serve as “quantum memory” nodes. 

The key is that the mediating photons were converted to a longer wavelength so that they could travel further through the fibers – their natural wavelength of 780 nanometers (nm) means they’d normally be lost after a few kilometers, so the team ran them through a device to convert them to a wavelength of 1,517 nm before their journey began. This is close to the 1,550-nm wavelength commonly used in fiber optics for telecoms, which reduces losses.

According to the team, this is an important step toward realizing a practical quantum internet. Such communication networks would be much faster and more secure than those currently in use, and this study demonstrates that they can operate using the existing fiber-optic infrastructure. This could be combined with satellite technology, which has previously demonstrated the ability to beam entangled photons thousands of kilometers.

Source: LMU

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