Scientists have been grappling with the strangeness of quantum entanglement for decades, and it’s almost as mysterious in 2022 as it was when Einstein famously dubbed the phenomenon “spooky action at a distance” in 1947. An experiment in Germany that set a new entanglement distance record — with atoms rather than photons — could help shed some light on this quirk of the universe.
Entanglement was initially proposed in the early 20th century as a consequence of quantum mechanics, but many scientists of the day, even Einstein himself, considered it to be impossible. However, many of the counterintuitive predictions of quantum mechanics have been verified over the years, including entanglement. As we’ve seen in numerous experiments, it is possible for particles to be “entangled” such that properties like position, momentum, spin, and polarization can be shared between them. A change in one is immediately reflected in its twin.
Scientists believe entanglement could form the basis for future communication systems that are faster and more secure than what we use today — if you measure the state of one entangled partner, you automatically know the state of the other, and this could be used to transmit data. You just need to separate the entangled pair to make it useful, and researchers from Ludwig-Maximilians-University Munich (LMU) and Saarland University have pushed that range much farther in the new experiment.
While it has been shown that photons can remain entangled at distances up to thousands of kilometers, that’s not the case with atoms. The goal of the new research was to entangle a pair of rubidium atoms. The two atoms started at a distance of 700m over an optical cable. The team excited the atoms with a laser pulse, which produced photons entangled with each atom. They traveled down the cable to a receiving station where they underwent a joint measurement, thus entangling them with each other. That caused the original atoms to become entangled with each other.
An experimental setup at Ludwig-Maximilians-University Munich.
The team then extended the distance between the entangled atoms by unspooling more fiber optic cable, eventually reaching 33 kilometers (20.5 miles). The key to this breakthrough was altering the wavelength of the photons, which have a natural frequency of 780 nm. This wavelength tends to dissipate after several kilometers passing through glass fibers, but shifting the frequency to 1517 nm achieved much better reliability. This is close to the telecom standard of 1550 nm, which ISPs use to reduce signal loss.
According to the study in the journal Nature, this is an important step toward making quantum communication practical. Rather than building new infrastructure, it may be possible to use existing fiber optic networks with entanglement-based systems.
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