_copy_1316x740.jpg)
Researchers in Germany have demonstrated quantum entanglement of two atoms
separated by 33 km (20.5 miles) of fiber optics. This is a record distance
for this kind of communication and marks a breakthrough towards a fast and
secure quantum internet.
Quantum entanglement is the uncanny phenomenon where two particles can
become so inextricably linked that examining one can tell you about the
state of the other. Stranger still, changing something about one particle
will instantly alter its partner, no matter how far apart they are. That
leads to the unsettling implication that information is being “teleported”
faster than the speed of light, an idea that was too much for even Einstein,
who famously described it as “spooky action at a distance.”
Despite its apparent impossibility, quantum entanglement has been
consistently demonstrated in experiments for decades, with scientists taking
advantage of its bizarre nature to quickly transmit data over long
distances. And in the new study, researchers from
Ludwig-Maximilians-University Munich (LMU) and Saarland University have now
broken a distance record for quantum entanglement between two atoms over
fiber optics.
In their experiments, the team entangled two rubidium atoms kept in optical
traps in two different buildings on the LMU campus. They were separated by
700 m (2,297 ft) of fiber optics, which was extended out to 33 km with extra
spools of cable. Each atom was excited with a laser pulse, which causes it
to emit a photon that’s quantum entangled with the atom.
The photons are then sent down the fiber optic cables to meet at a receiving
station in the middle. There, the photons undergo a joint measurement, which
entangles them – and because they’re each already entangled with their own
atom, the two atoms become entangled with each other as well.
While photons have been entangled over great distances before, this study
marks a new distance record for entangling two atoms, which could function
as “quantum memory” nodes, over fiber optics. The key is that the mediating
photons were converted into a longer wavelength so that they travel further
through the fibers – their natural wavelength of 780 nanometers (nm) means
they’d normally be lost after a few kilometers, so before their journey
started the team ran them through a device to convert them to a wavelength
of 1,517 nm. This is close to the 1,550-nm wavelength commonly used for
telecoms in fiber optics, which reduces losses.
The team says this is an important step on the way to realizing a practical
quantum internet. Such communications networks would be much faster and more
secure than those in use today and, importantly, this study shows that they
can operate using existing fiber optic infrastructure. This could be paired
with technologies like satellites, which have previously demonstrated the
ability to beam entangled photons over thousands of kilometers.
Reference:
van Leent, T., Bock, M., Fertig, F. et al. Entangling single atoms over 33 km
telecom fibre. Nature 607, 69–73 (2022).
DOI: 10.1038/s41586-022-04764-4
Tags:
Physics