Open-air quantum teleportation performed across a 97km lake

Open-air quantum teleportation performed across a 97km lake






Photo illustration of the beacon laser,
used to track an entangled photon signal across Qinghai Lake. The statue
is Padmasambhava at the Lotus Temple at Gangcha.



Chang Liu/Jian-Wei Pan


Sending signals through fiber optic cable is reliable and fast, but
because of internal absorption and other effects, they will lose
photons—which is a problem when the number of photons being sent is
small. This is of particular concern in quantum networks, which
typically involve a small number of entangled photons. Direct
transmission through free space (vacuum or air) experiences less photon
loss, but it's very difficult to align a distant receiver perfectly with
the transmitter so that photons arrive at their destination.

A group in China has made significant progress toward solving that
problem, via a high accuracy pointing and tracking system. Using this
method, Juan Yin and colleagues performed quantum teleportation (copying
of a quantum state) using multiple entangled photons through open air
between two stations 97 kilometers apart across a lake. Additionally,
they demonstrated entanglement between two receivers separated by
101.8km, transmitted by a station on an island roughly halfway between
them.

Though the authors do not make this clear in the paper, their method
is currently limited to nighttime communication. Nevertheless, their
results achieved larger distances for multi-photon teleportation and
three-point entanglement than before, and the tracking system used may
even enable ground-to-satellite quantum communication—at least if it
happens at night.

Quantum communication requires transmitting an arbitrary quantum
state between two points, similar to how ordinary communication sends
bits (voice or other data) across distances. However, a quantum state is
a small amount of information, typically carried by a single photon, so
many methods used in ordinary communication are out of the question
(including broadcasting).

In fiber optic quantum networks, photon loss is large over significant distances, requiring the use of quantum repeaters.
Point-to-point free-space transmission—either open-air or through the
vacuum of space—is better, though larger distances allow the beam of
photons to disperse. Atmospheric turbulence also contributes to photon
loss in the air, with the losses increasing the farther the signal must
travel.

One of the biggest challenges in point-to-point communication,
however, is target acquisition by the transmitter and/or receiver. If
the ground shifts slightly due to settling or tectonic activity, or
atmospheric turbulence makes the receiver appear to move, the laser
transmitting the signal can miss its target entirely. With few photons
to spare in quantum communication, real-time tracking and acquisition is
necessary. The researchers solved this problem using beacon lasers,
bright beams that carry no information, but can be used to aim both
transmitter and receiver, and wide-angle cameras.

As usual in quantum entanglement experiments, the group created
entangled photons by stimulating a crystal with ultraviolet light. This
produces a pair of photons with the same wavelength, but opposite (and
unknown) polarization values. These entangled photons were subsequently
sent to detectors, where their polarization quantum states were measured
and compared. In the first experiment, one photon was sent 97km across
Qinghai Lake (using a telescope to focus the beam), while the second was
analyzed locally. Using these photons, the researchers copied the
quantum state from the laboratory to the far station, achieving quantum
teleportation over a much larger distance than previously obtained.

However, quantum communication sometimes also requires coordination
between two distant receivers, so the researchers set up the transmitter
on an island in the lake. The receivers were 51.2 and 52.2 km from the
photon source respectively, on opposite shores of Qinghai lake, forming a
triangle with the transmitter. The distance between the
receivers—101.8km—was far enough to create a 3 microsecond delay between
measurements of the photon polarization.

Given this setup, there was no possible way for the two receiving
stations to communicate. Yet the photons they registered were
correlated, indicating entanglement was maintained.

These experiments provide not only a proof of principle for
free-space quantum communication, but also a means to test the
foundations of quantum theory over larger distances than before. With
very large detector separation, quantum entanglement experiments can
help differentiate between standard and alternative interpretations of
the quantum theory.

Though the long-distance aspect is promising, the fact that they set
up on the shores of a lake (where no intervening obstacles exist) and
that the experiment could only be performed successfully at night
indicate its limitations. Author Yuao Chen told Ars via e-mail that they
are working on solving the problem for daytime communication, but since
the signal consists of single photons, it's not clear how this will
work—the number of received photons fluctuated with the position of the
Moon, so noise appeared to be a significant problem for them.
Point-to-point communication will need to solve that problem as well
before satellite-to-ground quantum networks are practical.


Source:-
http://arstechnica.com/science/2012/08/open-air-quantum-teleportation-performed-across-a-97km-lake/