IEEE Spectrum January, 2016 - 12

Canary Islands of La Palma and Tenerife.
That distance is particularly significant,
because it's nearly as far as the boundary of low Earth orbit.
Second, they pulled off a similar feat-
although over a much shorter distance-
using twisted light, the kind featuring
photons having a property called orbital
angular momentum. Photonics experts
hope orbital angular momentum could
hugely increase the bandwidth of optical
telecommunications networks.
Entanglement is a quantum phenomenon. When a pair of particles, such as
photons, are created in a single physical
process or interact with each other in a
particular way, they become entangled-
that is, they start behaving like a single
particle, even when they become separated by any distance.
Teleportation of entanglement, also
known as entanglement swapping,
makes use of another curious phenomenon: It's also possible to entangle two photons by performing a joint
measurement on them, known as a
Bell-state measurement. Once these
photons are linked, switching the
polarization of one of them-say, from
up to down-causes an instantaneous
switch of the other photon's polarization, from down to up.

words, no physical link exists between
them. Now, assume you send photon 3
Tenerife
La Palma
from the transmitter to the receiver and
(receiver)
(sender)
perform
a Bell-state measurement simul143 kilometers
0
2
taneously on photon 3 and on photon 1.
Tenerife
La Palma
1
3(sender) As a result, 3 and 1 become entangled.
(receiver)
But surprisingly, photon 2, which stayed
143one
kilometers
Two pairs of photons,
pair (0,1) at a receiver
home, is now entangled with photon 0, at
0
Tenerife
La2Palma
0 and the other (2,3) at a transmitter
on
Tenerife
2
(receiver)
(sender)
the receiver. The entanglement between
on La Palma,
in the Canary Islands, are
1
1 with each
3
the two pairs has been swapped, and a
entangled
143other.
kilometers
0 3
2
quantum communication channel has
0
been established between photons 0 and
1
2 3
1 0
2,
although they've never been formally
2
0
introduced [see illustration, "Entangle3
2
1
ment Swapping"].
1
Entanglement swapping will be an
Photon33 is sent to Tenerife, and a measurement
0it and photon 1 simultaneously.
2
important component of future secure
is made on3
quantum links with satellites, says
1
Thomas Scheidl, a member of Zeilinger's
0
2
3
research group.
1
The team is working with a group at
the University of Science and Technol3
ogy of China on a satellite project led
Photon 1 and 3 are now entangled, and so are
by Zeilinger's former Ph.D. student Jian
photons 0 and 2-despite the fact that they
Wei Pan. Next year, when the Chinese
never came near each other.
Academy of Sciences launches its QuanHere's how the entanglement swap tum Science Satellite-which will have
works: Assume you have two pairs of an onboard source of entangled phoentangled photons, 0 and 1 in the receiv- tons-the satellite and ground stations
ing station and 2 and 3 in the transmitting in Europe and China will form the first
station. Both entangled pairs are com- space-Earth quantum network. They will
pletely unaware of each other; in other implement a quantum-key relay protocol,
securely linking the ground
stations in Europe and China,
ThE Coming CollidErs
says Scheidl.
With an upgraded large
a better understanding of the
life span, the buried ring of
The Quantum Science Satelhadron Collider (lhC) back to
higgs boson. now Chinese
superconductors-whose
smashing protons, it's time
scientists have revealed that
circumference is about twice
lite will be launched into a low
to think about what comes
they're planning something
that of the rings in the lhC-will
Earth orbit and will communinext. The international linear
even bigger. China's proposed
be upgraded to become the
cate with one ground station at
Collider (ilC), which had been
Circular Electron Positron
super Proton-Proton Collider,
the designated successor,
Collider would initially hit
a machine capable of 70
a time. "The satellite flies over
would impel particles with only 250 gigaelectron volts, or about teraelectron volts, or five times
a ground station in Europe and
a fraction of the energies of
one-fourth the ilC's energy. But as much as what the lhC can
establishes a quantum link to
the lhC, but it would still yield
at the end of this first phase's
produce.
the ground station, and you
generate a key between the sat2015
2025
ellite and the ground station in
large hadron Collider
Circular Electron
Europe. Then, some hours later,
27 kilometers
Positron Collider
13 teraelectron volts
54 kilometers
the satellite will pass a ground
250 gigaelectron volts
station in China and establish
a second quantum link and
2040
secure key with a ground stasuper Proton2025
tion in China," Scheidl explains.
Proton Collider
international linear Collider

EntanglEmEnt SwappIng

54.4 kilometers
70 teraelectron volts

31 kilometers
1 teraelectron volt

Jan 2016

nORTh aMERICan

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Table of Contents for the Digital Edition of IEEE Spectrum January, 2016