Cryptography Reference
In-Depth Information
3.2.6 Toward a Global Quantum Communication
Network
3.2.6.1 Free-Space Distribution of Quantum
Entanglement
In more recent years, work has begun on extending the reach of quantum
communication to longer and longer distances — after all, what good is a
quantum phone if you can only call across the room? Clearly, optical photons
are the ideal system for quantum communication over distances, owing to
their weak interaction with the environment (i.e., long decoherence times)
and high speed. The two methods for sharing photons over long distances
are through optical fibers or via free-space optical links. Previously, entan-
gled photons have been shared over long distances only in optical fiber up to
50 km [75]. Similar systems were used to perform a Bell inequality experiment
that closed the locality loophole [76]. Free-space optical links provide an excit-
ing alternative quantum channel when there is a direct line of sight between
two communicating parties. They consist of at least two telescopes — a trans-
mitter and a receiver — which are used to send light over large distances
through the air. Free-space links have been used in conjunction with faint
laser pulses to implement the BB84 quantum cryptography protocol up to a
distance of 23.4 km [77] and even at daylight [78,79,80]. Theoretical studies
have shown that quantum communication in optical fiber can be extended to
approximately 100 km before attenuation overwhelms the signal [81]. Recent
fiber-based experiments already reach this limit. Similar limitations are valid
for optical free-space links, which suffer from attenuation in the atmosphere
due to aerosols [82] and from atmospheric turbulences, which are eventually
limited by the Earth's curvature. Why is this distance of some hundred kilo-
meters not a limit in our optical networks of today? Quantum information
suffers from a fragility that is not present in its classical counterpart. For ex-
ample, classical optical pulses that encode 0's and 1's in an optical network
can be detected and regenerated or amplified every so often in repeater sta-
tions, effectively extending the range of optical communication indefinitely.
However, the polarization state of a single photon cannot be faithfully am-
plified — this can be seen as a consequence of the no-cloning theorem [83].
This makes the quantum analogue of repeaters much more complicated than
their classical counterparts. A quantum repeater [15] is in principle possible
with the use of quantum memories, entanglement purification [29,18], and
entanglement swapping [84,85]. In addition, free-space optical links may be
the way to increase significantly the present quantum communication dis-
tance limit: while earthbound free-space links are just as limited as fiber,
they have the advantage in that they can be combined with satellites. The
atmosphere is relatively thin, and most of the absorption takes place near
the Earth's surface. The attenuation experienced on a clear day at the Earth's
surface over approximately 4 km is roughly equivalent to that experienced
vertically through the atmosphere [86]. Transmitting entangled photons from
space to Earth will definitely allow us to overcome the current distance limits
