Cryptography Reference
In-Depth Information
photons are demonstrated to violate a Bell inequality by 14 standard devia-
tions. This confirms the high quality of the shared entanglement and it is an
encouraging step toward satellite-based distribution of quantum entangle-
ment and future intracity quantum networks.
3.2.6.2 Quantum Communications in Space
Although free-space optical links are in general superior to optical fibers
with respect to photon absorption, terrestrial free-space links will eventually
suffer from obstruction of objects in the line of sight, from possible severe
attenuation due to weather conditions and aerosols [82] from atmospheric
turbulence, and from the Earth's curvature. They are thus limited to rather
short distances. To exploit fully the advantages of free-space links, it will
be necessary to use space and satellite technology. By transmitting and/or
receiving either photons or entangled photon pairs to and/or from a satellite,
entanglement can be distributed over truly large distances and thus would
allow quantum communication applications on a global scale. Such a scenario
looks unrealistic at first sight, but we have recently shown that demonstrations
of quantum communication protocols using satellites are already feasible
today [91, 96, 101].
Based on present-day technology and assuming reasonable link parame-
ters, one can achieve enough entangled photons per receiver pair to demon-
strate several quantum communication protocols. For example, a single opti-
cal link between a satellite based transmitter terminal and an optical ground
station would suffice to establish a (single-photon) quantum cryptography
protocol such as BB84 and hence to generate a secure key between the satel-
lite and the ground station. If the same terminal generates another key with
another ground station (at an arbitrary distance from the first one), classi-
cal communication between the two ground stations suffices to establish a
secret key between them. In other words, satellite-based single-photon links
already allow quantum key distribution on a global scale. Note, however, that
in this scenario the security requirements on the satellite are as high as for
standard cryptography schemes. In contrast, these requirements are relaxed
if one can fully exploit an entangled source that distributes pairs of entangled
photons to two ground stations. For example, assuming a LEO based trans-
mitter terminal, a simultaneous link to two separate receiving ground stations
(see Figure 3.13) and a (conservatively estimated) total link attenuation of ap-
proximately 51 dB, one can expect a local count rate of approximately 2600
per second in total at each of the receiver terminals. The number of shared
entangled photon pairs is then expected to be approximately 4 per second. For
a link duration of 300 seconds, this accumulates to a net reception of 1200 en-
tangled qubits. One can expect erroneous detection events on the order of 7
per 100 seconds, which yields a bit error of approximately 2%. This would
already allow a quantum key distribution protocol between the two receiver
stations. It is thus clear that a demonstration of basic quantum communi-
cation protocols based on quantum entanglement can already be achieved
today. Furthermore, the possibility of distributing entangled particles over
