Chapter 14 Applications of Free Space Optical Communications in Hap Scenarios
Sub-Editor: M Knapek
Abstract: Relevant application scenarios for laser communication in HAP scenarios are shown. Main applications are backbone scenarios, where the high datarates of optical communication can fully develop their advantages. Laser links
would be used to interconnect networks of HAPs. Additional connections to
LEO/GEO satellites would provide the connection to international telecom networks. Links between HAPs and to satellites are not influenced by clouds, as they
are operated above the cloud layer. Therefore full availability can be provided. Laser downlinks from HAPs to ground stations suffer from a limited availability due
to clouds and ground fog. Improvement schemes like ground station diversity, i.e.
flexible links to several ground stations without cloud blockage, or hybrid microwave/laser connections are discussed.
A special application of optical communication is quantum key distribution
from HAPs. Quantum key distribution provides a method for absolutely secure
key exchange. HAPs could provide the ideal platform to distribute security keys to
separated regions as between cities or neighbouring countries.
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List of Authors:
M. Knapek
German Aerospace Center, Germany
Markus.Knapek@dlr.de
14.1, 14.2, 14.3
S. Arnon
Ben Gurion University, Israel
shlomi@ee.bgu.ac.il
14.4
D. Kedar
T. Sheidl
R. Ursin
ii
Table of Contents
Chapter 14 Applications of Free Space Optical Communications in Hap
ScenariosFormel-Kapitel 1 Abschnitt 1 ................................................................ i
14.
Applications of Free-Space Optical Communications in HAP Scenarios1
14.1. Introduction .......................................................................................... 1
14.2.
Networks above the Clouds – Inter-HAP Links ................................... 2
14.3.
HAPs as Relay Stations ........................................................................ 4
14.4.
Links
Hybrid RF & Optical Communication Systems for HAP-to-Ground
8
14.5.
Quantum Key Distribution from HAPs .............................................. 11
14.5.1.
Motivation ................................................................................ 11
14.5.2.
Introduction .............................................................................. 12
14.5.3.
Technical Concepts ................................................................... 13
BB84 Protocol with Weak Coherent Laser Pulses .................................. 13
Decoy State Protocol with Weak Coherent Laser Pulses........................ 14
Entanglement Based BB84 Protocol....................................................... 15
14.5.4.
State-of-the-Art Realizations .................................................... 16
Weak Coherent Laser Pulse Based Systems ........................................... 16
Entanglement Based Systems ................................................................. 16
Recent Results ........................................................................................ 17
14.5.5.
Preliminary Setup Design ......................................................... 17
Quantum Communication Module Onboard the HAP ............................ 17
Polarization Analysis .............................................................................. 18
Detector Module ..................................................................................... 19
14.5.6.
Challenges ................................................................................ 20
Polarization Issues .................................................................................. 20
Temporal Synchronization ...................................................................... 21
iii
14. Applications of Free-Space Optical Communications in HAP Scenarios
M. Knapek, S. Arnon, D. Kedar, T. Sheidl, R. Ursin
14.1. Introduction
High data-rates in the range of Gbps are the strongest motivation for the use of
free-space optical communication systems in comparison to microwave communication. Optical communication technology offers small terminal sizes and low
weight, very small aperture sizes and low power consumption. The small divergence angle of optical communication systems excludes in addition the danger of
eavesdroppers. However optical beams are blocked by clouds, which have to be
taken into consideration in the planning of such communication systems. Fig. 14.1
shows possible scenarios involving HAP optical communication links:




Inter-HAP optical connections to form a network of HAPs
HAP to satellite optical links to relay data via satellite to the ground
Satellite-to-HAP optical links to relay data via HAP to the ground
HAP-to-ground links by microwave and/or optical means. Hybrid schemes are
conceivable.
1
Fig. 14.1 HAP-scenarios involving optical communication links: Basis is an inter-platform
network indicated by three HAPs. The HAPs are connected by optical and RF downlinks to
the ground. In addition links to GEO and LEO satellites are shown.
14.2. Networks above the Clouds – Inter-HAP Links
Inter-HAP connections form the basis of this network. They allow the exchange of
date between HAPs over large distances respectively between regions. Cloud
blockage or atmospheric attenuation are not limiting factors for inter-platform and
platform-to-satellite links, as they operate above the cloud ceiling. HAP networks
would allow large scale diversity schemes to transmit data at cloud-free locations
by optical means to the ground. The network configuration would be dynamically
adapted to optimize the throughput. As the HAPs are interconnected by optical
links, there is no necessity of a terrestrial network. The optical links provide almost unlimited bandwidth to the network without the limiting aspects of spectrum
availability and spectrum regulations.
HAP networks would be installed to provide communication services with one
platform covering an extended area. These communication services could be required for urban areas, where the communication infrastructure is not fully developed, or in order to provide services for large rural, thinly populated, areas. HAPs
show the advantages of satellite services with a large coverage area, but they avoid
the link-budget problems of the larger satellite-to-ground distances. In this way
HAPs combine properties of terrestrial and satellite networks.
2
Fig. 14.2 Artists impression of a HAP network over the southern part of Africa with optical
interconnection links. Five HAPs with microwave payloads are shown with the indicated
footprint (gray).
Optical and microwave links interact with the atmosphere in different ways. It is
therefore important to investigate the effects of atmospheric structure, attenuation,
and turbulence on the propagation separately for an optical beam. In addition, geometrical concerns and background light levels will also affect the design of an
optical HAP-HAP link.
Stratospheric platforms operate at low air pressure due to the altitude. In the
consequence atmospheric attenuation effects (absorption, scattering) and index-ofrefraction effects are rather weak. Index-of-refraction effects are explained in the
next Chapter.
The earth's atmosphere consists of several distinct layers with the troposphere
at the bottom. Usually all weather phenomena (and thus cloud coverage) happens
inside the troposphere which has a negative temperature gradient on average –
typically 1 degree cooler per 100 m rise in altitude. The tropopause is defined by
the reversing of the temperature coefficient which then is positive inside the stratosphere. The lower bound of the stratosphere varies from below 8 km at the poles,
up to 18 km at the equator.
Giggenbach et al. [1] studied the maximal distance between two HAPs taking
into consideration the maximum height of clouds (cloud ceiling). In moderate latitudes clouds can be found up to 13 km altitude. This altitude increases to about
16 km closer to the equator. Only in rare weather conditions like severe thunderstorms might clouds rise above the tropopause.
Fig. 14.3 shows the link geometry between two HAPs, where the curvature of
the earth and the cloud altitude limit the link distance. If the grazing height of the
3
link drops below the upper limit of the clouds, the availability drops below 100%.
Fig. 14.4 shows the maximum inter-platform distance in relation to the height of
the cloud ceiling. Three different HAP altitudes are indicated. Assuming a typical
upper limit of the clouds of 13 km at moderate latitudes, link distances of about
600 km to 800 km are possible with full availability.
Fig. 14.3 Illustration of the influence of the curvature of the earth on the link distance. The
cloud altitude influences the maximum distance for inter-platform links avoiding cloud
blockage.
Fig. 14.4 Dependence of maximum link distance on the cloud top height (CTH) under the
assumption that both HAPs have the same height above ground.
14.3. HAPs as Relay Stations
HAPs could be additionally connected to GEO or LEO satellites. These links
could be used to relay data via satellite to ground. The relay of data via GEO satel-
4
lites would allow real-time data access from/to distant regions, which might be interesting for reconnaissance scenarios and for operations in emergency situations.
In the other direction HAPs might be used as relay stations above the clouds for
optical downlinks from Earth Observation (EO) satellites [2][3]. In this scenario
the link from the satellite to the HAP would be achieved by optical means. For the
last 20 - 60 km to the ground, a microwave or a hybrid optical/microwave scheme
would be used. Optical links could be only deployed during favorable weather
conditions but with higher data throughput. The availability of the optical link
would strongly depend on the location of the HAP operation (see section 15.5).
EO satellite missions produce a large amount of data for example using highresolution optical or radar sensors. During the last decades the amount of data has
steadily increased due to improved sensor technologies with increased temporal
resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna
sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps overcome the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability
through the atmosphere due to cloud blockage above the ground station. Apart
from diversity concepts with several ground stations or satellite networks, stratospheric HAPs could act as a relay station to forward the optical communication
beam over the last 20 km through the atmosphere to the ground station, where
short-range, high data-rate microwave systems are feasible or hybrid concepts
could be implemented. Decisive for such a scenario is the link capacity in comparison to direct satellite-to-ground microwave links.
The link capacity results from the number of satellite-HAP contacts and the duration of the contacts. The number of contacts and therefore the link duration per
day of a LEO satellite to a HAP strongly depends on the geographic latitude of the
HAP location and the orbit of the satellite. Fig. 14.5 shows link durations for TerraSAR-X as an example of a typical EO LEO satellite. TerraSAR-X has a nearly
polar orbit with 97.4° inclination and 508 km orbit height. The difference in the
link duration is mainly caused by the contact number satellite-to-HAP. For a nearly polar orbit there are about 3 contacts per day for a HAP position at low geographic latitudes and up to 15 close to the earth’s poles.
The link duration from a LEO satellite to the HAP or ground station also depends on the minimum elevation angle under which the satellite can be seen and a
communication link is possible. Larger elevation angles (5-10°) are required to establish a communication link for stations on the ground due to a limited horizontal
line-of-sight and atmospheric turbulence/attenuation effects. For links to HAPs elevation angles even below the horizon (-2°) are possible due to the elevated position of the platforms. The maximum link duration per pass is about 8 minutes at a
minimum angle of 10° and 13 minutes for -2° elevation.
5
Fig. 14.5 Link duration from an earth-observation satellite (TerraSAR-X) to ground stations at various latitudes. Link durations for minimum elevation angles of zero and 5 degrees as a constraint are shown. Link durations slightly change for a LEO-HAP link with
the HAP at 20km altitude.
The link capacity (e.g bits per day) from the LEO satellite to the ground depends
on several factors:
 Effective downlink bitrate (of the segment with the lowest capacity) in bits
per second. Modern optical systems like the Laser Communication Terminal
(LCT) on TerraSAR-X have bitrates of 5.6 Gbps over several 1000 km and
2.8 Gbps for GEO distances (40000 km). Microwave links operate at several
100 Mbps. Wavelength Division Multiplex (WDM) methods are envisioned for
optical links, which would significantly increase link capacities into the
100 Gbps region.
 Cumulative link duration per day (line of sight). The link duration for links
from LEO satellites to HAPs or ground stations mainly depends on the number
of satellite passes over a station and the minimum elevation angle.
 Cloud-free time is a crucial issue for optical links to the ground. Minimum
values of cloud coverage appear at about 20 degrees latitude north and south
(Chapter 15).
The transferable mean amount of data per day D for an optical HAP-to-ground
link is calculated from the effective downlink bitrate feff, the cumulative link duration T per day in dependence of the geographic latitude λgeo and the minimum elevation angle α, and the probability of cloud blockage pcloud, which depends mainly
on the latitude λ and the time of the year t.
6
D  f eff  T  geo ,    1  pclouds  geo , t 
(14.1)
Possible scenarios could be links from the LEO satellite via HAP to the ground,
but also links from the LEO satellite via a GEO satellite and a HAP to the ground.
For the link through the atmosphere a high capacity microwave link is assumed
with a bandwidth of 800 Mbps, since the link distance is only a few 10’s of km.
As the data rate to the ground is smaller than the rate from the satellite to the HAP,
data buffering on the HAP might be necessary. Data transmission would continue
from the buffer when the satellite would be already out of sight. Data transfer to
the ground would be delayed but the full optical link capacity could be used. Data
delay would be acceptable for EO missions, however not for real-time communication.
Fig. 14.6 shows the mean transmitted data volume from a LEO satellite for various transmission schemes. Example geographic locations (Tenerife, Calar Alto,
Weilheim, etc.) were taken to obtain mean cloud cover values, which resulted
from a study with satellite EO images [4]. A typical microwave X-Band LEO-toground (LEO-GND) downlink (DL) as on TerraSAR-X offers a data rate of
300 Mbps (Curve 1). This results in a downlink capacity of about 0.45 Tb to
2.25 Tb per day (1500 s-7500 s link duration per day in dependence of the latitude
at a minimum elevation of 5deg). In comparison the link via a HAP relay (LEOHAP-GND) increases the downlink rate to 1.9 - 8.0 Tb per day (unbuffered) at
800 Mbps (Curve 2), and 13.3 - 55.8 Tb per day (buffered) at 5.6 Gbps (Curve 4),
both with full availability due to the microwave link on the HAP-to-ground link
(800 Mbps). Typical link durations for LEO-to-HAP links are 2400 - 10000 s at a
minimum elevation angle of zero degrees elevation. Using an optical HAP-toground link at 5.6 Gbps provide mean daily transfer rates of 7.3 - 14.0 Tb (Curve
3), however the link might be blocked for longer periods due to clouds. The increase of data volume due to the larger contact time at higher latitudes is almost
defeated by the increased cloud coverage at these latitudes.
A future network of HAPs interconnected with optical links would eliminate
the cloud blockage problem of optical links by providing optical HAP-to-ground
links at different geographic locations. HAP constellations in favourable locations
achieve an overall link availability of over 95%. In addition the HAP network
would multiply the link duration LEO-to-HAP by the number of HAPs and therefore the transferable data volume. Optical HAP-to-ground links appear to be sensible for a HAP network of three or more stations, where the overall availability is
close to 100%. Details on diversity schemes are given in the next chapter. Also
hybrid systems with an optical and in parallel a microwave communication system
provide full availability and higher data rates.
7
Fig. 14.6 Mean daily transmitted data volume from a LEO satellite to the ground over the
elevation angle of the ground stations. Mean cloud coverage at the ground stations were
used.
Optical data transmission from LEO satellites via GEO relay satellites has the advantage of higher link durations as they cover nearly half of the LEO orbit, however, the overall system complexity significantly increases with the requirement of
an additional GEO satellite. Due to the larger link distances the terminal size,
power consumption and weight of optical LEO-to-GEO link terminals are higher,
which beyond that prevents the use of these terminals on small LEO satellites.
14.4. Hybrid RF & Optical Communication Systems for HAP-toGround Links
The high data-rates have made free-space optical (FSO) communication systems
the preferred technology for inter-satellite links, where the impediments of atmospheric propagation are absent. However, the high outage probabilities associated
with FSO links in an atmospheric channel have hindered their widespread terrestrial implementation. Only recently a growing interest in optical links in the terrestrial sphere is emerging as a method to boost traffic throughput where radio frequency infrastructure exists and to provide easily deployable and flexible
enhancements to wireless communication systems. These are hybrid systems
where optical and radio frequency systems work side-by-side and overall system
performance can be increased by the diversity offered.
Weather conditions such as fog and haze threaten the availability of an optic
link, while rainstorms can reduce power reception in a millimetre wave link to
prohibitively low levels. The presence of a hybrid optical/millimetre link offers
8
the possibility of reduced downtime by switching from one communication modality to the other as weather conditions change. When the propagation allows for
FSO links the high data rates offered by the optical method can be transmitted, increasing the throughput. When only lower data-rate (radio) transmissions are possible due to fog or haze, traffic can be prioritised with data transmissions that allow long latency (email, file downloads) buffered and sent later, while real-time
transmissions and high priority services (voice, high QoS transmissions) would be
sent at lower data rates via the radio link.
The addition of networking between HAPs and ground stations further promotes the implementation of hybrid RF-optical systems. A HAP-to-ground station
link that is temporarily unavailable for high data-rate optical transmissions can be
circumvented by transmitting the signal via inter-platform links to another HAP
and then sent to a nearby ground station, from which it can be transferred by an
optical fiber link to the original destination. This simple scenario is illustrated in
Fig. 14.7.
A more sophisticated optimisation of resources could be attained using the
emerging methodologies of cognitive radio to enhance system capacity, reduce
downtime and enable stratified service provision, with the accompanying revenue
gains.
Hybrid RF-optical IPLs
A
(three HAPs are networked)
B
Hybrid RF-optical
HAP-to-ground link
C
II
I
Optic fibre-linked ground
stations
(could also be wirelessly connected)
III
Fig. 14.7 Scheme of a HAP network with hybrid RF-optical IPLs and hybrid links to the
ground stations. The ground stations are interconnected with a terrestrial network.
In the figure we see three HAPs, A, B and C networked together with hybrid RFoptical IPLs. We assume that the optical links are operational most of the time and
the RF links act as a back-up in case of laser-link failure (due to wind-induced
misalignment, for instance). Each HAP is associated with a ground station in closest proximity to its location and communicates using a hybrid RF-optical link. The
9
choice of link, or distribution of traffic between the two links, is done in order to
optimize link performance. HAP A is associated with ground station I, HAP B is
associated with ground station II, and HAP C is associated with ground station III.
In spite of the association of the HAPs with a ground station, HAP A can communicate with ground station II as well (and possibly with additional ground stations). The three ground stations are also linked by an optical fiber network. In
remote terrain the ground stations may be linked wirelessly. We envisage a scenario in which HAP A is unable to transmit all its data to ground station I. This
may be due to adverse weather conditions or due to excessively high traffic demands. HAP A can utilize the additional HAP-to-ground link to ground station II,
using either the RF or the optical link – or, indeed, both links – to download the
excess data in its transmissions. The network between the ground stations facilitates transferring the data from ground station II to ground station I. Alternatively
(or additionally) data can be transmitted via the IPL to HAP B and from there to
ground station II and on to ground station I as desired. In this simple illustration
we observe the plethora of alternatives offered by the multiple-parameter diversity.
The functioning of a hybrid RF-optical communication system is illustrated
schematically in Fig. 14.8. Two HAPs (or a HAP and a ground station) are separated by an atmospheric channel that may be characterized by rainfall, indicating
preference for an optical link, or cloud, indicating a preference for an RF link.
Transceivers for FSO and RF are located both on the HAP and at the ground station. An intelligent controller distributes the traffic to one transceiver or both in
accordance with input data from the channel monitoring system and system performance requirements. System performance requirements include QoS parameters and acceptable bit-error-rates, and may include feedback from preceding
transmitted bit streams. The channel monitoring system may include meteorological data as well as on-line sensed data.
10
Fig. 14.8 Schematic of a hybrid RF-optical communication system.
The ultimate economic feasibility study of hybrid RF-optical networked HAPs and
ground stations remains to be executed in detail, but net savings in the number of
ground stations can be anticipated when optical links are added to boost capacity
and smart resource allocation methods are developed to adapt to changing traffic
and channel conditions.
A different application scenario of optical communication links appears for
low-altitude platforms, which operate mostly below the cloud layer. These platforms are quickly deployed on demand as for example in emergency rescue situations. Aerodynamic (fixed- or rotary-wing) as well aerostatic (lighter than air platforms, blimp-like) are conceivable as platforms. Their reduced field-of-regard is
rather suited for observing objects or events with known location (aircraft can
reach target areas very fast) than for general observation purposes. Depending on
the scenario these platforms would operate under the cloud layer, and the payload
data could be directly transmitted with almost full availability via optical communication to the ground. This is especially interesting for payloads generating
measurement data of the ground in the visual domain, like cameras, as they can
only be operated during sufficient visibility of the ground.
14.5. Quantum Key Distribution from HAPs
14.5.1. Motivation
In a modern information based society secure communication is of utmost importance. An overnight breakthrough in mathematics or computer science could
make current electronic money transfer encryption methods instantaneously vulnerable. This is because the security of classical cryptography relies on the computational difficulty of certain mathematical functions. Hence, classical cryptog-
11
raphy can neither provide any indication of eavesdropping nor guarantee absolute
security.
Modern quantum cryptography, often also called quantum key distribution
(QKD), provides both as it is based on theoretical and experimental proven laws
of nature [5]. However, with present fiber and detector technology terrestrial
quantum communication is limited to within some 100 of kilometers [6], well
within the radius a single person can travel within a reasonable time. A very convenient way to extend the distance for quantum communication between earth
based parties is the use of optical free-space links to and from HAPs. This will enable to perform quantum communication over distances currently not accessible
via fibers and is thus expected to be of high technological impact in the future.
The possibility to distribute an absolute secure key between globally separated
communication parties makes such a system highly marketable.
Within various feasibility studies [7, 8] and experimental tests [9, 10, 11, 12]
for adopting the concepts of fundamental quantum physics and quantum communications to a space infrastructure it was shown on a 144 km terrestrial free-space
channel that, with state-of-the-art technologies, quantum communication to and
from HAPs is possible. In the following, the basic principles of QKD, its implementation using various protocols and recent results from ESA funded studies
within this field will be described.
14.5.2. Introduction
Quantum cryptography has an important advantage over its classical counter part
because its security is based on fundamental quantum physical laws (i.e., the superposition principle and the no-cloning theorem [5]). The communicating parties
produce a key, which generally takes the form of a random bit string and if this
key is used only once (one-time-pad protocol [13]) it can be proven to be information theoretically unconditional secure (i.e., a produced hidden key cannot have
been read by any other than the authenticated participants). Unlike classical cryptography, which depends on unproven computational complexity of mathematical
techniques to restrict the possibility that an eavesdropper might learn the contents
of encrypted messages, quantum cryptography is based on the fact that naive attempts to read out quantum information will destroy the very same. Furthermore,
such an eavesdropping attempt introduces errors in the QKD protocol and can thus
be recognized by the communicating parties. Even when allowing a hypothetical
eavesdropper to have access to unlimited computational power, the laws of physics guarantee that the secret key exchange is secure. This holds also true for adversaries making use of future quantum computers [14, 15, 16, 17, 18]. The established key is then used for the encryption and decryption of the message that is
being sent over a public channel.
At present, the only suitable quantum systems for long-distance quantum communication are photons. Other systems such as atoms or ions are studied thoroughly; however, their applicability for quantum communication schemes seems
not to be feasible within the near future. The two most well-known concepts for
12
QKD with photons are based on weak coherent laser pulses (WCP) [19, 20, 9, 21]
and entanglement [22, 23, 10, 24, 25], respectively. Thereby, the two possible bit
values are usually encoded in the photon’s polarization degree of freedom. In contrast to classical laser communication, these schemes need to be able to randomly
switch between different polarization states that are transmitted. Therefore, usually the four linear polarization states 0°, 90°, 45° and 135° are used. The simplest
strategy an eavesdropper can pursue is to intercept the communication, measure
the photons and re-send the photons in the observed state. However, due to the superposition principle, this will introduce errors in the QKD protocol. Consequently, the amount of error in the generated key, i.e., the quantum bit error ratio
(QBER) gives the upper bound on the information an eavesdropper might have
gained [26]. If the QBER is below a critical value, classical error correction is
used to erase the information of the eavesdropper and to restore the security.
Not only an eavesdropper, but also transmission errors and detector dark counts
introduce errors in the generated key. As it cannot be distinguished whether the errors come from experimental imperfections or from eavesdropping activity, they
all must be attributed to an eavesdropping attack. On the basis of present fiber and
detector technology, it has been determined that absorptive losses and dark counts
in the detectors limit the distance for distributing quantum keys to the order of
100km [6]. Recent quantum cryptography experiments in fiber already come close
to such distances [27, 28, 29].
Other QKD systems make use of free-space links and experiments were performed already under real world conditions over distances up to 144km [9, 10].
This distance is already limited by the terrestrial available test-pads. To perform
QKD from air- or space-based platforms has special appeal to organizations,
which require an absolute security standard over global distances. HAPs can be
used in urban areas for example to distribute the secret key among users over distances of more than 500 km, clearly exceeding the possible distances for today's
fiber technology. Nevertheless, systems using moving platforms have to tackle
with additional challenges compared to terrestrial demonstrations.
The next section provides a summary of the most important QKD protocols followed by a short description of how these are commonly implemented in an experiment. Subsequently, recent results from ESA funded studies within this field
will be described. Finally, a preliminary setup design for the suggested experiments, including block diagrams for the HAP and the ground terminals will be
presented and specific problems for a quantum communication link (e.g. the automatic polarization tracking and the required synchronization of local time bases)
will be discussed.
14.5.3. Technical Concepts
BB84 Protocol with Weak Coherent Laser Pulses
The principle of the BB84 protocol was developed by Bennett and Brassard in
1984 [30] and it requires four different qubit states that are usually realized with
13
the four linear polarization states 0°, 90°, 45° and 135°. As schematically shown
in Fig. 14.9, Alice sends weak laser pulses randomly in one of these four states to
Bob. Bob receives and analyzes them with a two channel analyzer, again randomly in one of the two complementary polarization bases (i.e., the 0°/90°-basis and
the 45°/135°-basis). He records his measurement results together with the basis
the corresponding photon was analyzed in. After enough photons have been
transmitted, Bob communicates publicly with Alice and tells her which photons
actually arrived and the corresponding analyzing bases. In return, Alice tells Bob
when she has used the same bases to prepare them, because only in these cases
Bob obtains the correct result. Assigning the binary values ‘0’ and ‘1’ to the linear
polarization states 0°/45° and 90°/135°, respectively, leaves Alice and Bob with
an identical set of ‘0’s and ‘1’s.
Fig. 14.9 An illustration of the BB84 protocol using weak coherent laser pulses. Alice prepares the light pulses randomly in one of four linear polarization states and sends them to
Bob who measures, again randomly, in one of two complementary polarization bases.
Whenever the preparation and the measurement occurred in the same basis, the measurement result (either “0” or “1”) enters the cryptographic key while all the other measurement
events are disregarded.
This is called the sifted key. As explained above, the sifted key might be defective
because of transmission errors or eavesdropping attacks. The resulting QBER can
be estimated by comparing a part of Alice’s and Bob’s sifted key publicly. If the
QBER is below a certain value (approx. 11%) the final unconditional secure key
can be extracted from the defective sifted key by performing classical error correction (e.g. CASCADE [31]) and privacy amplification (e.g. using a hash function [32]).
Decoy State Protocol with Weak Coherent Laser Pulses
Within experimental realizations of the BB84 protocol, the mean photon number
per laser pulse is usually set well below 1 photon. Nevertheless, there exists the
possibility that a pulse contains even more than one photon due to the Poissonian
photon statistics. These multi-photon pulses enable an eavesdropper to perform a
very powerful attack – the so-called photon number splitting attack (PNS attack)
14
[33, 34, 35] – where one photon is split off a multi-photon pulse and measured by
an adversary.
An effective method to counteract this attack is to use decoy states [19, 36, 37].
These are states of various mean photon numbers which are added randomly to the
actual signal pulses. An eavesdropper cannot distinguish between signal and decoy pulses and thus, cannot act differently on them. Since the signal states and the
decoy states exhibit different photon number statistics, any photon-number dependent eavesdropping strategy (i.e., the PNS attack) has different effects on the
signal states and on the decoy states. Alice and Bob can now separately compute
the transmission probability of signal and decoy states and detect, with high probability, any photon number dependent attack.
Entanglement Based BB84 Protocol
In contrast to the prepare-and-measurement scheme of the original BB84 protocol,
in its entanglement based version (BBM92 protocol) Alice and Bob usually share
entangled photon pairs which are emitted by some source, as illustrated in Fig.
14.10. Using a two channel analyzer, each measures the incoming photons randomly (e.g. using a 50/50 beam splitter or an active switch) in either of the two
complementary polarization bases (i.e., the 0°/90°-basis and the 45°/135°-basis)
and records the results and the actual measurement bases.
Fig. 14.10 A schematic of the entanglement based QKD protocol. A source generates entangled photon-pairs, usually in the state ψ-= |↕↔|-|↔↕|, which are then sent to Alice and
Bob. Both measure the incoming photons randomly in one of two complementary polarization bases. Such as within the BB84 protocol, only those results enter the cryptographic
key, which are obtained from a measurement in the same basis.
Afterwards, they publicly communicate which photons they actually detected and
the corresponding measurement bases and discard those results in which they accidentally disagreed with the basis. Since the shared entangled state exhibits perfect polarization anticorrelations, Alice’s and Bob’s results are perfectly anticorrelated. After one of the communicating parties inverts the bit-sequence the sifted
key is obtained. Such as in the weak coherent pulse schemes described above, this
key might be defective due to transmission errors or eavesdropping attacks. The
QBER can again be estimated by comparing parts of the sifted key and, as long as
15
it is below a certain value (approx. 11%), an unconditional secure key can be extracted with the help of classical procedures.
14.5.4. State-of-the-Art Realizations
Weak Coherent Laser Pulse Based Systems
The four different polarization states can be generated with different methods. One
method is to use four separate linear polarized laser diodes that are rotated to the
desired angles. The individual signals are further combined (e.g. using a conical
mirror or similar) into a single spatial mode defined by a single mode optical fiber.
Another method is to split the signal of a single laser diode into four equally intense spatial modes using beam splitters and prepare the polarization in each mode
as desired. Recombining the four spatial modes and using active amplitude
switches such as Pockels Cells, any of the four polarization states can be prepared.
The laser diodes are driven with a short electrical pulse, and heavily attenuating
the optical output results in a weak laser pulse. The mean photon number per pulse
can be set by adjusting the strength of the electric pulse and the attenuation of the
optical signal.
Entanglement Based Systems
Entangled photon pairs are usually generated via spontaneous parametric down
conversion (SPDC). SPDC is a process in which an intense beam is incident on a
birefringent crystal where nonlinear effects lead to the conversion of pump photons into pairs of correlated down-converted (lower frequency) photons, usually
called signal and idler. If the down-converted photons have parallel polarization,
the process is called type-I, in the case of orthogonal polarization, type-II SPDC.
The most well known scheme is based on type-II SPDC in a BBO crystal,
where the signal and idler photons are emitted along separate cones that intersect
along two straight lines. Photon pairs that are emitted along these lines are entangled in their polarization since it can not be known to which cone they belong
without measuring their polarization. Using single mode optical fibers, exactly
these photon pairs can be collected and used for entanglement based QKD experiments.
Another very promising scheme for entangled photon generation is the socalled polarization Sagnac interferometer. It is an interferometric scheme that uses
a periodically poled (pp) nonlinear crystal (e.g. ppKTP) and collinear phase
matching to generate entangled photon pairs. Due to the collinear emission, all of
the down converted photon pairs are entangled. Together with the high nonlinearity of such periodically poled crystals the Sagnac source yields photon pair rates
approximately 10 times higher than can be achieved with common BBO based
sources. This is of utmost importance when very high channel losses should be
overcome within a QKD experiment.
16
Recent Results
In order to show the feasibility of QKD from HAPs or satellites in a low-earthorbit (LEO) various experiments have been carried out over a real distance of
144 km between the two Canary Islands La Palma and Tenerife. There, the atmospheric conditions and the channel attenuation (approximately 30 dB) are comparable to an optical link from HAPs or LEO satellites to ground based receivers.
One experiment [10] demonstrated entanglement based QKD. Entangled photon pairs were generated on the island of La Palma, one photon was measured locally whereas its partner was sent over a 144 km free-space link to Tenerife where
it was collected by ESA’s optical ground station (OGS). During a measurement
run of 75 s, 789 coincidences could be detected between the two islands. With the
obtained QBER of 4.8% this resulted in a secure key of 178 bits in total, corresponding to a secure key rate of 2 bits/s.
Another experiment [9] demonstrated decoy state QKD with weak coherent laser pulses on the very same optical link between La Palma and Tenerife. Due to
stray light and the high optical attenuation through the 144 km free-space channel,
secure key exchange would not be in connection with the standard BB84 protocol.
Thanks to the decoy-state analysis, the secrecy of the resulting quantum key could
be ensured despite the Poissonian nature of the emitted pulses and an averaged secure key rate of up to 42 bits/s could be obtained.
14.5.5. Preliminary Setup Design
Quantum Communication Module Onboard the HAP
The source module comprises the entangled photon source, the weak laser source
and the polarization controllers (see Fig. 14.11). The relative alignment of the UVlaser, the non-linear crystal and the single mode fibers must be kept at high mechanical stability.
17
Fig. 14.11 Overview of the complete quantum communication source module, which comprises the entangled photons source, the weak laser option, and the fiber polarization compensation for each of the photon channels.
Polarization Analysis
Two polarization analysis modules are needed (system A & B). The polarization
analyzer has a size of 30 x 30 x 30 mm3 and a mass less than 100 g.
Fig. 14.12 shows the in-and-outgoing (optical) interfaces:


one input window for the incoming signal beam and
four outgoing multi mode fibers (A, B represent each of the two modules).
18
Fig. 14.12 Polarization analyzer module. Input lens (L) collimates beam which is filtered
by filter F and split to two (alternatively three) analysis stages using beam splitter (BS).
Half-wave plate (HW) is used to set different analysis orientations. Analyzer comprises polarizing beam splitter and fiber couplers to multi-mode fibers.
Detector Module
Each of the two detector modules (see Fig. 14.13) has a size of 160 x 100 x
20 mm3. Each module consumes 3 W during operation. The avalanche photo diodes (APD) have to be cooled to an operating temperature of about -30°C and
must be stabilized to +/- 1°C. The required interfaces for each module are:





one in-and-outgoing interface for temperature stabilization for the APDs
(dotted frame),
one incoming power supply interface for the signal processing electronics,
one incoming interface for the control signal,
four incoming multimode fiber interfaces (from the polarization analyzer) to the APDs, and
four outgoing signal cables for the detection pulses (coaxial cables).
19
Fig. 14.13 The detector module consisting of four APDs, each with a passive quenching
circuit (PQ), a DC high voltage converter (HV) and an amplifier for signal configuration.
14.5.6. Challenges
Polarization Issues
Since in all the above described QKD protocols the qubit is encoded in the polarization of a photon, relative rotations of the polarization reference frame cause biterrors in the communication protocol and have thus to be corrected for. For example, a misalignment of the two complementary bases settings (i.e., the 0°/90°-basis
and the 45°/135°-basis) used in the analyzer modules of the communication parties of about 2° will result in a QBER of 0.5%.
Atmospheric effects, like scattering, turbulence and the Faraday effect due to
the Earth's magnetic field, affect only slightly the polarization state of light: for
typical experimental parameters these effects give an overall rotation of the linear
polarization plane smaller than 10-3rad [38]. Hence, the main source of polarization transformation is the path within the transmitter and receiver optics [39] as
well as the motion/rotation of the HAP. Due to the extremely low power of the
quantum signal, conceivable compensation solutions might be based on either a
polarization reference beacon (well isolated from the signal wavelengths) attached
to some pointing assembly, or on a computational prediction of the rotation angle
from accurate knowledge of the platforms trajectory and rotation.
The speed of the feedback loop is basically determined by the speed of the polarization drift and will probably not exceed the sub-Hz range (except in the case
20
of vibrational effects). Note, that the compensation has to be performed for both
basis-states, used in the respective QKD scheme, simultaneously. There are essentially three ways to compensate: First, using pre-compensation of the optical path
via in-fiber polarization controllers at the transmitter. Second, performing the polarization compensation on ground which would reduces the overall complexity of
the transmitter. The polarization drift of a polarized beacon is measured and a birefringent element in front of the quantum acquisition sensor is then actively adjusted such that the reference frame of the polarization stays the same [40]. Third,
computing the respective polarization shifts from the precise knowledge about the
optical components involved and calculating the necessary compensation operations using birefringent elements.
Temporal Synchronization
To perform quantum communication experiments it is essential to establish time
correlations between two time of arrival events on different locations (for entangled-photon based QKD) and the emission time and detection time in order to attribute the measurement results to certain states of polarization (for weak coherent
laser pulse based QKD). Entangled pairs are correlated to within their coherence
time to about some femto seconds, the accuracy is therefore limited by the timingjitter of the single photon detectors (i.e. approximately 0,5 ns for Si-APDs) and the
synchronization of the two local time bases. In [41, 42] the time-bases synchronization was achieved using a combination of global positioning system (GPS) and
software-driven time-correlation, which enabled to maintain a correlation window
of better than 0,8 ns over hours. The software was designed such, that even drifts
of the local timescales were compensated by maximizing the coincident events.
This requires accurate knowledge of the distance of the respective terminals in order to be able to set the correct offset corresponding to the (in-flight varying) timeof-flight of the quantum signal when performed with moving terminals. The position is required for the pointing and tracking, the distance can be calculated out of
this telemetry data. Alternatively, periodic bright beacon pulses at a different
wavelength could be employed to lock the timing. Thereby, varying Doppler shifts
due to the terminal motion will slowly change the repetition frequency whose
compensation can be calculated before the terminal pass from its telemetric data.
21
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