Quantum key distribution (QKD) is one of the most mature quantum technologies and can provide quantum-safe security in future communication networks. Since QKD in fiber is limited to a range of few hundred kilometers, one approach to bridge continental scale distances may be the use of high altitude pseudo satellites (HAPS) as mobile trusted nodes in the stratosphere. In parallel, free-space laser communication for high rate data transmission has been a subject of research and development for several decades and its commercialization is progressing rapidly. Important synergies exist between classical free-space communication and QKD systems since the quantum states are often implemented using the same degrees of freedom such as polarization or field amplitude and phase. These synergies can be used to benefit from the progress in classical free-space laser communication in QKD applications. In this paper, the use case of QKD in a stratospheric environment is described wherein HAPS may serve as relay station of secret keys and encrypted data. The mission scenario and HAPS capabilities are analyzed to derive special requirements on the stratospheric laser terminal, the link geometry and the ground segment with respect to a feasibility demonstration. To obtain a flexible and compatible system, discrete variable and continuous variable QKD protocols are considered to be implemented side by side in the HAPS payload. Depending on the system parameters, it can be beneficial to use the one or the other kind of protocol. Thus, a direct comparison of both in one and the same system is of scientific interest. Each of the protocols has particular requirements on coupling efficiency and implementation. Link budget calculations are performed to analyze possible distances, key rates and data transmission rates for the different schemes. In case of the QKD system, the mean coupling efficiency is of main interest, i.e. signal fluctuations arising from atmospheric turbulence must be taken into account in the security proof, but the buffered key generation relaxes real-time requirements. This is different to classical communications, where the corresponding fading loss must be assessed. A system architecture is presented that comprises the optical aircraft terminal, the optical ground terminal and the most important subsystems that enable implementation of the considered QKD protocols. The aircraft terminal is interfaced with the dedicated quantum transmitter module (Alice) and the ground station with the dedicated quantum receiver module (Bob). The optical interfaces are SMF couplings which put high requirements on the receiving optics, in particular the need for wave-front correction with adaptive optics. The findings of the system study are reviewed and necessary next steps pointed out.
The aim of the QIPS project (financed by ESA) is to explore quantum phenomena and to demonstrate quantum communication over long distances. Based on the current state-of-the-art a first study investigating the feasibility of space based quantum communication has to establish goals for mid-term and long-term missions, but also has to test the feasibility of key issues in a long distance ground-to-ground experiment. We have therefore designed a proof-of-concept demonstration for establishing single photon links over a distance of 144 km between the Canary Islands of La Palma and Tenerife to evaluate main limitations for future space experiments. Here we report on the progress of this project and present first measurements of crucial parameters of the optical free space link.
A worldwide growing interest in fast and secure data communications pushes technology development along two lines. While fast communications can be realized using laser communications in fiber and free-space, inherently secure communications can be achieved using quantum key distribution (QKD). By combining both technologies in a single device, many synergies can be exploited, therefore reducing size, weight and power of future systems. In recent experiments we demonstrated quantum communications over large distances as well as between an aircraft and a ground station which proved the feasibility of QKD between moving partners. Satellites thus may be used as trusted nodes in combination with QKD receiver stations on ground, thereby enabling fast and secure communications on a global scale. We discuss the previous experiment with emphasis on necessary developments to be done and corresponding ongoing research work of German Aerospace Center (DLR) and Ludwig Maximilians University Munich (LMU). DLR is performing research on satellite and ground terminals for the high-rate laser communication component, which are enabling technologies for the QKD link. We describe the concept and hardware of three generations of OSIRIS (Optical High Speed Infrared Link System) laser communication terminals for low Earth orbiting satellites. The first type applies laser beam pointing solely based on classical satellite control, the second uses an optical feedback to the satellite bus and the third, currently being in design phase, comprises of a special coarse pointing assembly to control beam direction independent of satellite orientation. Ongoing work also targets optical terminals for CubeSats. A further increase of beam pointing accuracy can be achieved with a fine pointing assembly. Two ground stations will be available for future testing, an advanced stationary ground station and a transportable ground station. In parallel the LMU QKD source size will be reduced by more than an order of magnitude thereby simplifying its integration into future free-space optical communication links with CubeSats.
Unlike currently implemented encryption schemes, Quantum Key Distribution provides a secure way of generating and distributing a key among two parties. Although a multitude of research platforms has been developed, the integration of QKD units within classical communication systems remains a tremendous challenge. The recently achieved maturity of integrated photonic technologies could be exploited to create miniature QKD add-ons that could extend the primary function of various existing systems such as mobile devices or optical stations. In this work we report on an integrated optics module enabling secure short-distance communication for, e.g., quantum access schemes. Using BB84-like protocols, Alice's mobile low-cost device can exchange secure key and information everywhere within a trusted node network. The new optics platform (35×20×8mm) compatible with current smartphone's technology generates NIR faint polarised laser pulses with 100MHz repetition rate. Fully automated beam tracking and live basis-alignment on Bob's side ensure user-friendly operation with a quantum link efficiency as high as 50% stable over a few seconds.
The resolution of optical scanning microscopes is limited by the spot size of the scanning beam. It has been proposed by Boto1 et al. and Nasr2 et al. that momentum entangled photons can be focused more tightly than classical light. Thus they should enable one to break the di ractive resolution limit. We report experiments measuring the spatial distribution of strong focussed momentum entangled photon pairs at a center wavelength of 812 nm from a spontaneous parametric down conversion source. Using a single gold stripe deposited on a glass surface as test object we observe a two-photon spot size as small as 194 nm, thereby beating the di raction limit even for conventional two-photon microscopy.
To enable global scale quantum key distribution1-3 (QKD), satellite based systems 4,5 are the most promising approach. So far, free-space QKD has already been demonstrated on communication channels with attenuation comparable to satellite downlinks,6 and classical laser communications with satellites and aircrafts is heavily explored.7-10 Here, combining both these challenges, we demonstrate an aircraft to ground QKD transmission obtaining a sifted key rate of 145 bit/s and a QBER, larglely dominated by background events and stray light, of 4:8 %.
Quantum Key Distribution (QKD), either fiber based or free-space, allows for provably secure key distribution solely
based on the laws of quantum mechanics. Feasibility of QKD systems in aircraft-ground links was demonstrated with a
successful key exchange. Experiment flights were undertaken during night time at the site of the German Aerospace
Center (DLR) Oberpfaffenhofen, Germany. The aircraft was a Dornier 228 equipped with a laser communication
terminal, originally designed for optical data downlinks with intensity modulation and direct detection. The counter
terminal on ground was an optical ground station with a 40 cm Cassegrain type receiver telescope. Alice and Bob, as the
transmitter and receiver systems usually are called in QKD, were integrated in the flight and ground terminals,
respectively. A second laser source with 1550 nm wavelength was used to transmit a 100 MHz signal for
synchronization of the two partners. The so called BB84 protocol, here implemented with faint polarization encoded
pulses at 850nm wavelength, was applied as key generation scheme. Within two flights, measurements of the QKD and
communication channel could be obtained with link distance of 20 km. After link acquisition, the tracking systems in the
aircraft and on ground were able to keep lock of the narrow QKD beam. Emphasis of this paper is put on presentation of
the link technology, i.e. link design and modifications of the communication terminals. First analysis of link attenuation,
performance of the QKD system and scintillation of the sync signal is also addressed.
Photon sources for multi-photon entanglement experiments are commonly based on the process of spontaneous
parametric down conversion. Due to the probabilistic photon production, such experiments suffer from low multiphoton
count rates. To increase this count rate, we present a novel SPDC pump source based on a femtosecond
UV enhancement cavity that increases the available pump power while maintaining a high repetition rate of
80MHz. We apply the cavity as photon source for realizing symmetric, multi-partite entangled Dicke states,
which are observed with a high rate and high fidelity. We characterize the observed Dicke states of up to six
photons using efficient tools exploiting the state's symmetries.
Entanglement between quantum objects can be used to enhance the sensitivity of measurements. We demonstrate this
effect by using entangled multi-photon states to go beyond the shot noise limit when observing polarization rotations.
We present strategies to obtain different classes of three and four photon entangled symmetric states from a
single experimental setup. The basic idea originates from the property of the symmetric Dicke state with two
excitations to connect the two inequivalent types of genuine tripartite entanglement. We experimentally confirm
the distinct types of entanglement of the observed states. We further propose an extension of the applied scheme
that allows one to obtain different classes of four-photon entanglement by adding a fifth photon. The requirement
of a single fifth photon is currently a technical challenge, and thus we consider the approach of using a strongly
attenuated weak coherent beam instead.
We report on the experimental implementation of a BB84-type quantum key distribution protocol over a 144 km free-space link using weak coherent laser pulses. The security was assured by employing decoy state analysis, and optimization of the link transmission was achieved with bi-directional active telescope tracking. This enabled us to distribute a secure key at a rate of 11 bits/s at an attenuation of about 35dB. Utilizing a simple transmitter setup and an optical ground station capable of tracking spacecraft in low earth orbit, this outdoor experiment demonstrates the feasibility of global key distribution via satellites.
Quantum key distribution (QKD)1 is the first method of quantum information science that will find its way into our everyday life. It employs fundamental laws of quantum physics to ensure provably secure symmetric key generation between two parties. The key can then be used to encrypt and decrypt sensitive data with unconditional security. Here, we report on a free space QKD implementation over a distance of 480 m using strongly attenuated laser pulses. It is designed to work continuously without human interaction. Until now, it produces quantum keys unattended at night for more than 12 hours with a sifted key rate of more than 50 kbit/s on average and a quantum bit error rate between 3% and 5%.
Coding data bits in the phase or polarization state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fiber and free-space quantum cryptography apparatus used for secure exchange of keys. Beyond this state of the art, various quantum communication schemes are being studied including entangled state key exchange quantum dense coding, state teleportation, and entanglement swapping. The feasibility, advantages and disadvantages of space based realisations of these novel schemes are discussed.
Multiphoton entanglement is the basis of many quantum
communication schemes, quantum cryptographic protocols, and
fundamental tests of quantum theory. Spontaneous parametric
down-conversion is the most effective source for polarization
entangled photon pairs. Here we show, that a entangled 4-photon
state can be directly created by parametric down-conversion. This
state exhibit perfect quantum correlations and a high robustness
of entanglement against photon loss. We have used this state for
four-particle test of local realistic theories. Therefore this
state can be used for new types of quantum communication. We also
report on possibilities for the experimentally realization of a
3-photon entangled state, the so called W-state, and discuss some
of its properties.
Quantum cryptography bases the security of key exchange on the laws of quantum physics and will become the first application of quantum information methods. Here we present the design of novel hardware components which enabled the demonstration of secure key exchange over a 23.4 km free-space link.