An Analysis of the Secure Key Distribution in Space Based Quantum Communication László Bacsárdi1,2, Máté Galambos1, Sándor Imre1, András Kiss2 1Department
of Telecommunications, Budapest University of Technology and Economics, Hungary 2Institute of Informatics and Economics, University of West Hungary bacsardi@hit.bme.hu, galambos@mcl.hu, imre@hit.bme.hu, kissa@gain.nyme.hu
Quantum Communications • From the engineering point of view, the quantum circuits built from different quantum gates provide many possibilities to perform computational calculations in a more efficient way than the nowadays used traditional computers. • Although quantum computers are still the tools of the future, there are promising quantum based applications, mainly in the field of communication. quantum based satellite communications
The free-space quantum communication can be extended to ground-tosatellite or satellite-satellite quantum communication, which could be an ideal application for global quantum cryptography.
Research problem • From mathematical point of view, the channel itself is an abstract transformation, which performs an identity transformation in an ideal case, or causes errors and decoherence in real world. But for engineers, the properties of the quantum channels are very important. For the quantum satellite communications a quantum channel with a length of 300–36,000 km is needed.
How the performance of quantum algorithms will be affected if the physical layer is changed from optical fiber to free-space, and the length of the channel increases dramatically?
Figure 1. Transmission of classical information through the satellite quantum channel. We encode the classical information into a quantum bit, then we send the quantum bit over the quantum channel. At the end, we have to decode and measure it to get back the classical information.
Modeling the quantum based satellite communications • We dealt with the theoretical analysis of weak laser signal based satellite communications. • We took our current technology into account, and we developed a physical model to describe the quantum communication over space-space, Earth-space and space-Earth links. The effects of aerosols and the optical turbulence of the atmosphere on quantum communication, and the finite size of the detectors and the beam spreading induced by diffraction were taken into account as well • Based on our model, we developed a simulator software named Quantum Satellite Communication Simulator. • We analyzed the performance of three quantum key distribution (QKD) protocols—BB84, B92, S09—on different Earth-space and space-space links.
Figure 2. Two user interfaces of our Quantum Satellite Communication Simulator program. In the right one, a quantum key distribution network built by different satellites is illustrated and analyzed
Figure 3. The Quantum Bit Error Rate (QBER) of the BB84 quantum key distribution protocol between a ground station and a satellite is illustrated in the function of the orbit. The uplink direction is represented by the dashed line, while downlink direction by the continuous line. The ground station is located in midlatitude climate, season is summer and weather is clear (visibility of 23 km).
Weather
Uplink/Downlink
Value
BB84
Protocol
clear
uplink
0.032
BB84
clear
downlink
0.0017
BB84
hazy
uplink
0.0604
BB84
hazy
downlink
0.0031
B92
clear
uplink
0.0639
B92
clear
downlink
0.0032
B92
hazy
uplink
0.1207
B92
hazy
0.006
S09
clear
S09
hazy
downlink uplink and downlink uplink and downlink
0.3754 0.8267
Table 1. Quantum Bit Error Rate (QBER) in B84, B92 and S09 protocol in clear and hazy weather conditions for given parameter set. The diameter of the aperture is 0.5 m, zenith angle is 20 degree.
Figure 4. QBER of the Bennett-Brassard 1984 (BB84) protocol in space–Earth communication at tropical climate for two weather conditions. Labels over the graphs indicate Bob’s detector diameter in meters. The satellite is orbiting at 300 km height. Clear weather: visibility of 23 km, hazy weather: visibility of 5 km. The condition of feasibility is that the QBER has to be lower than 11%.
Our research group at Mobile Communications and Quantum Technologies Laboratory
Future research • Based on our quantum channel model, we could analyze what parameters we need to implement a satellite quantum channel for Earth-satellite and satellite-satellite communication. • Another interesting question is how a real network can work in the future. A network of ground stations and satellites, which tries to enable global satellite based quantum key distribution.
• The research group consists of experts on quantumcomputation and communications and on engineering. The researcher team has published scientific results in the field of quantum computation and communications, quantum cryptography and quantum information theory. The members of the research group are young, ambitious and motivated. • Prof. Sándor Imre • Dr. László Bacsárdi • László Gyöngyösi, PhD candidate • Dóra Bányai, BSc student • Márton Bérces, PhD student • Máté Galambos, MSc student + András Kiss (University of West Hungary) Acknowledgement
The work is connected to the COST Action MP1006 Fundamental Problems in Quantum Physics.
2nd Virtual Nanotechnology Poster Conference, 10–14 September 2012, www.nanopaprika.eu