Best QBER graph obtained  

Quantum cryptography systems

a master thesis done in Quantum Cryptography group at the Department of Electronics and Telecommunications

Student:  Mikhail Chizhov,
— exchange from Radiophysics DepartmentSt. Petersburg State Polytechnical University
Supervisors (Norway):  Associate Professor Astrid Dyrseth,
Adjunct Professor Dag R. Hjelme
Supervisor (Russia):  Associate Professor Andrey Medvedev,
Delivered:  October 2004


This work consists of two separate parts, one done at the Radiophysics Department at SPbSPU and another done at the Department of Electronics and Telecommunications at NTNU. The chapters that belong to the first part are written in Russian while the ones that belong to the second part (chapters 2.2, 5—9) are written in English. The titles of the latter chapters are also given in English in the table of contents.

The entire thesis is available in PDF (22MB) or Word 97 (12MB) formats.

Source file of the interconnections diagram on page 121: Visio 2003 (290KB)


The part of the thesis written in English describes tests of fiber-optic quantum key distribution (QKD) system. Phase coding, BB84 protocol, active phase tracking in the interferometer and software-based afterpulse blocking for avalanche photo diode (APD) single-photon detector were implemented. Although the best recorded quantum bit error rate (QBER) was 4%, the system was unstable and QBER fluctuated in the 4% to 13% range during the experiments. Replacement of defective electronic and optical parts is necessary before a better QKD demonstration can be made. A detailed description of the set-up and software is included. Also, the principles of cryptography are reviewed, classical cryptosystems and QKD systems are compared, and the advantages of using phase-coding QKD set-up are explained.

The part of the thesis written in Russian describes design and implementation of a thermoelectric cooling system for fiber-pigtailed APDs. It occupies a standard 2U (3.5") rackmount (H89xW422xL606 mm, we used IPC-2025BK rackmount chassis by Skyhawk with pre-installed fans). The cooling system has a three-stage design with closed-loop pumped liquid heat exchanger between the first and second thermoelectric stages, and first stage cooled by fans. The temperature of two APDs (Epitaxx EPM239BA) can be maintained as low as -58°C and set with the accuracy of ±0.1°C, thanks to the use of an automatic temperature control system, which has the proportionally integrated regulator structure and whose design is detailed in the thesis. Tests have shown that the cooling system maintains the APD temperature with the specified accuracy, and attains the set temperature after power-up in less than 400 s.
Note #1: during the first tests, APD temperature of -50°C was attained with a thermoelectric cooler and a fan. Such a large rig with liquid coolant had to be built to bring the temperature an additional 8°C down, which resulted in noticeably better APD performance.
Note #2: this automatic temperature control system is made with analog electronics. However, plans were made for a computer-controlled temperature control system.