Free-Space Quantum Cryptography

Ivan Ordavo

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[...] The only encryption procedure which has been shown to be unbreakable is the one-time pad, a symmetric key algorithm. Unfortu...

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[...] The only encryption procedure which has been shown to be unbreakable is the one-time pad, a symmetric key algorithm. Unfortunately, some major drawbacks, key distribution above all, make this process hard to implement in the framework of classical information theory. Nevertheless, though this deficiency, it turns out that quantum information theory is able to provide a way-out to this problem. Unlike classical cryptography, which uses mathematical techniques to restrict the amount of eavesdropped information, quantum cryptography exploits the quantum character of nature to ensure secure communication between two trusted parties. This new issue, known under the name of Quantum Key Distribution (QKD), provides the two parties, Alice and Bob, with a setup to generate a secret key, which can be used afterwards in the encryption/decryption process of the secret message (e.g. with one-time pad). Within this scheme, single key bits are encoded in states of a quantum mechanical system (e.g. polarization states of a photon), and then distributed between two or more parties. If an eavesdropper would ever attempt to intercept key bits, he has to carry out a measurement on a quantum mechanical system, unavoidably changing its status, hence introducing errors. [...] In this work we present an experimental implementation of such a scheme, in which raw key bits are encoded in four different polarization states of photons. Using the first proposed quantum encoding protocol, the BB84, and weak coherent pulses from a laser source, we could realize a stable link between transmitter and receiver units over a free-space distance of 500 m. Software-based procedures for key extraction and privacy amplification lead to the final shared secure key. This thesis-work is articulated in four main sections: In the first (Chap. 2), we provide the reader with a wide overview of classical cryptographic methods, how they work and their security issues. The second (Chap. 3) illustrates the main concepts of QKD and the underlying physics involved in them. The third section (Chap. 4) deals with the description of our test-setup located in downtown Munich, with particular attention paid to transmitter/receiver units and source spectral selection. The last part describes the applied procedures for thermal management, which aims to stabilize the spectral characteristics of the source.