Quantum information emerged from some early findings on fundamental limits of quantum state processing (no-cloning theorem), yet innovative tasks that are not possible with classical information such as quantum teleportation, quantum key distribution, quantum error correction, and quantum algorithms themselves. Down listed are some projects I have worked on.

**Copy Resistance Encryption
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Alice and Bob, two parties can transmit an encrypted message such that Eve cannot copy the encrypted message. She will not get complete ciphertext, so she cannot do cryptanalysis. Thus, exposure of secret key sometime in future does Eve no good. This can be achieved by exploiting no-cloning property of quantum states usually represented as photon polarization in optical fibers. Much complicated protocols exists, and may not be easily implemented with present technology. The Uncloneable Encryption does that by means of quantum computing, however we were focused on information-theoretic proof/issues of our protocol. While it’s possible to do it with information-theoretic principles, the secure distance length heavily depends on physical imperfections which contribute additionally to other types of issues not seen in QKD setups. This work was done at the Fuji-Xerox Palo Alto Laboratory with Eleanor Rieffel and Wolfgang Polak [1,2]. They also have a new book on quantum information.

[1] Hrg, D., Rieffel, E. Copy Resistant Encryption. Palo Alto, USA, 2005. (In private communication)

[2] Hrg, D., Rieffel, E., Polak, W. Intern poster session: Exploring uncloneable encryption. Palo Alto Research Center, Palo Alto, CA, USA, 2005. (In private communication)

**Mathematical Modeling of Realistic Quantum Key Distribution (QKD) Platforms
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Realistic implementations of QKD protocols suffer from variety of problems that are mainly consisted of imperfect optical devices. In some way there is inherent stochastic component in each part, starting from fibers, detectors, to sources of photons. A Matlab GUI and simulator was implemented that outputs bit rates and error rates of different QKD protocols, taking into consideration realistic parameters such as photon sources (weak-coherent pulse and entangled based), fiber attenuation, detector efficiency, dark count probabilities, etc. Simplified visualization/modeling provided us with some interesting facts on bit and error rates as show in poster [3] and article on mathematical modeling [4]. This work was done at the quantum.at. Conceptually, it’s easy to understand how BB84 protocol works or what is the purpose of QKD. See for instance a paper for beginners [5].

[3] Hrg, D., Poppe, A., Fedrizzi, A., Blauenstainer, B., Huebel, H., Zeilinger, A. Poster: Security aspects and simulations of practical QKD platforms.,6th QIPC Workshop, Vienna, Austria, 2005.

[4] Hrg, D. Security aspects and simulations of practical QKD platfoms. 6th QIPC Workshop, Vienna, Austria, 2005.

[5] Hrg, D., Golub, M., Budin, L. Quantum cryptography and information security, pp. 269-275 in Proceedings of the 15th International Conference on Information and Intelligent Systems, IIS 2004, Varazdin, Croatia, September 22-24, 2004.

**Quantum Simulator & Foundations of Classical and Quantum Computing
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This simulator was done as a part of diploma work [6] where I was playing with Grover’s quantum algorithm and trying to devise novel searching strategies. Basically you can run Grover and visualize different states etc., while much of the work also focused on fundamentals, computability, computational complexity, and quantum mechanics. Actually, when looking it back now, it might serve as a nice course material for “Computability and Quantum Information Demistified”. There is also one beginner level presentation here.

[6] Hrg D. Simulator kvantnog računala. Diplomski rad. Zagreb, Fakultet elektrotehnike i računarstva, 2004. (on Croatian)