We show how the techniques developed for long distance quantum key distribution in op-tical fibers can be used to demonstrate other quantum information processing and com-munication protocols. We present a fiber optics realization of the Deutsch-Jozsa and Bernstein-Vazirani algorithms. We describe a method, called ”error filtration”, for re-ducing errors in quantum communication channels, and present an experimental imple-mentation thereof. We discuss the cryptographic primitive of string flipping, and present an experimental implementation which has higher security than achievable using any classical protocol.
We show how the techniques developed for long distance quantum key distribution in op-tical fibers can be used to demonstrate other quantum information processing and com-munication protocols. We present a fiber optics realization of the Deutsch-Jozsa and Bernstein-Vazirani algorithms. We describe a method, called ”error filtration”, for re-ducing errors in quantum communication channels, and present an experimental imple-mentation thereof. We discuss the cryptographic primitive of string flipping, and present an experimental implementation which has higher security than achievable using any classical protocol.
We briefly present three topics in quantum communication. First of all a method, called error filtration, to reduce errors during quantum communication. We apply this method to quantum cryptography in a noisy environment, and describe an experimental realisation thereof. Second we describe an experimental realisation of quantum string flipping, in which two parties that do not trust each other want to generate a string of random bits. Finally we present the experimental realisation of a source of photon pairs at telecommunication wavelengths based on parametric fluorescence in periodically poled twin hole fibers.
Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to noise. We have realized a fiber optics demonstration of this method and illustrated its potentialities by carrying out the optical part of a quantum key distribution scheme over a line whose phase noise is too high for a standard implementation of BB84 to be secure. By filtering out the noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.
In today's communication networks, the widespread use of optical fiber and passive optical elements allows to use quantum key distribution (QKD) in the current standard optical network infrastructure. In the past few years, quantum key distribution schemes have attracted much study. The security of modern cryptographic methods, like asymmetric cryptography, relies heavily on the problem of factoring large integers (Rivest et al., 1978), (Schneier, 1996). In the future, if quantum computers become reality, any information exchange using current classical cryptographic schemes will be immediately insecure (Shor, 1994), (Shor, 1997). Current classical cryptographic methods are not able to guarantee longterm security. Other cryptographic methods, with absolute security must be applied in the future.
Entanglement and superposition are foundations for the emerging field of quantum commu‐ nication and information processing. These two fundamental features of quantum mechanics have made quantum key distribution unconditionally secure (Scarani et al., 2009; Weedbrook et al., 2010) compared to communication based on classical key distribution. Currently, implementation of an optical quantum communication is mainly based on discrete and continuous quantum variables. They are usually generated through nonlinear interaction processes in χ(2) (Kwiat et al., 1995) and χ(3) (Lee et al., 2006,2009) media. Discrete-variable qubit based implementations using polarization (Liang et al., 2006, 2007; Chen et al. 2007, 2008; Sharping et al., 2006) and time-bin (Brendel et al., 1999; Tittel et al., 1998, 1999) entanglement have difficulty to obtain unconditional-ness, and also usually have low optical data-rate because of post-selection technique with low probability of success in a low efficient single photon detector at telecom-band (Liang et al., 2005, 2006, 2007). Continuous-variable imple‐ mentations using quadrature entanglement (Yonezawa et al., 2004; Bowen et al., 2003; Silberhorn et al., 2002) and polarization squeezing (Korolkova et al., 2002) can have high efficiency and high optical data-rate because of available high speed and efficient homodyne detection. However, the quality of quadrature entanglement is very sensitive to loss, which is imperfect for implementing any entanglement based quantum protocols over long distance. Continuous-variable protocols that do not rely on entanglement, for instance, coherent-state based quantum communication (Yuen, 2004; Corndorf et al., 2003; Barbosa et al., 2003; Grosshans et al., 2002, 2003; Qi et al., 2007; Wilde Qi et al., 2008), are perfect for long distance optical communication. Several experimental approaches were taken to resolve transmission loss for long distance optical communication by using coherent light source. Optical wave
We review recent advances in the field of quantum cryptography, focusing in particular on practical implementations of two central protocols for quantum network applications, namely key distribution and coin flipping. The former allows two parties to share secret messages with information-theoretic security, even in the presence of a malicious eavesdropper in the communication channel, which is impossible with classical resources alone. The latter enables two distrustful parties to agree on a random bit, again with information-theoretic security, and with a cheating probability lower than the one that can be reached in a classical scenario. Our implementations rely on continuous-variable technology for quantum key distribution and on a plug and play discrete-variable system for coin flipping, and necessitate a rigorous security analysis adapted to the experimental schemes and their imperfections. In both cases, we demonstrate the protocols with provable security over record long distances in optical fibers and assess the performance of our systems as well as their limitations. The reported advances offer a powerful toolbox for practical applications of secure communications within future quantum networks.
Quantum key distribution is an application of quantum physics that allows two distant partners to securely distribute shared random secrets which they can use for secure message encryption, authentication or other communication tasks. In this thesis, I study and implement several concepts, protocols and their components, in particular single photon detectors. I evaluate its compatibility with fiber infrastructures with the aim to improve applicability, performance and profitability of quantum key distribution in different scenarios. I present superconducting single photon detectors implemented for long distance quantum communication, as well as semiconductor detectors employing rapid sine gating schemes. Wavelength-multiplexing of quantum and classical communication channels is demonstrated to stabilize a polarization-encoded quantum channel against birefringence fluctuations, and to realize quantum and encrypted communication over one fiber. An implementation of the coherent one-way protocol is presented which features a hardware distillation engine, wavelength-multiplexing, and an analysis of finite key effects and authentication costs.