New methods in optical communications rely on the quantum properties of photons to assure link security and data privacy. However, optical quantum communications make several assumptions, among them that the quantum properties of photons are preserved over many-kilometer long continuous fiber, which additionally is free from absorbing and scattering centers. In this paper we review quantum communication methods based on photon polarization, single and entangled, and we investigate the impact of noise sources on quantum signal propagation.
New methods in optical communications rely on the quantum properties of photons to assure link security and data privacy. However, optical quantum communications make several assumptions, among them that the quantum properties of photons are preserved over many-kilometer long continuous fiber, which additionally is free from absorbing and scattering centers. In this paper we review quantum communication methods based on photon polarization, single and entangled, and we investigate the impact of noise sources on quantum signal propagation.
Quantum laws can be used to implement secure communication channels; this has been named quantum cryptography. In quantum cryptography the security does not depend of limited computational power, but is inherent to the laws that govern the propagation and detection of single and entangled photons. We show how single and entangled photon-pairs can be efficiently generated using four-wave mixing in optical fibers. We analyze the source statistics, degree of entanglement and impact of spontaneous Raman scattering. By coding information in the photons polarization we are able to transmit quantum information over 20 km of standard single mode fiber.
The last two decades have witnessed an exciting advanced research field that stems from non-classical atomic theory, the quantum mechanics. This research promises an interesting applicability in computation, known as quantum computation, and also in secure data communications, known as quantum cryptography. Quantum cryptography capitalizes on the inherent random polarization state of single photons, which are associated with binary logic values. Because the polarization state of a photon is not reproducible by an eavesdropper between the source and the destination, polarized photons are used with an intelligent algorithm to disseminate the cryptographic key with high security from the source to the destination, a process known as quantum key distribution. However, although the polarization state of a photon remains intact in free-space propagation, it does not remain so in dielectric medium and thus quantum cryptography is not problem-free. In this paper we review quantum cryptography and we identify the various steps in the quantum key identification process. We then analyze and discuss issues related to quantum key distribution that rise in pragmatic fiber-optic transmission and in communication network topologies. In addition, we identify a major weakness of the method that is prone to attacking and which incapacitates quantum cryptography in fiber communications.
Noise present in an environment has significant impacts on a quantum system affecting properties like coherence, entanglement and other metrological features of a quantum state. In this dissertation, we address the effects of different types of noise that are present in a communication channel (or medium) and an interferometric setup, and analyze their effects in the contexts of preserving coherence and entanglement, phase sensitivity, and limits on rate of communication through noisy channels. We first consider quantum optical phase estimation in quantum metrology when phase fluctuations are introduced in the system by its interaction with a noisy environment. By considering path-entangled dual-mode photon Fock states in a Mach-Zehnder optical interferometric configuration, we show that such phase fluctuations affect phase sensitivity and visibility by adding noise to the phase to be estimated. We also demonstrate that the optimal detection strategy for estimating a phase in the presence of such phase noise is provided by the parity detection scheme. We then investigate the random birefringent noise present in an optical fiber affecting the coherence properties of a single photon polarization qubit propagating through it. We show that a simple but effective control technique, called dynamical decoupling, can be used to suppress the effects of the dephasing noise, thereby preserving its ability to carry the encoded quantum information in a long-distance optical fiber communication system. Optical amplifiers and attenuators can also add noise to an entangled quantum system, deteriorating the non-classical properties of the state. We show this by considering a two-mode squeezed vacuum state, which is a Gaussian entangled state, propagating through a noisy medium, and characterizing the loss of entanglement in the covariance matrix and the symplectic formalism for this state. Finally, we discuss limits on the rate of communication in the context of sending messages through noisy optical quantum communication channels. In particular, we prove that a strong converse theorem holds under a maximum photon number constraint for these channels, guaranteeing that the success probability in decoding the message vanishes in the asymptotic limit for the rate exceeding the capacity of the channels.
Quantum communications can provide almost perfect security through the use of quantum laws to detect any possible leak of information. We discuss critical issues in the implementation of quantum communication systems over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers, and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse. We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing. We report results for different type of fibers to approach the maximum value of the Bell inequality. We model the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to increase the degree of entanglement. We encode information in the photons polarization and assess the use of wavelength and time division multiplexing based control systems to compensate for the random rotation of the polarization during transmission. We show that time division multiplexing systems provide a more robust solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed WDM optical communication systems without strongly penalizing the performance of the quantum channel. We discuss the process of retrieving information from the photons polarization. We identify the major impairments that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present results of an experimental performance assessment of the system quality through QBER measurements.
Quantum-augmented networks aim to use quantum phenomena to improve detection and protection against malicious actors in a classical communication network. This may include multiplexing quantum signals into classical fiber optical channels and incorporating purely quantum links alongside classical links in the network. In such hybrid networks, quantum protocols based on single photons become a bottleneck for transmission distances and data speeds, thereby reducing entire network performance. Furthermore, many of the security assumptions of the single-photon protocols do not hold up in practice because of the impossibility of manufacturing single-photon emitters. Multi-photon quantum protocols, on the other hand, are designed to operate under practical assumptions and do not require single photon emitters. As a result, they provide higher levels of security guarantees and longer transmission distances. However, the effect of channel and device noise on multiphoton protocols in terms of security, transmission distances, and bit rates has not been investigated. In this paper, we focus on channel noise and present our observations on the effect of various types of noise on multi-photon protocols. We also investigate the effect of topologies such as ring, star, and torus on the noise characteristics of the multi-photon protocols. Our results show the possible advantages of switching to multi-photon protocols and give insights into the repeater placement and topology choice for quantum-augmented networks.
The last two decades have witnessed an exciting advanced research field that stems from non-classical atomic theory, the quantum mechanics. This research promises an interesting applicability in computation known as quantum computation, and also in secure data communications, known as quantum cryptography Quantum cryptography capitalizes on the inherent random polarization state of single photons, which are associated with binary logic values. Because the polarization state of a photon is not reproducible by an eavesdropper between the source and the destination polarized photons are used with an intelligent algorithm to disseminate the cryptographic key with high security from he source to the destination, a process known as quantum key distribution. However, although the polarization state of a photon remains intact in free- space propagation, it does not remain so in dielectric medium and thus quantum cryptography is not problem- free. In this paper we review quantum cryptography and we identify the various steps in the quantum key identification process. We then analyze and discuss issues related to quantum key distribution that rise in pragmatic fiberoptic transmission and in communication network topologies. In addition, we identify a major weakness of the method that is prone to attacking and which incapacitates quantum cryptography in fiber communications.