Quantum effects and noise in optical communications

Colin J. McKinstrie

The quantum properties of optical devices are important • In conventional optical communication systems, quantum effects are responsible for noise inherent in the si (by amplifiers and attenuators) and noise in the detectors. gnals, noise added during transmission • Noise obscures the signals and lowers the information capacities. • There are many schemes for quantum communication and computation that rely on the quantum properties of light. • • Examples include quantum key distribution (QKD) and linear optical quantum computation (LOQC). • Common requirements for quantum information experiments include photon generators (singlet or pair) and (distortionless) photon frequency-convertors.

C. Bendjaballah, Osamu Hirota, Serge Reynaud

Stamatios V. Kartalopoulos

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.

Philippe B. Gallion, Francisco Javier Mendieta, Shifeng Jiang

Konrad Banaszek

Bhaskar Roy Bardhan

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.