Communication protocols based on quantum states of light have attracted increasing interest in the recent years, since they offer the possibility of enhancing either the communication capacity or the security by exploiting the very quantum nature of the information carriers. While first theoretical ideas and practical realizations started from discrete variables (DC) coding (practically based on the weak laser beams and SPDC-generated photon pairs), the continuous variables (CV) approach (supposing the use of coherent beams and entangled beam pairs) to the quantum information processing is aimed at the increase of the effectiveness and reliability of the quantum information protocols. Though being the subject to the environmental influence and thus performing always imperfectly, the CV-based quantum systems require the study of the effect the environment has on the quantum information processing schemes which is done by analyzing the perturbance of the corresponding quantum states. In this work we address M-ary communication channels based on photon-number continuous-variable multiphoton entangled states (PNES) in the conditions of the realistic lossy optical media.
Communication protocols based on quantum states of light have attracted increasing interest in the recent years, since they offer the possibility of enhancing either the communication capacity or the security by exploiting the very quantum nature of the information carriers. While first theoretical ideas and practical realizations started from discrete variables (DC) coding (practically based on the weak laser beams and SPDC-generated photon pairs), the continuous variables (CV) approach (supposing the use of coherent beams and entangled beam pairs) to the quantum information processing is aimed at the increase of the effectiveness and reliability of the quantum information protocols. Though being the subject to the environmental influence and thus performing always imperfectly, the CV-based quantum systems require the study of the effect the environment has on the quantum information processing schemes which is done by analyzing the perturbance of the corresponding quantum states. In this work we address M-ary communication channels based on photon-number continuous-variable multiphoton entangled states (PNES) in the conditions of the realistic lossy optical media.
We address binary and quaternary communication channels based on correlated multiphoton two-mode states of radiation in the presence of losses. The protocol are based on photon number correlations and realized upon choosing a shared set of thresholds to convert the outcome of a joint photon number measurement into a symbol from a discrete alphabet. In particular, we focus on channels built using feasible photon-number entangled states (PNES) as two-mode coherently-correlated (TMC) or twin-beam (TWB) states and compare their performances with that of channels built using feasible classically correlated (separable) states. We found that PNES provide larger channel capacity in the presence of loss, and that TWB-based channels may transmit a larger amount of information than TMC-based ones at fixed energy and overall loss. Optimized bit discrimination thresholds, as well as the corresponding maximized mutual information, are explicitly evaluated as a function of the beam intensity and the loss parameter. The propagation of TMC and TWB in lossy channels is analyzed and the joint photon number distribution is evaluated, showing that the beam statistics, either sub-Poissonian for TMC or super-Poissonian for TWB, is not altered by losses. Although entanglement is not strictly needed to establish the channels, which are based on photon-number correlations owned also by separable mixed states, purity of the support state is relevant to increase security. The joint requirement of correlation and purity individuates PNES as a suitable choice to build effective channels. The effects of losses on channel security are briefly discussed.
Distributing and sharing entanglement at a distance is a key ingredient in many future quantum communication protocols, however entanglement is a fragile resource and can break down upon interacting with the environment. Within this thesis we present two possible entanglement swapping protocols, and show that these protocols are resilient to small levels of photonic losses. We propose the use of these protocols in quantum communication schemes that require shared entangled qubits, in the form of a Bell state. \nThe input states to our proposed protocols are hybrid entangled states, which are discrete-variable and continuous-variable entangled states. We use the vacuum and \nsingle photon Fock state as our discrete half, which is stationary in our entanglement swapping protocol, whereas the continuous variable half is modelled as travelling through lossy optical fibre before being measured. The first protocol uses coherent states in a superposition as propagating modes in our entanglement swapping setup, \nwhereas the second, more complicated, protocol uses superposed cat states. \nWe model photonic losses by applying a beam-splitter of transmission T to our propagating continuous variable modes, along with an input vacuum state. We also model the more realistic circumstance in which the losses in these two continuous variable modes are not equal. We then detect these continuous variable modes using a vacuum projective measurement and balanced homodyne detection. We also investigate homodyne measurement imperfections and non-ideal outcomes, as well as success probabilities of these measurement schemes. We calculate the entanglement negativity and linear entropy of our final two qubit state, as well as fidelity against the |\\Phi^+> Bell state in the coherent state protocol, and a phase-rotated |\\Phi^+(\\alpha)> Bell state in the cat state regime. \nWe demonstrate that a small amount of loss mismatch does not destroy the overall entanglement, thus demonstrating the physical practicality of this protocol.
We address M -ary communication channels based on entangled two-mode states of radiation in the presence of losses. In particular we focus on channels build by two-mode coherently-correlated (TMC) or twin-beam (TWB) states. Optimized bit discrimination thresholds, as well as the corresponding maximized mutual information, are explicitly evaluated as a function of beam intensities and loss parameters for binary and quaternary alphabets. The evolution of the two entangled support states in lossy channels is analyzed and the joint photon number distribution is evaluated, showing that the beam statistics (either sub-Poissonian for TMC or super-Poissonian for TWB) is not altered by channel losses. The effects of losses on the channel security is discussed.
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
Quantum communication enables various technological possibilities that are hard or impossible with classical communication. These include secure distribution of keys, transfer of quantum information and distributed quantum computation and sensing. Quantum communication relies on the ability to transmit a quantum state over some distance. Exponential photon loss from fibre inevitably limits the distance one would be able to achieve this over and therefore make use of these promising new technologies.Quantum repeaters have been proposed as a solution to this problem. Elementary repeater protocols involve dividing the channel into smaller segments with repeater nodes, distributing entanglement between these nodes and then performing swapping operations to connect entangled pairs and purification to correct operation errors. In the years since these elementary protocols, quantum repeaters have been through extensive theoretical improvements including utilising the benefits of quantum error correction to improve efficiency.However, the vast majority of these protocols are for discrete encodings of quantum information. There is significant interest in using continuous variable encodings of quantum information for quantum communication applications. This is motivated in part by the ease of generating, manipulating and detecting quantum states and additionally the increased compatibility with existing infrastructure. In this thesis we present one of the first quantum repeater protocols for continuous variable encodings of quantum information.The continuous variable quantum repeater we present in this thesis is based on concatenated error correction protocols that work to correct loss on any optical field state. We thoroughly investigate operation of this error correction protocol including the trade-offs associated with real implementations, various modifications to the error correction protocol as well as whether it can be implemented with current technology.We illustrate how these error correction protocols may be structured to form a quantum repeater that inhibits the exponential photon loss problem with a resource cost that scales polynomially with distance. We show that our continuous variable repeater is able to beat the fundamental bound on quantum communication without a repeater. Finally, we compare the performance of our first generation continuous variable repeater to existing first generation discrete variable protocols.
Quantum communication can realize secure information transmission based on the fundamental principles of quantum mechanics. Photon is a crucial information carrier in quantum communication. The photonic quantum communication protocols require the transmission of photons or photonic entanglement between communicating parties. However, in this process, photon transmission loss inevitably occurs due to environmental noise. Photon transmission loss significantly reduces the efficiency of quantum communication and even threatens its security, so that it becomes a major obstacle for practical long-distance quantum communication. Quantum noiseless linear amplification (NLA) is a promising method for mitigating photon transmission loss. Through local operations and post-selection, NLA can effectively increase the fidelity of the target state or the average photon number in the output state while perfectly preserving the encoded information of the target state. As a result, employing NLA technology can effectively overcome photon transmission loss and extend the secure communication distance. In this paper, the existing NLA protocols are categorized into two types, i.e. the NLA protocols in DV quantum systems and CV quantum systems. A detailed introduction is given to the quantum scissor (QS)-based NLA protocols for single photons, single-photon polarization qubits, and single-photon spatial entanglement in the DV quantum systems. The QS-based NLA can effectively increase the fidelity of the target states while perfectly preserving its encodings. In recent years, researchers have made efforts to study various improvements to the QS-based NLA protocols. In the CV quantum systems, researchers have adopted parallel multiple QS structure and generalized QS to increase the average photon numbers of the Fock states, coherent states and two-mode squeezed vacuum states. In addition to theoretical advancements, significant progress has also been made in the experimental implementations of NLA. The representative experimental demonstrations of QS-based NLA protocols are summarized. Finally, the future development directions for NLA to facilitate its practical applications are presented. This review can provide theoretical support for practically developing NLA in the future.