Quantum cryptography converts structural restrictions on quantum information into operational secrecy. This article develops the chain connecting nonorthogonal-state discrimination, trace distance and fidelity, the Helstrom bound, no-cloning, no-broadcasting, information-disturbance relations, conditional entropy, and privacy amplification. A linearity argument proves that no universal device can copy arbitrary pure states, while the mixed-state no-broadcasting theorem identifies pairwise commutativity as the exact boundary of exact broadcastability. These results explain why an eavesdropper cannot retain a perfect replica of every signal and wait for later basis disclosure. They are not, however, complete security proofs. Modern quantum key distribution must cover coherent attacks, quantum memory, public reconciliation leakage, imperfect sources and detectors, finite samples, authentication, and composition with later applications. The review connects state discrimination to adversarial guessing probability through conditional min-entropy and explains how entropic uncertainty and entanglement-based reductions transform observed statistics into a lower bound on extractable key. Approximate cloning, unambiguous discrimination, channel loss, side channels, and device-independent certification are examined as qualifications to elementary intuition. The central result is a hierarchy: nonorthogonality restricts distinguishability; data processing forbids perfect information amplification; parameter estimation bounds an adversary’s conditional states; and quantum-proof hashing yields a key close in trace distance to an ideal independent key. Cryptographic security therefore depends on quantum limitations embedded within a fully specified protocol and adversarial model.
Quantum cryptography converts structural restrictions on quantum information into operational secrecy. This article develops the chain connecting nonorthogonal-state discrimination, trace distance and fidelity, the Helstrom bound, no-cloning, no-broadcasting, information-disturbance relations, conditional entropy, and privacy amplification. A linearity argument proves that no universal device can copy arbitrary pure states, while the mixed-state no-broadcasting theorem identifies pairwise commutativity as the exact boundary of exact broadcastability. These results explain why an eavesdropper cannot retain a perfect replica of every signal and wait for later basis disclosure. They are not, however, complete security proofs. Modern quantum key distribution must cover coherent attacks, quantum memory, public reconciliation leakage, imperfect sources and detectors, finite samples, authentication, and composition with later applications. The review connects state discrimination to adversarial guessing probability through conditional min-entropy and explains how entropic uncertainty and entanglement-based reductions transform observed statistics into a lower bound on extractable key. Approximate cloning, unambiguous discrimination, channel loss, side channels, and device-independent certification are examined as qualifications to elementary intuition. The central result is a hierarchy: nonorthogonality restricts distinguishability; data processing forbids perfect information amplification; parameter estimation bounds an adversary’s conditional states; and quantum-proof hashing yields a key close in trace distance to an ideal independent key. Cryptographic security therefore depends on quantum limitations embedded within a fully specified protocol and adversarial model.
We develop a unified, operational, and falsifiable framework for quantum communication, quantum networks, and quantum cryptography as a conservative extension of three physical axioms: microscopic ontic definiteness with finite localization, the physical reality of the spacetime–vacuum substrate, and persistent causal retarded source–response coupling. Quantum states, density operators, entanglement, channels, and information are treated as mathematical or operational representations rather than additional fundamental ontologies. The framework derives completely positive trace-preserving maps from unitary system–environment dynamics and examines Kraus, Choi, Liouville, Stinespring, complementary-channel, and process-tensor representations. It establishes explicit interfaces for initial system–environment correlations, non-Markovian memory, channel discrimination, diamond-norm certification, and causal multitime processes. Classical, quantum, private, and entanglement-assisted capacities are distinguished from finite-blocklength and resource-normalized throughput. Optical-fiber, free-space, atmospheric, satellite, deep-space, microwave, atomic, spin, and continuous-variable links are incorporated through complete source–channel–receiver–detector forward models. Quantum networks are formulated as stochastic renewal, memory, swapping, purification, routing, and queueing systems subject to cut-set bounds, decoherence, synchronization error, congestion, latency, and energy constraints. For quantum key distribution, the security chain proceeds from source characterization and parameter estimation through smooth min-entropy, entropic uncertainty, error correction, authentication, privacy amplification, and composable key certification. BB84, decoy-state, entanglement-based, measurement-device-independent, device-independent, and continuous-variable protocols are treated with explicit finite-size and assumption boundaries. The theory preserves operational no-signaling, finite propagation speed, passivity, and complete energy accounting. Every performance or security claim requires independent calibration, nuisance covariance, held-out prediction, finite-sample validation, matched classical comparison, and an explicit rejection criterion. **Keywords** Quantum communication; quantum channels; quantum networks; quantum cryptography; quantum key distribution; open quantum systems; channel capacity; continuous-variable communication; quantum repeaters; process tensors; non-Markovian dynamics; composable security; finite-key analysis; no-signaling; detector forward models; experimental identifiability.
Quantum information theory is a modern branch of theoretical physics. One of its main goals is to interpret concepts of quantum physics. This leads to a deeper understanding of quantum theory. The most common examples of practical applications of basic quantum theory are quantum computation and quantum cryptography. Quantum cryptography provides secure communication between legitimate users even in the presence of an adversary by making possible the distribution of a secret key. It then allows error correction and privacy amplification, which is elimination of adversary information, through classical communication. In this thesis two important aspects of quantum key distribution are covered, namely robustness bounds with respect to provable entanglement for ideal protocols and practical quantum key distribution using two-way classical communication. In part one of the thesis, ideal quantum key distribution protocols and their robustness in terms of provable entanglement are discussed. The robustness bounds are proved for most general coherent attacks. These bounds for provable entanglement are already known to be 25% for the four-state protocol and 33% for the six-state protocol. We anticipate to provide a region in which the legitimate users share entanglement. This region is large for the four-state protocol and is reduced to a smaller region for the six-state protocol because of additional constraint on it. We also investigate the information cost which the adversary has to pay in order to reach these bounds. In part two we adopt a more practical approach. We investigate the limitation on distance of secure communication because of practical restrictions. In particular we investigate the restrictions due to the lack of single photon sources, the lossy channel and faulty detectors. These practical limitations have already been observed using one-way classical communication between legitimate users. It has been observed that it is actually the dark count rate that limit the distance up to which legitimate users can share a secret key. We have used two-way classical communication to postpone the effect of dark counts and increase the distance to considerable amount. For the purpose we have considered an optimal attack with respect to the disturbance that an eavesdropper creates while attacking. Any other format of attacking will increase the disturbance. We show that using two-way classical communication for post processing we can increase the distance of secure communication considerably.
We examine public broadcast, forward conceptual, and backward conceptual, Quantum channels in the context of communication protocols that are independent of secret keys. Given research directions of interest previously identified in arXiv: 1804.01797, besides converse upper bounds on the bit transmission rate obtained by the author in recent work (arXiv: 2507.03035), additional possibilities remain, including: (1) determining whether aspects of QKD dependent protocols can be incorporated into steps of QKD independent protocols; (2) whether there would be any amplification to the Quantum-classical performance gap that Alice and Bob can exploit towards prospective Quantum advantage; (3) formulating the conditions under which secrecy and authentication can be simultaneously achieved. To characterize the conditions for which secrecy can be achieved with high probability, we argue that there not only exists suitable protocols which enable Alice and Bob to map into the authenticated space of bit codewords with high probability, but also that forward conceptual channels, through cascading, can significantly increase Eve's probability of false acceptance. Albeit the fact that secrecy, along with conceputalizations of the public broadcast channel, were initially discussed by Maurer for QKD dependent protocols, determining whether aspects of such protocols can be adapted for unconditional security without the use of a secret key is of great interest to explore. We demonstrate that Eve's error probability, through the cascading procedure, can be analyzed with the Holevo information under an optimal decoder. Furthermore, through post-processing of the outputs of a Completely Positive Trace Preserving (CPTP) map, we also demonstrate how to decrease Holevo sum quantities with data-processing and entropy-continuity bounds.
Quantum cryptography is one of the most important quantum information applications. The present thesis covers several topics on quantum cryptography, such as the security analysis of quantum channels for key distribution protocols and the study of quantum cloning. First, we introduce a general formalism to characterize the cryptographic properties of quantum channels in the realistic scenario where the two honest parties employ prepare and measure protocols and the known two-way communication reconciliation techniques. We derive a necessary and sufficient condition to distill a secret key using this type of schemes for arbitrary bipartite quantum systems of finite dimension. The obtained results suggest that there may exist weakly entangling channels useless for key distribution using prepare and measure schemes. Next, we consider Gaussian states and Gaussian operations for cryptographic tasks and derive a new security condition. As it happens for quantum systems of finite dimension, our results suggest that there may also exist weakly entangled Gaussian states useless for key distribution, using Gaussian operations. Finally, we study the connection between cloning and state estimation. It was a long-standing problem to show whether state estimation becomes equivalent to quantum cloning in the asymptotic limit of an infinite number of clones. The equivalence is proven here using two known results in quantum information theory, the monogamy of quantum states and the properties of entanglement-breaking channels.
The proof of the security of quantum key distribution is a rather complex problem. Security is defined in terms different from the requirements imposed on keys in classical cryptography. In quantum cryptography, the security of keys is expressed in terms of the closeness of the quantum state of an eavesdropper after key distribution to an ideal quantum state that is uncorrelated to the key of legitimate users. A metric of closeness between two quantum states is given by the trace metric. In classical cryptography, the security of keys is understood in terms of, say, the complexity of key search in the presence of side information. In quantum cryptography, side information for the eavesdropper is given by the whole volume of information on keys obtained from both quantum and classical channels. The fact that the mathematical apparatuses used in the proof of key security in classical and quantum cryptography are essentially different leads to misunderstanding and emotional discussions [1]. Therefore, one should be able to answer the question of how different cryptographic robustness criteria are related to each other. In the present study, it is shown that there is a direct relationship between the security criterion in quantum cryptography, which is based on the trace distance determining the distinguishability of quantum states, and the criterion in classical cryptography, which uses guesswork on the determination of a key in the presence of side information.
The counter-intuitive features of Quantum Mechanics make it possible to solve problems and perform tasks that are beyond the abilities of classical computers and classical communication devices. The area of quantum information processing studies how representing information by quantum states can help achieving such improvements.In this research, we use basic notions of quantum information (mainly entanglement, Bloch sphere, and geometrical distances between quantum states) for analyzing relations of quantum states to each other and quantum cryptographic protocols.Entanglement is an important feature of quantum states. Intuitively (and, partly, inaccurately), entanglement represents quantum (non-classical) correlations between several different quantum systems. Entanglement is one of the most important quantum phenomena, and it has many uses in quantum information, quantum communication, and quantum computing.Some quantum states can be geometrically represented by the Bloch sphere: the unit sphere in the three-dimensional Euclidean space. The "standard" quantum states, to which the laws of Quantum Mechanics directly apply, are called pure states. Other states are the mixed states: probability distributions ("mixtures") of several pure states. The points on the Bloch sphere are the pure states, and those inside the Bloch sphere are the mixed states. This geometrical representation is useful and intuitive for many purposes.We provide a geometrical analysis of entanglement for all the quantum mixed states of rank 2 (all the mixtures of exactly two pure states): for any such state (in any dimension), we define a generalized Bloch sphere by using the two pure states, and we analyze this state and its neighbor states inside this Bloch sphere. We look at the set of non-entangled states ("separable states") in the Bloch sphere and characterize it into exactly five possible classes. We give examples for each class and prove that there are no other classes. In addition, we suggest possible definitions of "entanglement measures" by using the "trace distance" from the nearest separable state.Many types of distances between quantum states can be defined. One of the most useful distances is the trace distance, which bounds the "distinguishability" between the states. The trace distance is very useful in quantum information and quantum cryptography, and it also has a simple geometrical interpretation: it is half of the Euclidean distance between the states in the Bloch sphere.Quantum key distribution (QKD) protocols make it possible for two participators to achieve the classically-impossible task of generating a secret random shared key even if their adversary is computationally unlimited. Several important QKD protocols, including the first protocol of Bennett and Brassard (BB84), have their unconditional security proved against adversaries performing the most general attacks in a theoretical (idealized) setting. We discuss a slightly different protocol, named "BB84-INFO-z", and prove it secure against a broad class of attacks (the collective attacks). Moreover, we make use of the "trace distance" for making our security proof more "composable" than similar security proofs for BB84: namely, for making a step towards proving that the secret key remains secret even when the two participators actually use it for cryptographic purposes.