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We investigate the security against the intercept/resend and translucent attacks on the quantum key distribution protocol based on the pre- and post-selection effect. In 2001, Bub proposed the quantum cryptography scheme, which was an application of the so-called mean king's problem. We evaluate a probability that legitimate users cannot detect eavesdropper's malicious acts for Bub's protocol. We also estimate a probability that the eavesdropper guesses right at the random secret key one of the legitimate users tries to share with the other one. From rigorous mathematical and numerical analyses, we conclude that Bub's protocol is weaker than the Bennett–Brassard protocol of 1984 (BB84) against both the intercept/resend and translucent attacks. Because Bub's protocol uses a two-way quantum channel, the analyses of its security are tough to accomplish. We refer to their technical points accurately in the current paper. For example, we impose some constraints upon the eavesdropper's strategies in order to let their degrees of freedom be small. [ABSTRACT FROM AUTHOR]

Decoherence of a two-qubit system is studied in terms of an exactly solvable dephasing model. In this model, the two qubits are influenced by the random telegraph noises (RTNs) which are dynamically independent but initially correlated, where the initial correlation means that joint probability of the two RTNs is not factorizable at an initial time. It is shown that the purely classical initial correlation between the RTNs can create interesting effects on the time evolution of the two-qubit system. For this purpose, two-qubit coherence, entanglement and other correlations are investigated in detail. It is found that the initial correlation can suppress the decoherence of the two-qubit system. The most important result is that the time evolution of the two-qubit system can become non-Markovian in the presence of the initial correlation, even if it is Markovian in the absence of the initial correlation. Furthermore, it is shown that the initial correlation between the RTNs can enhance quantum coherence and correlation of the two-qubit system. [ABSTRACT FROM AUTHOR]

Quantumness is studied for single and bipartite states of qubits which interact with the identical structured reservoirs in indefinite causal order. Coherence and temporal steerability are investigated for a single-qubit state. Entanglement, spatial steerability and the Bell-CHSH nonlocality are examined for a bipartite-qubit state. It is shown that the indefinite causal order can reduce the decay of the quantumness of qubit states. • Quantumness is studied for qubits interacting with two structure reservoirs in indefinite causal order. • Coherence, entanglement, steerability and nonlocality are investigated. • The indefinite causal order can reduce the degradation of quantumness. [ABSTRACT FROM AUTHOR]

A transient linear response to a phase modulating field is studied for a two-level system which is placed under the influence of a surrounding environment. Both initial correlation and detuning between a two-level system and environment are taken into account. An exact expression of the transient linear response function is derived when the whole system contains no more than single excitation. If the reduced time evolution is non-Markovian, the transient response strongly depends on initial correlation, detuning and initial coherence. The detuning and initial coherence are more important than the initial correlation in the transversal response while the detuning and the initial correlation become significant in the longitudinal response. In particular, it is found that the longitudinal response is greatly enhanced and its decay becomes sufficiently slow under certain conditions. [ABSTRACT FROM AUTHOR]