We define threshold disentanglement as the level above which entanglement must be maintained for later use and calculate the values of quantum system parameters at t = 0 that will ensure that the threshold is not crossed in a specified time. An important issue is the possibility of non-smooth, typically non-exponential, decay of entanglement into the state of complete disentanglement. Given a specified operation time τ, we determine phase diagrams showing entanglement preservation during the evolution of a generic two-qubit mixed state as a function of selected initial parameters such as concurrence and excitation probabilities. In other words, disentanglement can be pre-managed through these parameters. Three common relaxation processes—amplitude, phase and depolarization—are considered.
We define threshold disentanglement as the level above which entanglement must be maintained for later use and calculate the values of quantum system parameters at t = 0 that will ensure that the threshold is not crossed in a specified time. An important issue is the possibility of non-smooth, typically non-exponential, decay of entanglement into the state of complete disentanglement. Given a specified operation time τ, we determine phase diagrams showing entanglement preservation during the evolution of a generic two-qubit mixed state as a function of selected initial parameters such as concurrence and excitation probabilities. In other words, disentanglement can be pre-managed through these parameters. Three common relaxation processes—amplitude, phase and depolarization—are considered.
The entanglement of two qubits, each defined as an effective two-level, spin 1/2 system, is investigated for the case that the qubits interact via a Heisenberg XY interaction and are subject to decoherence due to population relaxation and thermal effects. For zero temperature, the time-dependent concurrence is studied analytically and numerically for some typical initial states, including a separable (unentangled) initial state. An analytical formula for non-zero steady state concurrence is found for any initial state, and optimal parameter values for maximizing steady state concurrence are given. The steady state concurrence is found analytically to remain non-zero for low, finite temperatures. We also identify the contributions of global and local coherence to the steady state entanglement.
Quantum communication in free space will be disturbed by natural environment, such as fog, dust, and rain, which is a difficult problem in the construction of quantum communication system. In order to solve this problem and improve the survivability of quantum communication system, we propose an adaptive parameter adjustment strategy for free-space quantum communication based on software-defined quantum communication (SDQC). Firstly, we propose a software-defined quantum communication model based on the idea of software defined networks. The architecture of SDQC is divided into four layers: transport layer, access layer, control layer, and management layer. The SDQC system sends the link information to the preset program at a management level through the real-time monitoring of channel state by the access layer. According to the link information, the management level issues instructions to the control layer to adjust the parameters such as the initial quantum state and the existence time of single quantum state, in order to improve the quantum entanglement and fidelity. Secondly, we analyze the relationship between quantum fidelity and parameters in SDQC system under three noise channels, i.e. depolarization channel, spontaneous amplitude decay channel, and phase damping channel. In the depolarized channel, the quantum fidelity F decreases with the increase of the error probability Pd of the qubit. When the error probability of qubit is certain, the system has the maximum quantum fidelity with the value of parameter x is 0.5. In the spontaneous amplitude decay channel, the quantum fidelity F decreases with the increase of the quantum state transition probability pt. When the transition probability of quantum state is certain, the higher the value of parameter x, the higher the fidelity will be. In the phase-damped channel, the quantum fidelity F decreases with the increase of the probability pc with which the qubit and the background interference equivalent quantum state have complete elastic scattering. When the probability is certain, the larger the value of |1/2 – x|, the higher the quantum fidelity of the system will be. Finally, we study the optimal values of SDQC system parameters under different environmental disturbances. The simulation results show that the optimal parameters of SDQC system are different when the parameters of three noise channels, namely depolarization, spontaneous amplitude decay and phase damping, are different. The system adaptively selects the initial quantum state and the existence time of single quantum state according to the environmental change and business demand, so that the quantum fidelity is always at the peak in the communication process. This strategy effectively improves the adaptability and comprehensive immunity of the quantum communication system.
The distribution of entangled quantum systems among the nodes of a network is a key task at the basis of the development of quantum technologies, e.g., quantum communication, quantum computation, etc. Many efforts have been devoted to identify strategies, based on pre- and post-processing operations or decoherence-free subspaces, to prevent the deterioration of such exotic correlations. However, all these approaches loose their usefulness when the noise level affecting the system surpasses a certain minimal threshold that leads to an entanglement-breaking dynamics. Here we attack this problem in the context of discrete- and continuous-time description of the system dynamics, providing some explicit examples in the context of qubit channels.
For a solvable pure-decoherence model, we confirm by an explicit model calculation that the decay of entanglement of two two-state systems (two qubits) is approximately governed by the product of the suppression factors describing decoherence of the subsystems, provided that they are subject to uncorrelated sources of quantum noise. This demonstrates an important physical property that separated open quantum systems can evolve quantum mechanically on time scales larger than the times for which they remain entangled.
We systematically characterize quantum entanglement in all sixteen combinations of two-qubit Bell states across three operational regimes: pure superposition, noiseless mixing, and asymmetric mixing of one noisy and one pure Bell state. For pure superpositions, we calculate the von Neumann entropy and show that entanglement depends critically on the relative phase phi between superposed states. For mixed states, we use concurrence C under three physically motivated decoherence channels. The Depolarizing (DP) and Phase Damping (PD) channels are modeled as global, collective operations on the two-qubit Hilbert space, while Amplitude Damping (AD) is treated as independent local decay on each qubit, reflecting the asymmetric, energy-dissipating nature of spontaneous emission. A central and counterintuitive result is that increasing noise can raise concurrence in specific parameter regimes across all three channels. We further map the boundary between mathematical entanglement (C > 0) and operational quantum non-locality using the Horodecki criterion, finding this boundary to be strongly channel-dependent. Under Phase Damping, the Bell-local entanglement regime vanishes entirely for identical and same-bases mixtures, meaning any surviving entanglement guarantees a CHSH violation. Under Depolarizing noise, a large Bell-local region requires mixing probability r > 1/sqrt(2) to recover non-locality at maximum noise. For Amplitude Damping, certain mixtures exhibit a non-monotonic Horodecki parameter M(T): concurrence decreases monotonically with noise, yet the capacity for CHSH violation is lost at intermediate noise levels and restored at high noise, as maximal damping repurifies the noisy branch. These results provide a unified analytical reference for entanglement and non-locality management across Bell-state combinations and decoherence channels relevant to NISQ architectures.
We theoretically study the non-Markovian disentanglement dynamics of a two-qubit system coupled to nonequilibrium environments with nonstationary and non-Markovian random telegraph noise statistical properties. The reduced density matrix of the two-qubit system can be expressed as the Kraus representation in terms of the tensor products of the single qubit Kraus operators. We derive the relation between the entanglement and nonlocality of the two-qubit system which are both closely associated with the decoherence function. We identify the threshold values of the decoherence function to ensure the existences of the concurrence and nonlocal quantum correlations for an arbitrary evolution time when the two-qubit system is initially prepared in the composite Bell states and the Werner states, respectively. It is shown that the environmental nonequilibrium feature can suppress the disentanglement dynamics and reduce the entanglement revivals in non-Markovian dynamics regime. In addition, the environmental nonequilibrium feature can enhance the nonlocality of the two-qubit system. Moreover, the entanglement sudden death and rebirth phenomena and the transition between quantum and classical nonlocalities closely depend on the parameters of the initial states and the environmental parameters in nonequilibrium environments.