Coherence and entanglement in a two-qubit system

We study the evolution of the entanglement of noninteracting qubits coupled to reservoirs under monitoring of the reservoirs by means of continuous measurements. We calculate the average of the concurrence of the qubits wavefunction over all quantum trajectories. For two qubits coupled to independent baths subjected to local measurements, this average decays exponentially with a rate depending on the measurement scheme only. This contrasts with the known disappearance of entanglement after a finite time for the density matrix in the absence of measurements. For two qubits coupled to a common bath, the mean concurrence can vanish at discrete times. Our analysis applies to arbitrary quantum jump or quantum state diffusion dynamics in the Markov limit. We discuss the best measurement schemes to protect entanglement in specific examples.

“…3. For any initial state, C(t) converges at large times t ≫ γ −1 to the same asymptotic value C ∞ = |c − | 2 /2 as the concurrence C ρ(t) [15,27].…”

Section: Qubits Coupled To a Common Bathmentioning

confidence: 90%

Average entanglement for Markovian quantum trajectories

Vogelsberger

Spehner

2010

“…The concept of thermal entanglement was proposed in a spin chain system at thermal equilibrium [40][41][42][43][44]. The dynamical evolution of the entanglement was explored for two uncoupled qubits interacting with two reservoirs [45][46][47][48][49]. The coupled qubit system has also been investigated when time-dependent external driving is present [60][61][62].…”

Section: Introductionmentioning

confidence: 99%

Steady-state entanglement and coherence of two coupled qubits in equilibrium and nonequilibrium environments

Wang

2019

We analytically and numerically investigate the steady-state entanglement and coherence of two coupled qubits each interacting with a local boson or fermion reservoir, based on the Bloch-Redfield master equation beyond the secular approximation. We find that there is non-vanishing steady-state coherence in the nonequilibrium scenario, which grows monotonically with the nonequilibrium condition quantified by the temperature difference or chemical potential difference of the two baths. The steady-state entanglement, in general, is a non-monotonic function of the nonequilibrium condition as well as the bath parameters in the equilibrium setting. We also discover that weak inter-qubit coupling and high base temperature or chemical potential of the baths can strongly suppress the steady-state entanglement and coherence, regardless of the strength of the nonequilibrium condition. On the other hand, the energy detuning of the two qubits, when used in a compensatory way with the nonequilibrium condition, can lead to significant enhancement of the steady-state entanglement in some parameter regimes. In addition, the qubits typically have a stronger steady-state entanglement when coupled to fermion baths exchanging particles with the system than boson baths exchanging energy with the system, under similar conditions. We also identify a close connection between the energy current flowing through the system and the steady-state coherence. Preliminary investigations suggest that these results are insensitive to the form of the reservoir spectral densities in the Markovian regime. Feasible experimental realization of measuring the steady-state entanglement and coherence is discussed for the coupled qubit system in nonequilibrium environments. Our findings offer some general guidelines for optimizing the steady-state entanglement and coherence in the coupled qubit system and may find potential applications in quantum information technology.

“…According to the definition of concurrence [50,51] 4) arranged in a descending order is the square root of the eigenvalues of ρ abρab withρ ab = (σ y ⊗ σ y )ρ * ab (σ y ⊗ σ y ), and σ y being the Pauli operator, in our case the concurrence is calculated as follows [52]:…”

Section: Analysis and Discussionmentioning

confidence: 99%

Optical-frequency-comb generation and entanglement with low-power optical input in a photonic molecule

Yu¹,

Ding²,

Wang³

et al. 2014

Optical-frequency combs consisting of equally spaced sharp lines in frequency space have triggered substantial advances in optical-frequency metrology and precision measurements and in applications such as laser-based gas sensing and molecular fingerprinting. Here, we propose a scheme to generate a type of optical-frequency combs and convert them from one cavity to the other in a hybrid optical system composed of a pair of coupled photonic crystal cavities called a photonic molecule (PM) and a single semiconductor quantum dot (QD) embedded in one cavity of the molecule. Optical-frequency combs are formed by the interaction between a cavity mode and a continuous-wave (CW) two-tone driving laser consisting of a pump field and a seed field via QD-induced strong nonlinearity. In this situation, the initial input pump and seed CW lasers can interact among each other and produce optical higher-order sidebands with equal spacing via parametric frequency conversion provided by QD-induced nonlinear optical effects. Using numerical simulations, it is clearly shown that the beat frequency of the two-tone components plays an important role in determining the comb spacing and matched frequency combs can be formed in the PM. We also demonstrate that the present interacting QD-PM system can serve as a platform to generate large-scale quantum entanglement between two comb modes. The results obtained here may be useful for real experiments in a photonic crystal platform.