Delocalization and Quantum Entanglement in Physical Systems (original) (raw)
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Environment-Assisted Quantum Coherence in Photosynthetic Complex
The Journal of Physical Chemistry Letters, 2017
Recent experiments [Nature, 2007, 446, 782-786] revealed the existence of surprisingly long lived quantum coherence in noisy biological environment of photosynthetic Fenna-Matthews-Olson (FMO) complex. Such coherence can clearly play important role in facilitating efficient energy transfer. The occurrence of quantum coherence in quantum transport is also implicated in excitation transport processes in conjugated polymers [Science, 2009, 323, 369-373]. Even though these systems are strongly correlated, most theoretical studies invoke Markovian approximation where the temporal correlation of bath fluctuations is neglected. We use an elegant non-perturbative method based on Kubo's quantum stochastic Liouville equation (QSLE) to study the effects of correlated non-Markovian bath fluctuations in several different limits and find the interesting result that fluctuations not only destroy coherence but under appropriate conditions can also facilitate it. We show that temperature has the most pronounced effect in the intermediate coupling limit where it can promote transition from coherent to incoherent transfer.
We investigate a system constituted by two interacting qubits having one of them isolated and the other coupled to a thermal reservoir. We analyze the dynamics of the system considering two different models of system-reservoir interaction: i) a "microscopic" model, in which the master equation is derived taking into account the interaction between the two subsystems (qubits); ii) a "phenomenological" model, in which the master equation consists of a dissipative term simply added to the unitary evolution term. We show that in the strong coupling regime for the qubit-qubit interaction, a thermal equilibrium steady state for the two-qubit density operator is not achieved within the framework of the phenomenological approach. However, according to the microscopic model, the system is driven to a thermal equilibrium state, instead. We compare the time evolution of the concurrence (between the two qubits) and the linear entropy (of the isolated qubit) in both models, for different temperatures of the thermal bath. We find that the two models predict (with small differences) the phenomenon of stationary entanglement for the two qubits. We also show that in the weak coupling regime for the qubit-qubit interaction, although both models provide the same results if the reservoir is at T = 0K, there are significant differences if the reservoir is at finite temperature. While the evolution of bipartite entanglement is very similar in both cases, we find contrasting results for the (isolated qubit) linear entropy evolution. Namely, while according to the microscopic model the isolated qubit would approach a maximally mixed state faster for higher temperatures, the phenomenological model gives just the opposite behavior, i.e., it would take longer for the qubit state to become maximally mixed for higher temperatures of the reservoir.
Dynamic entanglement in oscillating molecules
We demonstrate that entanglement can persistently recur in an oscillating two-spin molecule that is coupled to a hot and noisy environment, in which no static entanglement can survive. The system represents a non-equilibrium quantum system which, driven through the oscillatory motion, is prevented from reaching its (separable) thermal equilibrium state. Environmental noise, together with the driven motion, plays a constructive role by periodically resetting the system, even though it will destroy entanglement as usual. As a building block, the present simple mechanism supports the perspective that entanglement can exist also in systems which are exposed to a hot environment and to high levels of de-coherence, which we expect e.g. for biological systems. Our results furthermore suggest that entanglement plays a role in the heat exchange between molecular machines and environment. Experimental simulation of our model with trapped ions is within reach of the current state-of-the-art quantum technologies.
Temperature dependence of quantum correlations in 1D macromolecular chains
Наносистемы: физика, химия, математика, 2019
We investigate the problem of generating quantum correlations between different sites of a macromolecular chain by vibronic excitation depending on the temperature. The influence of temperature on the model dynamics is taken into account by employing the partial-dressing method based on the modified LangFirsov unitary transformation under the assumption that the chain collective oscillations are in the thermal equilibrium state. To describe quantum correlations between the chain sites in the case of the initial single-vibronic excitation, we use two-time correlation functions of the second order and the logarithmic negativity as the degree of entanglement. We find that at certain temperatures for various model parameters time-stable entanglement can occur in the chain.
The European Physical Journal D, 2017
In this paper, we investigate preservation of quantum coherence of a single-qubit interacting with a zero-temperature thermal reservoir through the addition of noninteracting qubits in the reservoir. Moreover, we extend this scheme to preserve quantum entanglement between two and three distant qubits, each of which interacts with a dissipative reservoir independently. At the limit t → ∞, we obtained analytical expressions for the coherence measure and the concurrence of two and three qubits in terms of the number of additional qubits. It is observed that, by increasing the number of additional qubits in each reservoir, the initial coherence and the respective entanglements are completely protected in both Markovian and non-Markovian regimes. Interestingly, the protection of entanglements occurs even under the individually different behaviors of the reservoirs.
arXiv: Statistical Mechanics, 2018
The presence of static off-diagonal disorder promotes coherent exciton transport while diffusive motion can be recovered in the presence of fluctuations in the diagonal and off-diagonal elements of the Hamiltonian. Here we study the crossover induced by correlated dynamical disorder.We uncover a novel role of the excited bath states (ExBS) in dictating quantum coherence and quantum transport in dissipative quantum systems interacting with correlated bath.We solve both analytically and numerically the temperature dependent Quantum Stochastic Liouville equation (TD-QSLE) to study temperature dependence of quantum coherence in both linear chains and cyclic trimer (first three subunits of Fenna-Matthews-Olson(FMO) and also heptamer) complexes, using Haken-Strobl-Reineker Hamiltonian. In the non-Markovian limit where the lowering of temperature induces long-lasting quantum coherences, ExBSnot only determines the lifetime of coherences but also dictates the long time population distributi...
Quantum Coherence Conservation by Growth in Environmental Dissipation Rate
2007
Quantum coherence conservation is shown to be achieved by a very high rate of dissipation of an environmental system coupled with a principal system. This effect is not in the list of previously-known strategies of noise suppression, such as Zeno effect, dynamical decoupling, quantum error correction code, and decoherence free subspace. An analytical solution is found for a simplified model of a single qubit coupled with an environmental single qubit dissipating rapidly. We also show examples of coherence conservation in a spin-boson linear coupling model with a numerical evaluation.
Dynamic entanglement in oscillating molecules and potential biological implications
Physical Review E, 2010
We demonstrate that entanglement can persistently recur in an oscillating two-spin molecule that is coupled to a hot and noisy environment, in which no static entanglement can survive. The system represents a non-equilibrium quantum system which, driven through the oscillatory motion, is prevented from reaching its (separable) thermal equilibrium state. Environmental noise, together with the driven motion, plays a constructive role by periodically resetting the system, even though it will destroy entanglement as usual. As a building block, the present simple mechanism supports the perspective that entanglement can exist also in systems which are exposed to a hot environment and to high levels of de-coherence, which we expect e.g. for biological systems. Our results furthermore suggest that entanglement plays a role in the heat exchange between molecular machines and environment. Experimental simulation of our model with trapped ions is within reach of the current state-of-the-art quantum technologies.
Entanglement and quantal coherence: Study of two limiting cases of rapid system-bath interactions
Physical Review A, 2002
Pacs 3.65w, 3.67a, 5.40a We consider the dynamics of a system coupled to a thermal bath, going beyond the standard two-level system through the addition of an energy excitation degree of freedom. Further extensions are to systems containing many fermions, with the master equations modified to take Fermi-Dirac statistics into account, and to potentials with a time-dependent bias that induce resonant avoided crossing transitions. The limit Q → ∞, where the interaction rate with the bath is much greater than all free oscillation rates for the system, is interrogated. Two behaviors are possible: freezing (quantum Zeno effect) or synchronization (motional narrowing). We clarify the conditions that give rise to each possibility, making an explicit connection with quantum measurement theory. We compare the evolution of quantal coherence for the two cases as a function of Q, noting that full coherence is restored as Q → ∞. Using an extended master equation, the effect of system-bath interactions on entanglement in bipartite system states is computed. In particular, we show that the sychronization case sees bipartite system entanglement fully preserved in the large Q limit.