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arXiv:1610.00730 · Physical Review A

Scale-invariant freezing of entanglement

Titas Chanda, Tamoghna Das, Debasis Sadhukhan, Amit Kumar Pal, Aditi Sen, Ujjwal Sen

Published Jun 15, 2018• 14 citations
arXivHTMLPDF

Abstract

We show that bipartite entanglement in an one-dimensional quantum spin model undergoing time evolution due to local Markovian environments can be frozen over time. We demonstrate this by using a number of paradigmatic quantum spin models including the anisotropic $XY$ model in the presence of a uniform and an alternating transverse magnetic field (ATXY), the $XXZ$ model, the $XYZ$ model, and the ${J}_{1}\ensuremath{-}{J}_{2}$ model involving the next-nearest-neighbor interactions. We show that the length of the freezing interval, for a chosen pair of nearest-neighbor spins, may remain independent of the length of the spin chain, for example, in paramagnetic phases of the ATXY model, indicating a scale invariance. Freezing of entanglement is found to be robust against a change in the environment temperature, the presence of disorder in the system, and whether the noise is dissipative or not dissipative. Moreover, we connect the freezing of entanglement with the propagation of information through a quantum many-body system, as considered in the Lieb-Robinson theorem. We demonstrate that the variation of the freezing duration exhibits a quadratic behavior against the distance of the nearest-neighbor spin pair from the noise source, obtained from exact numerical simulations, in contrast to the linear one as predicted by the Lieb-Robinson theorem.

DOI
10.1103/physreva.97.062324

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We show that bipartite entanglement in an one-dimensional quantum spin model undergoing time evolution due to local Markovian environments can be frozen over time. We demonstrate this by using a number of paradigmatic quantum spin models including the anisotropic XYXYXY model in the presence of a uniform and an alternating transverse magnetic field (ATXY), the XXZXXZXXZ model, the XYZXYZXYZ model, and the J1\ensuremath−J2{J}_{1}\ensuremath{-}{J}_{2}J1​\ensuremath−J2​ model involving the next-nearest-neighbor interactions. We show that the length of the freezing interval, for a chosen pair of nearest-neighbor spins, may remain independent of the length of the spin chain, for example, in paramagnetic phases of the ATXY model, indicating a scale invariance. Freezing of entanglement is found to be robust against a change in the environment temperature, the presence of disorder in the system, and whether the noise is dissipative or not dissipative. Moreover, we connect the freezing of entanglement with the propagation of information through a quantum many-body system, as considered in the Lieb-Robinson theorem. We demonstrate that the variation of the freezing duration exhibits a quadratic behavior against the distance of the nearest-neighbor spin pair from the noise source, obtained from exact numerical simulations, in contrast to the linear one as predicted by the Lieb-Robinson theorem.
We investigate the behavior of entanglement in the ground state of a doped one-dimensional lattice, where the particles interact via the quantum t-J model, which can be obtained from the Hubbard Hamiltonian with large onsite interactions. For different values of the electron concentration, the rich phase diagram exhibits both polynomial and exponential decay of bipartite quantum entanglement, with increasing lattice distance. This respectively characterizes the properties of the Luttinger liquid and the electron-hole phase separation regions of the phase diagram. Interestingly, at low electron concentration, where the spin-gap opens, the ground state turns out to be a long-ranged resonating valence bond gas. We observe that the phase diagram remains qualitatively unchanged even when additional next-nearest-neighbor spin couplings are introduced, though the phase boundaries are dependent on the relative strength between the nearest and next-nearest neighbor interactions, which the decay patterns of entanglement can capture. A key finding of the study relates to the genuine multipartite entanglement of the ground state of the model at low electron densities. We observe that for fixed values of the electron density, multipartite entanglement remains immutable under perturbative or sudden changes of system parameters, a phenomenon termed as adiabatic freezing. The phenomenon is absent in the anisotropic undoped limit of the system. It is to be noted that multipartite entanglement, in general, is sensitive to external perturbation, as observed in several systems, and hitherto, no freezing behavior has been reported.
We consider an infinite one-dimensional anisotropic XY\mathit{XY}XY spin chain with a nearest-neighbor time-dependent Heisenberg coupling J(t)J(t)J(t) between the spins in presence of a time-dependent magnetic field h(t)h(t)h(t). We discuss a general solution for the system and present an exact solution for particular choice of JJJ and hhh of practical interest. We investigate the dynamics of entanglement for different degrees of anisotropy of the system and at both zero and finite temperatures. We find that the time evolution of entanglement in the system shows nonergodic and critical behavior at zero and finite temperatures and different degrees of anisotropy. The asymptotic behavior of entanglement at the infinite time limit at zero temperature and constant JJJ and hhh depends only the parameter $\ensuremath{\lambda}=J/h$ rather than the individual values of JJJ and hhh for all degrees of anisotropy but changes for nonzero temperature. Furthermore, the asymptotic behavior is very sensitive to the initial values of JJJ and hhh and for particular choices we may create finite asymptotic entanglement regardless of the final values of JJJ and hhh. The persistence of quantum effects in the system as it evolves and as the temperature is raised is studied by monitoring the entanglement. We find that the quantum effects dominate within certain regions of the kT\mathit{kT}kT-$\ensuremath{\lambda}$ space that vary significantly depending on the degree of the anisotropy of the system. Particularly, the quantum effects in the Ising model case persist in the vicinity of both its critical phase transition point and zero temperature as it evolves in time. Moreover, the interplay between the different system parameters to tune and control the entanglement evolution is explored.
In a system where two identical two-level atoms interact with their common one-mode cavity field, it is shown that entanglement can become abruptly frozen in time, remaining at a constant value for a period of time until it begins to thaw from this value from the entanglement sharing perspective [Ding et al., Phys. Rev. A 103, 032418 (2021)]. We generalize this exotic behavior of entanglement sharing dynamics to more general systems with arbitrary N qubits, instead of restricting to the atom-cavity mode interaction system. We also demonstrate methods to control the entanglement freezing time and freezing value, and we discover a nontrivial dynamics where entanglement is frozen permanently. In addition, we show that this phenomenon is not a coincidence but a universal feature in a variety of systems with a geometric explanation of the mechanisms.
Recently new novel magnetic phases were shown to exist in the asymptotic steady states of spin systems coupled to dissipative environments at zero temperature. Tuning the different system parameters led to quantum phase transitions among those states. We study, here, a finite two-dimensional Heisenberg triangular spin lattice coupled to a dissipative Markovian Lindblad environment at finite temperature. We show how applying an inhomogeneous magnetic field to the system at different degrees of anisotropy may significantly affect the spin states, and the entanglement properties and distribution among the spins in the asymptotic steady state of the system. In particular, applying an inhomogeneous field with an inward (growing) gradient toward the central spin is found to considerably enhance the nearest neighbor entanglement and its robustness against the thermal dissipative decay effect in the completely anisotropic (Ising) system, whereas the beyond nearest neighbor ones vanish entirely. The spins of the system in this case reach different steady states depending on their positions in the lattice. However, the inhomogeneity of the field shows no effect on the entanglement in the completely isotropic (XXX) system, which vanishes asymptotically under any system configuration and the spins relax to a separable (disentangled) steady state with all the spins reaching a common spin state. Interestingly, applying the same field to a partially anisotropic (XYZ) system does not just enhance the nearest neighbor entanglements and their thermal robustness but all the long-range ones as well, while the spins relax asymptotically to very distinguished spin states, which is a sign of a critical behavior taking place at this combination of system anisotropy and field inhomogeneity.
We report all phases and corresponding critical lines of the quantum anisotropic transverse XY model with uniform and alternating transverse magnetic fields (ATXY) in the presence of the Dzyaloshinskii-Moriya (DM) interaction by using appropriately chosen order parameters. We prove that when DM interaction is weaker than the anisotropy parameter, it has no effect at all on the zero-temperature states of the XY model with uniform transverse magnetic field (UXY), which is not the case for the ATXY model. However, when DM interaction is stronger than the anisotropy parameter, we show the appearance of a new gapless chiral phase---in the XY model with uniform as well as alternating field. We further observe that first derivatives of nearest neighbor two-site entanglement with respect to magnetic fields can detect all the critical lines present in the system. We also find that the factorization surface at zero temperature present in this model without DM interaction becomes a volume on the introduction of the latter. Moreover, DM interaction turns out to be good to generate bipartite entanglement sustainable at large times, leading to a proof of ergodic nature of bipartite entanglement in this system.
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