Quantum correlations and entanglement in far-from-equilibrium spin systems (original) (raw)

Emergent timescales in entangled quantum dynamics of ultracold molecules in optical lattices

New Journal of Physics, 2009

We derive a novel lattice Hamiltonian, the Molecular Hubbard Hamiltonian (MHH), which describes the essential many body physics of closed-shell ultracold heteronuclear molecules in their absolute ground state in a quasi-one-dimensional optical lattice. The MHH is explicitly time-dependent, making a dynamic generalization of the concept of quantum phase transitions necessary. Using the Time-Evolving Block Decimation (TEBD) algorithm to study entangled dynamics, we demonstrate that, in the case of hard core bosonic molecules at half filling, the MHH exhibits an emergent time scale over which spatial entanglement grows, crystalline order appears, and oscillations between rotational states self-damp into an asymptotic superposition. We show that this time scale is a non-monotonic function of the physical parameters describing the lattice. We also point out that experimental mapping of the static phase boundaries of the MHH can be used to measure the molecular polarizability tensor.

Dynamics of quantum information

Nature Reviews Physics, 2019

The ability to harness the dynamics of quantum information and entanglement is necessary for the development of quantum technologies and the study of complex quantum systems. On the theoretical side the dynamics of quantum information is a topic that is helping us unify and confront common problems in otherwise disparate fields in physics, such as quantum statistical mechanics and cosmology. On the experimental side the impressive developments on the manipulation of neutral atoms and trapped ions are providing new capabilities to probe their quantum dynamics. Here, we overview and discuss progress in characterizing and understanding the dynamics of quantum entanglement and information scrambling in quantum many-body systems. The level of control attainable over both the internal and external degrees of freedom of individual particles in these systems provides great insight into the intrinsic connection between entanglement and thermodynamics, bounds on information transport and computational complexity of interacting systems. In turn this understanding should enable the realization of quantum technologies.

Measurement induced dynamics for quantum communication in strongly correlated systems

Physical Review A

We show that a single qubit measurement followed by a unitary operation which is determined by the random outcome of the measurement can be used for both encoding the information and performing a quantum quench in a strongly correlated anti-ferromagnetic many-body system. The proposed mechanism, does not need any local control over the interaction of the system and instead exploits local operations on the quantum state of the system and harnesses the inherent entanglement in the ground state of the strongly correlated system. The performance remains always better than the classical threshold for chains up to length 50. Cold atoms in optical lattices are ideal physical setup for realizing our proposal and all the ingredients needed for local measurements and unitary operations are already available in the experiments.

Entanglement, Nonlinear Dynamics, and the Heisenberg Limit

Physical Review Letters, 2009

We show that the quantum Fisher information provides a sufficient condition to recognize multiparticle entanglement in a N qubit state. The same criterion gives a necessary and sufficient condition for sub shot-noise phase sensitivity in the estimation of a collective rotation angle θ. The analysis therefore singles out the class of entangled states which are useful to overcome classical phase sensitivity in metrology and sensors. We finally study the creation of useful entangled states by the non-linear dynamical evolution of two decoupled Bose-Einstein condensates or trapped ions.

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.

Simulating the out-of-equilibrium dynamics of local observables by trading entanglement for mixture

The fact that the computational cost of simulating a many-body quantum system on a computer increases with the amount of entanglement has been considered as the major bottleneck for simulating its out-of-equilibrium dynamics. Some aspects of the dynamics are, nevertheless, robust under appropriately devised approximations. Here we present a possible algorithm that allows to systematically approximate the equilibration value of local operators after a quantum quench. At the core of our proposal there is the idea to transform entanglement between distant parts of the system into mixture, and at the same time preserving the local reduced density matrices of the system. We benchmark the resulting algorithm by studying quenches of quadratic Fermionic Hamiltonians.

Measurement-induced, spatially-extended entanglement in a hot, strongly-interacting atomic system

Nature Communications

Quantum technologies use entanglement to outperform classical technologies, and often employ strong cooling and isolation to protect entangled entities from decoherence by random interactions. Here we show that the opposite strategy—promoting random interactions—can help generate and preserve entanglement. We use optical quantum non-demolition measurement to produce entanglement in a hot alkali vapor, in a regime dominated by random spin-exchange collisions. We use Bayesian statistics and spin-squeezing inequalities to show that at least 1.52(4) × 1013 of the 5.32(12) × 1013 participating atoms enter into singlet-type entangled states, which persist for tens of spin-thermalization times and span thousands of times the nearest-neighbor distance. The results show that high temperatures and strong random interactions need not destroy many-body quantum coherence, that collective measurement can produce very complex entangled states, and that the hot, strongly-interacting media now in us...

Initial-phase spectroscopy as a control of entangled systems

… of Physics B: Atomic, Molecular and …, 2010

We introduce the concept of initial-phase spectroscopy as a control of the dynamics of entangled states encoded into a two-atom system interacting with a broadband squeezed vacuum field. We illustrate our considerations by examining the transient spectrum of the field emitted by two systems, the small sample (Dicke) and the spatially extended (non-Dicke) models. It is found that the shape of the spectral components depends crucially on the relative phase between the initial entangled state and the squeezed field. We follow the temporal evolution of the spectrum and show that depending on the relative phase a hole burning can occur in one of the two spectral lines. We compare the transient behavior of the spectrum with the time evolution of the initial entanglement and find that the hole burning can be interpreted as a manifestation of the phenomenon of entanglement sudden death. In addition, we find that in the case of the non-Dicke model, the collective damping rate may act like an artificial tweezer that rotates the phase of the squeezed field.

Entanglement in Far From Equilibrium Stationary States

We present four estimators of the entanglement (or interdepency) of ground-states in which the coefficients are all real nonnegative and therefore can be interpreted as probabilities of configurations. Such ground-states of hermitian and non-hermitian Hamiltonians can be given, for example, by superpositions of valence bond states which can describe equilibrium but also stationary states of stochastic models. We consider in detail the last case. Using analytical and numerical methods we compare the values of the estimators in the directed polymer and the raise and peel models which have massive, conformal invariant and non-conformal invariant massless phases. We show that like in the case of the quantum problem, the estimators verify the area law and can therefore be used to signal phase transitions in stationary states. Comment: 4 pages 3figures