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arXiv:1703.08602

Coherent collective dynamics and entanglement evolution of polar molecules on 1D lattices

V. C. Olaya-Agudelo, Karen Rodríguez

Published Mar 24, 2017• 0 citations
arXivHTMLPDF

Abstract

We study a LiCs strongly-interacting molecular gas loaded into an one-dimensional optical lattice at quarter filling. The molecules are in the lowest electronic and vibrational state, $X^{1}Σ$ ($ν=0$). Due to the large intermolecular distance and low filling, dipole-dipole interaction in the nearest-neighbor approximation governs the dynamics of the rotational excitations. For low DC electric field strengths, the full set of rotational levels $N=0,1$ must be taken into account, nevertheless, our calculations show that very weak fields act as field-selectors disclosing two- and three-level systems out of the original four-level one. The dynamics and the generated von Neumann entanglement entropy among the internal rotational states throughout the evolution are presented for low, moderate and strong fields . We observe a sharp and monotonous growth of the entanglement as the dynamics take place showing the potential of these molecular systems to be used in quantum information protocols. The numerical simulations are performed by means of the Time-Evolving Block Decimation algorithm based on the Matrix Product State formalism and the Susuki-Trotter decomposition.

DOI
10.48550/arxiv.1703.08602

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We study a LiCs strongly-interacting molecular gas loaded into an one-dimensional optical lattice at quarter filling. The molecules are in the lowest electronic and vibrational state, X1ΣX^{1}ΣX1Σ (ν=0ν=0ν=0). Due to the large intermolecular distance and low filling, dipole-dipole interaction in the nearest-neighbor approximation governs the dynamics of the rotational excitations. For low DC electric field strengths, the full set of rotational levels N=0,1N=0,1N=0,1 must be taken into account, nevertheless, our calculations show that very weak fields act as field-selectors disclosing two- and three-level systems out of the original four-level one. The dynamics and the generated von Neumann entanglement entropy among the internal rotational states throughout the evolution are presented for low, moderate and strong fields . We observe a sharp and monotonous growth of the entanglement as the dynamics take place showing the potential of these molecular systems to be used in quantum information protocols. The numerical simulations are performed by means of the Time-Evolving Block Decimation algorithm based on the Matrix Product State formalism and the Susuki-Trotter decomposition.
We study a LiCs strongly-interacting molecular gas loaded on an one-dimensional optical lattice keeping one particle per site.The molecules are in the lowest electronic and vibrational (ν=0) state, Χ 1 Σ.Here, coherent transfers of rotational excitations (N={0,1}) along the lattice are shown for several lattice sizes and initial conditions.Due to the large intermolecular distance and low filling, the dipoledipole interaction in the nearest-neighbor approximation governs the dynamics of the rotational excitations.For low field strengths, the full set of internal rotational projections must be taken into account.The generated von Neumann entanglement entropy throughout the dynamics of the energy transfer are also presented.A monotonic growth for short times and a saturation towards the steady regime is observed, showing the potential of these molecular systems to be used in quantum information protocols.The numerical simulations are performed by means of the Time-Evolving Block Decimation algorithm based on the Matrix Product State formalism and the Susuki-Trotter decomposition.
We investigate the dynamical formation of crystalline states with systems of polar molecules or Rydberg atoms loaded into a deep optical lattice. External fields in these systems can be used to couple the atoms or molecules between two internal states: one that is weakly interacting and one that exhibits a strong dipole–dipole interaction. By appropriate time variation of the external fields, we show that it is possible to produce crystalline states of the strongly interacting states with high filling fractions chosen via the parameters of the coupling. We study the coherent dynamics of this process in one dimension (1D) using a modified form of the time-evolving block decimation (TEBD) algorithm, and obtain crystalline states for system sizes and parameters corresponding to realistic experimental configurations. For polar molecules these crystalline states will be long-lived, assisting in a characterization of the state via the measurement of correlation functions. We also show that as the coupling strength increases in the model, the crystalline order is broken. This is characterized in 1D by a change in density–density correlation functions, which decay to a constant in the crystalline regime, but show different regions of exponential and algebraic decay for larger coupling strengths.
Abstract : With the production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In ordinary atomic systems, where contact interactions require wavefunction overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as superexchange; however, the coupling is relatively weak and is limited to nearest-neighbour interactions. In contrast, dipolar interactions exist even in the absence of tunnelling and extend beyond nearest neighbours. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. Measured effects of dipolar interactions in ultracold molecular gases have been limited to the modification of inelastic collisions and chemical reactions. Here we use dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model. Spin is encoded in rotational states of molecules that are prepared and probed by microwaves. Resonant exchange of rotational angular momentum between two molecules realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling factor and the effect of a multipulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a continuous quantum Zeno mechanism. Measurements of these tunnelling-induced losses allow us to determine the lattice filling factor independently.
Ultracold polar molecules, because of their long-range, spatially anisotropic interactions, are a new quantum system in which to study novel many-body phenomena. In our lab, we have produced the first quantum gas of 40K87Rb polar molecules. These molecules were found to undergo exothermic chemical reactions, and this led to interesting studies of chemistry near absolute zero. By creating the molecules at individual sites of a 3D optical lattice, we completely suppress these chemical reactions, and the polar molecule gas becomes stable and lives for tens of seconds. This thesis documents our efforts to explore coherent, many-body phenomena resulting from long-range dipolar interactions in the lattice. By encoding a spin-1=2 system in the rotational states of the molecules, we were able to realize spin-exchange interactions based on a spin Hamiltonian, which is one of the first steps in studying quantum magnetism with polar molecules. While this study was the first realization of such coherent dipolar interactions with polar molecules in a lattice, its full potential was limited by the low lattice filling fractions. Using our ability to exquisitely control the initial atomic gas mixture, we loaded a Mott insulator of Rb and a band insulator of K into the lattice. This quantum synthesis approach led to significantly higher molecular filling fractions and represents the first fully connected system of polar molecules in an optical lattice. This low-entropy quantum gas of polar molecules opens the door to interesting quantum simulations, which should be attainable in the next generation of the experiment.
We derive a novel lattice Hamiltonian, the molecular Hubbard Hamiltonian , 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 molecular Hubbard Hamiltonian is explicitly time dependent, making a dynamic generalization of the concept of quantum phase transitions necessary. Using the time-evolving block decimation algorithm to study entangled dynamics, we demonstrate that, in the case of hard-core bosonic molecules at half-filling, the molecular Hubbard Hamiltonian exhibits emergent timescales over which spatial entanglement grows, crystalline order appears and oscillations between rotational states self-damp into an asymptotic superposition. We show that these timescales are non-monotonic functions of the physical parameters describing the lattice.
We study the evolution of one-dimensional quantum lattice systems when the ground state is perturbed by altering one site in the middle of the chain. For a large class of models, we observe a similar pattern of entanglement growth during the evolution, characterized by a moderate increase of significant Schmidt coefficients in all relevant bipartite decompositions of the state. As a result, the evolution can be accurately described by a matrix product state and efficiently simulated using the time-evolving block decimation algorithm.