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Register a new userDisorder-assisted distribution of entanglement in $XY$ spin chains
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Guilherme Martins Alves De Almeida
Publication date
2017
fields
Physics
and research's language is
English
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We study the creation and distribution of entanglement in disordered $XY$-type spin-$1/2$ chains for the paradigmatic case of a single flipped spin prepared on a fully polarized background. The local magnetic field is set to follow a disordered long-range-correlated sequence with power-law spectrum. Depending on the degree of correlations of the disorder, a set of extended modes emerge in the middle of the band yielding an interplay between localization and delocalization. As a consequence, a rich variety of entanglement distribution patterns arises, which we evaluate here through the concurrence between two spins. We show that, even in the presence of disorder, the entanglement wave can be pushed to spread out reaching distant sites and also enhance pairwise entanglement between the initial site and the rest of the chain. We also study the propagation of an initial maximally-entangled state through the chain and show that correlated disorder improves the transmission quite significantly when compared with the uncorrelated counterpart. Our work contributes in designing solid-state devices for quantum information processing in the realistic setting of correlated static disorder.
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Se estudia en este trabajo la capacidad de generar entrelazamiento de una cadena de espines con acoplamiento de Heisenberg XY y un campo magnetico uniforme a partir de un estado inicial en el que los espines estan completamente alineados. Se encuentra que la capacidad de generar estados entrelazados no muestra un comportamiento monotono con el campo presentando, en cambio, plateaus y resonancias. Tambien se muestra que, a pesar de que la anisotropia es necesaria para que se generen estados entrelazados, una mayor anisotropia no implica necesariamente mejores condiciones para generar entrelazamiento que sirva para usarse en una computadora cuantica. Inclusive, se observa que, se genera una cantidad finita de entrelazamiento en el limite de pequena anisotropia. (The maximum entanglement reached by an initially fully aligned state evolving in a XY Heisenberg spin chain placed in a uniform transverse magnetic field is studied. It is shown that the capacity to create entangled states (both of one qubit with the rest of the chain and pairwise between two adjacent qubits) is not a monotonous function of the magnetic field: it presents plateaus and resonances. It is also shown that, though anisotropy in the interaction is necessary for entanglement generation, the best conditions for generating entanglement that may be suitable for use in quantum computation is not that of biggest anisotropy. Moreover finite amounts of entanglement are generated in the small anisotropy limit.)
We propose a protocol for state transfer and entanglement generation between two distant spin qubits (sender and receiver) that have different energies. The two qubits are permanently coupled to a far off-resonant spin-chain, and the qubit of the sender is driven by an external field, which provides the energy required to bridge the energy gap between the sender and the receiver. State transfer and entanglement generation are achieved via virtual single-photon and multi-photon transitions to the eigenmodes of the channel.
The entanglement dynamics of arrays of qubits is analysed in the presence of some general sources of noise and disorder. In particular, we consider linear chains of Josephson qubits in experimentally realistic conditions. Electromagnetic and other (spin or boson) fluctuations due to the background circuitry and surrounding substrate, finite temperature in the external environment, and disorder in the initial preparation and the control parameters are embedded into our model. We show that the amount of disorder that is typically present in current experiments does not affect the entanglement dynamics significantly, while the presence of noise can have a drastic influence on the generation and propagation of entanglement. We examine under which circumstances the system exhibits steady-state entanglement for both short (N < 10) and long (N > 30) chains and show that, remarkably, there are parameter regimes where the steady-state entanglement is strictly non-monotonic as a function of the noise strength. We also present optimized schemes for entanglement verification and quantification based on simple correlation measurements that are experimentally more economic than state tomography.
We study an entanglement transfer protocol in a two-leg ladder spin-1/2 chain in the presence of disorder. In the regime where on-site energies and the intrachain couplings follow aproximately constant proportions locally, we set up a scheme for high-fidelity state transfer via a disorder-protected subspace wherein fluctuations in the parameters do not depend on the global disorder of the system, accounted by $W$. Moreover, we find that the leakage of information from that subspace is actually suppressed upon increasing $W$ and thus the transfer fidelity, evaluated through the entanglement concurrence at the other end of the chain, builds up with the disorder strength.
Entanglement in quantum XY spin chains of arbitrary length is investigated via a recently-developed global measure suitable for generic quantum many-body systems. The entanglement surface is determined over the phase diagram, and found to exhibit structure richer than expected. Near the critical line, the entanglement is peaked (albeit analytically), consistent with the notion that entanglement--the non-factorization of wave functions--reflects quantum correlations. Singularity does, however, accompany the critical line, as revealed by the divergence of the field-derivative of the entanglement along the line. The form of this singularity is dictated by the universality class controlling the quantum phase transition.
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