Ravindra W. Chhajlany

Analytical progress on symmetric geometric discord: Measurement-based upper bounds

Quantum correlations may be measured by means of the distance of the state to the subclass ofstates having well defined classical properties. In particular, a geometric measure of asymmetricdiscord [Daki´c et al., Phys. Rev. Lett. 105, 190502 (2010)] was recently defined as the Hilbert-Schmidt distance of a given two-qubit state to the closest classical-quantum (CQ) correlated state.We analyze a geometric measure of symmetric discord defined as the Hilbert-Schmidt distance ofa given state to the closest classical-classical (CC) correlated state. The optimal member of isjust specially measured original state both for the CQ and CC discords. This implies that thismeasure is equal to quantum deficit of post-measurement purity. We discuss some general relationsbetween the CC discords and explain why an analytical formula for the CC discord, contrary to theCQ discord, can hardly be found even for a general two-qubit state. Instead of such exact formula,we find simple analytical measurement-based upper bounds for the CC discord which, as we show,are very efficient in the case of two qubits and may serve as independent indicators of two-partyquantum correlations. In particular, we propose an adaptive upper bound, which corresponds to theoptimal states induced by single-party measurements: optimal measurement on one of the partiesdetermines an optimal measurement on the other party. We discuss how to refine the adaptiveupper bound by nonoptimal single-party measurements and by an iterative procedure which usuallyrapidly converges to the CC discord. We also raise the question of optimality of the symmetricmeasurements realising the CC discord on symmetric states, and give partial answer for the qubitcase.

Entanglement in the Majumdar-Ghosh model

We present an analysis of the entanglement characteristics in the Majumdar-Ghosh (MG) or J{sub 1}-J{sub 2} Heisenberg model. For a system consisting of up to 28 spins, there is a deviation from the scaling behavior of the entanglement entropy characterizing the unfrustrated Heisenberg chain above J{sub 2}{approx_equal}0.25. This feature can be used as an indicator of the dimer phase transition occurring in this model. Additionally, we also consider entanglement at the MG point J{sub 2}=0.5J{sub 1}.

Synthetic magnetic fluxes and topological order in one-dimensional spin systems

Engineering topological quantum order has become a major field of physics. Many advances have been made by synthesizing gauge fields in cold atomic systems. Here we carry over these developments to other platforms which are extremely well suited for quantum engineering, namely, trapped ions and nano-trapped atoms. Since these systems are typically one-dimensional, the action of artificial magnetic fields has so far received little attention. However, exploiting the long-range nature of interactions, loops with nonvanishing magnetic fluxes become possible even in one-dimensional settings. This gives rise to intriguing phenomena, such as fractal energy spectra, flat bands with localized edge states, and topological many-body states. We elaborate on a simple scheme for generating the required artificial fluxes by periodically driving an XY spin chain. Concrete estimates demonstrating the experimental feasibility for trapped ions and atoms in wave guides are given.

Generation of Inequivalent Generalized Bell Bases

AbstractThe notion of equivalence of maximally entangled bases of bipartite d–dimensional Hilbert spaces ℋd ⊗ ℋd is introduced. An explicit method of inequivalent bases construction is presented. PACS: 03.67.-a

Sensitivity of entanglement decay of quantum-dot spin qubits to the external magnetic field

Pawe l Mazurek, 2 Katarzyna Roszak, Ravindra W. Chhajlany, 5 and Pawe l Horodecki 6 National Quantum Information Centre of Gdańsk, 81-824 Sopot, Poland Institute for Theoretical Physics and Astrophysics, University of Gdańsk, 80-952 Gdańsk, Poland Institute of Physics, Wroc law University of Technology, 50-370 Wroc law, Poland Faculty of Physics, Adam Mickiewicz University, 61-614 Poznań, Poland ICFO Institut de Ciéncies Fotóniques, Mediterranean Technology Park, E-08860 Castelldefels, Barcelona, Spain Faculty of Applied Physics and Mathematics, Gdańsk University of Technology, 80-952 Gdańsk, Poland (Dated: May 10, 2014)

Terahertz field control of in-plane orbital order in La0.5Sr1.5MnO4

In-plane anisotropic ground states are ubiquitous in correlated solids such as pnictides, cuprates and manganites. They can arise from doping Mott insulators and compete with phases such as superconductivity; however, their origins are debated. Strong coupling between lattice, charge, orbital and spin degrees of freedom results in simultaneous ordering of multiple parameters, masking the mechanism that drives the transition. Here we demonstrate that the orbital domains in a manganite can be oriented by the polarization of a pulsed THz light field. Through the application of a Hubbard model, we show that domain control can be achieved by enhancing the local Coulomb interactions, which drive domain reorientation. Our results highlight the key role played by the Coulomb interaction in the control and manipulation of orbital order in the manganites and demonstrate a new way to use THz to understand and manipulate anisotropic phases in a potentially broad range of correlated materials.

Stabilized Parametric Cooper-Pair Pumping in a Linear Array of Coupled Josephson Junctions

We present an experimentally realizable stabilized charge pumping scheme in a linear array of Cooper-pair boxes. The system design intrinsically protects the pumping mechanism from severe errors, especially current reversal and spontaneous charge excitation. The quantum Zeno effect is implemented to further diminish pumping errors. The characteristics of this scheme are considered from the perspective of improving the current standard. Such an improvement bears relevance to the closure of the so-called measurement triangle (see [D. Averin, Nature (London) 434, 285 (2005)).

Mean-field approaches to the Bose?Hubbard model with three-body local interaction

The zero temperature properties of the generalized Bose‐Hubbard model including three-body interactions are studied on a mean-field level. We obtain analytical results using the so-called perturbative mean-field method and more detailed numerical results using the Gutzwiller product state variational ansatz. These two approaches yield equivalent results which compare well on a qualitative level with recent exact results obtained in the literature.

Hubbard-I approach to the Mott transition

We expose the relevance of double occupancy conservation symmetry in application of the Hubbard-I approach to strongly correlated electron systems. We propose the utility of a composite method, viz. the Hubbard-I method in conjunction with strong coupling perturbation expansion, for studying systems violating the afore--mentioned symmetry. We support this novel approach by presenting a first successful Hubbard-I type calculation for the description of the metal-insulator Mott transition in a strongly correlated electron system with conserved double occupancies, which is a constrained Hubbard Hamiltonian equivalent to the Hubbard bond charge Hamiltonian with

Quantum superadditivity in linear optics networks: Sending bits via multiple-access Gaussian channels

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