D. O. Soares-Pinto

Environment-Induced Sudden Transition in Quantum Discord Dynamics

Nonclassical correlations play a crucial role in the development of quantum information science. The recent discovery that nonclassical correlations can be present even in separable (nonentangled) states has broadened this scenario. This generalized quantum correlation has been increasing in relevance in several fields, among them quantum communication, quantum computation, quantum phase transitions, and biological systems. We demonstrate here the occurrence of the sudden-change phenomenon and immunity against some sources of noise for the quantum discord and its classical counterpart, in a room temperature nuclear magnetic resonance setup. The experiment is performed in a decohering environment causing loss of phase relations among the energy eigenstates and exchange of energy between system and environment, resulting in relaxation to the Gibbs ensemble.

Quantum discord determines the interferometric power of quantum states

Quantum metrology exploits quantum mechanical laws to improve the precision in estimating technologically relevant parameters such as phase, frequency, or magnetic fields. Probe states are usually tailored to the particular dynamics whose parameters are being estimated. Here we consider a novel framework where quantum estimation is performed in an interferometric configuration, using bipartite probe states prepared when only the spectrum of the generating Hamiltonian is known. We introduce a figure of merit for the scheme, given by the worst-case precision over all suitable Hamiltonians, and prove that it amounts exactly to a computable measure of discord-type quantum correlations for the input probe. We complement our theoretical results with a metrology experiment, realized in a highly controllable room-temperature nuclear magnetic resonance setup, which provides a proof-of-concept demonstration for the usefulness of discord in sensing applications. Discordant probes are shown to guarantee a nonzero phase sensitivity for all the chosen generating Hamiltonians, while classically correlated probes are unable to accomplish the estimation in a worst-case setting. This work establishes a rigorous and direct operational interpretation for general quantum correlations, shedding light on their potential for quantum technology.

Experimentally Witnessing the Quantumness of Correlations

The quantification of quantum correlations (other than entanglement) usually entails labored numerical optimization procedures also demanding quantum state tomographic methods. Thus it is interesting to have a laboratory friendly witness for the nature of correlations. In this Letter we report a direct experimental implementation of such a witness in a room temperature nuclear magnetic resonance system. In our experiment the nature of correlations is revealed by performing only few local magnetization measurements. We also compared the witness results with those for the symmetric quantum discord and we obtained a fairly good agreement.

Observation of environment-induced double sudden transitions in geometric quantum correlations.

Correlations in quantum systems exhibit a rich phenomenology under the effect of various sources of noise. We investigate theoretically and experimentally the dynamics of quantum correlations and their classical counterparts in two nuclear magnetic resonance setups, as measured by geometric quantifiers based on trace norm. We consider two-qubit systems prepared in Bell diagonal states, and perform the experiments in real decohering environments resulting from Markovian local noise which preserves the Bell diagonal form of the states. We then report the first observation of environment-induced double sudden transitions in the geometric quantum correlations, a genuinely nonclassical effect not observable in classical correlations. The evolution of classical correlations in our physical implementation reveals in turn the finite-time relaxation to a pointer basis under nondissipative decoherence, which we characterize geometrically in full analogy with predictions based on entropic measures.

Nonclassical correlation in NMR quadrupolar systems

The existence of quantum correlation (as revealed by quantum discord), other than entanglement and its role in quantum-information processing (QIP), is a current subject for discussion. In particular, it has been suggested that this nonclassical correlation may provide computational speedup for some quantum algorithms. In this regard, bulk nuclear magnetic resonance (NMR) has been successfully used as a test bench for many QIP implementations, although it has also been continuously criticized for not presenting entanglement in most of the systems used so far. In this paper, we report a theoretical and experimental study on the dynamics of quantum and classical correlations in an NMR quadrupolar system. We present a method for computing the correlations from experimental NMR deviation-density matrices and show that, given the action of the nuclear-spin environment, the relaxation produces a monotonic time decay in the correlations. Although the experimental realizations were performed in a specific quadrupolar system, the main results presented here can be applied to whichever system uses a deviation-density matrix formalism.

Measuring Bipartite Quantum Correlations of an Unknown State

We report the experimental measurement of bipartite quantum correlations of an unknown two-qubit state. Using a liquid state Nuclear Magnetic Resonance setup and employing geometric discord, we evaluate the quantum correlations of a state without resorting to prior knowledge of its density matrix. The method is applicable to any 2 ⊗ d system and provides, in terms of number of measurements required, an advantage over full state tomography scaling with the dimension d of the unmeasured subsystem. The negativity of quantumness is measured as well for reference. We also observe the phenomenon of sudden transition of quantum correlations when local phase and amplitude damping channels are applied to the state.

Generalized simulated annealing applied to protein folding studies

During the last few years, computational simulations based on the atomic description of biological molecules have resulted in significant advances in the comprehension of biological processes. It is well known, however, that a molecular system may have a great number of conformations due to the large number of rotation of degrees of freedom around chemical bonds, leading to several local minima on the energy hypersurface. It has been proposed though, that proteins express their biological function when their structure is close to a conformation with energy global minimum. To help solve the protein‐folding problem, we use a new strategy based on Simulated Annealing methods. These methods have been well suited for a large extent of optimization problems, especially those containing many local minima. In fact, this work applies the Generalized Simulated Annealing method (GSA) coupled to the GROMOS96 Molecular Force Field to research the minimum energy conformation of 18‐alanine. We show that the qT GSA parameter can be used to control the freezing process during the annealing procedure, and to avoid polypeptide chains to be trapped in energy local minima. We scanned the q‐values for visiting (qV) and accepting (qA) functions for qT values ranging from 1 to 3, and found the best values to obtain an α‐helix conformation for the polyalanine peptide, which is the conformation with energy global minimum. Global optimization methods also exemplify a class of applications that requires a large amount of computational resources, being suitable for Grid computing. To implement a Grid computing platform, we developed and tested a Grid environment based on MYGRID middleware, which is a technology that can employ all machines accessed by the user to run the application.

Experimental determination of thermal entanglement in spin clusters using magnetic susceptibility measurements

The present work reports an experimental observation of thermal entanglement in a clusterized spin chain formed in the compound

Geometric lower bound for a quantum coherence measure

{\mathrm{Na}}_{2}{\mathrm{Cu}}_{5}{\mathrm{Si}}_{4}{\mathrm{O}}_{14}

Experimental demonstration of information to energy conversion in a quantum system at the Landauer limit

. The presence of entanglement was investigated through two measured quantities, an entanglement witness and the entanglement of formation, both derived from the magnetic susceptibility. It was found that pairwise entanglement exists below