30 research outputs found
The Loschmidt Echo as a robust decoherence quantifier for many-body systems
We employ the Loschmidt Echo, i.e. the signal recovered after the reversal of
an evolution, to identify and quantify the processes contributing to
decoherence. This procedure, which has been extensively used in single particle
physics, is here employed in a spin ladder. The isolated chains have 1/2 spins
with XY interaction and their excitations would sustain a one-body like
propagation. One of them constitutes the controlled system S whose reversible
dynamics is degraded by the weak coupling with the uncontrolled second chain,
i.e. the environment E. The perturbative SE coupling is swept through arbitrary
combinations of XY and Ising like interactions, that contain the standard
Heisenberg and dipolar ones. Different time regimes are identified for the
Loschmidt Echo dynamics in this perturbative configuration. In particular, the
exponential decay scales as a Fermi golden rule, where the contributions of the
different SE terms are individually evaluated and analyzed. Comparisons with
previous analytical and numerical evaluations of decoherence based on the
attenuation of specific interferences, show that the Loschmidt Echo is an
advantageous decoherence quantifier at any time, regardless of the S internal
dynamics.Comment: 12 pages, 6 figure
Non-Markovian decay and dynamics of decoherence in private and public environments
We study the decay process in an open system, emphasizing on the relevance of
the environment's spectral structure. Non-Markovian effects are included to
quantitatively analyze the degradation rate of the coherent evolution. The way
in which a two level system is coupled to different environments is
specifically addressed: multiple connections to a single bath (public
environment)or single connections to multiple baths (private environments). We
numerically evaluate the decay rate of a local excitation by using the Survival
Probability and the Loschmidt Echo. These rates are compared to analytical
results obtained from the standard Fermi Golden Rule (FGR) in Wide Band
Approximation, and a Self-Consistent evaluation that accounts for the bath's
memory in cases where an exact analytical solution is possible. We observe that
the correlations appearing in a public bath introduce further deviations from
the FGR as compared with a private bath.Comment: 18 pages, 7 figures. Accepted for publication in Physical Review
13C dynamic nuclear polarization in diamond via a microwave-free 'integrated' cross effect
Color-center-hosting semiconductors are emerging as promising source
materials for low-field dynamic nuclear polarization (DNP) at or near room
temperature, but hyperfine broadening, susceptibility to magnetic field
heterogeneity, and nuclear spin relaxation induced by other paramagnetic
defects set practical constraints difficult to circumvent. Here, we explore an
alternate route to color-center-assisted DNP using nitrogen-vacancy (NV)
centers in diamond coupled to substitutional nitrogen impurities, the so-called
P1 centers. Working near the level anti-crossing condition - where the P1
Zeeman splitting matches one of the NV spin transitions - we demonstrate
efficient microwave-free 13C DNP through the use of consecutive magnetic field
sweeps and continuous optical excitation. The amplitude and sign of the
polarization can be controlled by adjusting the low-to-high and high-to-low
magnetic field sweep rates in each cycle so that one is much faster than the
other. By comparing the 13C DNP response for different crystal orientations, we
show that the process is robust to magnetic field/NV misalignment, a feature
that makes the present technique suitable to diamond powders and settings where
the field is heterogeneous. Applications to shallow NVs could capitalize on the
greater physical proximity between surface paramagnetic defects and outer
nuclei to efficiently polarize target samples in contact with the diamond
crystal
The ARROWS project: Adapting and developing robotics technologies for underwater archaeology
ARchaeological RObot systems for the World's Seas (ARROWS) EU Project proposes to adapt and develop low-cost Autonomous Underwater Vehicle (AUV) technologies to significantly reduce the cost of archaeological operations, covering the full extent of archaeological campaign. ARROWS methodology is to identify the archaeologists requirements in all phases of the campaign and to propose related technological solutions. Starting from the necessities identified by archaeological project partners in collaboration with the Archaeology Advisory Group, a board composed of European archaeologists from outside ARROWS, the aim is the development of a heterogeneous team of cooperating AUVs capable of comply with a complete archaeological autonomous mission. Three new different AUVs have been designed in the framework of the project according to the archaeologists' indications: MARTA, characterized by a strong hardware modularity for ease of payload and propulsion systems configuration change; U-C AT, a turtle inspired bio-mimetic robot devoted to shipwreck penetration and A-Size AUV, a vehicle of small dimensions and weight easily deployable even by a single person. These three vehicles will cooperate within the project with AUVs already owned by ARROWS partners exploiting a distributed high-level control software based on the World Model Service (WMS), a storage system for the environment knowledge, updated in real-time through online payload data process, in the form of an ontology. The project includes also the development of a cleaning tool for well-known artifacts maintenance operations. The paper presents the current stage of the project that will lead to overall system final demonstrations, during Summer 2015, in two different scenarios, Sicily (Italy) and Baltic Sea (Estonia
Orientation-independent room temperature optical C-13 hyperpolarization in powdered diamond
Dynamic nuclear polarization via contact with electronic spins has emerged as an attractive route to enhance the sensitivity of nuclear magnetic resonance beyond the traditional limits imposed by magnetic field strength and temperature. Among the various alternative implementations, the use of nitrogen vacancy (NV) centers in diamond—a paramagnetic point defect whose spin can be optically polarized at room temperature—has attracted widespread attention, but applications have been hampered by the need to align the NV axis with the external magnetic field. We overcome this hurdle through the combined use of continuous optical illumination and a microwave sweep over a broad frequency range. As a proof of principle, we demonstrate our approach using powdered diamond with which we attain bulk 13C spin polarization in excess of 0.25% under ambient conditions. Remarkably, our technique acts efficiently on diamond crystals of all orientations and polarizes nuclear spins with a sign that depends exclusively on the direction of the microwave sweep. Our work paves the way toward the use of hyperpolarized diamond particles as imaging contrast agents for biosensing and, ultimately, for the hyperpolarization of nuclear spins in arbitrary liquids brought in contact with their surface
Role of energy uncertainties in ergodicity breaking induced by competing interactions and disorder. A dynamical assessment through the Loschmidt echo
A local excitation in a quantum many-particle system evolves deterministically. A time-reversal procedure, involving the invertion of the signs of every energy and interaction, should produce an excitation revival: the Loschmidt echo (LE). If somewhat imperfect, only a fraction of the excitation will refocus. We use such a procedure to show how non-inverted weak disorder and interactions, when assisted by the natural reversible dynamics, fully degrade the LE. These perturbations enhance diffusion and evenly distribute the excitation throughout the system. Such a dynamical paradigm, called ergodicity, breaks down when either the disorder or the interactions are too strong. These extreme regimes give rise to the well known Anderson localization and Mott insulating phases, where quantum diffusion becomes restricted. Accordingly, regardless of the kinetic energy terms, the excitation remains mainly localized and out-of-equilibrium, and the system behaves non-ergodically. The LE constitutes a fair dynamical witness for the whole phase diagram since it evidences a surprising topography in which ergodic and non-ergodic phases interpenetrate each other. Furthermore, we provide an estimation for the critical lines separating the ergodic and non-ergodic phases around the Mott and Anderson transitions. The energy uncertainties introduced by disorder and interaction shift these thresholds towards stronger perturbations. Remarkably, the estimations of the critical lines are in good agreement with the phase diagram derived from the LE dynamics.
Received: 20 Novembre 2014, Accepted: 29 June 2015; Edited by: C. A. Condat, G. J. Sibona; Reviewed by: A. De Luca, Laboratoire de Physique Theorique, ENS & Institut Philippe Meyer, Paris, France; DOI: http://dx.doi.org/10.4279/PIP.070012
Cite as: P R Zangara, P R Levstein, H M Pastawski, Papers in Physics 7, 070012 (2015