Quantum phases of dimerized and frustrated Heisenberg spin chains with s=1/2, 1 and 3/2: an entanglement entropy and fidelity study

We study here different regions in phase diagrams of the spin-1/2, spin-1 and spin-3/2 one-dimensional antiferromagnetic Heisenberg systems with frustration (next-nearest-neighbor interaction J(2)) and dimerization (delta). In particular, we analyze the behaviors of the bipartite entanglement entropy and fidelity at the gapless to gapped phase transitions and across the lines separating different phases in the J(2)-delta plane. All the calculations in this work are based on numerical exact diagonalizations of finite systems.

Renormalization of noncommutative quantum field theories

We report on a comprehensive analysis of the renormalization of noncommutative phi(4) scalar field theories on the Groenewold-Moyal plane. These scalar field theories are twisted Poincare invariant. Our main results are that these scalar field theories are renormalizable, free of UV/IR mixing, possess the same fixed points and beta-functions for the couplings as their commutative counterparts. We also argue that similar results hold true for any generic noncommutative field theory with polynomial interactions and involving only pure matter fields. A secondary aim of this work is to provide a comprehensive review of different approaches for the computation of the noncommutative S-matrix: noncommutative interaction picture and noncommutative Lehmann-Symanzik-Zimmermann formalism. DOI: 10.1103/PhysRevD.87.064014

Poincare invariant quantum field theories with twisted internal symmetries

Following up the work of 1] on deformed algebras, we present a class of Poincare invariant quantum field theories with particles having deformed internal symmetries. The twisted quantum fields discussed in this work satisfy commutation relations different from the usual bosonic/fermionic commutation relations. Such twisted fields by construction are nonlocal in nature. Despite this nonlocality we show that it is possible to construct interaction Hamiltonians which satisfy cluster decomposition principle and are Lorentz invariant. We further illustrate these ideas by considering global SU(N) symmetries. Specifically we show that twisted internal symmetries can provide a natural-framework for the discussion of the marginal deformations (beta-deformations) of the N = 4 SUSY theories.

Electron irradiation effects on TGA-capped CdTe quantum dots

8MeV electron irradiation effects on thioglycolic acid (TGA)-capped CdTe quantum dots (QD) are discussed in this study. CdTe QDs were characterized using x-ray diffraction (XRD), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS). Steady-state and time-resolved emission spectroscopy and UV-visible absorption spectroscopy were performed before and after irradiation with 8MeV electrons. XRD and TEM confirm the growth of TGA-capped CdTe QDs. The photoemission wavelength, intensity and lifetimes were found to vary with electron dose. At lower doses, they were found to be increasing (red-shift of photoluminescence (PL) peak and intensity) while the intensity decreased at higher electron doses. The observed changes in PL property, XPS and XRD analysis suggest possible epitaxial growth of the CdS shell on the CdTe core. This work demonstrates electron beam induced formation of the CdS layer on the CdTe core, which is a key step towards growth of the water soluble CdTe/CdS core-shell structure for biomedical labelling applications.

Electronic energy transfer between long-range resonance states of 3-mercaptopropionic acid capped CdTe quantum dots in aqueous media

We demonstrate electronic energy transfer between resonance states of 2 and 2.8 nm CdTe quantum dots in aqueous media using steady-state photoluminescence spectroscopy without using any external linker molecule. With increasing concentration of larger dots, there is subsequent quenching of luminescence in smaller dots accompanied by the enhancement of luminescence in larger dots. Our experimental evidence suggests that there is long-range resonance energy transfer among electronic excitations, specifically from the electronically confined states of the smaller dots to the higher excited states of the larger dots.

Quantum phases and phase transitions of frustrated hard-core bosons on a triangular ladder

Kinetically frustrated bosons at half filling in the presence of a competing nearest-neighbor repulsion support a wide supersolid regime on the two-dimensional triangular lattice. We study this model on a two-leg ladder using the finite-size density-matrix renormalization-group method, obtaining a phase diagram which contains three phases: a uniform superfluid (SF), an insulating charge density wave (CDW) crystal, and a bond ordered insulator (BO). We show that the transitions from SF to CDW and SF to BO are continuous in nature, with critical exponents varying continuously along the phase boundaries, while the transition from CDW to BO is found to be first order. The phase diagram is also found to contain an exactly solvable Majumdar Ghosh point, and reentrant SF to CDW phase transitions.

Inversion of moments to retrieve joint probabilities in quantum sequential measurements

A sequence of moments obtained from statistical trials encodes a classical probability distribution. However, it is well known that an incompatible set of moments arises in the quantum scenario, when correlation outcomes associated with measurements on spatially separated entangled states are considered. This feature, viz., the incompatibility of moments with a joint probability distribution, is reflected in the violation of Bell inequalities. Here, we focus on sequential measurements on a single quantum system and investigate if moments and joint probabilities are compatible with each other. By considering sequential measurement of a dichotomic dynamical observable at three different time intervals, we explicitly demonstrate that the moments and the probabilities are inconsistent with each other. Experimental results using a nuclear magnetic resonance system are reported here to corroborate these theoretical observations, viz., the incompatibility of the three-time joint probabilities with those extracted from the moment sequence when sequential measurements on a single-qubit system are considered.

Ancilla-assisted quantum state tomography in multiqubit registers

The standard method of quantum state tomography (QST) relies on the measurement of a set of noncommuting observables, realized in a series of independent experiments. Ancilla-assisted QST (AAQST) proposed by Nieuwenhuizen and co-workers Phys. Rev. Lett. 92, 120402 (2004)] greatly reduces the number of independent measurements by exploiting an ancilla register in a known initial state. In suitable conditions AAQST allows mapping out density matrix of an input register in a single experiment. Here we describe methods for explicit construction of AAQST experiments in multiqubit registers. We also report nuclear magnetic resonance studies on AAQST of (i) a two-qubit input register using a one-qubit ancilla in an isotropic liquid-state system and (ii) a three-qubit input register using a two-qubit ancilla register in a partially oriented system. The experimental results confirm the effectiveness of AAQST in such multiqubit registers.

Plasmonic tuning of photoluminescence from semiconducting quantum dot assemblies

We report tuning of photoluminescence enhancement and quenching from closed packed monolayers of cadmium selenide quantum dots doped with gold nanoparticles. Plasmon-mediated control of the emission intensity from the monolayers is achieved by varying the size and packing density of the quantum dots as well as the doping concentration of gold nanoparticles. We observe a unique packing density dependent crossover from enhancement to quenching and vice versa for fixed size of quantum dots and doping concentration of gold nanoparticles. We suggest that this behavior is indicative of a crossover from single particle to collective emission from quantum dots mediated by gold nanoparticles.

Micro-Raman and photoluminescence studies of self-assembled quantum well microtubes on GaAs substrate

The Semiconductor Quantum Well (QW) microtubes have been fabricated by strain-induced self assembling technique. Three types of multilayer structures have consisted of GaAs/InxGa1-xAs strained layers containing with various thickness of Monolayers of (GaAs/AlGaAs) QW were grown by Varian Gen II Molecular Beam Epitaxy (MBE) on the GaAs (100) substrate. The shape of the rolled up microtubes provide a clear idea about the formation of three dimensional micro- and nanostructures. Micro-Raman and photoluminescence (PL) studies were performed to the QW microtubes and as compared with their grown area on the GaAs substrate. The results of Raman spectra show the frequency shift of phonon modes measured in tube and compared with the grown area due to residual strain. The PL peaks of the microtube were red-shifted due to the strain effect and transition of bandgap from Type-II to Type-I. (C) 2013 Elsevier B.V. All rights reserved.

Quantum entropic ambiguities: Ethylene

In a quantum system, there may be many density matrices associated with a state on an algebra of observables. For each density matrix, one can compute its entropy. These are, in general, different. Therefore, one reaches the remarkable possibility that there may be many entropies for a given state R. Sorkin (private communication)]. This ambiguity in entropy can often be traced to a gauge symmetry emergent from the nontrivial topological character of the configuration space of the underlying system. It can also happen in finite-dimensional matrix models. In the present work, we discuss this entropy ambiguity and its consequences for an ethylene molecule. This is a very simple and well-known system, where these notions can be put to tests. Of particular interest in this discussion is the fact that the change of the density matrix with the corresponding entropy increase drives the system towards the maximally disordered state with maximum entropy, where Boltzman's formula applies. Besides its intrinsic conceptual interest, the simplicity of this model can serve as an introduction to a similar discussion of systems such as colored monopoles and the breaking of color symmetry.

Thermodynamics of quantum heat engines

We consider a recently proposed four-level quantum heat engine (QHE) model to analyze the role of quantum coherences in determining the thermodynamic properties of the engine, such as flux, output power, and efficiency. A quantitative analysis of the relative effects of the coherences induced by the two thermal baths is brought out. By taking account of the dissipation in the cavity mode, we define useful work obtained from the QHE and present some analytical results for the optimal values of relative coherences that maximizes flux (hence output power) through the engine. We also analyze the role of quantum effects in inducing population inversion (lasing) between the states coupled to the cavity mode. The universal behavior of the efficiency at maximum power (EMP) is examined. In accordance with earlier theoretical predictions, to leading order, we find that EMP similar to eta(c)/2, where eta(c) is Carnot efficiency. However, the next higher order coefficient is system dependent and hence nonuniversal.

Multipartite quantum correlations reveal frustration in a quantum Ising spin system

We report a nuclear magnetic resonance experiment, which simulates the quantum transverse Ising spin system in a triangular configuration, and further demonstrate that multipartite quantum correlations can be used to distinguish between the frustrated and the nonfrustrated regimes in the ground state of this system. Adiabatic state preparation methods are used to prepare the ground states of the spin system. We employ two different multipartite quantum correlation measures to analyze the experimental ground state of the system in both the frustrated and the nonfrustrated regimes. As expected from theoretical predictions, the experimental data confirm that the nonfrustrated regime shows higher multipartite quantum correlations compared to the frustrated one.

Photoluminescence decay rate engineering of CdSe quantum dots in ensemble arrays embedded with gold nano-antennae

We discuss experimental results on the ability to significantly tune the photoluminescence decay rates of CdSe quantum dots embedded in an ordered template, using lightly doped small gold nanoparticles (nano-antennae), of relatively low optical efficiency. We observe both enhancement and quenching of photoluminescence intensity of the quantum dots varying monotonically with increasing volume fraction of added gold nanoparticles, with respect to undoped quantum dot arrays. However, the corresponding variation in lifetime of photoluminescence spectra decay shows a hitherto unobserved, non-monotonic variation with gold nanoparticle doping. We also demonstrate that Purcell effect is quite effective for the larger (5 nm) gold nano-antenna leading to more than four times enhanced radiative rate at spectral resonance, for largest doping and about 1.75 times enhancement for off-resonance. Significantly for spectral off-resonance samples, we could simultaneously engineer reduction of non-radiative decay rate along with increase of radiative decay rate. Non-radiative decay dominates the system for the smaller (2 nm) gold nano-antenna setting the limit on how small these plasmonic nano-antennae could be to be effective in engineering significant enhancement in radiative decay rate and, hence, the overall quantum efficiency of quantum dot based hybrid photonic assemblies.

Effect of Stress and Interface defects on Photo Luminescence of Si nano-crystals embedded in SiO2

Effect of stress and interface defects on photo luminescence property of a silicon nano-crystal (Si-nc) embedded in amorphous silicon dioxide (a-SiO2) are studied in this paper using a self-consistent quantum-continuum based modeling framework. Si-ncs or quantum dots show photoluminescence at room temperature. Whether its origin is due to Si-nc/a-SiO2 interface defects or quantum confinement of carriers in Si-nc is still an outstanding question. Earlier reports have shown that stresses greater than 12 GPa change the indirect energy band gap structure of bulk Si to a direct energy band gap structure. Such stresses are observed very often in nanostructures and these stresses influence the carrier confinement energy significantly. Hence, it is important to determine the effect of stress in addition to the structure of interface defects on photoluminescence property of Si-nc. In the present work, first a Si-nc embedded in a-SiO2 is constructed using molecular dynamics simulation framework considering the actual conditions they are grown so that the interface and residual stress in the structure evolves naturally during formation. We observe that the structure thus created has an interface of about 1 nm thick consisting of 41.95% of defective states mostly Sin+ (n = 0 to 3) coordination states. Further, both the Si-nc core and the embedding matrix are observed to be under a compressive strain. This residual strain field is applied in an effective mass k.p Hamiltonian formulation to determine the energy states of the carriers. The photo luminescence property computed based on the carrier confinement energy and interface energy states associated with defects will be analysed in details in the paper.