1,537 research outputs found
Double symmetry breaking and 2D quantum phase diagram in spin-boson systems
The quantum ground state properties of two independent chains of spins
(two-levels systems) interacting with the same bosonic field are theoretically
investigated. Each chain is coupled to a different quadrature of the field,
leading to two independent symmetry breakings for increasing values of the two
spin-boson interaction constants and . A phase diagram is
provided in the plane (,) with 4 different phases that can
be characterized by the complex bosonic coherence of the ground states and can
be manipulated via non-abelian Berry effects. In particular, when
and are both larger than two critical values, the fundamental
subspace has a four-fold degeneracy. Possible implementations in
superconducting or atomic systems are discussed
Heisenberg Uncertainty Principle as Probe of Entanglement Entropy: Application to Superradiant Quantum Phase Transitions
Quantum phase transitions are often embodied by the critical behavior of
purely quantum quantities such as entanglement or quantum fluctuations. In
critical regions, we underline a general scaling relation between the
entanglement entropy and one of the most fundamental and simplest measure of
the quantum fluctuations, the Heisenberg uncertainty principle. Then, we show
that the latter represents a sensitive probe of superradiant quantum phase
transitions in standard models of photons such as the Dicke Hamiltonian, which
embodies an ensemble of two-level systems interacting with one quadrature of a
single and uniform bosonic field. We derive exact results in the thermodynamic
limit and for a finite number N of two-level systems: as a reminiscence of the
entanglement properties between light and the two-level systems, the product
diverges at the quantum critical point as . We
generalize our results to the double quadrature Dicke model where the two
quadratures of the bosonic field are now coupled to two independent sets of two
level systems. Our findings, which show that the entanglement properties
between light and matter can be accessed through the Heisenberg uncertainty
principle, can be tested using Bose-Einstein condensates in optical cavities
and circuit quantum electrodynamicsComment: 7 pages, 3 figures. Published Versio
Common gauge origin of discrete symmetries in observable sector and hidden sector
An extra Abelian gauge symmetry is motivated in many new physics models in
both supersymmetric and nonsupersymmetric cases. Such a new gauge symmetry may
interact with both the observable sector and the hidden sector. We
systematically investigate the most general residual discrete symmetries in
both sectors from a common Abelian gauge symmetry. Those discrete symmetries
can ensure the stability of the proton and the dark matter candidate. A hidden
sector dark matter candidate (lightest U-parity particle or LUP) interacts with
the standard model fields through the gauge boson Z', which may selectively
couple to quarks or leptons only. We make a comment on the implications of the
discrete symmetry and the leptonically coupling dark matter candidate, which
has been highlighted recently due to the possibility of the simultaneous
explanation of the DAMA and the PAMELA results. We also show how to construct
the most general U(1) charges for a given discrete symmetry, and discuss the
relation between the U(1) gauge symmetry and R-parity.Comment: Version to appear in JHE
Chaotic quantum ratchets and filters with cold atoms in optical lattices: Properties of Floquet states.
The -kicked rotor is a paradigm of quantum chaos. Its realisation with clouds of cold atoms in pulsed optical lattices demonstrated the well-known quantum chaos phenomenon of 'dynamical localisation'. In those experi ments by several groups world-wide, the £-kicks were applied at equal time intervals. However, recent theoretical and experimental work by the cold atom group at UCL Monteiro et al 2002, Jonckheere et al 2003, Jones et al 2004 showed that novel quantum and classical dynamics arises if the atomic cloud is pulsed with repeating sequences of unequally spaced kicks. In Mon teiro et al 2002 it was found that the energy absorption rates depend on the momentum of the atoms relative to the optical lattice hence a type of chaotic ratchet was proposed. In Jonckheere et al and Jones et al, a possible mechanism for selecting atoms according to their momenta (velocity filter) was investigated. The aim of this thesis was to study the properties of the underlying eigen values and eigenstates. Despite the unequally-spaced kicks, these systems are still time-periodic, so we in fact investigated the Floquet states, which are eigenstates of U(T), the one-period time evolution operator. The Floquet states and corresponding eigenvalues were obtained by diagonalising a ma trix representation of the operator U(T). It was found that the form of the eigenstates enables us to analyse qual itatively the atomic momentum probability distributions, N(p) measured experimentally. In particular, the momentum width of the individual eigen states varies strongly with as expected from the theoretical and ex- perimental results obtained previously. In addition, at specific close to values which in the experiment yield directed motion (ratchet transport), the probability distribution of the individual Floquet states is asymmetric, mirroring the asymmetric N(p) measured in clouds of cesium atoms. In the penultimate chapter, the spectral fluctuations (eigenvalue statis tics) are investigated for one particular system, the double-delta kicked rotor. We computed Nearest Neighbour Spacing (NNS) distributions as well as the number variances (E2 statistics). We find that even in regimes where the corresponding classical dynamics are fully chaotic, the statistics are, unex pectedly, intermediate between fully chaotic (GOE) and fully regular (Pois- son). It is argued that they are analogous to the critical statistics seen in the Anderson metal-insulator transition
Majorana Spin Liquids, Topology and Superconductivity in Ladders
We theoretically address spin chain analogs of the Kitaev quantum spin model
on the honeycomb lattice. The emergent quantum spin liquid phases or Anderson
resonating valence bond (RVB) states can be understood, as an effective model,
in terms of p-wave superconductivity and Majorana fermions. We derive a
generalized phase diagram for the two-leg ladder system with tunable
interaction strengths between chains allowing us to vary the shape of the
lattice (from square to honeycomb ribbon or brickwall ladder). We evaluate the
winding number associated with possible emergent (topological) gapless modes at
the edges. In the Az phase, as a result of the emergent Z2 gauge fields and
pi-flux ground state, one may build spin-1/2 (loop) qubit operators by analogy
to the toric code. In addition, we show how the intermediate gapless B phase
evolves in the generalized ladder model. For the brickwall ladder, the
phase is reduced to one line, which is analyzed through perturbation theory in
a rung tensor product states representation and bosonization. Finally, we show
that doping with a few holes can result in the formation of hole pairs and
leads to a mapping with the Su-Schrieffer-Heeger model in polyacetylene; a
superconducting-insulating quantum phase transition for these hole pairs is
accessible, as well as related topological properties.Comment: 25 pages, 10 figures, final version - to be published in PR
Smearing of charge fluctuations in a grain by spin-flip assisted tunneling
We investigate the charge fluctuations of a grain (large dot) coupled to a
lead via a small quantum dot in the Kondo regime. We show that the strong
entanglement of charge and spin flips in this setup can result in a stable
SU(4) Kondo fixed point, which considerably smears out the Coulomb staircase
behavior already in the weak tunneling limit. This behavior is robust enough to
be experimentally observable.Comment: 4 pages, 1 figure, final version for PRB Rapid Com
Reconstruction of plasma density profiles by measuring spectra of radiation emitted from oscillating plasma dipoles
We suggest a new method for characterising non-uniform density distributions of plasma by measuring the spectra of radiation emitted from a localised plasma dipole oscillator excited by colliding electromagnetic pulses. The density distribution can be determined by scanning the collision point in space. Two-dimensional particle-in-cell simulations demonstrate the reconstruction of linear and nonlinear density profiles corresponding to laser-produced plasma. The method can be applied to a wide range of plasma, including fusion and low temperature plasmas. It overcomes many of the disadvantages of existing methods that only yield average densities along the path of probe pulses, such as interferometry and spectroscopy
Turing machines can be efficiently simulated by the General Purpose Analog Computer
The Church-Turing thesis states that any sufficiently powerful computational
model which captures the notion of algorithm is computationally equivalent to
the Turing machine. This equivalence usually holds both at a computability
level and at a computational complexity level modulo polynomial reductions.
However, the situation is less clear in what concerns models of computation
using real numbers, and no analog of the Church-Turing thesis exists for this
case. Recently it was shown that some models of computation with real numbers
were equivalent from a computability perspective. In particular it was shown
that Shannon's General Purpose Analog Computer (GPAC) is equivalent to
Computable Analysis. However, little is known about what happens at a
computational complexity level. In this paper we shed some light on the
connections between this two models, from a computational complexity level, by
showing that, modulo polynomial reductions, computations of Turing machines can
be simulated by GPACs, without the need of using more (space) resources than
those used in the original Turing computation, as long as we are talking about
bounded computations. In other words, computations done by the GPAC are as
space-efficient as computations done in the context of Computable Analysis
Coulomb drag between two spin incoherent Luttinger liquids
In a one dimensional electron gas at low enough density, the magnetic (spin)
exchange energy between neighboring electrons is exponentially suppressed
relative to the characteristic charge energy, the Fermi energy . At
non-zero temperature , the energy hierarchy can be
reached, and we refer to this as the spin incoherent Lutinger liquid state. We
discuss the Coulomb drag between two parallel quantum wires in the spin
incoherent regime, as well as the crossover to this state from the low
temperature regime by using a model of a fluctuating Wigner solid. As the
temperature increases from zero to above for a fixed electron density, the
oscillations in the density-density correlations are lost. As a result,
the temperature dependence of the Coulomb drag is dramatically altered and
non-monotonic dependence may result. Drag between wires of equal and unequal
density are discussed, as well as the effects of weak disorder in the wires. We
speculate that weak disorder may play an important role in extracting
information about quantum wires in real drag experiments.Comment: 19 pages, 10 figure
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