10,766 research outputs found
Incommensurability Effects in Odd Length J_1-J_2 Quantum Spin Chains: On-site magnetization and Entanglement
For the antiferromagnetic J_1-J_2 quantum spin chain with an even number of
sites, the point J_2^d=1/2 J_1 is a disorder point. It marks the onset of
incommensurate real space correlations for J_2>J_2^d. At a distinct larger
value of J_2^L=0.52036(6)J_1, the Lifshitz point, the peak in the static
structure factor begins to move away from k=\pi. Here, we focus on chains with
an odd number of sites. In this case the disorder point is also at J_2^d=1/2
J_1, but the behavior close to the Lifshitz point, J_2^L approx. 0.538 J_1, is
quite different: starting at J_2^L, the ground-state goes through a sequence of
level crossings as its momentum changes away from k=\pi/2. An even length
chain, on the other hand, is gapped for any J_2>0.24J_1 and has the
ground-state momentum k=0. This gradual change in the ground-state wave
function for chains with an odd number of sites is reflected in a dramatic
manner directly in the ground-state on-site magnetization as well as in the
bi-partite von Neumann entanglement entropy. Our results are based on DMRG
calculations and variational calculations performed in a restricted Hilbert
space defined in the valence bond picture. In the vicinity of the point J_2=1/2
J_1, we expect the variational results to be very precise.Comment: Accepted for publication in Phys. Rev.
Efficient atomic clocks operated with several atomic ensembles
Atomic clocks are typically operated by locking a local oscillator (LO) to a
single atomic ensemble. In this article we propose a scheme where the LO is
locked to several atomic ensembles instead of one. This results in an
exponential improvement compared to the conventional method and provides a
stability of the clock scaling as with being the number
of atoms in each of the ensembles and is a constant depending on
the protocol being used to lock the LOComment: 10 pages, 8 figure
An efficient quantum memory based on two-level atoms
We propose a method to implement a quantum memory for light based on
ensembles of two-level atoms. Our protocol is based on controlled reversible
inhomogeneous broadening (CRIB), where an external field first dephases the
atomic polarization and thereby stores an incoming light pulse into collective
states of the atomic ensemble, and later a reversal of the applied field leads
to a rephasing of the atomic polarization and a reemission of the light. As
opposed to previous proposals for CRIB based quantum memories we propose to
only apply the broadening for a short period after most of the pulse has
already been absorbed by the ensemble. We show that with this procedure there
exist certain modes of the incoming light field which can be stored with an
efficiency approaching 100% in the limit of high optical depth and long
coherence time of the atoms. These results demonstrate that it is possible to
operate an efficient quantum memory without any optical control fields
Floquet quantum simulation with superconducting qubits
We propose a quantum algorithm for simulating spin models based on periodic
modulation of transmon qubits. Using Floquet theory we derive an effective
time-averaged Hamiltonian, which is of the general XYZ class, different from
the isotropic XY Hamiltonian typically realised by the physical setup. As an
example, we provide a simple recipe to construct a transverse Ising Hamiltonian
in the Floquet basis. For a 1D system we demonstrate numerically the dynamical
simulation of the transverse Ising Hamiltonian and quantum annealing to its
ground state. We benchmark the Floquet approach with a digital simulation
procedure, and demonstrate that it is advantageous for limited resources and
finite anharmonicity of the transmons. The described protocol can serve as a
simple yet reliable path towards configurable quantum simulators with currently
existing superconducting chips.Comment: 6+12 pages, 4+5 figure
Three-dimensional theory of stimulated Raman scattering
We present a three-dimensional theory of stimulated Raman scattering
(SRS) or superradiance. In particular we address how the spatial and temporal
properties of the generated SRS beam, or Stokes beam, of radiation depends on
the spatial properties of the gain medium. Maxwell equations for the Stokes
field operators and of the atomic operators are solved analytically and a
correlation function for the Stokes field is derived. In the analysis we
identify a superradiating part of the Stokes radiation that exhibit beam
characteristics. We show how the intensity in this beam builds up in time and
at some point largely dominates the total Stokes radiation of the gain medium.
We show how the SRS depends on geometric factors such as the Fresnel number and
the optical depth, and that in fact these two factors are the only factors
describing the coherent radiation.Comment: 21 pages 14 figure
DARCOF II. Danish research in Organic Food and Farming systems 2000-2005
The aim of this book is to present a comprehensive overview of the 41 research projects undertaken in the period 2000-2005 in the research programme DARCOF II.For each project there is a description of its background and objective in terms of which issues gave rise to the project and what the project aims to achieve. This is followed by a short description of the experiments or investigations that have been undertaken in the project. The general and applicable results derived from the project are finally described. For each project there is a reference to a project home page on www.darcof.dk. Via this page there is direct access to "Organic Eprints", which is the site containing all the project publications – both technical and scientific
ICROFS news 2/2009 - newsletter from ICROFS
News from ICROFS:
CORE Organic ERA-net proposal is formulated, FAO side-event was a success, upcoming course in media handling
Articles:
J. Eriksen, M. Askegaard & K. Søegaard: Nitrogen management on large organic daity farms
C. Daugbjerg & K. M. Sønderskov: Organic labelling systems and consumer confidence
G. T. Svendsen: Organic farmers can gain from Green House Gas trade
H. Egelyng: Certified Organic Agriclture: Policy Instrument for Sustainable Development?
M.S. Carter & N. Chirinda: No effect of cropping system on the greenhouse gas N2O
J.H. Ingemann: Economics, Policy, and Organic Agriculture
Brief news:
TP ORganics needs you!, New publication: The World of ORganic Agriculture: Statistics and emerging trends, NJF seminar, Organic farmers bite back!, International conference on organic agriculture in Scope of environmental problems, Expo - MENOPE: 7th Middle East Natural and Organic Product
Quantum networks with chiral light--matter interaction in waveguides
We propose a scalable architecture for a quantum network based on a simple
on-chip photonic circuit that performs loss-tolerant two-qubit measurements.
The circuit consists of two quantum emitters positioned in the arms of an
on-chip Mach-Zehnder interferometer composed of waveguides with chiral
light--matter interfaces. The efficient chiral light--matter interaction allows
the emitters to perform high-fidelity intranode two-qubit parity measurements
within a single chip, and to emit photons to generate internode entanglement,
without any need for reconfiguration. We show that by connecting multiple
circuits of this kind into a quantum network, it is possible to perform
universal quantum computation with heralded two-qubit gate fidelities achievable in state-of-the-art quantum dot systems.Comment: 5 pages plus supplementary materia
High dimensional measurement device independent quantum key distribution on two dimensional subspaces
Quantum key distribution (QKD) provides ultimate cryptographic security based
on the laws of quantum mechanics. For point-to-point QKD protocols, the
security of the generated key is compromised by detector side channel attacks.
This problem can be solved with measurement device independent QKD (mdi-QKD).
However, mdi-QKD has shown limited performances in terms of the secret key
generation rate, due to post-selection in the Bell measurements. We show that
high dimensional (Hi-D) encoding (qudits) improves the performance of current
mdi-QKD implementations. The scheme is proven to be unconditionally secure even
for weak coherent pulses with decoy states, while the secret key rate is
derived in the single photon case. Our analysis includes phase errors,
imperfect sources and dark counts to mimic real systems. Compared to the
standard bidimensional case, we show an improvement in the key generation rate.Comment: 6 pages, 3 figure
Elementary test for non-classicality based on measurements of position and momentum
We generalise a non-classicality test described by Kot et al. [Phys. Rev.
Lett. 108, 233601 (2010)], which can be used to rule out any classical
description of a physical system. The test is based on measurements of
quadrature operators and works by proving a contradiction with the classical
description in terms of a probability distribution in phase space. As opposed
to the previous work, we generalise the test to include states without
rotational symmetry in phase space. Furthermore, we compare the performance of
the non-classicality test with classical tomography methods based on the
inverse Radon transform, which can also be used to establish the quantum nature
of a physical system. In particular, we consider a non-classicality test based
on the so-called filtered back-projection formula. We show that the general
non-classicality test is conceptually simpler, requires less assumptions on the
system and is statistically more reliable than the tests based on the filtered
back-projection formula. As a specific example, we derive the optimal test for
a quadrature squeezed single photon state and show that the efficiency of the
test does not change with the degree of squeezing
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