9,640 research outputs found
Network Synthesis of Linear Dynamical Quantum Stochastic Systems
The purpose of this paper is to develop a synthesis theory for linear
dynamical quantum stochastic systems that are encountered in linear quantum
optics and in phenomenological models of linear quantum circuits. In
particular, such a theory will enable the systematic realization of
coherent/fully quantum linear stochastic controllers for quantum control,
amongst other potential applications. We show how general linear dynamical
quantum stochastic systems can be constructed by assembling an appropriate
interconnection of one degree of freedom open quantum harmonic oscillators and,
in the quantum optics setting, discuss how such a network of oscillators can be
approximately synthesized or implemented in a systematic way from some linear
and non-linear quantum optical elements. An example is also provided to
illustrate the theory.Comment: Revised and corrected version, published in SIAM Journal on Control
and Optimization, 200
The Las Campanas Infra-red Survey. V. Keck Spectroscopy of a large sample of Extremely Red Objects
(Abridged) We present deep Keck spectroscopy, using the DEIMOS and LRIS
spectrographs, of a large and representative sample of 67 ``Extremely Red
Objects'' (EROs) to H=20.5, with I-H>3.0, in three of the Las Campanas Infrared
Survey fields. Spectroscopic redshifts are determined for 44 sources, of which
only two are contaminating low mass stars. When allowance is made for
incompleteness, the spectroscopic redshift distribution closely matches that
predicted earlier on the basis of photometric data. Our spectra are of
sufficient quality that we can address the important question of the nature and
homogeneity of the z>0.8 ERO population. A dominant old stellar population is
inferred for 75% of our spectroscopic sample; a higher fraction than that seen
in smaller, less-complete samples with broader photometric selection criteria
(e.g. R-K). However, only 28% have spectra with no evidence of recent star
formation activity, such as would be expected for a strictly passively-evolving
population. More than ~30% of our absorption line spectra are of the `E+A' type
with prominent Balmer absorption consistent, on average, with mass growth of
5-15% in the past Gyr. We use our spectroscopic redshifts to improve earlier
estimates of the spatial clustering of this population as well as to understand
the significant field-to-field variation. Our spectroscopy enables us to
pinpoint a filamentary structure at z=1.22 in the Chandra Deep Field South.
Overall, our study suggests that the bulk of the ERO population is an
established population of clustered massive galaxies undergoing intermittent
activity consistent with continued growth over the redshift interval 0.8<z<1.6.Comment: 27 pages, including 14 figures and appendix of spectra (at low
resolution). Full resolution paper can be found at
http://www.ast.cam.ac.uk/~md . To appear in MNRA
Quantum Computation as Geometry
Quantum computers hold great promise, but it remains a challenge to find
efficient quantum circuits that solve interesting computational problems. We
show that finding optimal quantum circuits is essentially equivalent to finding
the shortest path between two points in a certain curved geometry. By recasting
the problem of finding quantum circuits as a geometric problem, we open up the
possibility of using the mathematical techniques of Riemannian geometry to
suggest new quantum algorithms, or to prove limitations on the power of quantum
computers.Comment: 13 Pages, 1 Figur
Extremal Quantum Correlations and Cryptographic Security
We investigate a fundamental property of device independent security in
quantum cryptography by characterizing probability distributions which are
necessarily independent of the measurement results of any eavesdropper. We show
that probability distributions that are secure in this sense are exactly the
extremal quantum probability distributions. This allows us to give a
characterization of security in algebraic terms. We apply the method to common
examples for two-party as well as multi-party setups and present a scheme for
verifying security of probability distributions with two parties, two
measurement settings, and two outcomes.Comment: 7 pages, 2 figures, revised version, accepted for publication in
Phys. Rev. Let
NHEJ protects mycobacteria in stationary phase against the harmful effects of desiccation
The physiological role of the non-homologous end-joining (NHEJ) pathway in the repair of DNA double-strand breaks (DSBs) was examined in Mycobacterium smegmatis using DNA repair mutants (DeltarecA, Deltaku, DeltaligD, Deltaku/ligD, DeltarecA/ku/ligD). Wild-type and mutant strains were exposed to a range of doses of ionizing radiation at specific points in their life-cycle. NHEJ-mutant strains (Deltaku, DeltaligD, Deltaku/ligD) were significantly more sensitive to ionizing radiation (IR) during stationary phase than wild-type M. smegmatis. However, there was little difference in IR sensitivity between NHEJ-mutant and wild-type strains in logarithmic phase. Similarly, NHEJ-mutant strains were more sensitive to prolonged desiccation than wild-type M. smegmatis. A DeltarecA mutant strain was more sensitive to desiccation and IR during both stationary and especially in logarithmic phase, compared to wild-type strain, but it was significantly less sensitive to IR than the DeltarecA/ku/ligD triple mutant during stationary phase. These data suggest that NHEJ and homologous recombination are the preferred DSB repair pathways employed by M. smegmatis during stationary and logarithmic phases, respectively
Semiclassical theory of cavity-assisted atom cooling
We present a systematic semiclassical model for the simulation of the
dynamics of a single two-level atom strongly coupled to a driven high-finesse
optical cavity. From the Fokker-Planck equation of the combined atom-field
Wigner function we derive stochastic differential equations for the atomic
motion and the cavity field. The corresponding noise sources exhibit strong
correlations between the atomic momentum fluctuations and the noise in the
phase quadrature of the cavity field. The model provides an effective tool to
investigate localisation effects as well as cooling and trapping times. In
addition, we can continuously study the transition from a few photon quantum
field to the classical limit of a large coherent field amplitude.Comment: 10 pages, 8 figure
Assignment of the NV0 575 nm zero-phonon line in diamond to a 2E-2A2 transition
The time-averaged emission spectrum of single nitrogen-vacancy defects in
diamond gives zero-phonon lines of both the negative charge state at 637 nm
(1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs
through photo-conversion between the two charge states. Due to strain in the
diamond the zero-phonon lines are split and it is found that the splitting and
polarization of the two zero-phonon lines are the same. From this observation
and consideration of the electronic structure of the nitrogen-vacancy center it
is concluded that the excited state of the neutral center has A2 orbital
symmetry. The assignment of the 575 nm transition to a 2E - 2A2 transition has
not been established previously.Comment: 5 pages, 5 figure
Quantum Heating of a nonlinear resonator probed by a superconducting qubit
We measure the quantum fluctuations of a pumped nonlinear resonator, using a
superconducting artificial atom as an in-situ probe. The qubit excitation
spectrum gives access to the frequency and temperature of the intracavity field
fluctuations. These are found to be in agreement with theoretical predictions;
in particular we experimentally observe the phenomenon of quantum heating
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