1,904 research outputs found
Particle Gibbs with Ancestor Sampling
Particle Markov chain Monte Carlo (PMCMC) is a systematic way of combining
the two main tools used for Monte Carlo statistical inference: sequential Monte
Carlo (SMC) and Markov chain Monte Carlo (MCMC). We present a novel PMCMC
algorithm that we refer to as particle Gibbs with ancestor sampling (PGAS).
PGAS provides the data analyst with an off-the-shelf class of Markov kernels
that can be used to simulate the typically high-dimensional and highly
autocorrelated state trajectory in a state-space model. The ancestor sampling
procedure enables fast mixing of the PGAS kernel even when using seemingly few
particles in the underlying SMC sampler. This is important as it can
significantly reduce the computational burden that is typically associated with
using SMC. PGAS is conceptually similar to the existing PG with backward
simulation (PGBS) procedure. Instead of using separate forward and backward
sweeps as in PGBS, however, we achieve the same effect in a single forward
sweep. This makes PGAS well suited for addressing inference problems not only
in state-space models, but also in models with more complex dependencies, such
as non-Markovian, Bayesian nonparametric, and general probabilistic graphical
models
Photon-Number Squeezing in Circuit Quantum Electrodynamics
A superconducting single-electron transistor (SSET) coupled to an anharmonic
oscillator, e.g., a Josephson junction-L-C circuit, can drive the latter to a
nonequilibrium photon number state. By biasing the SSET in a regime where the
current is carried by a combination of inelastic quasiparticle tunneling and
coherent Cooper-pair tunneling (Josephson quasiparticle cycle), cooling of the
oscillator as well as a laser like enhancement of the photon number can be
achieved. Here we show, that the cut-off in the quasiparticle tunneling rate
due to the superconducting gap, in combination with the anharmonicity of the
oscillator, may create strongly squeezed photon number distributions. For low
dissipation in the oscillator nearly pure Fock states can be produced.Comment: 5 pages, 5 figure
Properties of the energy landscape of network models for covalent glasses
We investigate the energy landscape of two dimensional network models for
covalent glasses by means of the lid algorithm. For three different particle
densities and for a range of network sizes, we exhaustively analyse many
configuration space regions enclosing deep-lying energy minima. We extract the
local densities of states and of minima, and the number of states and minima
accessible below a certain energy barrier, the 'lid'. These quantities show on
average a close to exponential growth as a function of their respective
arguments. We calculate the configurational entropy for these pockets of states
and find that the excess specific heat exhibits a peak at a critical
temperature associated with the exponential growth in the local density of
states, a feature of the specific heat also observed in real glasses at the
glass transition.Comment: RevTeX, 19 pages, 7 figure
Measuring current by counting electrons in a nanowire quantum dot
We measure current by counting single electrons tunneling through an InAs
nanowire quantum dot. The charge detector is realized by fabricating a quantum
point contact in close vicinity to the nanowire. The results based on electron
counting compare well to a direct measurements of the quantum dot current, when
taking the finite bandwidth of the detector into account. The ability to detect
single electrons also opens up possibilities for manipulating and detecting
individual spins in nanowire quantum dots
Detecting THz current fluctuations in a quantum point contact using a nanowire quantum dot
We use a nanowire quantum dot to probe high-frequency current fluctuations in
a nearby quantum point contact. The fluctuations drive charge transitions in
the quantum dot, which are measured in real-time with single-electron detection
techniques. The quantum point contact (GaAs) and the quantum dot (InAs) are
fabricated in different material systems, which indicates that the interactions
are mediated by photons rather than phonons. The large energy scales of the
nanowire quantum dot allow radiation detection in the long-wavelength infrared
regime
Zero-bias anomaly in cotunneling transport through quantum-dot spin valves
We predict a new zero-bias anomaly in the differential conductance through a
quantum dot coupled to two ferromagnetic leads with antiparallel magnetization.
The anomaly differs in origin and properties from other anomalies in transport
through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade
valleys with an unpaired dot electron. It is a consequence of the interplay of
single- and double-barrier cotunneling processes and their effect on the spin
accumulation in the dot. The anomaly becomes significantly modified when a
magnetic field is applied.Comment: 4 pages, 3 figure
Sequential Generation of Matrix-Product States in Cavity QED
We study the sequential generation of entangled photonic and atomic
multi-qubit states in the realm of cavity QED. We extend the work of C. Schoen
et al. [Phys. Rev. Lett. 95, 110503 (2005)], where it was shown that all states
generated in a sequential manner can be classified efficiently in terms of
matrix-product states. In particular, we consider two scenarios: photonic
multi-qubit states sequentially generated at the cavity output of a
single-photon source and atomic multi-qubit states generated by their
sequential interaction with the same cavity mode.Comment: 11 page
The role of damping for the driven anharmonic quantum oscillator
For the model of a linearly driven quantum anharmonic oscillator, the role of
damping is investigated. We compare the position of the stable points in phase
space obtained from a classical analysis to the result of a quantum mechanical
analysis. The solution of the full master equation shows that the stable points
behave qualitatively similar to the classical solution but with small
modifications. Both the quantum effects and additional effects of temperature
can be described by renormalizing the damping.Comment: 4 pages, 2 figures; submitted to "Journal of Physics: Conference
Series
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