175 research outputs found
Solid-state quantum optics with quantum dots in photonic nanostructures
Quantum nanophotonics has become a new research frontier where quantum optics
is combined with nanophotonics in order to enhance and control the interaction
between strongly confined light and quantum emitters. Such progress provides a
promising pathway towards quantum-information processing on an all-solid-state
platform. Here we review recent progress on experiments with single quantum
dots in nanophotonic structures. Embedding the quantum dots in photonic
band-gap structures offers a way of controlling spontaneous emission of single
photons to a degree that is determined by the local light-matter coupling
strength. Introducing defects in photonic crystals implies new functionalities.
For instance, efficient and strongly confined cavities can be constructed
enabling cavity-quantum-electrodynamics experiments. Furthermore, the speed of
light can be tailored in a photonic-crystal waveguide forming the basis for
highly efficient single-photon sources where the photons are channeled into the
slowly propagating mode of the waveguide. Finally, we will discuss some of the
surprises that arise in solid-state implementations of quantum-optics
experiments in comparison to their atomic counterparts. In particular, it will
be shown that the celebrated point-dipole description of light-matter
interaction can break down when quantum dots are coupled to plasmon
nanostructures.Comment: Review. 15 pages, 9 figure
Efficient Minimization of Decomposable Submodular Functions
Many combinatorial problems arising in machine learning can be reduced to the
problem of minimizing a submodular function. Submodular functions are a natural
discrete analog of convex functions, and can be minimized in strongly
polynomial time. Unfortunately, state-of-the-art algorithms for general
submodular minimization are intractable for larger problems. In this paper, we
introduce a novel subclass of submodular minimization problems that we call
decomposable. Decomposable submodular functions are those that can be
represented as sums of concave functions applied to modular functions. We
develop an algorithm, SLG, that can efficiently minimize decomposable
submodular functions with tens of thousands of variables. Our algorithm
exploits recent results in smoothed convex minimization. We apply SLG to
synthetic benchmarks and a joint classification-and-segmentation task, and show
that it outperforms the state-of-the-art general purpose submodular
minimization algorithms by several orders of magnitude.Comment: Expanded version of paper for Neural Information Processing Systems
201
Interfacing single photons and single quantum dots with photonic nanostructures
Photonic nanostructures provide means of tailoring the interaction between
light and matter and the past decade has witnessed a tremendous experimental
and theoretical progress in this subject. In particular, the combination with
semiconductor quantum dots has proven successful. This manuscript reviews
quantum optics with excitons in single quantum dots embedded in photonic
nanostructures. The ability to engineer the light-matter interaction strength
in integrated photonic nanostructures enables a range of fundamental
quantum-electrodynamics experiments on, e.g., spontaneous-emission control,
modified Lamb shifts, and enhanced dipole-dipole interaction. Furthermore,
highly efficient single-photon sources and giant photon nonlinearities may be
implemented with immediate applications for photonic quantum-information
processing. The review summarizes the general theoretical framework of photon
emission including the role of dephasing processes, and applies it to photonic
nanostructures of current interest, such as photonic-crystal cavities and
waveguides, dielectric nanowires, and plasmonic waveguides. The introduced
concepts are generally applicable in quantum nanophotonics and apply to a large
extent also to other quantum emitters, such as molecules, nitrogen vacancy
ceters, or atoms. Finally, the progress and future prospects of applications in
quantum-information processing are considered.Comment: Updated version resubmitted to Reviews of Modern Physic
Mapping the local density of optical states of a photonic crystal with single quantum dots
We use single self-assembled InGaAs quantum dots as internal probes to map
the local density of optical states of photonic crystal membranes. The employed
technique separates contributions from non-radiative recombination and
spin-flip processes by properly accounting for the role of the exciton fine
structure. We observe inhibition factors as high as 55 and compare our results
to local density of optical states calculations available from the literature,
thereby establishing a quantitative understanding of photon emission in
photonic crystal membranes.Comment: 4 pages, 3 figure
Unraveling the mesoscopic character of quantum dots in nanophotonics
We provide a microscopic theory for semiconductor quantum dots that explains
the pronounced deviations from the prevalent point-dipole description that were
recently observed in spectroscopic experiments on quantum dots in photonic
nanostructures. At the microscopic level the deviations originate from
structural inhomogeneities generating a large circular quantum current density
that flows inside the quantum dot over mesoscopic length scales. The model is
supported by the experimental data, where a strong variation of the multipolar
moments across the emission spectrum of quantum dots is observed. Our work
enriches the physical understanding of quantum dots and is of significance for
the fields of nanophotonics, quantum photonics, and quantum-information
science, where quantum dots are actively employed.Comment: 6 pages, 5 figure
Two mechanisms of disorder-induced localization in photonic-crystal waveguides
Unintentional but unavoidable fabrication imperfections in state-of-the-art
photonic-crystal waveguides lead to the spontaneous formation of
Anderson-localized modes thereby limiting slowlight propagation and its
potential applications. On the other hand, disorder-induced cavities offer an
approach to cavity-quantum electrodynamics and random lasing at the nanoscale.
The key statistical parameter governing the disorder effects is the
localization length, which together with the waveguide length determines the
statistical transport of light through the waveguide. In a disordered
photonic-crystal waveguide, the localization length is highly dispersive, and
therefore, by controlling the underlying lattice parameters, it is possible to
tune the localization of the mode. In the present work, we study the
localization length in a disordered photonic-crystal waveguide using numerical
simulations. We demonstrate two different localization regimes in the
dispersion diagram where the localization length is linked to the density of
states and the photon effective mass, respectively. The two different
localization regimes are identified in experiments by recording the
photoluminescence from quantum dots embedded in photonic-crystal waveguides.Comment: Accepted for publication in Physical Review
Probing electric and magnetic vacuum fluctuations with quantum dots
The electromagnetic-vacuum-field fluctuations are intimately linked to the
process of spontaneous emission of light. Atomic emitters cannot probe
electric- and magnetic-field fluctuations simultaneously because electric and
magnetic transitions correspond to different selection rules. In this paper we
show that semiconductor quantum dots are fundamentally different and are
capable of mediating electric-dipole, magnetic-dipole, and electric-quadrupole
transitions on a single electronic resonance. As a consequence, quantum dots
can probe electric and magnetic fields simultaneously and can thus be applied
for sensing the electromagnetic environment of complex photonic nanostructures.
Our study opens the prospect of interfacing quantum dots with optical
metamaterials for tailoring the electric and magnetic light-matter interaction
at the single-emitter level.Comment: 6 pages, 4 figure
Exciton spin-flip rate in quantum dots determined by a modified local density of optical states
The spin-flip rate that couples dark and bright excitons in self-assembled
quantum dots is obtained from time-resolved spontaneous emission measurements
in a modified local density of optical states. Employing this technique, we can
separate effects due to non-radiative recombination and unambiguously record
the spin-flip rate. The dependence of the spin-flip rate on emission energy is
compared in detail to a recent model from the literature, where the spin flip
is due to the combined action of short-range exchange interaction and acoustic
phonons. We furthermore observe a surprising enhancement of the spin-flip rate
close to a semiconductor-air interface, which illustrates the important role of
interfaces for quantum dot based nanophotonic structures. Our work is an
important step towards a full understanding of the complex dynamics of quantum
dots in nanophotonic structures, such as photonic crystals, and dark excitons
are potentially useful for long-lived coherent storage applications.Comment: 5 pages, 4 figure
Utopisches Denken bei V. Chlebnikov
Die vorliegende Arbeit setzt sich mit dem Begriff Utopie bei dem russischen Schriftsteller Velimir Chlebnikov (1885 - 1922) auseinander. Das Thema wird allerdings nicht unter dem Aspekt der Sozialutopie, sondern unter dem Gesichtspunkt des utopischen Denkens behandelt. Hier sind vor allem die Überlegungen Chlebnikovs zur Zahl, bzw. zur Zeit als Ausdruck eines zyklischen Verlaufs der Geschichte, sowie die Versuche, eine transkulturelle Sprache ("zvesdnyj jazik", "zaum") zu schaffen im Zusammenhang mit dem Konzept des "budetljanstvo" als ein utopisches Konzept einer zukünftigen Welt von Bedeutung
Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy
We investigate the optical emission and decay dynamics of excitons confined
in large strain-free GaAs quantum dots grown by droplet epitaxy. From
time-resolved measurements combined with a theoretical model we show that
droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an
oscillator strength between 8 and 10. The quantum dots are found to be fully
described by a model for strongly-confined excitons, in contrast to the
theoretical prediction that excitons in large quantum dots exhibit the
so-called giant oscillator strength. We attribute these findings to localized
ground-state excitons in potential minima created by material intermixing
during growth. We provide further evidence for the strong-confinement regime of
excitons by extracting the size of electron and hole wavefunctions from the
phonon-broadened photoluminescence spectra. Furthermore, we explore the
temperature dependence of the decay dynamics and, for some quantum dots,
observe a pronounced reduction in the effective transition strength with
temperature. We quantify and explain these effects as being an intrinsic
property of large quantum dots owing to thermal excitation of the ground-state
exciton. Our results provide a detailed understanding of the optical properties
of large quantum dots in general, and of quantum dots grown by droplet epitaxy
in particular.Comment: 13 pages, 7 figure
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