225 research outputs found
Large quantum dots with small oscillator strength
We have measured the oscillator strength and quantum efficiency of excitons
confined in large InGaAs quantum dots by recording the spontaneous emission
decay rate while systematically varying the distance between the quantum dots
and a semiconductor-air interface. The size of the quantum dots is measured by
in-plane transmission electron microscopy and we find average in-plane
diameters of 40 nm. We have calculated the oscillator strength of excitons of
that size and predict a very large oscillator strength due to Coulomb effects.
This is in stark contrast to the measured oscillator strength, which turns out
to be much below the upper limit imposed by the strong confinement model. We
attribute these findings to exciton localization in local potential minima
arising from alloy intermixing inside the quantum dots.Comment: 4 pages, 3 figures, submitte
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
Quantum properties of transverse pattern formation in second-harmonic generation
We investigate the spatial quantum noise properties of the one dimensional
transverse pattern formation instability in intra-cavity second-harmonic
generation. The Q representation of a quasi-probability distribution is
implemented in terms of nonlinear stochastic Langevin equations. We study these
equations through extensive numerical simulations and analytically in the
linearized limit. Our study, made below and above the threshold of pattern
formation, is guided by a microscopic scheme of photon interaction underlying
pattern formation in second-harmonic generation. Close to the threshold for
pattern formation, beams with opposite direction of the off-axis critical wave
numbers are shown to be highly correlated. This is observed for the fundamental
field, for the second harmonic field and also for the cross-correlation between
the two fields. Nonlinear correlations involving the homogeneous transverse
wave number, which are not identified in a linearized analysis, are also
described. The intensity differences between opposite points of the far fields
are shown to exhibit sub-Poissonian statistics, revealing the quantum nature of
the correlations. We observe twin beam correlations in both the fundamental and
second-harmonic fields, and also nonclassical correlations between them.Comment: 18 pages, 17 figures, submitted to Phys. Rev.
Entanglement properties of a quantum-dot biexciton cascade in a chiral nanophotonic waveguide
We analyse the entanglement properties of deterministic path-entangled
photonic states generated by coupling the emission of a quantum-dot biexciton
cascade to a chiral nanophotonic waveguide, as implemented by {\O}stfeldt et
al. [PRX Quantum 3, 020363 (2022)]. We model the degree of entanglement through
the concurrence of the two-photon entangled state in the presence of realistic
experimental imperfections. The model accounts for imperfect chiral
emitter-photon interactions in the waveguide and the asymmetric coupling of the
exciton levels introduced by fine-structure splitting along with time-jitter in
the detection of photons. The analysis shows that the approach offers a
promising platform for deterministically generating entanglement in integrated
nanophotonic systems in the presence of realistic experimental imperfections.Comment: 12 pages, 5 figure
Single-photon nonlinear optics with a quantum dot in a waveguide
Strong nonlinear interactions between photons enable logic operations for
both classical and quantum-information technology. Unfortunately, nonlinear
interactions are usually feeble and therefore all-optical logic gates tend to
be inefficient. A quantum emitter deterministically coupled to a propagating
mode fundamentally changes the situation, since each photon inevitably
interacts with the emitter, and highly correlated many-photon states may be
created . Here we show that a single quantum dot in a photonic-crystal
waveguide can be utilized as a giant nonlinearity sensitive at the
single-photon level. The nonlinear response is revealed from the intensity and
quantum statistics of the scattered photons, and contains contributions from an
entangled photon-photon bound state. The quantum nonlinearity will find
immediate applications for deterministic Bell-state measurements and
single-photon transistors and paves the way to scalable waveguide-based
photonic quantum-computing architectures
Experimental realization of highly-efficient broadband coupling of single quantum dots to a photonic crystal waveguide
We present time-resolved spontaneous emission measurements of single quantum
dots embedded in photonic crystal waveguides. Quantum dots that couple to the
photonic crystal waveguide are found to decay up to 27 times faster than
uncoupled quantum dots. From these measurements -factors of up to 0.89
are derived, and an unprecedented large bandwidth of 20 nm is demonstrated.
This shows the promising potential of photonic crystal waveguides for efficient
single-photon sources. The scaled frequency where the enhancement is observed
is in excellent agreement with theory taking into account that the light-matter
coupling is strongly enhanced due to the significant slow-down of light in the
photonic crystal waveguide.Comment: 4 pages, 4 figures, submitted to PR
Size-Dependence of the Wavefunction of Self-Assembled Quantum Dots
The radiative and non-radiative decay rates of InAs quantum dots are measured
by controlling the local density of optical states near an interface. From
time-resolved measurements we extract the oscillator strength and the quantum
efficiency and their dependence on emission energy. From our results and a
theoretical model we determine the striking dependence of the overlap of the
electron and hole wavefunctions on the quantum dot size. We conclude that the
optical quality is best for large quantum dots, which is important in order to
optimally tailor quantum dot emitters for, e.g., quantum electrodynamics
experiments.Comment: 5 pages, 3 figure
Spin-photon interface and spin-controlled photon switching in a nanobeam waveguide
Access to the electron spin is at the heart of many protocols for integrated
and distributed quantum-information processing [1-4]. For instance, interfacing
the spin-state of an electron and a photon can be utilized to perform quantum
gates between photons [2,5] or to entangle remote spin states [6-9].
Ultimately, a quantum network of entangled spins constitutes a new paradigm in
quantum optics [1]. Towards this goal, an integrated spin-photon interface
would be a major leap forward. Here we demonstrate an efficient and optically
programmable interface between the spin of an electron in a quantum dot and
photons in a nanophotonic waveguide. The spin can be deterministically prepared
with a fidelity of 96\%. Subsequently the system is used to implement a
"single-spin photonic switch", where the spin state of the electron directs the
flow of photons through the waveguide. The spin-photon interface may enable
on-chip photon-photon gates [2], single-photon transistors [10], and efficient
photonic cluster state generation [11]
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