45 research outputs found
Highly indistinguishable single photons from incoherently and coherently excited GaAs quantum dots
Semiconductor quantum dots are converging towards the demanding requirements
of photonic quantum technologies. Among different systems, quantum dots with
dimensions exceeding the free-exciton Bohr radius are appealing because of
their high oscillator strengths. While this property has received much
attention in the context of cavity quantum electrodynamics, little is known
about the degree of indistinguishability of single photons consecutively
emitted by such dots and on the proper excitation schemes to achieve high
indistinguishability. A prominent example is represented by GaAs quantum dots
obtained by local droplet etching, which recently outperformed other systems as
triggered sources of entangled photon pairs. On these dots, we compare
different single-photon excitation mechanisms, and we find (i) a "phonon
bottleneck" and poor indistinguishability for conventional excitation via
excited states and (ii) photon indistinguishablilities above 90% for both
strictly resonant and for incoherent acoustic- and optical-phonon-assisted
excitation. Among the excitation schemes, optical phonon-assisted excitation
enables straightforward laser rejection without a compromise on the source
brightness together with a high photon indistinguishability
Fabrication And Optical Properties Of Strain-free Self-assembled Mesoscopic Gaas Structures
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES)We use a combined process of Ga-assisted deoxidation and local droplet etching to fabricate unstrained mesoscopic GaAs/AlGaAs structures exhibiting a high shape anisotropy with a length up to 1.2 mu m and a width of 150 nm. We demonstrate good controllability over size and morphology of the mesoscopic structures by tuning the growth parameters. Our growth method yields structures, which are coupled to a surrounding quantum well and present unique optical emission features. Microscopic and optical analysis of single structures allows us to demonstrate that single structure emission originates from two different confinement regions, which are spectrally separated and show sharp excitonic lines. Photoluminescence is detected up to room temperature making the structures the ideal candidates for strain-free light emitting/detecting devices.12SisNano (MCTI Brazil)FAPESP [2012/11382-9, 2014/17141-9, 2015/08344-6, 2016/14001-7]CNPq [482729/2013-9, 305769/2015-4, 475343/2013-1]CAPESFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES
Strain-Tunable GaAs Quantum dot: A Nearly Dephasing-Free Source of Entangled Photon Pairs on Demand
Entangled photon generation from semiconductor quantum dots via the
biexciton-exciton cascade underlies various decoherence mechanisms related to
the solid-state nature of the quantum emitters. So far, this has prevented the
demonstration of nearly-maximally entangled photons without the aid of
inefficient and complex post-selection techniques that are hardly suitable for
quantum communication technologies. Here, we tackle this challenge using
strain-tunable GaAs quantum dots driven under two-photon resonant excitation
and with strictly-degenerate exciton states. We demonstrate experimentally that
our on-demand source generates polarization-entangled photons with fidelity of
0.978(5) and concurrence of 0.97(1) without resorting to post-selection
techniques. Moreover, we show that the remaining decoherence mechanisms can be
overcome using a modest Purcell enhancement so as to achieve a degree of
entanglement >0.99. Our results highlight that GaAs quantum dots can be readily
used in advanced communication protocols relying on the non-local properties of
quantum entanglement
On-demand generation of background--free single photons from a solid-state source
True on--demand high--repetition--rate single--photon sources are highly
sought after for quantum information processing applications. However, any
coherently driven two-level quantum system suffers from a finite re-excitation
probability under pulsed excitation, causing undesirable multi--photon
emission. Here, we present a solid--state source of on--demand single photons
yielding a raw second--order coherence of
without any background subtraction nor data processing. To this date, this is
the lowest value of reported for any single--photon source even
compared to the previously best background subtracted values. We achieve this
result on GaAs/AlGaAs quantum dots embedded in a low--Q planar cavity by
employing (i) a two--photon excitation process and (ii) a filtering and
detection setup featuring two superconducting single--photon detectors with
ultralow dark-count rates of and , respectively. Re--excitation processes are dramatically suppressed by
(i), while (ii) removes false coincidences resulting in a negligibly low noise
floor
Intermediate Field Coupling of Single Epitaxial Quantum Dots to Plasmonic Waveguides
Key requirements for quantum plasmonic nanocircuits are reliable
single-photon sources, high coupling efficiency to the plasmonic structures and
low propagation losses. Self-assembled epitaxially grown GaAs quantum dots are
close to ideal stable, bright and narrowband single-photon emitters. Likewise,
wet-chemically grown monocrystalline silver nanowires are among the best
plasmonic waveguides. However, large propagation losses of surface plasmons on
the high-index GaAs substrate prevent their direct combination. Here, we show
by experiment and simulation that the best overall performance of the quantum
plasmonic nanocircuit based on these building blocks is achieved in the
intermediate field regime with an additional spacer layer between the quantum
dot and the plasmonic waveguide. High-resolution cathodoluminescence
measurements allow a precise determination of the coupling distance and support
a simple analytical model to explain the overall performance. The coupling
efficiency is increased up to four times by standing wave interference near the
end of the waveguide.Comment: Accepted at ACS Nano Letters; contains main text and supporting
informatio
Entanglement swapping with photons generated on-demand by a quantum dot
Photonic entanglement swapping, the procedure of entangling photons without
any direct interaction, is a fundamental test of quantum mechanics and an
essential resource to the realization of quantum networks. Probabilistic
sources of non-classical light can be used for entanglement swapping, but
quantum communication technologies with device-independent functionalities
demand for push-button operation that, in principle, can be implemented using
single quantum emitters. This, however, turned out to be an extraordinary
challenge due to the stringent requirements on the efficiency and purity of
generation of entangled states. Here we tackle this challenge and show that
pairs of polarization-entangled photons generated on-demand by a GaAs quantum
dot can be used to successfully demonstrate all-photonic entanglement swapping.
Moreover, we develop a theoretical model that provides quantitative insight on
the critical figures of merit for the performance of the swapping procedure.
This work shows that solid-state quantum emitters are mature for quantum
networking and indicates a path for scaling up.Comment: The first four authors contributed equally to this work. 17 pages, 3
figure
GaAs quantum dots under quasi-uniaxial stress: experiment and theory
The optical properties of excitons confined in initially-unstrained
GaAs/AlGaAs quantum dots are studied as a function of a variable quasi-uniaxial
stress. To allow the validation of state-of-the-art computational tools for
describing the optical properties of nanostructures, we determine the quantum
dot morphology and the in-plane components of externally induced strain tensor
at the quantum dot positions. Based on these experimentally determined
parameters, we calculate the strain-dependent excitonic emission energy, degree
of linear polarization, and fine-structure splitting using a combination of
eight-band formalism with multiparticle corrections using
the configuration interaction method. The presented experimental observations
are quantitatively well reproduced by our calculations
Resonance fluorescence of GaAs quantum dots with near-unity photon indistinguishability
Photonic quantum technologies call for scalable quantum light sources that
can be integrated, while providing the end user with single and entangled
photons on-demand. One promising candidate are strain free GaAs/AlGaAs quantum
dots obtained by droplet etching. Such quantum dots exhibit ultra low
multi-photon probability and an unprecedented degree of photon pair
entanglement. However, different to commonly studied InGaAs/GaAs quantum dots
obtained by the Stranski-Krastanow mode, photons with a near-unity
indistinguishability from these quantum emitters have proven to be elusive so
far. Here, we show on-demand generation of near-unity indistinguishable photons
from these quantum emitters by exploring pulsed resonance fluorescence. Given
the short intrinsic lifetime of excitons confined in the GaAs quantum dots, we
show single photon indistinguishability with a raw visibility of
, without the need for Purcell enhancement. Our
results represent a milestone in the advance of GaAs quantum dots by
demonstrating the final missing property standing in the way of using these
emitters as a key component in quantum communication applications, e.g. as an
entangled source for quantum repeater architectures
Collective Excitation of Spatio-Spectrally Distinct Quantum Dots Enabled by Chirped Pulses
For a scalable photonic device producing entangled photons, it is desirable
to have multiple quantum emitters in an ensemble that can be collectively
excited, despite their spectral variability. For quantum dots, Rabi rotation,
the most popular method for resonant excitation, cannot assure a universal,
highly efficient excited state preparation, because of its sensitivity to the
excitation parameters. In contrast, Adiabatic Rapid Passage (ARP), relying on
chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here,
we advocate the robustness of ARP for simultaneous excitation of the biexciton
states of multiple quantum dots. For positive chirps, we find that there is
also regime of phonon advantage that widens the tolerance range of spectral
detunings. Using the same laser pulse we demonstrate the simultaneous
excitation of energetically and spatially distinct quantum dots. Being able to
generate spatially multiplexed entangled photon pairs is a big step towards the
scalability of photonic devices