22 research outputs found
Spectral broadening in self-assembled GaAs quantum dots with narrow size distribution
The control over the spectral broadening of an ensemble of emitters, mainly
attributable to the size and shape dispersion and the homogenous broadening
mechanisms, is crucial to several applications of quantum dots. We present a
convenient self-assembly approach to deliver strain-free GaAs quantum dots with
size distribution below 15%, due to the control of the growth parameters during
the preliminary formation of the Ga droplets. This results in an ensemble
photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the
absence of a wetting layer promoting dot-dot coupling allow us to deconvolve
the contribution of phonon broadening in the ensemble photoluminescence and
study it in a wide temperature range.Comment: 9 pages, 4 figure
High-temperature droplet epitaxy of symmetric GaAs/AlGaAs quantum dots
We introduce a high-temperature droplet epitaxy procedure, based on the
control of the arsenization dynamics of nanoscale droplets of liquid Ga on
GaAs(111)A surfaces. The use of high temperatures for the self-assembly of
droplet epitaxy quantum dots solves major issues related to material defects,
introduced during the droplet epitaxy fabrication process, which limited its
use for single and entangled photon sources for quantum photonics applications.
We identify the region in the parameter space which allows quantum dots to
self-assemble with the desired emission wavelength and highly symmetric shape
while maintaining a high optical quality. The role of the growth parameters
during the droplet arsenization is discussed and modelled.Comment: 18 pages, 5 figure
Optically controlled dual-band quantum dot infrared photodetector
We present the design for a novel type of dual-band photodetector in the
thermal infrared spectral range, the Optically Controlled Dual-band quantum dot
Infrared Photodetector (OCDIP). This concept is based on a quantum dot ensemble
with a unimodal size distribution, whose absorption spectrum can be controlled
by optically-injected carriers. An external pumping laser varies the electron
density in the QDs, permitting to control the available electronic transitions
and thus the absorption spectrum. We grew a test sample which we studied by AFM
and photoluminescence. Based on the experimental data, we simulated the
infrared absorption spectrum of the sample, which showed two absorption bands
at 5.85 um and 8.98 um depending on the excitation power
High-yield fabrication of entangled photon emitters for hybrid quantum networking using high-temperature droplet epitaxy
Several semiconductor quantum dot techniques have been investigated for the
generation of entangled photon pairs. Among the other techniques, droplet
epitaxy enables the control of the shape, size, density, and emission
wavelength of the quantum emitters. However, the fraction of the
entanglement-ready quantum dots that can be fabricated with this method is
still limited to around 5%, and matching the energy of the entangled photons to
atomic transitions (a promising route towards quantum networking) remains an
outstanding challenge.
Here, we overcome these obstacles by introducing a modified approach to
droplet epitaxy on a high symmetry (111)A substrate, where the fundamental
crystallization step is performed at a significantly higher temperature as
compared to previous reports. Our method drastically improves the yield of
entanglement-ready photon sources near the emission wavelength of interest,
which can be as high as 95% due to the low values of fine structure splitting
and radiative lifetime, together with the reduced exciton dephasing offered by
the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in
the operating spectral region of Rb-based slow-light media, providing a viable
technology for quantum repeater stations.Comment: 14 pages, 3 figure
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
Hyperfine-interaction limits polarization entanglement of photons from semiconductor quantum dots
Excitons in quantum dots are excellent sources of polarization-entangled
photon pairs, but a quantitative understanding of their interaction with the
nuclear spin bath is still missing. Here we investigate the role of hyperfine
energy shifts using experimentally accessible parameters and derive an upper
limit to the achievable entanglement fidelity. Our results are consistent with
all available literature, indicate that spin-noise is often the dominant
process limiting the entanglement in InGaAs quantum dots, and suggest routes to
alleviate its effect
Experimental Multi-state Quantum Discrimination in the Frequency Domain with Quantum Dot Light
The quest for the realization of effective quantum state discrimination
strategies is of great interest for quantum information technology, as well as
for fundamental studies. Therefore, it is crucial to develop new and more
efficient methods to implement discrimination protocols for quantum states.
Among the others, single photon implementations are more advisable, because of
their inherent security advantage in quantum communication scenarios. In this
work, we present the experimental realization of a protocol employing a
time-multiplexing strategy to optimally discriminate among eight non-orthogonal
states, encoded in the four-dimensional Hilbert space spanning both the
polarization degree of freedom and photon energy. The experiment, built on a
custom-designed bulk optics analyser setup and single photons generated by a
nearly deterministic solid-state source, represents a benchmarking example of
minimum error discrimination with actual quantum states, requiring only linear
optics and two photodetectors to be realized. Our work paves the way for more
complex applications and delivers a novel approach towards high-dimensional
quantum encoding and decoding operations
Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly-Pumped Quantum Dots
Two-photon resonant excitation of the biexciton-exciton cascade in a quantum
dot generates highly polarization-entangled photon pairs in a
near-deterministic way. However, there are still open questions on the ultimate
level of achievable entanglement. Here, we observe the impact of the
laser-induced AC-Stark effect on the spectral emission features and on
entanglement. A shorter emission time, longer laser pulse duration, and higher
pump power all result in lower values of concurrence. Nonetheless, additional
contributions are still required to fully account for the observed below-unity
concurrence.Comment: 7 pages, 3 figure
A multipair-free source of entangled photons in the solid state
Unwanted multiphoton emission commonly reduces the degree of entanglement of
photons generated by non-classical light sources and, in turn, hampers their
exploitation in quantum information science and technology. Quantum emitters
have the potential to overcome this hurdle but, so far, the effect of
multiphoton emission on the quality of entanglement has never been addressed in
detail. Here, we tackle this challenge using photon pairs from a
resonantly-driven quantum dot and comparing quantum state tomography and
second-order coherence measurements as a function of the excitation power. We
observe that the relative (absolute) multiphoton emission probability is as low
as () at the maximum
source brightness, values that lead to a negligible effect on the degree of
entanglement. In stark contrast with probabilistic sources of entangled
photons, we also demonstrate that the multiphoton emission probability and the
degree of entanglement remain practically unchanged against the excitation
power for multiple Rabi cycles, despite we clearly observe oscillations in the
second-order coherence measurements. Our results, explained by a theoretical
model that we develop to estimate the actual multiphoton contribution in the
two-photon density matrix, highlight that quantum dots can be regarded as a
multipair-free source of entangled photons in the solid state