18 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
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
Resolving the temporal evolution of line broadening in single quantum emitters
Funding: H2020 European Research Council (679183); Austrian Science Fund (P29603); Seventh Framework Programme (601126); Central European Institute of Technology (7AMB17AT044); Horizon 2020 Framework Programme (731473); European Metrology Programme for Innovation and Research (17FUN06); Bundesministerium fĂŒr Wissenschaft, Forschung und Wirtschaft (CZ 07 / 2017); Ministerstvo Ć kolstvĂ, MlĂĄdeĆŸe a TelovĂœchovy; QuantERA (Hyper-U-P-S); QuantERA (CUSPIDOR); Linz Institute of Technology (LIT); LIT Secure and Correct Systems Lab; Bayerisches Staatsministerium fĂŒr Bildung und Kultus, Wissenschaft und Kunst; Deutsche Forschungsgemeinschaft (SCHN1376 5.1).Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in a line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening in two prominent systems: GaAs and In(Ga)As quantum dots. Differently from previous experiments, the emitters are driven with short laser pulses as required for the generation of high-purity single photons, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as ⌠2”eV. We find pronounced differences in the temporal evolution of different optical transition lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing.Publisher PDFPeer reviewe
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
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
Post-fabrication tuning of circular Bragg resonators for enhanced emitter-cavity coupling
Solid-state quantum emitters embedded in circular Bragg resonators are
attractive due to their ability to emit quantum states of light with high
brightness and low multi-photon probability. As for any emitter-microcavity
system, fabrication imperfections limit the spatial and spectral overlap of the
emitter with the cavity mode, thus limiting their coupling strength. Here, we
show that an initial spectral mismatch can be corrected after device
fabrication by repeated wet chemical etching steps. We demonstrate ~16 nm
wavelength tuning for optical modes in AlGaAs resonators on oxide, leading to a
4-fold Purcell enhancement of the emission of single embedded GaAs quantum
dots. Numerical calculations reproduce the observations and suggest that the
achievable performance of the resonator is only marginally affected in the
explored tuning range. We expect the method to be applicable also to circular
Bragg resonators based on other material platforms, thus increasing the device
yield of cavity-enhanced solid-state quantum emitters
A source of entangled photons based on a cavity-enhanced and strain-tuned GaAs quantum dot
A quantum-light source that delivers photons with a high brightness and a
high degree of entanglement is fundamental for the development of efficient
entanglement-based quantum-key distribution systems. Among all possible
candidates, epitaxial quantum dots are currently emerging as one of the
brightest sources of highly entangled photons. However, the optimization of
both brightness and entanglement currently requires different technologies that
are difficult to combine in a scalable manner. In this work, we overcome this
challenge by developing a novel device consisting of a quantum dot embedded in
a circular Bragg resonator, in turn, integrated onto a micromachined
piezoelectric actuator. The resonator engineers the light-matter interaction to
empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator
manipulates strain fields that tune the quantum dot for the generation of
entangled photons with fidelities up to 0.96(1). This hybrid technology has the
potential to overcome the limitations of the key rates that plague current
approaches to entanglement-based quantum key distribution and
entanglement-based quantum networks. Introductio