14 research outputs found
Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots
Semiconductor quantum dots have emerged as an especially promising platform
for the generation of polarization-entangled photon pairs. However, it was
demonstrated recently that the two-photon excitation scheme employed in
state-of-the-art experiments limits the achievable degree of entanglement by
introducing which-path information. In this work, the combined impact of
two-photon excitation and longitudinal acoustic phonons on photon pairs emitted
by strongly-confining quantum dots is investigated. It is found that phonons
further reduce the achievable degree of entanglement even in the limit of
vanishing temperature due to phonon-induced pure dephasing and phonon-assisted
one-photon processes, which increase the reexcitation probability. In addition,
the degree of entanglement, as measured by the concurrence, decreases with
rising temperature and/or pulse duration, even if the excitonic fine-structure
splitting is absent and when higher electronic states are out of reach.
Furthermore, in the case of finite fine-structure splittings, phonons enlarge
the discrepancy in concurrence for different laser polarizations.Comment: 10 pages, 3 figure
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
Temperature-independent almost perfect photon entanglement from quantum dots via the SUPER scheme
Entangled photon pairs are essential for quantum communication technology. They can be generated on-demand by semiconductor quantum dots, but several mechanisms are known to reduce the degree of entanglement. While some obstacles like the finite fine-structure splitting of the exciton states can currently be overcome, the excitation scheme itself can impair the entanglement fidelity. Here, we demonstrate that the swing-up of quantum emitter population (SUPER) scheme, using two red-detuned laser pulses applied to a quantum dot in a cavity, yields almost perfectly entangled photons. The entanglement remains robust against phonon influences even at elevated temperatures, due to decoupling of the excitation and emission process. With this achievement, quantum dots are ready to be used as entangled photon pair sources in applications requiring high degrees of entanglement up to temperatures of approximately 80 K
Temperature-independent almost perfect photon entanglement from quantum dots via the SUPER scheme
Entangled photon pairs are essential for quantum communication technology. They can be generated on-demand by semiconductor quantum dots, but several mechanisms are known to reduce the degree of entanglement. While some obstacles like the finite fine-structure splitting of the exciton states can currently be overcome, the excitation scheme itself can impair the entanglement fidelity. Here, we demonstrate that the swing-up of quantum emitter population (SUPER) scheme, using two red-detuned laser pulses applied to a quantum dot in a cavity, yields almost perfectly entangled photons. The entanglement remains robust against phonon influences even at elevated temperatures, due to decoupling of the excitation and emission process. With this achievement, quantum dots are ready to be used as entangled photon pair sources in applications requiring high degrees of entanglement up to temperatures of approximately 80 K
Different Types of Photon Entanglement from a Constantly Driven Quantum Emitter Inside a Cavity
Bell states are the most prominent maximally entangled photon states. In a
typical four-level emitter, like a semiconductor quantum dot, the photon states
exhibit only one type of Bell state entanglement. By adding an external driving
to the emitter system, also other types of Bell state entanglement are
reachable without changing the polarization basis. In this paper, we show under
which conditions the different types of entanglement occur and give analytical
equations to explain these findings. We further identify special points, where
the concurrence, being a measure for the degree of entanglement, drops to zero,
while the coherences between the two-photon states stay strong. Results of this
work pave the way to achieve a controlled manipulation of the entanglement type
in practical devices.Comment: 14 pages, 7 figure
Swing-Up of Quantum Emitter Population Using Detuned Pulses
The controlled preparation of the excited state in a quantum emitter is a
prerequisite for its usage as single-photon sources - a key building block for
quantum technologies. In this paper we propose a coherent excitation scheme
using off-resonant pulses. In the usual Rabi scheme, these pulses would not
lead to a significant occupation. This is overcome by using a frequency
modulated pulse to swing up the excited state population. The same effect can
be obtained using two pulses with different strong detunings of the same sign.
We theoretically analyze the applicability of the scheme to a semiconductor
quantum dot. In this case the excitation is several meV below the band gap,
i.e., far away from the detection frequency allowing for easy spectral
filtering, and does not rely on any auxiliary particles such as phonons. Our
scheme has the potential to lead to the generation of close-to-ideal photons.Comment: 10 pages, 7 figure