491 research outputs found
Adaptive tracking of a time-varying field with a quantum sensor
Sensors based on single spins can enable magnetic field detection with very
high sensitivity and spatial resolution. Previous work has concentrated on
sensing of a constant magnetic field or a periodic signal. Here, we instead
investigate the problem of estimating a field with non-periodic variation
described by a Wiener process. We propose and study, by numerical simulations,
an adaptive tracking protocol based on Bayesian estimation. The tracking
protocol updates the probability distribution for the magnetic field, based on
measurement outcomes, and adapts the choice of sensing time and phase in real
time. By taking the statistical properties of the signal into account, our
protocol strongly reduces the required measurement time. This leads to a
reduction of the error in the estimation of a time-varying signal by up to a
factor 4 compared to protocols that do not take this information into account.Comment: 10 pages, 6 figure
Extending qubit coherence by adaptive quantum environment learning
Decoherence, resulting from unwanted interaction between a qubit and its
environment, poses a serious challenge towards the development of quantum
technologies. Recently, researchers have started analysing how real-time
Hamiltonian learning approaches, based on estimating the qubit state faster
than the environmental fluctuations, can be used to counteract decoherence. In
this work, we investigate how the back-action of the quantum measurements used
in the learning process can be harnessed to extend qubit coherence. We propose
an adaptive protocol that, by learning the qubit environment, narrows down the
distribution of possible environment states. While the outcomes of quantum
measurements are random, we show that real-time adaptation of measurement
settings (based on previous outcomes) allows a deterministic decrease of the
width of the bath distribution, and hence an increase of the qubit coherence.
We numerically simulate the performance of the protocol for the electronic spin
of a nitrogen-vacancy centre in diamond subject to a dilute bath of C
nuclear spin, finding a considerable improvement over the performance of
non-adaptive strategies
Aberration cancellation in quantum interferometry
We report the first experimental demonstration of even-order aberration
cancellation in quantum interferometry. The effect is a spatial counterpart of
the spectral group velocity dispersion cancellation, which is associated with
spectral entanglement. It is manifested in temporal interferometry by virtue of
the multi-parameter spatial-spectral entanglement. Spatially-entangled photons,
generated by spontaneous parametric down conversion, were subjected to spatial
aberrations introduced by a deformable mirror that modulates the wavefront. We
show that only odd-order spatial aberrations affect the quality of quantum
interference
Optical modes in oxide-apertured micropillar cavities
We present a detailed experimental characterization of the spectral and
spatial structure of the confined optical modes for oxide-apertured micropillar
cavities, showing good-quality Hermite-Gaussian profiles, easily mode-matched
to external fields. We further derive a relation between the frequency
splitting of the transverse modes and the expected Purcell factor. Finally, we
describe a technique to retrieve the profile of the confining refractive index
distribution from the spatial profiles of the modes.Comment: 4 pages, 3 figure
Independent electrical tuning of separated quantum dots in coupled photonic crystal cavities
Systems of photonic crystal cavities coupled to quantum dots are a promising
architecture for quantum networking and quantum simulators. The ability to
independently tune the frequencies of laterally separated quantum dots is a
crucial component of such a scheme. Here, we demonstrate independent tuning of
laterally separated quantum dots in photonic crystal cavities coupled by
in-plane waveguides by implanting lines of protons which serve to electrically
isolate different sections of a diode structure.Comment: 3 pages, 3 figure
Strain-tuning of quantum dot optical transitions via laser-induced surface defects
We discuss the fine-tuning of the optical properties of self-assembled
quantum dots by the strain perturbation introduced by laser-induced surface
defects. We show experimentally that the quantum dot transition red-shifts,
independently of the actual position of the defect, and that such frequency
shift is about a factor five larger than the corresponding shift of a
micropillar cavity mode resonance. We present a simple model that accounts for
these experimental findings.Comment: 9 pages, 6 figures. To appear in Phys. Rev.
Tuning micropillar cavity birefringence by laser induced surface defects
We demonstrate a technique to tune the optical properties of micropillar
cavities by creating small defects on the sample surface near the cavity region
with an intense focused laser beam. Such defects modify strain in the
structure, changing the birefringence in a controllable way. We apply the
technique to make the fundamental cavity mode polarization-degenerate and to
fine tune the overall mode frequencies, as needed for applications in quantum
information science.Comment: RevTex, 7 pages, 4 figures (accepted for publication in Applied
Physics Letters
CNOT and Bell-state analysis in the weak-coupling cavity QED regime
We propose an interface between the spin of a photon and the spin of an
electron confined in a quantum dot embedded in a microcavity operating in the
weak coupling regime. This interface, based on spin selective photon reflection
from the cavity, can be used to construct a CNOT gate, a multi-photon entangler
and a photonic Bell-state analyzer. Finally, we analyze experimental
feasibility, concluding that the schemes can be implemented with current
technology.Comment: 4 pages, 2 figure
Atomically-thin quantum dots integrated with lithium niobate photonic chips
The electro-optic, acousto-optic and nonlinear properties of lithium niobate
make it a highly versatile material platform for integrated quantum photonic
circuits. A prerequisite for quantum technology applications is the ability to
efficiently integrate single photon sources, and to guide the generated photons
through ad-hoc circuits. Here we report the integration of quantum dots in
monolayer WSe2 into a Ti in-diffused lithium niobate directional coupler. We
investigate the coupling of individual quantum dots to the waveguide mode,
their spatial overlap, and the overall efficiency of the hybrid-integrated
photonic circuit
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