1,373 research outputs found
Evolution of locally excited avalanches in semiconductors
We show that semiconductor avalanche photodiodes can exhibit diminutive
amplification noise during the early evolution of avalanches. The noise is so
low that the number of locally excited charges that seed each avalanche can be
resolved. These findings constitute an important first step towards realization
of a solid-state noiseless amplifier. Moreover, we believe that the
experimental setup used, \textit{i.e.}, time-resolving locally excited
avalanches, will become a useful tool for optimizing the number resolution
Efficient photon number detection with silicon avalanche photodiodes
We demonstrate an efficient photon number detector for visible wavelengths
using a silicon avalanche photodiode. Under subnanosecond gating, the device is
able to resolve up to four photons in an incident optical pulse. The detection
efficiency at 600 nm is measured to be 73.8%, corresponding to an avalanche
probability of 91.1% of the absorbed photons, with a dark count probability
below 1.1x10^{-6} per gate. With this performance and operation close to room
temperature, fast-gated silicon avalanche photodiodes are ideal for optical
quantum information processing that requires single-shot photon number
detection
Continuous operation of high bit rate quantum key distribution
We demonstrate a quantum key distribution with a secure bit rate exceeding 1
Mbit/s over 50 km fiber averaged over a continuous 36-hours period. Continuous
operation of high bit rates is achieved using feedback systems to control path
length difference and polarization in the interferometer and the timing of the
detection windows. High bit rates and continuous operation allows finite key
size effects to be strongly reduced, achieving a key extraction efficiency of
96% compared to keys of infinite lengths.Comment: four pages, four figure
Probing higher order correlations of the photon field with photon number resolving avalanche photodiodes
We demonstrate the use of two high speed avalanche photodiodes in exploring
higher order photon correlations. By employing the photon number resolving
capability of the photodiodes the response to higher order photon coincidences
can be measured. As an example we show experimentally the sensitivity to higher
order correlations for three types of photon sources with distinct photon
statistics. This higher order correlation technique could be used as a low cost
and compact tool for quantifying the degree of correlation of photon sources
employed in quantum information science
Controlling the polarisation correlation of photon pairs from a charge-tuneable quantum dot
Correlation between the rectilinear polarisations of the photons emitted from
the biexciton decay in a single quantum dot is investigated in a device which
allows the charge-state of the dot to be controlled. Optimising emission from
the neutral exciton states maximises the operating efficiency of the biexciton
decay. This is important for single dot applications such as a triggered source
of entangled photons. As the bias on the device is reduced correlation between
the two photons is found to fall dramatically as emission from the negatively
charged exciton becomes significant. Lifetime measurements demonstrate that
electronic spin-scattering is the likely cause.Comment: 3 figure
Inversion of exciton level splitting in quantum dots
The demonstration of degeneracy of exciton spin states is an important step toward the production of entangled photon pairs from the biexciton cascade. We measure the fine structure of exciton and biexciton states for a large number of single InAs quantum dots in a GaAs matrix; the energetic splitting of the horizontally and vertically polarized components of the exciton doublet is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Thermal annealing is shown to reduce the exciton confinement, thereby increasing the number of dots with splitting close to zero
Multi-dimensional photonic states from a quantum dot
Quantum states superposed across multiple particles or degrees of freedom offer an advantage in the development of quantum technologies. Creating these states deterministically and with high efficiency is an ongoing challenge. A promising approach is the repeated excitation of multi-level quantum emitters, which have been shown to naturally generate light with quantum statistics. Here we describe how to create one class of higher dimensional quantum state, a so called W-state, which is superposed across multiple time bins. We do this by repeated Raman scattering of photons from a charged quantum dot in a pillar microcavity. We show this method can be scaled to larger dimensions with no reduction in coherence or single-photon character. We explain how to extend this work to enable the deterministic creation of arbitrary time-bin encoded qudits
Ramsey interference in a multilevel quantum system
We report Ramsey interference in the excitonic population of a negatively charged quantum dot measured in resonant fluorescence. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing the laser detuning to reveal two-, three-, and four-level system behavior. We show that under finite field the phase-sensitive control of two optical pulses from a single laser can be used to prepare both population and spin states simultaneously. We also demonstrate the coherent optical manipulation of a trapped spin in a quantum dot in a Faraday geometry magnetic field
An avalanche-photodiode-based photon-number-resolving detector
Avalanche photodiodes are widely used as practical detectors of single
photons.1 Although conventional devices respond to one or more photons, they
cannot resolve the number in the incident pulse or short time interval.
However, such photon number resolving detectors are urgently needed for
applications in quantum computing,2-4 communications5 and interferometry,6 as
well as for extending the applicability of quantum detection generally. Here we
show that, contrary to current belief,3,4 avalanche photodiodes are capable of
detecting photon number, using a technique to measure very weak avalanches at
the early stage of their development. Under such conditions the output signal
from the avalanche photodiode is proportional to the number of photons in the
incident pulse. As a compact, mass-manufactured device, operating without
cryogens and at telecom wavelengths, it offers a practical solution for photon
number detection.Comment: 12 pages, 4 figure
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Radiative recombination mechanisms in polar and non-polar InGaN/GaN quantum well LED structures
We study the photoluminescence internal quantum efficiency (IQE) and recombination dynamics in a pair of polar and non-polar InGaN/GaN quantum well (QW) light-emitting diode (LED) structures as a function of excess carrier density and temperature. In the polar LED at 293 K, the variation of radiative and non-radiative lifetimes is well described by a modified ABC type model which accounts for the background carrier concentration in the QWs due to unintentional doping. As the temperature is reduced, the sensitivity of the radiative lifetime to excess carrier density becomes progressively weaker. We attribute this behaviour to the reduced mobility of the localised electrons and holes at low temperatures, resulting in a more monomolecular like radiative process. Thus we propose that in polar QWs, the degree of carrier localisation determines the sensitivity of the radiative lifetime to the excess carrier density. In the non-polar LED, the radiative lifetime is independent of excitation density at room temperature, consistent with a wholly excitonic recombination mechanism. These findings have significance for the interpretation of LED efficiency data within the context of the ABC recombination model
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