5,134 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
A New Parameter In Accretion Disk Model
Taking optically thin accretion flows as an example, we investigate the
dynamics and the emergent spectra of accretion flows with different outer
boundary conditions (OBCs) and find that OBC plays an important role in
accretion disk model. This is because the accretion equations describing the
behavior of accretion flows are a set of {\em differential} equations,
therefore, accretion is intrinsically an initial-value problem. We argue that
optically thick accretion flow should also show OBC-dependent behavior. The
result means that we should seriously consider the initial physical state of
the accretion flow such as its angular momentum and its temperature. An
application example to Sgr A is presented.Comment: 6 pages, 4 figures, to appear in the Proceeding of "Pacific Rim
Conference on Stellar Astrophysics", Aug. 1999, HongKong, Chin
Directly phase-modulated light source
The art of imparting information onto a light wave by optical signal modulation is fundamental to all forms of optical communication. Among many schemes, direct modulation of laser diodes stands out as a simple, robust, and cost-effective method. However, the simultaneous changes in intensity, frequency, and phase have prevented its application in the field of secure quantum communication. Here, we propose and experimentally demonstrate a directly phase-modulated light source which overcomes the main disadvantages associated with direct modulation and is suitable for diverse applications such as coherent communications and quantum cryptography. The source separates the tasks of phase preparation and pulse generation between a pair of semiconductor lasers leading to very pure phase states. Moreover, the cavity-enhanced electro-optic effect enables the first example of subvolt half-wave phase modulation at high signal rates. The source is compact, stable, and versatile, and we show its potential to become the standard transmitter for future quantum communication networks based on attenuated laser pulses
Manipulating photon coherence to enhance the security of distributed phase reference quantum key distribution
Distributed-phase-reference (DPR) systems were introduced as a method of decreasing the complexity of quantum key distribution systems for practical use. However, their information-theoretic security has only been proven when the added requirement of block-wise phase randomisation is met. Realisation of this with a conventional approach would result in a cumbersome transmitter, removing any practical advantage held by DPR systems. Here we solve this problem using a light source that allows the coherence between pulses to be controlled on a pulse-by-pulse basis without the need for additional bulky components. The system is modulator-free, does not require a complex receiver, and features an excellent stability without an active stabilisation mechanism. We achieve megabit per second key rates that are almost three times higher than those obtained with the standard Bennet-Brassard 1984 (BB84) protocol
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Gigahertz-gated InGaAs/InP single-photon detector with detection efficiency exceeding 55% at 1550 nm
We report on a gated single-photon detector based on InGaAs/InP avalanche
photodiodes (APDs) with a single-photon detection efficiency exceeding 55% at
1550 nm. Our detector is gated at 1 GHz and employs the self-differencing
technique for gate transient suppression. It can operate nearly dead time free,
except for the one clock cycle dead time intrinsic to self-differencing, and we
demonstrate a count rate of 500 Mcps. We present a careful analysis of the
optimal driving conditions of the APD measured with a dead time free detector
characterization setup. It is found that a shortened gate width of 360 ps
together with an increased driving signal amplitude and operation at higher
temperatures leads to improved performance of the detector. We achieve an
afterpulse probability of 7% at 50% detection efficiency with dead time free
measurement and a record efficiency for InGaAs/InP APDs of 55% at an afterpulse
probability of only 10.2% with a moderate dead time of 10 ns.L. C. Comandar acknowledges personal support via the EPSRC funded CDT in Photonics System Development.This is the author accepted manuscript. The final version is available via AIP at http://scitation.aip.org/content/aip/journal/jap/117/8/10.1063/1.4913527
Best-Practice Criteria for Practical Security of Self-Differencing Avalanche Photodiode Detectors in Quantum Key Distribution
Fast gated avalanche photodiodes (APDs) are the most commonly used single
photon detectors for high bit rate quantum key distribution (QKD). Their
robustness against external attacks is crucial to the overall security of a QKD
system or even an entire QKD network. Here, we investigate the behavior of a
gigahertz-gated, self-differencing InGaAs APD under strong illumination, a
tactic Eve often uses to bring detectors under her control. Our experiment and
modelling reveal that the negative feedback by the photocurrent safeguards the
detector from being blinded through reducing its avalanche probability and/or
strengthening the capacitive response. Based on this finding, we propose a set
of best-practice criteria for designing and operating fast-gated APD detectors
to ensure their practical security in QKD
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