Aging-induced double ferroelectric hysteresis loops in BiFeO₃multiferroic ceramic

Author name used in this publication: Siu Wing Or2007-2008 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

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

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

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

Low-temperature switching fatigue behavior of ferroelectric SrBi₂Ta₂O[sub 9] thin films

Author name used in this publication: Z. G. LiuAuthor name used in this publication: H. L. W. ChanAuthor name used in this publication: C. L. Choy2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Two diterpenes and three diterpene glucosides from Phlogacanthus curviflorus

Two new diterpene lactones, phlogacantholides B (1) and C (2), and three new diterpene lactone glucosides, phlogacanthosides A (3), B (4), and C (5), together with lupeol, beta-sitosterol, betulin, P-daucosterol, (+)syringaresinol, and (+)-syringaresinol-4-O-beta-D-glucopyranoside, were isolated from the roots of Phlogacanthus curviflorus. Their structures were elucidated by chemical and spectroscopic evidence. The structure, including the relative configuration of phlogacantholide B (1), was confirmed by X-ray crystallographic analysis of its diacetate (6)

Intensity modulation as a preemptive measure against blinding of single-photon detectors based on self-differencing cancellation

Quantum key distribution is rising as an important cryptographic primitive for protecting the communication infrastructure in the digital era. However, its implementation security is often weakened by components whose behavior deviates from what is expected. Here we analyze the response of a self-differencing avalanche photodiode, a key enabler for high speed quantum key distribution, to intense light shone from a continuous-wave laser. Under incorrect settings, the cancellation entailed by the self-differencing circuitry can make the detector insensitive to single photons. However, we experimentally demonstrate that even in such cases intensity modulation can be used as an effective measure to restore the detector's expected response to the input light.A.K.-S. gratefully acknowledges financial support from Toshiba Research Europe Ltd. and the Engineering and Physical Sciences Research Council (EPSRC) through an Industrial CASE studentship Grant No. NMZE\187 (RG84410)

Combined acoustic radiation force impulse, aminotransferase to platelet ratio index and Forns index assessment for hepatic fibrosis grading in hepatitis B

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Setting best practice criteria for self-differencing avalanche photodiodes in quantum key distribution

In recent years, the security of avalanche photodiodes as single photon detectors for quantum key distribution has been subjected to much scrutiny. The most prominent example of this surrounds the vulnerability of such devices to blinding under strong illumination. We focus on self-differencing avalanche photodiodes, single photon detectors that have demonstrated count rates exceeding 1 GCounts/s resulting in secure key rates over 1 MBit/s. These detectors use a passive electronic circuit to cancel any periodic signals thereby enhancing detection sensitivity. However this intrinsic feature can be exploited by adversaries to gain control of the devices using illumination of a moderate intensity. Through careful experimental examinations, we define here a set of criteria for these detectors to avoid such attacks.EPSRC Toshiba Research Europe Lt