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Quantum key distribution (QKD) promises information theoretic secure key as long as the device performs as assumed in the theoretical model. One of the assumptions is an absence of information leakage about individual photon detection outcomes of the receiver unit. Here we investigate the information leakage from a QKD receiver due to photon emission caused by detection events in single-photon detectors (backflash). We test commercial silicon avalanche photodiodes and a photomultiplier tube, and find that the former emit backflashes. We study the spectral, timing and polarization characteristics of these backflash photons. We experimentally demonstrate on a free-space QKD receiver that an eavesdropper can distinguish which detector has clicked inside it, and thus acquire secret information. A set of countermeasures both in theory and on the physical devices are discussed., Comment: 9 pages, 7 figures

A new implementation of a Mach-Zehnder interferometer is presented. Aimed at facilitating coherent optical wavelength conversion, the interferometer utilizes a novel double displacement technique that eliminates dispersion induced phase discrepancies between its input and output arms. To demonstrate the design, the interferometer was incorporated into a source of polarization entangled photon pairs. The source produced on average 2-3 million photon pairs per second per mW of pump power, the pairs emitted being maximally entangled in the polarization degree of freedom with a fidelity of ≥98%_. The new interferometer implementation is simple and robust and promises to become a design benchmark for polarization entangled photon sources.

Creating miniature chip scale implementations of optical quantum information protocols is a dream for many in the quantum optics community. This is largely because of the promise of stability and scalability. Here we present a monolithically integratable chip architecture upon which is built a photonic device primitive called a Bragg reflection waveguide (BRW). Implemented in gallium arsenide, we show that, via the process of spontaneous parametric down conversion, the BRW is capable of directly producing polarization entangled photons without additional path difference compensation, spectral filtering or post-selection. After splitting the twin-photons immediately after they emerge from the chip, we perform a variety of correlation tests on the photon pairs and show non-classical behaviour in their polarization. Combined with the BRW's versatile architecture our results signify the BRW design as a serious contender on which to build large scale implementations of optical quantum processing devices.