Thermal blinding of gated detectors in quantum cryptography

Lars Lydersen, Carlos Wiechers, Christoffer Wittmann, Dominique Elser, Johannes Skaar, Vadim Makarov

It has previously been shown that the gated detectors of two commercially available quantum key distribution (QKD) systems are blindable and controllable by an eavesdropper using continuous-wave illumination and short bright trigger pulses, manipulating voltages in the circuit [Nat. Photonics 4, 686 (2010)]. This allows for an attack eavesdropping the full raw and secret key without increasing the quantum bit error rate (QBER). Here we show how thermal effects in detectors under bright illumination can lead to the same outcome. We demonstrate that the detectors in a commercial QKD system Clavis2 can be blinded by heating the avalanche photo diodes (APDs) using bright illumination, so-called thermal blinding. Further, the detectors can be triggered using short bright pulses once they are blind. For systems with pauses between packet transmission such as the plug-and-play systems, thermal inertia enables Eve to apply the bright blinding illumination before eavesdropping, making her more difficult to catch. (C) 2010 Optical Society of America

Comparison of three digital fringe signal processing methods in a ballistic free-fall absolute gravimeter

S. Svitlov, P. Maslyk, Ch Rothleitner, H. Hu, L. J. Wang

This paper reports results of comparison of three digital fringe signal processing methods implemented in the same free-fall absolute gravimeter. A two-sample zero-crossing method, a windowed second-difference method and a method of non-linear least-squares adjustment on the undersampled fringe signal are compared in numerical simulations, hardware tests and actual measurements with the MPG-2 absolute gravimeter, developed at the Max Planck Institute for the Science of Light, Germany. The two-sample zero-crossing method realizes data location schemes that are both equally spaced in distance and equally spaced in time (EST) along the free-fall trajectory. The windowed second-difference method and the method of non-linear least-squares adjustment with complex heterodyne demodulation operate with the EST data. Results of the comparison verify an agreement of the three methods within one part in 10(9) of the measured gravity value, provided a common data location scheme is considered.

Comment on 'Evaluation of the local value of the Earth gravity field in the context of the new definition of the kilogram'

S. M. Svitlov

A recent paper (Baumann et al 2009 Metrologia 46 178-86) presents a method to evaluate the free-fall acceleration at a desired point in space, as required for the watt balance experiment. The claimed uncertainty of their absolute gravity measurements is supported by two bilateral comparisons using two absolute gravimeters of the same type. This comment discusses the case where absolute gravity measurements are traceable to a key comparison reference value. Such an approach produces a more complete uncertainty budget and reduces the risk of the results of different watt balance experiments not being compatible.

Tuning the structural properties of femtosecond-laser-induced nanogratings

Lourdes Patricia R. Ramirez, Matthias Heinrich, Soeren Richter, Felix Dreisow, Robert Keil, Alexander V. Korovin, Ulf Peschel, Stefan Nolte, Andreas Tuennermann

We present the results of our investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters, thereby identifying ways to systematically control the grating properties. Nanogratings, self-organized nanostructures with subwavelength periodicity, are formed in certain parameter ranges during femtosecond-laser processing of transparent materials, resulting in characteristic birefringent modifications. They provide the opportunity for the fabrication of arbitrary three-dimensional birefringent elements with position-dependent retardation. Based on our findings, we were able to fabricate birefringent elements with various precise retardations in otherwise isotropic fused silica.

Hacking commercial quantum cryptography systems by tailored bright illumination

Lars Lydersen, Carlos Wiechers, Christoffer Wittmann, Dominique Elser, Johannes Skaar, Vadim Makarov

The peculiar properties of quantum mechanics allow two remote parties to communicate a private, secret key, which is protected from eavesdropping by the laws of physics(1-4). So-called quantum key distribution (QKD) implementations always rely on detectors to measure the relevant quantum property of single photons(5). Here we demonstrate experimentally that the detectors in two commercially available QKD systems can be fully remote-controlled using specially tailored bright illumination. This makes it possible to tracelessly acquire the full secret key; we propose an eavesdropping apparatus built from off-the-shelf components. The loophole is likely to be present in most QKD systems using avalanche photodiodes to detect single photons. We believe that our findings are crucial for strengthening the security of practical QKD, by identifying and patching technological deficiencies.

Silver Coated Platinum Core-Shell Nanostructures on Etched Si Nanowires: Atomic Layer Deposition (ALD) Processing and Application in SERS

Vladimir A. Sivakov, Katja Hoeflich, Michael Becker, Andreas Berger, Thomas Stelzner, Kai-Erik Elers, Viljami Pore, Mikko Ritala, Silke H. Christiansen

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface-enhanced Raman spectroscopy (SERS)-based sensing. As host material for the plasmonically active nanostructures we use dense single-crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core-shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core-shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning- and transmission electron microscopy. Optimized core-shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.

The quantum vacuum at the foundations of classical electrodynamics

G. Leuchs, A. S. Villar, L. L. Sanchez-Soto

In the classical theory of electromagnetism, the permittivity epsilon (0) and the permeability mu (0) of free space are constants whose magnitudes do not seem to possess any deeper physical meaning. By replacing the free space of classical physics with the quantum notion of the vacuum, we speculate that the values of the aforementioned constants could arise from the polarization and magnetization of virtual pairs in vacuum. A classical dispersion model with parameters determined by quantum and particle physics is employed to estimate their values. We find the correct orders of magnitude. Additionally, our simple assumptions yield an independent estimate for the number of charged elementary particles based on the known values of epsilon (0) and mu (0) and for the volume of a virtual pair. Such an interpretation would provide an intriguing connection between the celebrated theory of classical electromagnetism and the quantum theory in the weak-field limit.

Transverse angular momentum of photons

Andrea Aiello, Christoph Marquardt, Gerd Leuchs

We develop the quantum theory of transverse angular momentum of light beams. The theory applies to paraxial and quasiparaxial photon beams in vacuum and reproduces the known results for classical beams when applied to coherent states of the field. Both the Poynting vector, alias the linear momentum, and the angular-momentum quantum operators of a light beam are calculated including contributions from first-order transverse derivatives. This permits a correct description of the energy flow in the beam and the natural emergence of both the spin and the angular momentum of the photons. We show that for collimated beams of light, orbital angular-momentum operators do not satisfy the standard commutation rules. Finally, we discuss the application of our theory to some concrete cases.

Phase-shifting point-diffraction interferometry with common-path and in-line configuration for microscopy

Peng Gao, Irina Harder, Vanusch Nercissian, Klaus Mantel, Baoli Yao

A new common-path and in-line point-diffraction interferometer for quantitative phase microscopy is proposed. The interferometer is constructed by introducing a grating pair into the point-diffraction interferometer, thus forming a common-path and in-line configuration for object and reference waves. Achromatic phase shifting is implemented by linearly moving one of the two gratings in its grating vector direction. The feasibility of the proposed configuration is demonstrated by theoretical analysis and experiments. (C) 2010 Optical Society of America

Atmospheric channel characteristics for quantum communication with continuous polarization variables

B. Heim, D. Elser, T. Bartley, M. Sabuncu, C. Wittmann, D. Sych, C. Marquardt, G. Leuchs

We investigate the properties of an atmospheric channel for free space quantum communication with continuous polarization variables. In our prepare-and-measure setup, coherent polarization states are transmitted through an atmospheric quantum channel of 100 m length on the flat roof of our institute's building. The signal states are measured by homodyne detection with the help of a local oscillator (LO) which propagates in the same spatial mode as the signal, orthogonally polarized to it. Thus the interference of signal and LO is excellent and atmospheric fluctuations are auto-compensated. The LO also acts as a spatial and spectral filter, which allows for unrestrained daylight operation. Important characteristics for our system are atmospheric channel influences that could cause polarization, intensity and position excess noise. Therefore we study these influences in detail. Our results indicate that the channel is suitable for our quantum communication system in most weather conditions.