Impact of turbulence in long range quantum and classical communications

The study of the free-space distribution of quantum correlations is necessary for any future application of quantum as classical communication aiming to connect two remote locations. Here we study the propagation of a coherent laser beam over 143 Km (between Tenerife and La Palma Islands of the Canary archipelagos). By attenuating the beam we also studied the propagation at the single photon level. We investigated the statistic of arrival of the incoming photons and the scintillation of the beam. From the analysis of the data, we propose the exploitation of turbulence to improve the SNR of the signal.Comment: 5 Pages, 5 figures, 1 Table, revtex

Recursive Imaging with Multiply-Scattered Waves Using Partial Image Regularization: A North Sea Case Study

As more resources are directed toward reverse-time migration an accurate velocity model, including strong reflectors, is necessary to form a clear image of the subsurface. This is of particular importance in the vicinity of salt, where singly-scattered waves are often not ideal for imaging the salt flanks. This has led to interest in processing doubly-scattered waves (also called duplex or prismatic waves) for imaging salt flanks and thus improving the location of salt boundaries in a velocity model. We present a case study in which we use doubly-scattered waves in a two-pass one-way method to image salt flanks in a North Sea data set. By working in the one-way framework we are able to separately construct images with singly, doubly, and triply scattered waves. We illustrate a multi-step imaging process that includes multiply-scattered waves by using an imaged reflector to fix one (or more) of the scattering points, allowing for multiply-scattered energy from several reflectors, potentially with poor continuity, to be included without picking each reflector individually. With this method we are able to image the flank of a North Sea salt body.Norwegian State Oil CompanyNorwegian Research CouncilGeo-Mathematical Imaging GroupTOTAL (Firm)Massachusetts Institute of Technology. Earth Resources Laborator

Seismic Imaging and Illumination with Internal Multiples

If singly scattered seismic waves illuminate the entirety of a subsurface structure of interest, standard methods can be applied to image it. It is generally the case, however, that with a combination of restricted acquisition geometry and imperfect velocity models, it is not possible to illuminate all structures with only singly scattered waves. We present an approach to use multiply scattered waves to illuminate structures not sensed by singly scattered waves. It can be viewed as a refinement of past work in which a method to predict artifacts in imaging with multiply scattered waves was developed. We propose an algorithm and carry out numerical experiments, representative of imaging of the bottom and flanks of salt, demonstrating the effectiveness of our approach.Dutch National Science Foundation (grant number NWO:VIVI865.03.007)StatoilHydroNorwegian Research Council (ROSE project)Geo-Mathematical Imaging Grou

Full-field implementation of a perfect eavesdropper on a quantum cryptography system

Quantum key distribution (QKD) allows two remote parties to grow a shared secret key. Its security is founded on the principles of quantum mechanics, but in reality it significantly relies on the physical implementation. Technological imperfections of QKD systems have been previously explored, but no attack on an established QKD connection has been realized so far. Here we show the first full-field implementation of a complete attack on a running QKD connection. An installed eavesdropper obtains the entire 'secret' key, while none of the parameters monitored by the legitimate parties indicate a security breach. This confirms that non-idealities in physical implementations of QKD can be fully practically exploitable, and must be given increased scrutiny if quantum cryptography is to become highly secure.Comment: Revised after editorial and peer-review feedback. This version is published in Nat. Commun. 8 pages, 6 figures, 1 tabl

Attacks on quantum key distribution protocols that employ non-ITS authentication

We demonstrate how adversaries with unbounded computing resources can break Quantum Key Distribution (QKD) protocols which employ a particular message authentication code suggested previously. This authentication code, featuring low key consumption, is not Information-Theoretically Secure (ITS) since for each message the eavesdropper has intercepted she is able to send a different message from a set of messages that she can calculate by finding collisions of a cryptographic hash function. However, when this authentication code was introduced it was shown to prevent straightforward Man-In-The-Middle (MITM) attacks against QKD protocols. In this paper, we prove that the set of messages that collide with any given message under this authentication code contains with high probability a message that has small Hamming distance to any other given message. Based on this fact we present extended MITM attacks against different versions of BB84 QKD protocols using the addressed authentication code; for three protocols we describe every single action taken by the adversary. For all protocols the adversary can obtain complete knowledge of the key, and for most protocols her success probability in doing so approaches unity. Since the attacks work against all authentication methods which allow to calculate colliding messages, the underlying building blocks of the presented attacks expose the potential pitfalls arising as a consequence of non-ITS authentication in QKD-postprocessing. We propose countermeasures, increasing the eavesdroppers demand for computational power, and also prove necessary and sufficient conditions for upgrading the discussed authentication code to the ITS level.Comment: 34 page

Controlling passively-quenched single photon detectors by bright light

Single photon detectors based on passively-quenched avalanche photodiodes can be temporarily blinded by relatively bright light, of intensity less than a nanowatt. I describe a bright-light regime suitable for attacking a quantum key distribution system containing such detectors. In this regime, all single photon detectors in the receiver Bob are uniformly blinded by continuous illumination coming from the eavesdropper Eve. When Eve needs a certain detector in Bob to produce a click, she modifies polarization (or other parameter used to encode quantum states) of the light she sends to Bob such that the target detector stops receiving light while the other detector(s) continue to be illuminated. The target detector regains single photon sensitivity and, when Eve modifies the polarization again, produces a single click. Thus, Eve has full control of Bob and can do a successful intercept-resend attack. To check the feasibility of the attack, 3 different models of passively-quenched detectors have been tested. In the experiment, I have simulated the intensity diagrams the detectors would receive in a real quantum key distribution system under attack. Control parameters and side effects are considered. It appears that the attack could be practically possible.Comment: Experimental results from a third detector model added. Minor corrections and edits made. 11 pages, 10 figure