Instability of frozen-in states in synchronous Hebbian neural networks

The full dynamics of a synchronous recurrent neural network model with Ising binary units and a Hebbian learning rule with a finite self-interaction is studied in order to determine the stability to synaptic and stochastic noise of frozen-in states that appear in the absence of both kinds of noise. Both, the numerical simulation procedure of Eissfeller and Opper and a new alternative procedure that allows to follow the dynamics over larger time scales have been used in this work. It is shown that synaptic noise destabilizes the frozen-in states and yields either retrieval or paramagnetic states for not too large stochastic noise. The indications are that the same results may follow in the absence of synaptic noise, for low stochastic noise.Comment: 14 pages and 4 figures; accepted for publication in J. Phys. A: Math. Ge

Fundamentally secure data with the help of quantum key distribution on CubeSats

With the uprise of worldwide satellite communication networks, data security is a critical issue. This issue is being addressed in the QUBE project, which proposes a CubeSat for quantum cryptography experiments. The satellite and its subsystems are currently being developed and will be used for the downlink of individual photons, or strongly attenuated light pulses, containing encoded quantum information, which can then be employed for the exchange of encryption keys. The launch of the 3U Nanosatellite is planned for early 2020. It will be built using the UNISEC-Europe standard, which has demonstrated to be able to provide a robust structure for increased reliability in CubeSat missions. In addition to state-of-the-art reaction wheels for precision pointing, the satellite will be bringing the OSIRIS optical downlink system from DLR as well as two dedicated payloads for testing components required for quantum key distribution. A sequence of numbers will be created by a miniaturized quantum random number generator (QRNG), which will be used to set the quantum states of the light pulses. These pulses will then be downlinked to the optical ground station (OGS) at DLR in Oberpfaffenhofen, Germany. The ground station is also equipped with the corresponding components for receiving individual quantum states. In addition, the random numbers will be made available via an RF downlink. The photon states received by the optical ground station will then be compared to the previously generated numbers. Due to the underlying quantum mechanics, any attempt of reading the quantum states will alter them, which makes interceptions easily detectable. These quantum key distribution experiments will evaluate whether secure communication links are possible even on a CubeSat scale. A major challenge for building the proposed CubeSat is the attitude determination and control system that will provide precise pointing. This work will outline detailed mission requirements as well as the chosen subsystems for tackling these challenges in order to achieve a successful mission and prepare for future data security

Gram-Scale Synthesis of the (-)-Sparteine Surrogate and (-)-Sparteine

An 8-step, gram-scale synthesis of the (-)-sparteine surrogate (22% yield, with just 3 chromatographic purifications) and a 10-step, gram-scale synthesis of (-)-sparteine (31% yield) are reported. Both syntheses proceed with complete diastereocontrol and allow access to either antipode. Since the syntheses do not rely on natural product extraction, our work addresses long-term supply issues relating to these widely used chiral ligands

QUBE-II - Quantum Key Distribution with a CubeSat

The digitization of our everyday lives is omnipresent. Secure data transmission is therefore of enormous importance in almost all areas of our society. The cryptographic processes for encrypting transmitted messages used today are based on algorithms relying on the limited computing power of today's computers. However, data intercepted today can be stored, decrypted and altered in the future with more powerful or even quantum computers, which are currently under development. However, the use of quantum states as carriers of information makes physically secure communication possible. The laws of quantum mechanics guarantee that data cannot be intercepted or stored unnoticedly. The security against eavesdropping is based on fundamental laws of nature and therefore cannot be overcome even by future technologies. One approach for the global distribution of quantum keys is communication via satellite. This enables a greater range than fiber-optic links, which are currently limited to a few 100 kilometers due to losses along the line. The exchange of secret keys between several ground stations via satellite thus enables global, secure communication. The QUBE-II group is working on the development of a novel miniature satellite capable of complete quantum key exchange. The platform for this is formed by low-cost miniature satellites, so-called CubeSats. New technologies for generating quantum keys on the CubeSat platform in combination with powerful optical communication systems will enable a fully functional system in a 3x2 cube form factor. Building on the predecessor project QUBE, miniaturized quantum components are being developed that can withstand the extreme temperature and radiation loads in space. These will then be integrated into a miniature satellite weighing about 10 kilograms. The research team will use the Optical Ground Station Oberpfaffenhofen near Munich as receiver station and upgrade it accordingly for reception of QUBE quantum states and implementation of the needed classical free-space optical communication links. This paper provides insight into the structure of the QUBE-II project and lays out the challenges of a successful key exchange between CubeSat and ground station. Thereby, especially the future improvements and innovations compared to the predecessor project QUBE will be discussed