Yaara Aviad

Synchronization of random bit generators based on coupled chaotic lasers and application to cryptography.

Random bit generators (RBGs) constitute an important tool in cryptography, stochastic simulations and secure communications. The later in particular has some difficult requirements: high generation rate of unpredictable bit strings and secure key-exchange protocols over public channels. Deterministic algorithms generate pseudo-random number sequences at high rates, however, their unpredictability is limited by the very nature of their deterministic origin. Recently, physical RBGs based on chaotic semiconductor lasers were shown to exceed Gbit/s rates. Whether secure synchronization of two high rate physical RBGs is possible remains an open question. Here we propose a method, whereby two fast RBGs based on mutually coupled chaotic lasers, are synchronized. Using information theoretic analysis we demonstrate security against a powerful computational eavesdropper, capable of noiseless amplification, where all parameters are publicly known. The method is also extended to secure synchronization of a small network of three RBGs.

Synchronization in small networks of time-delay coupled chaotic diode lasers

Topologies of two, three and four time-delay-coupled chaotic semiconductor lasers are experimentally and theoretically found to show new types of synchronization. Generalized zero-lag synchronization is observed for two lasers separated by long distances even when their self-feedback delays are not equal. Generalized sub-lattice synchronization is observed for quadrilateral geometries while the equilateral triangle is zero-lag synchronized. Generalized zero-lag synchronization, without the limitation of precisely matched delays, opens possibilities for advanced multi-user communication protocols.

Spiking optical patterns and synchronization.

We analyze the time resolved spike statistics of a solitary and two mutually interacting chaotic semiconductor lasers whose chaos is characterized by apparently random, short intensity spikes. Repulsion between two successive spikes is observed, resulting in a refractory period, which is largest at laser threshold. For time intervals between spikes greater than the refractory period, the distribution of the intervals follows a Poisson distribution. The spiking pattern is highly periodic over time windows corresponding to the optical length of the external cavity, with a slow change of the spiking pattern as time increases. When zero-lag synchronization between two lasers is established, the statistics of the nearly perfectly matched spikes are not altered. The similarity of these features to those found in complex interacting neural networks, suggests the use of laser systems as simpler physical models for neural networks.

Towards the generation of random bits at terahertz rates based on a chaotic semiconductor laser

Random bit generators (RBGs) are important in many aspects of statistical physics and crucial in Monte-Carlo simulations, stochastic modeling and quantum cryptography. The quality of a RBG is measured by the unpredictability of the bit string it produces and the speed at which the truly random bits can be generated. Deterministic algorithms generate pseudo-random numbers at high data rates as they are only limited by electronic hardware speed, but their unpredictability is limited by the very nature of their deterministic origin. It is widely accepted that the core of any true RBG must be an intrinsically non-deterministic physical process, e.g. measuring thermal noise from a resistor. Owing to low signal levels, such systems are highly susceptible to bias, introduced by amplification, and to small nonrandom external perturbations resulting in a limited generation rate, typically less than 100M bit/s. We present a physical random bit generator, based on a chaotic semiconductor laser, having delayed optical feedback, which operates reliably at rates up to 300Gbit/s. The method uses a high derivative of the digitized chaotic laser intensity and generates the random sequence by retaining a number of the least significant bits of the high derivative value. The method is insensitive to laser operational parameters and eliminates the necessity for all external constraints such as incommensurate sampling rates and laser external cavity round trip time. The randomness of long bit strings is verified by standard statistical tests.

Chaos synchronization in networks of semiconductor superlattices

Chaos synchronization has been demonstrated as a useful building block for various tasks in secure communications, including a source of all-electronic ultrafast physical random number generators based on room temperature spontaneous chaotic oscillations in a DC-biased weakly coupled GaAs/Al0.45Ga0.55As semiconductor superlattice (SSL). Here, we experimentally demonstrate the emergence of several types of chaos synchronization, e.g. leader-laggard, face-to-face and zero-lag synchronization in network motifs of coupled SSLs consisting of unidirectional and mutual coupling as well as self-feedback coupling. Each type of synchronization clearly reflects the symmetry of the topology of its network motif. The emergence of a chaotic SSL without external feedback and synchronization among different structured SSLs open up the possibility for advanced secure multi-user communication methods based on large networks of coupled SSLs.

Chaotic Current Self-oscillations in Doped, Weakly Coupled Semiconductor Superlattices for True Random Number Generation

A weakly coupled semiconductor superlattice (SL) represents an almost ideal one-dimensional nonlinear dynamical system, the nonlinearity of which is due to sequential resonant tunneling between adjacent quantum wells. A great richness of nonlinear transport behavior has been observed in weakly coupled SLs, including periodic as well as quasi-period current self-oscillations and even driven as well as undriven chaos. Recently, spontaneous chaotic and quasi-periodic self-oscillations have been observed in doped GaAs/(Al,Ga)As SLs with 45% Al at room temperature. Based on this type of SLs, an all-electronic true random number generator has been demonstrated at room temperature with achievable bit rates of up to 80 Gbit/s, about two orders of magnitude larger than typical bit rates for currently available all-electronic true random number generators. The synchronization of chaos using these SLs has been demonstrated as a useful building block for various tasks in secure communications. The realization of chaotic SLs without external feedback and the synchronization among differently structured SLs open up the possibility for advanced secure multi-user communication methods based on large networks of coupled SLs.

Throwing Dice Really Fast

Working at the noise level of an optical signal, we can engineer unexpected performance.

Synchronized optical spiking

Diode laser with optical feedback can show chaotic intensity fluctuations in the form of short intensity spikes. Two such lasers can be synchronized to give identical isochronal spiking patterns.