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In delay-coupled semiconductor lasers (SLs) network, we have established a scheme to separate SLs clusters into distinct groups in which the stability of cluster synchronization in each group is completely independent from that of others. The synchronization stability of these dynamical groups, termed as independently synchronizable groups , are theoretically and numerically investigated. The influences of crucial parameters in SLs networks on the stability of independently synchronizable groups are systematically evaluated. We also numerically discuss the robustness of synchronization stability against parameter mismatch, and demonstrate that the parameter mismatch between different SLs within an independent group did not affect the synchrony of other groups. Moreover, it is shown that modulation of parameters for specific group can significantly improve synchronization performance. Our results offer a new insight to explore new regimes of synchronization in SLs networks.

The cluster synchronization of mutually-coupled semiconductor lasers (SLs) network with complex topology is theoretically and numerically investigated, in which stable zero-lag synchronization could be achieved between different SLs within same cluster. It is shown that the symmetry of SLs network topology is an underlying mechanism for the formation of SLs clusters. Different SLs clusters can exhibit different local dynamics and, most importantly, synchronized SLs clusters could coexist with unsynchronized ones. We also discussed the influence of the significant parameters of SLs on the stability of cluster synchronization. The numerical results show that two key internal parameters of SLs, the linewidth-enhancement factor and gain saturation coefficient, play important roles on the SLs network dynamics. It is also shown that, from the point view of symmetry, the self-feedback strength of SLs can play a particularly important role on the formation of SLs cluster. Moreover, the existence of symmetry-induced cluster synchronization is verified in different types of SLs networks topologies (random network and Nepal power grid network), indicating the universality of proposed results. Our results suggest a new insight into the research of synchronized dynamics in complex delay-coupled SLs networks.

Quantum random number is a particularly important physical resource both for quantum communication and quantum cryptography. Quantum randomness amplification, as a key technology in quantum random number generation, has a significant counterintuitive effect: one can amplify weak randomness to almost perfect randomness by quantum systems, which is impossible in classical cryptography. In this letter, we propose an explicit quantum randomness amplification scheme with two untrusted devices, from which we could extract one single perfect random bit from weak random bits. An explicit non-malleable two-source extractor is introduced to extract perfect randomness from two independent min-entropy sources, which are derived from a Bell test with two untrusted devices. The universally composable security of the proposed protocol is proved.

Multi-bit extraction schemes for fast random bit generation using a semiconductor laser subject to delayed optical feedback are numerically investigated. The simulation shows that the statistical properties of the chaotic signal significantly influence the generation of random bit sequences. More importantly, bit sequences with verified randomness at hundreds of gigabit per second, even up to the order of terabit per second could be numerically generated utilizing some effective postprocessing techniques even though the original statistical distribution substantially differs from a symmetric distribution. Such numerical results frequently required for developing ultrafast random bit generators as the first stage help to evaluate the experimental implementations of the corresponding schemes.

This paper presents detailed numerical investigations of quantifying the complexity of the chaotic intensity obtained from the well-known Lang-Kobayashi model for an external-cavity semiconductor laser (ECSL) using sample entropy (SampEn). We demonstrate that the modified SampEn could be an alternative to quantify the underlying dynamics of an ECSL under the condition that the dimension, radius, and time delay of the delayed vectors are properly selected. The numerical results are supported by the earlier numerical studies using the permutation entropy and Kolmogorov-Sinai entropy. Furthermore, we also confirm that the SampEn shows certain robustness to the additive observational noise.

The time delay (TD) signatures of chaotic signals in semiconductor lasers (SLs) with dual-path injection (DPI) are investigated numerically and experimentally based on the autocorrelation function and the permutation entropy function, and are compared with those for chaotic signals in SLs with single-path injection (SPI). Both the intensity and phase chaos are considered. The regions of injection strength and frequency detuning contributing to TD concealment are identified. The optimal selection of injection delays for the DPI case is also suggested. It is found that, for both intensity and phase chaos, the TD signature for the DPI case can be better concealed in wider parameter regions than that for the SPI case, which is highly desirable for security-enhanced chaotic communication systems as well as high-speed random number generators based on chaotic SLs.

The randomness and bandwidth enhancement of chaotic signals in optically injected semiconductor lasers (SLs) with dual-path injection from a single master laser are investigated experimentally and numerically. It is shown that both the randomness and bandwidth of chaotic signals generated in a slave SL (SSL) can be enhanced significantly, especially for positive frequency detuning, by simply adding an additional injection path in the conventional master-slave configuration. The region of injection parameter space leading to wideband randomness-enhanced chaos in SSL can be greatly broadened compared to the SSL with single-path injection. This low-cost and simple configuration for wideband randomness-enhanced chaotic source is highly desirable for high-speed random number generators based on chaotic SLs.

Photonic generation of wideband chaotic signals with time delay signature elimination is investigated experimentally and numerically based on a semiconductor laser (slave laser) with chaotic optical injection from a master laser. The master laser is subject to moderate optical feedback where the feedback strength and external-cavity length are fixed, while the slave laser stands alone. The experimental results show that wideband chaotic signals with successful time delay concealment can be generated in the slave laser by simply adjusting the coupling strength and frequency detuning between the two lasers. Furthermore, the numerical results are in accordance with the experimental observations. Finally, we propose a simple method for simultaneously generating multiple streams of high-quality chaotic signals using multichaotic lasers, where the time delay is effectively concealed in the autocorrelation function and delayed mutual information calculated from the chaotic time series.

In this letter, we quantitatively investigate the time-delay (TD) signature of chaotic outputs generated by mutually coupled vertical-cavity surface-emitting lasers (VCSELs) with variable polarization optical injection (VPOI). The research is performed using a novel estimator, which calculates the peak-to-mean ratio on the basis of the autocorrelation function (ACF). It is shown that the TD signature of mutually coupled VCSELs with VPOI at intermediate polarizer angles can be better concealed than that of mutually coupled VCSELs with conventional polarization-selected optical injection. The effects of bias current and injection delay are also taken into account. The system is further discussed by applying a recent approach that computes the ACF from a phase of chaotic outputs.