Coherent Generation Research Articles

Quantum coherence induced by a flux qubit coupled by a resonator coherent field through a two-photon interaction

The intrinsic decoherence effects on a flux qubit coupled to a resonator through a two-photon interaction where the resonator field is initially in coherent and even coherent states are investigated. The qubit-resonator entanglement and coherence loss (mixedness) of the system and its subsystems are examined using entropy and negativity. The ability of the qubit-resonator interaction to generate quantum coherence (qubit-resonator entanglement and mixedness) is shown to be dependent on the initial cavity non-classicality, detuning, and decoherence. For larger values of the qubit-resonator detuning, the initial resonator non-classicality can enhance the generation and stability of quantum coherence. The decoherence degrades the qubit-resonator entanglement and destroys the sudden death-birth entanglement.

Mid-IR supercontinuum generation initiated by two-cascade stimulated Raman scattering in D2-filled revolver fibre

We report a numerical study of coherent mid-IR supercontinuum generation at wavelengths above 2.4 μm, initiated by two-cascade stimulated Raman scattering in a deuterium-filled revolver fibre pumped by positively chirped picosecond pulses at a wavelength of 1.03 μm. It is shown that the highest efficiency of conversion to the second Stokes is ensured by fibre lengths at which the spectrum is strongly broadened by Kerr nonlinear effects. A range of optimal parameters (deuterium pressure, fibre length, pulse energy, and pulse duration) is found that ensure quantum efficiency at a level of 50 % and a mid-IR supercontinuum width above 1000 nm. The coherence properties of the supercontinuum are shown to be determined by the pulse duration, the magnitude of the pulse chirp, and dispersion characteristics in each transmission band of the fibre. We demonstrate the feasibility of generating a single compressed mid-IR pulse of 20-fs duration with an energy of 1.9 μJ.

Coherent tunable diffractional pulse shaping and generation of the 0π-pulse in Rb vapor

We have experimentally studied for the first time a new operation principle of the coherent diffractional pulse shaper (Rabi shaper). In the experiment, we observed an effect of tunable pulse shaping of nanosecond semiconductor laser pulse during the resonant pumping of the D2 line (780.24 nm) of 87Rb vapor in the range of self-diffraction angles φ = ± 4°. We observed the synthesis of nanosecond 0π-pulses at the small length of the nonlinear interaction 0.1…1 mm. We propose to use the Rabi shaper as an energy efficient tunable shaper of classical and single-photon wave packets. We analyze a possibility of the Rabi shaper operation in quantum systems with feedback.

Temporal loss boundary engineered photonic cavity

Losses are ubiquitous and unavoidable in nature inhibiting the performance of most optical processes. Manipulating losses to adjust the dissipation of photons is analogous to braking a running car that is as important as populating photons via a gain medium. Here, we introduce the transient loss boundary into a photon populated cavity that functions as a ‘photon brake’ and probe photon dynamics by engineering the ‘brake timing’ and ‘brake strength’. Coupled cavity photons can be distinguished by stripping one photonic mode through controlling the loss boundary, which enables the transition from a coupled to an uncoupled state. We interpret the transient boundary as a perturbation by considering both real and imaginary parts of permittivity, and the dynamic process is modeled with a temporal two-dipole oscillator: one with the natural resonant polarization and the other with a frequency-shift polarization. The model unravels the underlying mechanism of concomitant coherent spectral oscillations and generation of tone-tuning cavity photons in the braking process. By synthesizing the temporal loss boundary into a photon populated cavity, a plethora of interesting phenomena and applications are envisioned such as the observation of quantum squeezed states, low-loss nonreciprocal waveguides and ultrafast beam scanning devices.

Highly coherent visible supercontinuum generation in a micrometer-core borosilicate glass photonic crystal fiber

Highly coherent visible supercontinuum (SC) sources are demanded for many applications such as bio-sensing and imaging. Either dispersion management on the fiber or optimizing ultrafast pulsed pump parameters can work for achieving broadband coherent fiber SC. Normally, highly coherent SC with medium bandwidth can be obtained in an all-normal-dispersion (ANDi) nonlinear photonic crystal fiber (PCF), which has normal dispersion for all the wavelengths by tailoring the structure parameters of the microstructured cladding to sub-wavelength scale. Hence, such an ANDi fiber requires a submicron core diameter, and it makes precise fabrication challenging. Instead, using a standard anomalous dispersion pumping scheme and an ultrafast pulsed pump source, broadband SC with a high degree of coherence can also be obtained. In this study, we report broadband visible SC with a high degree of coherence approaching unity in a low-index borosilicate glass air-suspended PCF with a micrometer-core diameter, by the anomalous dispersion pumping scheme. The pulse duration of the 800 nm Ti:sapphire femtosecond laser is 29 fs. The experimental results are in good agreement with the numerical simulations, indicating that ultrashort pump pulses are the primary cause for the high degree of the generated visible SC, while the weak Raman effect of the borosilicate glass host plays a non-negligible but secondary role in the procedure of coherent SC generation.

Low pump power coherent supercontinuum generation in heavy metal oxide solid-core photonic crystal fibers infiltrated with carbon tetrachloride covering 930-2500 nm.

All-normal dispersion supercontinuum (ANDi SC) generation in a lead-bismuth-gallate glass solid-core photonic crystal fiber (PCF) with cladding air-holes infiltrated with carbon tetrachloride (CCl4) is experimentally investigated and numerically verified. The liquid infiltration results in additional degrees of freedom that are complimentary to conventional dispersion engineering techniques and that allow the design of soft-glass ANDi fibers with an exceptionally flat near-zero dispersion profile. The unique combination of high nonlinearity and low normal dispersion enables the generation of a coherent, low-noise SC covering 0.93-2.5 µm requiring only 12.5 kW of pump peak power delivered by a standard ultrafast erbium-fiber laser with 100 MHz pulse repetition rate (PRR). This is a much lower peak power level than has been previously required for the generation of ANDi SC with bandwidths exceeding one octave in silica- or soft-glass fibers. Our results show that liquid-composite fibers are a promising pathway for scaling the PRR of ANDi SC sources by making the concept accessible to pump lasers with hundreds of megahertz of gigahertz PRR that have limited peak power per pulse but are often required in applications such as high-speed nonlinear imaging, optical communications, or frequency metrology. Furthermore, due to the overlap of the SC with the major gain bands of many rare-earth fiber amplifiers, our source could serve as a coherent seed for low-noise ultrafast lasers operating in the short-wave infrared spectral region.

Recent advances in supercontinuum generation in specialty optical fibers [Invited

The physics and applications of fiber-based supercontinuum (SC) sources have been a subject of intense interest over the last decade, with significant impact on both basic science and industry. New uses for SC sources are also constantly emerging due to their unique properties that combine high brightness, multi-octave frequency bandwidth, fiber delivery, and single-mode output. The last few years have seen significant research efforts focused on extending the wavelength coverage of SC sources towards the 2 to 20 µ m molecular fingerprint mid-infrared (MIR) region and in the ultraviolet (UV) down to 100 nm, while also improving stability, noise and coherence, output power, and polarization properties. Here we review a selection of recent advances in SC generation in a range of specialty optical fibers, including fluoride, chalcogenide, telluride, and silicon-core fibers for the MIR; UV-grade silica fibers and gas-filled hollow-core fibers for the UV range; and all-normal dispersion fibers for ultralow-noise coherent SC generation.

Long-range atomic correlations as a source of coherent light generation.

In this Letter, we give a new, to the best of our knowledge, perspective on the origin of light coherence in lasers. We demonstrate that a coherence appears below the lasing threshold and manifests itself as long-range correlations between polarizations of active medium atoms. These correlations contribute to the formation of a collective state of atomic polarizations and electromagnetic field modes, which interacts more effectively with the active medium and lases when pumping exceeds the lasing threshold. We demonstrate that inhibiting these atomic correlations leads to the destruction of the collective state and suppression of lasing. The obtained results open up new ways to control coherence.

Coherent light wave generation in an incoherently pumped fiber optical parametric oscillator

We present a numerical study on the coherent sideband generation from an incoherently pumped fiber optical parametric oscillator (FOPO). We revisit the fiber dispersion condition in a parametric process that fulfills the convection-induced phase-locking mechanism when pumping incoherently. Particularly, there exist analytical solutions for phase-matching frequencies in a convection-induced phase-locking process when using a parametric gain fiber with two zero dispersion wavelengths. With the optical feedback, we manage to circulate and amplify the convection sideband in a singly resonant FOPO. The autocorrelation shows that the coherence of the oscillating convection sideband can be strongly influenced by the cavity feedback fraction.

Real-time transient stability prediction and coherency identification in power systems using Koopman mode analysis

In this paper, we propose a novel methodology based on Koopman mode analysis to predict the transient stability of a power system in real-time. The method assesses the stability of the system based on a sliding sampling window of PMU measurements, and it detects the evolving instabilities by predicting future samples and investigating the computed Koopman eigenvalues. This approach is also able to identify alarm conditions, which include slowly evolving instabilities that may not be detected by predicting future samples in a limited time horizon. Identifying these conditions provides additional time to prepare a proper set of emergency control actions to be performed when necessary. Using the proposed method, groups of coherent generators that play roles in the evolving instabilities can also be identified, contributing to the design of a defensive islanding scheme for unstable cases. The efficacy of the proposed approach is demonstrated by simulating its performance with three test systems of different scales.

A new intelligent time series prediction technique for coherency identification performance enhancement

The manuscript proposes a time series prediction technique to enhance the response time of the coherency identification technique. The proposed methodology utilizes the nonlinear autoregressive exogenous neural network (NARX) algorithm to predict the generator speed deviations following a disturbance in the system. Consequently, the coherency identification technique based on independent component analysis (ICA) is utilized on the predicted system responses. The effectiveness of the proposed approach is demonstrated on of the IEEE 16-generator 68-bus test system model. The result shows that the proposed technique can predict 0:2s following a disturbance in the system accurately. Therefore, the NARX allows a 0:2s head start for the ICA to determine the real-time coherent generator group in the system. Furthermore, the result shows that the proposed approach can identify the coherent group of generators based on the predicted generator speed deviation in all cases considered in this study, accurately. Conclusively, the result implies that the proposed technique can speed up the overall coherency identification process in a power system operation.

Coherent Dialog Generation with Query Graph

Learning to generate coherent and informative dialogs is an enduring challenge for open-domain conversation generation. Previous work leverage knowledge graph or documents to facilitate informative dialog generation, with little attention on dialog coherence. In this article, to enhance multi-turn open-domain dialog coherence, we propose to leverage a new knowledge source, web search session data, to facilitate hierarchical knowledge sequence planning, which determines a sketch of a multi-turn dialog. Specifically, we formulate knowledge sequence planning or dialog policy learning as a graph grounded Reinforcement Learning (RL) problem. To this end, we first build a two-level query graph with queries as utterance-level vertices and their topics (entities in queries) as topic-level vertices. We then present a two-level dialog policy model that plans a high-level topic sequence and a low-level query sequence over the query graph to guide a knowledge aware response generator. In particular, to foster forward-looking knowledge planning decisions for better dialog coherence, we devise a heterogeneous graph neural network to incorporate neighbouring vertex information, or possible future RL action information, into each vertex (as an RL action) representation. Experiment results on two benchmark dialog datasets demonstrate that our framework can outperform strong baselines in terms of dialog coherence, informativeness, and engagingness.

Observation of Quasiparticle Pair Production and Quantum Entanglement in Atomic Quantum Gases Quenched to an Attractive Interaction.

We report observations of quasiparticle pair production by a modulational instability in an atomic superfluid and present a measurement technique that enables direct characterization of quasiparticle quantum entanglement. By quenching the atomic interaction to attractive and then back to weakly repulsive, we produce correlated quasiparticles and monitor their evolution in a superfluid through evaluating the insitu density noise power spectrum, which essentially measures a "homodyne" interference between ground-state atoms and quasiparticles of opposite momenta. We observe large amplitude growth in the power spectrum and subsequent coherent oscillations in a wide spatial frequency band within our resolution limit, demonstrating coherent quasiparticle generation and evolution. The spectrum is observed to oscillate below a quantum limit set by the Peres-Horodecki separability criterion of continuous-variable states, thereby confirming quantum entanglement between interaction quench-induced quasiparticles.

Compact dual-crystal Tm,Ho:YLF laser with balanced orthogonal polarization output power.

The compact dual-crystal orthogonally polarized Tm,Ho:YLF laser with balanced orthogonal polarization output power was reported for the first time, to the best of our knowledge. Two pieces of a-cut Tm,Ho:YLF crystals were placed compactly with their c-axis perpendicular. The maximum multimode total output power of 460 mW was obtained with the slope efficiency of 28.2%, and the M2 factor value was measured to be 1.07. By adjusting the pump focus position along the optical axis in the process of changing pump power, the balanced output powers in the two orthogonal polarization directions were successfully achieved, and the maximum output powers of the P-polarization and S-polarization states were both 215 mW. Inserting two uncoated solid state etalons into the resonator, the dual-wavelength single-longitudinal-mode laser with orthogonal polarization was obtained. The output powers at P-polarized 2052.1 nm and S-polarized2065 nm were 71mW and 62mW, respectively, and the M2 factor was 1.26. The orthogonally polarized dual-wavelength laser at 2 µm can be used as a seed laser for differential absorption lidars and coherent THz wave generation.

A low-noise photonic heterodyne synthesizer and its application to millimeter-wave radar

Microwave photonics offers transformative capabilities for ultra-wideband electronic signal processing and frequency synthesis with record-low phase noise levels. Despite the intrinsic bandwidth of optical systems operating at ~200 THz carrier frequencies, many schemes for high-performance photonics-based microwave generation lack broadband tunability, and experience tradeoffs between noise level, complexity, and frequency. An alternative approach uses direct frequency down-mixing of two tunable semiconductor lasers on a fast photodiode. This form of optical heterodyning is frequency-agile, but experimental realizations have been hindered by the relatively high noise of free-running lasers. Here, we demonstrate a heterodyne synthesizer based on ultralow-noise self-injection-locked lasers, enabling highly-coherent, photonics-based microwave and millimeter-wave generation. Continuously-tunable operation is realized from 1-104 GHz, with constant phase noise of -109 dBc/Hz at 100 kHz offset from carrier. To explore its practical utility, we leverage this photonic source as the local oscillator within a 95-GHz frequency-modulated continuous wave (FMCW) radar. Through field testing, we observe dramatic reduction in phase-noise-related Doppler and ranging artifacts as compared to the radar’s existing electronic synthesizer. These results establish strong potential for coherent heterodyne millimeter-wave generation, opening the door to a variety of future applications including high-dynamic range remote sensing, wideband wireless communications, and THz spectroscopy.

Spectrum Broadening of Supercontinuum Generation by fill Styrene in core of Photonic Crystal Fibers

In this paper, a new photonic crystal fiber with hollow core filled by styrene (PCFS) has been designed for coherent supercontinuum generation. Its main optical characterisitcs are simulated for different lattice microstructure. The PCFS 2,0.3 with lattice pitch L=2 mm, filling factor d/L=0.3 and styrene core of diameter of 3.34 mm, owning the flat and anomalous dispersion in the near infrared range from 1.33 mm, lower confinement loss, higher effective refractive index at pump wavelength, and smaller effective mode area is chosen to invetigate the supercontinuum spectrum (SC). The spectrum broadening of SC depending on the fiber’s length, pump duration and pump energy is simulated and discussed in comperison to that obtained in the photonic crystal fibers with hollow core filled with toluene (PCFT 2,0.3 ).

Quantum-like model for unconscious–conscious interaction and emotional coloring of perceptions and other conscious experiences

Quantum measurement theory is applied to quantum-like modeling of coherent generation of perceptions and emotions and generally for emotional coloring of conscious experiences. In quantum theory, a system should be separated from an observer. The brain performs self-measurements. To model them, we split the brain into two subsystems, unconsciousness and consciousness. They correspond to a system and an observer. The states of perceptions and emotions are described through the tensor product decomposition of the unconscious state space; similarly, there are two classes of observables, for conscious experiencing of perceptions and emotions, respectively. Emotional coloring is coupled to quantum contextuality: emotional observables determine contexts. Such contextualization reduces degeneration of unconscious states. The quantum-like approach should be distinguished from consideration of the genuine quantum physical processes in the brain (cf. Penrose and Hameroff). In our approach the brain is a macroscopic system which information processing can be described by the formalism of quantum theory. The paper is concluded with experimental test of contextual emotional coloring of conscious experiences based on Bell type inequalities which are treated in the contextual framework.

Prospecting the Effect of Topic Modeling in Information Retrieval

Enormous records and data are gathered every day. Organization of this data is a challenging task. Topic modeling provides a way to categorize these documents, where high dimensionality of the corpus affects the result of topic model, making it important to apply feature selection or information retrieval process for dimensionality reduction. The requirement for efficient topic modeling includes the removal of unrelated words that might lead to specious coexistence of the unrelated words. This paper proposes an efficient framework for the generation of better topic coherence, where term frequency-inverse document frequency (TF-IDF) and parsimonious language model (PLM) are used for the information retrieval task. PLM extracts the important information and expels the general words from the corpus, whereas TF-IDF re-estimates the weightage of each word in the corpus. The work carried out in this paper improved the topic coherence measure to provide a better correlation among the actual topic and the topics generated from PLM.

Quantum Irreversibility in a Misaligned Spin System

A single spin that is misaligned with respect to the static external magnetic field is investigated as a toy model to clarify the nature of irreversibility in terms of inner friction and irreversible work. The coherence generation and the effects of unwanted transitions are analyzed in detail. The behavior of inner friction and irreversible work as a function of protocol time are analyzed for a finite-time unitary transformation. The coherence generation is shown to be the common sign for the inner friction and irreversible work. The excess energy sourced by the unwanted transitions for a quasistatic transformation is found to be the only sign for irreversible work. The angle The angle dependencies of the inner friction and irreversible work are also analyzed explicitly. The selected model and the considered realistic parameters are available to be implemented for the finite-time operations on the nuclear magnetic resonance setups.

Design of step-index-microstructured hybrid fiber for coherent supercontinuum generation

In this paper, a step-index-microstructured hybrid fiber (SIMHF) with near-zero dispersion at the communication band is proposed. In the SIMHF, a germania-doped core in the center, air holes and germania-doped inclusions arrangement in the cladding are used to control the property of dispersion. Through optimized design, the dispersion of the SIMHF can be maintained within −0.3 to 0 ps/nm/km from 1.49 µm to 1.85 µm wavelength. Taking the advantage of this near-zero-flattened normal dispersion, the SIMHF shows great potential on coherent supercontinuum (SC) generation. By injecting the pulse with peak power of 1 kW and full width at half maximum (FWHM) of 500 fs into the fiber, SC that covers the wavelength range of 1.35– 2.02 µm can be generated.