arXiv subjects

Xianfeng Chen

Publications and source records attributed to Xianfeng Chen.

At least 91 records · Page 5Linked to original sources

arXiv2018

Experimental demonstration of nonbilocality with truly independent sources and strict locality constraints

Entanglement swapping entangles two particles that have never interacted[1], which implicitly assumes that each particle carries an independent local hidden variable, i.e., the presence of bilocality[2]. Previous experimental studies of bilocal hidden variable models did not fulfill the central requirement that the assumed two local hidden variable models must be mutually independent and hence their conclusions are flawed on the rejection of local realism[3-5]. By harnessing the laser phase randomization[6] rising from the spontaneous emission to the stimulated emission to ensure the independence between entangled photon-pairs created at separate sources and separating relevant events spacelike to satisfy the no-signaling condition, for the first time, we simultaneously close the loopholes of independent source, locality and measurement independence in an entanglement swapping experiment in a network. We measure a bilocal parameter of 1.181$\pm$0.004 and the CHSH game value of 2.652$\pm$0.059, indicating the rejection of bilocal hidden variable models by 45 standard deviations and local hidden variable models by 11 standard deviations. We hence rule out local realism and justify the presence of quantum nonlocality in our network experiment. Our experimental realization constitutes a fundamental block for a large quantum network. Furthermore, we anticipate that it may stimulate novel information processing applications[7,8].

quant-ph↗

arXiv2018

Mode surgery of LN micro-resonator by femtosecond laser irradiation

Lithium niobate(LN) microresonator is a key component in photonic integrated circuits. An intriguing phenomenon was observed that the quality factor for specific modes of the resonator can be further improved by femtosecond laser irradiation. The high repetition laser pulses scatters into the cavity by forming a defect on the surface and is confined in the edge. A localized coupled laser refining process will then repair the lattice disorders and tiny burrs on the periphery. The intrinsic quality factor of a measured mode can be promoted from 2.17$\times$10$^5$ to 1.84$\times$10$^6$ indicating one order of magnitude improvement. This highly controllable method has its own unique advantages in improving the quality of an integrated resonator which opens new prospects for single component engineering of LN photonic integrated circuits.

physics.app-ph↗

arXiv2018

Bloch oscillations in arrays of helical waveguides

We study optical Bloch oscillations in the one- and two-dimensional arrays of helical waveguides with transverse refractive index gradient. Longitudinal rotation of waveguides may lead to notable variations of the width of the band of quasi-energies and even its complete collapse for certain radii of the helix. This drastically affects the amplitude and direction of Bloch oscillations. Thus, they can be completely arrested for certain helix radii or their direction can be reversed. If the array of helical waveguides is truncated and near-surface waveguide is excited, helix radius determines whether periodic Bloch oscillations persist or replaced by the irregular near-surface oscillations.

physics.optics↗

arXiv2018

Broadband quasi-phase matching in MgO: PPLN thin film

Future quantum information networks operated on telecom channels require qubit transfer between different wavelengths while preserving quantum coherence and entanglement. Qubit transfer is a nonlinear optical process, but currently the types of atoms used for quantum information processing and storage are limited by the narrow bandwidth of up-conversion available. Here we present the first experimental demonstration of broadband and high-efficiency quasi-phase matching second harmonic generation (SHG) in a chip-scale periodically poled lithium niobate thin film. We achieve large bandwidth of up to 2 THz for SHG by satisfying quasi-phase matching and group-velocity matching simultaneously. Furthermore, by changing film thickness, the central wavelength of quasi-phase matching SHG bandwidth can be modulated from 2.70 um to 1.44 um. The reconfigurable quasi-phase matching lithium niobate thin film provides a significant on-chip integrated platform for photonics and quantum optics.

physics.optics↗

arXiv2018

Lieb polariton topological insulators

We predict that the interplay between the spin-orbit coupling, stemming from the TE-TM energy splitting, and the Zeeman effect in semiconductor microcavities supporting exci- ton-polariton quasi-particles results in the appearance of unidirectional linear topological edge states when the top microcavity mirror is patterned to form a truncated dislocated Lieb lattice of cylindrical pillars. Periodic nonlinear edge states are found to emerge from the linear ones. They are strongly localized across the interface and they are remarkably robust in comparison to their counterparts in hexagonal lattices. Such robustness makes possible the existence of nested unidirectional dark solitons that move steadily along the lattice edge.

cond-mat.mes-hall↗

arXiv2017

Quantum mechanical analysis of nonlinear optical response of interacting graphene nanoflakes

We propose a distant-neighbor quantum-mechanical (DNQM) approach to study the linear and nonlinear optical properties of graphene nanoflakes (GNFs). In contrast to the widely used tight-binding description of the electronic states that considers only the nearest-neighbor coupling between the atoms, our approach is more accurate and general, as it captures the electron-core interactions between all atoms in the structure. Therefore, as we demonstrate, the DNQM approach enables the investigation of the optical coupling between two closely separated, but chemically unbound GNFs. We also find that the optical response of GNFs depends crucially on their shape, size, and symmetry properties. Specifically, increasing the size of nanoflakes is found to shift their accommodated quantum plasmon oscillations to lower frequency. Importantly, we show that by embedding a cavity into GNFs, one can change their symmetry properties, tune their optical properties, or enable otherwise forbidden second-harmonic generation processes.

cond-mat.mes-hall↗

arXiv2017

PT-symmetry in nonlinear twisted multi-core fibers

We address propagation of light in nonlinear twisted multi-core fibers with alternating amplifying and absorbing cores that are arranged into the PT - symmetric structure. In this structure, the coupling strength between neighboring cores and global energy transport can be controlled not only by the nonlinearity strength, but also by gain and losses and by the fiber twisting rate. The threshold level of gain/losses, at which PT -symmetry breaking occurs, is a non-monotonic function of the fiber twisting rate and it can be reduced nearly to zero or, instead, notably increased just by changing this rate. Nonlinearity usually leads to the monotonic reduction of the symmetry breaking threshold in such fibers.

physics.optics↗

arXiv2017

Observation of Gain Spiking and Nonlinear Beating of Optical Frequency Comb in a Microcavity

Optical frequency combs are crucial for both fundamental science and applications demanding wide frequency spanning and ultra-precision resolutions. Recent advancements of nonlinear Kerr effect based optical frequency combs in microcavities open up new opportunities in a compact platform, however, internal cavity-enhanced nonlinearities are still unclear. Here we demonstrate transient nonlinear dynamics during optical frequency comb formation inside a Kerr microcavity. We show that gain spiking forms due to nonlinear phase modulation, causing comb lines' self-detuning nearby a cavity resonance, this introduces one key mechanism to stabilize optical frequency combs. Moreover, nonlinear beating has be observed by injecting an external probe to exam nonlinear cross-phase modulation between comb lines. Nonlinear transient dynamics here reveal the hidden nonlinear features of Kerr based optical frequency combs, leading to a new direction for ultrawide, ultrastable and ultrafast frequency comb generation in microcavities.

physics.optics↗

arXiv2017

Time delay signature elimination of chaos in a semiconductor laser by dispersive feedback from a chirped FBG

Time delay signature (TDS) of a semiconductor laser subject to dispersive optical feedback from a chirped fiber Bragg grating (CFBG) is investigated experimentally and numerically. Different from mirror, CFBG provides additional frequency-dependent delay caused by dispersion, and thus induces external-cavity modes with irregular mode separation rather than a fixed separation induced by mirror feedback. Compared with mirror feedback, the CFBG feedback can greatly depress and even eliminate the TDS, although it leads to a similar quasi-period route to chaos with increases of feedback. In experiments, by using a CFBG with dispersion of 2000ps/nm, the TDS is decreased by 90% to about 0.04 compared with mirror feedback. Furthermore, both numerical and experimental results show that the TDS evolution is quite different: the TDS decreases more quickly down to a lower plateau (even background noise level of autocorrelation function) and never rises again. This evolution tendency is also different from that of FBG feedback, of which the TDS first decreases to a minimal value and then increases again as feedback strength increases. In addition, the CFBG feedback has no filtering effects and does not require amplification for feedback light.

physics.optics↗

arXiv2016

Enhancement of UV Second-Harmonic Radiation at Nonlinear Interfaces with Discontinuous Second-order Susceptibilities

We investigate the generation of ultraviolet (UV) second-harmonic radiation on the boundary of a UV transparent crystal, which is derived from the automatic partial phase matching of the incident wave and the total internal reflection. By adhering to another UV non-transparency crystal with larger second-order nonlinear coefficient χ^{(2)}, an nonlinear interface with large disparity in χ^{(2)} is formed and the enhancement of UV second-harmonic radiation is observed experimentally. The intensity of enhanced second harmonic wave generated at the nonlinear interface was up to 11.6 times at the crystal boundary. As a tunable phase-matching method, it may suggest potential applications in the UV, even vacuum-UV, spectral region.

physics.optics↗

arXiv2016

Surface enhanced nonlinear Cherenkov radiation in one-dimensional nonlinear photonic crystal

We study the configuration of efficient nonlinear Cerenkov diffraction generated from a one-dimensional nonlinear photonic crystal surface, which underlies the incorporation of both quasi-phase-matching and total internal reflection by the crystal surface. Multidirectional radiation spots with different Cerenkov angles are demonstrated experimentally, which results from different orders of reciprocal vectors. At specific angles, the incident light and total internal reflect light associating with quasi-phase-matching format completely phase-matching scheme, leading to great enhancement of harmonic efficiency.

physics.optics↗

arXiv2016

Spectral compression of single-photon-level laser pulse

We experimentally demonstrate that the bandwidth of single photons laser pulse is compressed by a factor of 58 in a periodically poled lithium niobate (PPLN) waveguide chip. A chirped single photons laser pulse and an oppositely chirped classical laser pulse are employed to produce a narrowband single photon pulse with new frequency through sum-frequency generation. In our experiment, the frequency and bandwidth of single photons at 1550 nm are simultaneously converted. Our results mark a critical step towards the realization of coherent photonic interface between quantum communication at 1550 nm and quantum memory in the near-visible window.

physics.optics↗

arXiv2016

Second-harmonic focusing by nonlinear turbid medium via feedback-based wavefront shaping

Scattering has usually be considered as detrimental for optical focusing or imaging. Recently, more and more research has shown that strongly scattering materials can be utilized to focus coherent light by controlling or shaping the incident light. Here, purposeful focusing of second-harmonic waves, which are generated and scattered from nonlinear turbid media via feedback-based wavefront shaping is presented. This work shows a flexible manipulation of both disordered linear and nonlinear scattering signals, indicating more controllable degrees of freedom for the description of turbid media. This technique also provides a possible way to an efficient transmission of nonlinear signal at a desired location in the form of a focal point or other patterns. With the combination of random nonlinear optics and wavefront shaping methods, more interesting applications are expected in the future, such as nonlinear transmission matrix, multi-frequency imaging and phase-matching-free nonlinear optics.

physics.optics↗

arXiv2016

Random laser action with coherent feedback via second-harmonic generation

The random laser action with coherent feedback by second-harmonic generation (SHG) was experimentally demonstrated in this paper. Compared with the conventional random laser action based on photoluminescence effect, which needs strong photoresponse in the active medium and has a fixed response waveband due to the inherent energy level structure of the material, this random SHG laser action indicates a possible confinement of the nonlinear signal with ring cavities and widens the response waveband due to the flexible frequency conversion in nonlinear process. The combination of coherent random laser and nonlinear optics will provide us another possible way to break phase-matching limitations, with fiber or feedback-based wavefront shaping method to transmit the emission signal directionally. This work suggests potential applications in band-tunable random laser, phase-matching-free nonlinear optics and even brings in new consideration about random nonlinear optics (RNO).

physics.optics↗

arXiv2016

Localized waves supported by the rotating waveguide array

We show that truncated rotating square waveguide arrays support new types of localized modes that exist even in the linear case, in complete contrast to localized excitations in nonrotating arrays requiring nonlinearity for their existence and forming above the energy flow threshold. These new modes appear either around array center, since rotation leads to the emergence of the effective attractive potential with a minimum at the rotation axis, or in the array corners, in which case localization occurs due to competition between centrifugal force (in terms of quasi-particle analogy) and total internal reflection at the interface of the truncated array. The degree of localization of the central and corner modes mediated by rotation increases with rotation frequency. Stable rotating soliton families bifurcating from linear modes are analyzed in both focusing and defocusing media.

physics.optics↗

arXiv2016

Entanglement Swapping with Independent Sources over an Optical Fibre Network

Teleportation of an entangled state, known as entanglement swapping, plays an essential role in quantum communication and network.Here we report a field-test entanglement swapping experiment with two independent telecommunication band entangled photon-pair sources over the optical fibre network of Hefei city. The two sources are located at two nodes 12 km apart and the Bell-state measurement is performed in a third location which is connected to the two source nodes with 14.7 km and 10.6 km optical fibres. An average visibility of 79.9+/-4.8% is observed in our experiment, which is high enough to infer a violation of Bell inequality. With the entanglement swapping setup, we demonstrate a source independent quantum key distribution, which is also immune to any attack against detection in the measurement site.

quant-ph↗

arXiv2016

Quantum teleportation with independent sources over an optical fibre network

Quantum teleportation faithfully transfers a quantum state between distant nodes in a network, enabling revolutionary information processing applications. Here we report teleporting quantum states over a 30 km optical fiber network with the input single photon state and the EPR state prepared independently. By buffering photons in 10 km coiled optical fiber, we perform Bell state measurement after entanglement distribution. With active feed-forward operation, the average quantum state fidelity and quantum process fidelity are measured to be 0.85 and 0.77, exceeding classical limits of 0.67 and 0.5, respectively. The statistical hypothesis test shows that the probability of a classical process to predict an average state fidelity no less than the one observed in our experiment is less than 2.4E-14, confirming the quantum nature of our quantum teleportation experiment. Our experiment marks a critical step towards the realization of quantum internet in the future.

quant-ph↗

arXiv2016

Optically Induced Transparency in a Micro-cavity

Electromagnetically induced transparency has the unique ability to optically control transparency windows with low light in atomic systems. However, its practical applications in quantum physics and information science are limited due to rigid experimental requirements. Here, we demonstrate a new mechanism of optically induced transparency in a micro-cavity by introducing a four-wave mixing gain to nonlinearly couple two separated resonances of the micro-cavity in an ambient environment. A signature Fano-like resonance was observed owing to the nonlinear interference of the two coupled resonances. Moreover, we show that the unidirectional gain of the four-wave mixing can lead to the remarkable effect of non-reciprocal transmission at the transparency windows. Optically induced transparency may offer a unique platform for a compact, integrated solution to all-optical and quantum information.

physics.optics↗