Integrated quantum photonics is a promising strand of research that involves guiding light in submicron-scale structures and utilizing the quantum properties of the fundamental particle of light—the photon. One of these quantum properties, the indistinguishability of the photons, is used to achieve high-visibility quantum interference, which is one of the key resources in photonic quantum information processing. Here, we demonstrate that the faster and simpler heralded and unheralded second-order correlation functions ($g^{(2)}$) estimate the visibility (indistinguishability) well without the need to perform laborious and time-consuming quantum-interference measurements.

We report on terahertz (THz) emission from a single-layer ferromagnet, which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating THz radiation. The THz emission can be either enhanced or suppressed or the polarity can be reversed by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require an additional nonmagnetic layer or Rashba interface.

An on-chip light source is one of the essential components for integrated photonic circuits and quantum-information processing chips. To date, it has been a great challenge to construct an on-chip light source with high directionality, high collection efficiency, and ultrasmall feature size simultaneously in the optical communication range. Here, an on-chip light source, consisting of a circular paraboloid etched in an aluminum-doped zinc-oxide film, which is an epsilon-near-zero material, is experimentally realized at 1550 nm. Photons emitted from the a $\mathrm{PbS}$ quantum-dot cluster placed near the focal point of this paraboloid are reflected into a directional parallel beam due to the large impedance mismatch between aluminum-doped zinc oxide and ambient medium. An ultrasmall emission divergence angle of only $\pm 3^{\circ }$, and an ultrahigh directional emission contrast ratio of 44 dB are achieved from the ultracompact device with a feature size of only $1.5\mu \mathrm{m}$. Also, an ultrahigh collection efficiency of up to 92% is predicted by simulation. This work not only opens an alternative way for the realization of integrated photonic devices based on epsilon-near-zero materials, but also provides another method for the precise assembling of composite functional nanostructures.

Generation and control of quantum states of light on an integrated platform have become essential tools for scalable quantum technologies. Chip-scale sources such as nonlinear optical microcavities have been demonstrated to efficiently generate entangled biphoton states. However, these systems have little control over the continuous-variable time-energy entanglement of the photons. We demonstrate such control by preparing biphoton states with asymmetric temporal wave functions. This is done by selectively modifying the density of states of the cavity modes participating in the interaction using Rayleigh-scattering-induced strong coupling of optical modes of a resonator. These states reveal exotic coherence properties and show a path forward for continuous-variable quantum-state engineering on a chip.

Information reconciliation is crucial for continuous-variable quantum key distribution (CV QKD) because its performance affects the secret key rate and maximal secure transmission distance. Fixed-rate error-correction codes limit the potential applications of the CV QKD because of the difficulty of optimizing such codes for different low SNRs. In this Paper, we propose a rateless reconciliation protocol combined multidimensional scheme with Raptor codes that not only maintains the rateless property but also achieves high efficiency in different SNRs using just one degree distribution. It significantly decreases the complexity of optimization and increases the robustness of the system. Using this protocol, the CV QKD system can operate with the optimal modulation variance, which maximizes the secret key rate. Simulation results show that the proposed protocol can achieve reconciliation efficiency of more than 95% within the range of SNR from $-20$ to 0 dB. It also shows that we can obtain a high secret key rate at arbitrary distances in a certain range and achieve a secret key rate of about $5\times {10}^{-4}$ bits/pulse at a maximum distance of 132 km (corresponding SNR is $-20$ dB) that is higher than previous works. The proposed protocol can maintain highly efficient key extraction under the wide range of SNRs and paves the way toward the practical application of CV QKD systems in flexible scenarios.

Realization of magnetic-free nonreciprocal transmission for two or more wavelengths simultaneously is in great demand for multiwavelength optical communication and signal processing. Here, making use of optical pumping, we propose and experimentally demonstrate a magnetic-free multiwavelength isolator based on warm V-type atoms with the assistance of the Doppler effect. Applying ${}^{85}\mathrm{Rb}$ atoms, isolation ratios of 25 and 28 dB are obtained simultaneously for signal fields at 795 and 780 nm with transmissions higher than 0.85. This work provides an effective way to realize a multiwavelength optical isolator for optical information processing applications.

Subwavelength plasmonic nanostructures have demonstrated many interesting phenomena and applications in recent decades. However, plasmonic resonance and associated properties, such as strong field enhancement, are limited to the visible regime due to the quasistatic nature of subwavelength plasmonic nanoparticles. Here, we numerically demonstrate spectral resonance tuning of subwavelength nanoparticles with a high-index core and a plasmonic shell while maintaining a sharp spectral peak. These core-shell-based nanoparticles have predominantly nonradiative loss and strong near-field enhancement due to their subwavelength feature size. This scheme can be further extended for magnetic and chiral meta-atoms. We suggest that these features of subwavelength core-shell-based nanoparticles are advantageous for many interesting applications including photothermal devices, enhanced light-matter interaction, and photonic circuitry.

High-power and broadband dual-comb sources in the terahertz frequency regime are much in demand for fast and high-resolution spectroscopies. The terahertz quantum cascade laser (QCL) is a powerful radiation source in the frequency range between 1 and 5 THz, which is an ideal candidate for frequency-comb and dual-comb generations in this frequency regime. In this work, we report on broadband on-chip dual-comb sources based on terahertz QCLs emitting at approximately 4.2 THz with a single plasmon waveguide configuration. Due to the fast intersubband transitions in QCLs, the dual-comb lines can be detected by employing the laser self-detection scheme. A finite-element analysis shows that a weak optical coupling between the two laser combs is sufficient for generating dual-comb lines via the nonlinear multiheterodyne frequency beatings. The noise-equivalent power of the QCL detector is also evaluated for different optical powers; furthermore, a microwave double injection is employed to stabilize the comb frequency and broaden the optical comb bandwidth. We find that even with weak radio-frequency powers, the optical comb bandwidth can be significantly broadened from 86 to 166 GHz. The double injection also allows us to investigate the carrier offset noise of QCL combs.

Heralding techniques are useful in quantum communication to circumvent losses without resorting to error-correction schemes or quantum repeaters. Such techniques are realized, for example, by monitoring for photon loss at the receiving end of the quantum channel while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a direct quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit quantum electrodynamics and is fully microwave controlled, making it an interesting candidate for future modular quantum-computing architectures.

We propose a theoretical scheme to achieve strong photon blockade via a single atom in a cavity. By utilizing the optical Stark shift, the dressed-state splitting between the higher and lower branches is enhanced, which results in a significant increase in the lower (higher) branch and a decrease in the higher (lower) branch for a negative (positive) Stark shift, and dominates the time evolution of the photon-number oscillations as well. Furthermore, two-photon excitation is suppressed via quantum interference for the optimal phase and Rabi frequency of an external microwave field. It is shown that the interplay between quantum interference and the enhanced vacuum Rabi splitting gives rise to a strong photon blockade that can be used to realize a high-quality single-photon source beyond the regime of strong atom-cavity coupling. In particular, by tuning the optical Stark shift, the second-order correlation function in our scheme can be made three orders of magnitude smaller than in the Jaynes-Cummings model, and, correspondingly, a large cavity photon number appears as well. Our proposal may imply exciting opportunities for potential applications in quantum networks and quantum information processing.

Circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors yet is less studied in graphene due to the relatively weak light-matter interaction and is submerged by other nonlinear optical effects. We give experimental evidence of CPDE in vertically grown graphene (VGG) by terahertz (THz) emission spectroscopy. The emitted THz polarization states can be tuned to linear, left-handed, and right-handed elliptical polarizations by changing the helicity of the pump laser. Polarity reversal of the THz time-domain signal occurs with the opposite helicity of pump laser excitation due to the CPDE. Theory analysis suggests that both the linear photon drag effect and CPDE-induced transient photocurrents contribute to the THz emission from which the contribution weight of CPDE can be tuned by different elliptical states of the excitation light. The photon-helicity-dependent THz emission from VGG based on CPDE offers an alternative thought of graphene-based polarization sensitive THz sources for chiral analysis in the THz field.

Bendable signal transmission based on accelerating beams has become an increasingly interesting topic in recent years due to not only its fascinating propagation characteristics but also the potential to evade or resist obstructions within the transmission link. In this work, we propose a type of image-signal transmission based on nonconvex accelerating beams in contrast to convex accelerating beams such as Airy beams, which is expected to be more resilient to obstruction due to the special spectrum-position mapping characteristic. A constructed helical beam is used as the nonconvex accelerating beam for demonstration, and an image encoded in its angular spectrum is successfully retrieved after multiple self-bending without and with obstruction in the transmission link, which confirms the above idea and may open a new door for image-signal transmission in the fields of imaging, signal processing, and free-space communication.

The recently discovered phenomena of photomagnetic recording using femtosecond laser pulses in magnetic dielectrics opens up intriguing opportunities for ultrafast least-dissipative magnetic writing. Fundamental questions remain, however, about the processes, which limit the repetition rate of such writing. We report the magnetization dynamics and switching caused by a double-pulse excitation with various orientations of pump polarization. In particular, we study write and erase sequences, showing that mutual interference of the effects caused by two laser pulses can occur if the pulses are temporally separated by less than 60 ps. This imposes the fundamental limit for the repetition rate of writing (20 GHz), but this could be increased even further by strengthening the photoinduced magnetic anisotropy.

Photoelectron emission from solids, as driven by high-intensity lasers, offers a platform for the coherent control of electron motion on ultrashort spatiotemporal scales. By solving the time-dependent Schrödinger equation, we present an exact analytical solution for the nonlinear ultrafast electron emission from a dc-biased metal surface illuminated by two-color laser fields. We systematically examine the combined effects of a dc electric field and two-color laser fields. In addition to the remarkable tunability of electron emission processes due to interference from two-color laser fields, we find that a strong dc electric field not only opens up more tunneling emission channels, but also introduces intense modulation to the emission current. We find surprising results that strong current modulation (with respect to the phase difference of the two-color lasers) persists (>70%), even with a large dc bias (i.e., ratio of the electric fields for dc : fundamental laser : second-harmonic laser $\cong 1:0.5:0.07$). In the meantime, the average emission current level increases by about three orders of magnitude relative to the case of zero dc bias. Application of our model to time-resolved photoelectron spectroscopy is exemplified and shows that the dynamics of the n-photon excited states depends strongly on the applied dc field. Our study suggests a practical way to maintain a strong modulation to high current photoemission, by the addition of a large dc bias for two-color laser-induced electron emission.

We develop a concept for a traveling-wave parametric amplifier driven by a magnetic flux wave. The circuit consists of a serial array of symmetric dc superconducting quantum-interference devices coupled inductively to a separate superconducting $LC$ transmission line carrying the pump wave. The adjusted phase velocity of the pump flux wave of frequency ${\omega }_p$ ensures amplification of the signal (${\omega }_s$) and idler (${\omega }_i$) waves, with frequencies obeying the relation ${\omega }_s+{\omega }_i={\omega }_p$. The advantage of the proposed flux-driven linear circuit includes a large gain over a wide frequency range and overcoming of the pump depletion problem. Unlike conventional traveling-wave amplifiers, the signal and pump in the proposed circuit are applied to different ports, what can greatly simplify the whole measurement setup. The experimental parameters and characteristics of this amplifier are evaluated and show promise for applications in quantum-information single-photon circuits.

Existing thermal metamaterials are mostly designed to work at room temperature, where thermal conduction is the dominant method of heat transfer. Unfortunately, as the temperature increases, thermal radiation becomes non-negligible, and hence these thermal metamaterials can no longer work. The inability to handle thermal radiation largely limits practical applications at high temperatures, such as thermal protection. To solve this problem, here we propose an effective-medium theory to manipulate thermal radiation with the Rosseland diffusion approximation. This theory helps to design three radiative metamaterials even with anisotropic geometries, including transparency, cloak, and expander. Theoretical analyses are further confirmed by finite-element simulations, which indicate that these radiative metamaterials behave well in both steady and transient states. This work not only offers an effective-medium theory to manipulate thermal radiation but also designs three radiative metamaterials. These results may provide hints for novel thermal management and have potential applications in radiative camouflage, radiative diodes, etc.

Using a time-resolved optically pumped scanning-optical-microscopy technique, we demonstrate the laser-driven excitation and propagation of spin waves in a 20-nm film of a ferromagnetic metallic alloy Galfenol epitaxially grown on a $\mathrm{Ga}\mathrm{As}$ substrate. In contrast to previous all-optical studies of spin waves, we employ laser-induced thermal changes of magnetocrystalline anisotropy as an excitation mechanism. A tightly focused 70-fs laser pulse excites packets of magnetostatic surface waves with an $e^{-1}$-propagation length of $3.4\mu \mathrm{m}$, which is comparable with that of permalloy. As a result, laser-driven magnetostatic spin waves are clearly detectable at distances in excess of $10\mu \mathrm{m}$, which promotes epitaxial Galfenol films to the limited family of materials suitable for magnonic devices. A pronounced in-plane magnetocrystalline anisotropy of the Galfenol film offers an additional degree of freedom for manipulating the spin waves’ parameters. Reorientation of an in-plane external magnetic field relative to the crystallographic axes of the sample tunes the frequency, amplitude, and propagation length of the excited waves.

Over the past decades, geometric phases have brought profound insights in both quantum and classical physics. Although metasurfaces based on the Pancharatnam-Berry phase have made great achievements in polarization optics, chiral holograms, and even quantum entanglement, the construction of other types of geometric phases remains unclear. We propose to mimic a single ballistic Aharonov-Bohm ring using meta-atoms in both spin direction and polarization state changes during spin-material interactions, resulting in an analog of the Aharonov-Anandan phase of fermion particles. By combining the Aharonov-Anandan and Pancharatnam-Berry phases, the symmetric restriction of the geometric phase is finally broken. In this way, desired phase profiles can be easily imposed on arbitrary polarization states without restricting the orthogonal limitations. The proposed geometric phases allow flexible manipulations of the photonic spin-Hall effect and enable the superposition of arbitrary ${\mathrm{spin}}_{1/2}$ charges, thus breaking the conjugate constraints of the output orbital angular momentum modes. The results of this work expand the categories of geometric phases in metasurfaces and may find applications in polarization control and spin-enabled optics.

Quantum plasmonics experiments have on multiple occasions resulted in the observation of quantum coherence of discrete plasmons, which exhibit remarkable preservation of quantum interference visibility, a seemingly surprising feature for systems mixing light and matter with high Ohmic losses during propagation. However, most experiments to date used essentially weakly confined plasmons, which experience limited light-matter hybridization, thus limiting the potential for decoherence. In this paper, we investigate experimentally the robustness of coherence preservation in a plasmonic system: our setup is based on a hole-array chip supporting plasmons near the surface plasma frequency, where plasmonic dispersion and confinement are much stronger than in previous experiments, making the plasmons much more susceptible for decoherence processes. We, however, report preservation of quantum coherence even in these extreme conditions. We generate polarization-entangled pairs of photons using type-I spontaneous parametric down-conversion and transmit one of the photons through a plasmonic hole array that is numerically designed to convert incident single photons into highly dispersive single surface-plasmon polaritons. Our results show that the quality of photon entanglement after the plasmonic channel is unperturbed by the introduction of a highly dispersive plasmonic element. Our findings provide a lower bound of 100 fs for the pure dephasing time for dispersive plasmons in gold, and show that even in a highly dispersive regime surface plasmons preserve quantum mechanical correlations, making possible harnessing of the power of extreme light confinement for integrated quantum photonics.

The photophysical properties of emitting charge-transfer states are often reported to be profoundly influenced by the dielectric environment that surrounds the emitter. This poses a technological challenge because chromophores based on thermally activated delayed fluorescence used in organic light-emitting diodes feature a strong charge-transfer character of the emitting state, and their photophysics can be greatly altered by the properties of the host material in the emitting layer. In this work, three isomers with different degrees of steric hindrance against the rotation around the donor-acceptor connection are investigated. Furthermore, a continuous variation of the dielectric constant is achieved by our mixing two solvents, and the time-resolved fluorescence is analyzed to understand how the ratio of the delayed fluorescence to the prompt fluorescence varies. The computation of the excitonic structure in media of different dielectric constants using an optimally tuned range-separated hybrid functional allows us to precisely interpret the experimental results and to pinpoint the participation of a higher triplet in the reverse intersystem crossing as the key factor causing the peculiar increase of delayed fluorescence at low dielectric constant for one of the model compounds.