We report a classical molecular-dynamics simulation of models of silicon nanoparticles and bulk silicon, in both the crystalline and the amorphous phase, to investigate their vibrational properties. By using a dynamical-matrix approach and a bond-polarizability model, together with a Raman-decomposition approach [Phys. Rev. B 100, 134309 (2019)], we present a comprehensive analysis of the vibrational spectra. In particular, the dependence of the high-frequency range of the Raman spectra on the nanoparticle size is studied. The results are in good agreement with Raman measurements on crystalline nanoparticles and explain the role of the nanoparticle surface, which is responsible for a shift in the Raman spectrum dependent on the particle size. In the low-frequency range, our Raman calculations reproduce well the Lamb-mode signatures, which obey the selection rules deduced by Duval [Phys. Rev. B 46, 5795 (1992)]. As a result of systematic Raman modeling, we confirm the scaling of the main signatures (ascribed to the Lamb modes with $l=0,2$) with respect to the nanoparticle size. By using the Raman-decomposition approach, we demonstrate that only a thin surface layer several angstroms in thickness contributes to the low-frequency Raman signature regardless of the nanoparticle size in the case of both the amorphous and the crystalline phase. Finally, we study the role of the coordination number of the atoms in the surface layer of a nanoparticle in order to explain the difference between the crystalline and amorphous vibrational spectra. The approach developed provides knowledge necessary for the correct interpretation of Raman spectra of nanoparticles, which opens up the possibility of quantitative control of surface-induced effects that may be relevant to various applications.

We demonstrate a method to generate multiparameter controllable chiral optical patterns. In the method, we introduce a phase calculation method of the rotation matrix and the transformation matrix. The controllable characteristics of the chiral optical patterns, which are generated by an annular subzone vortex phase plate, are demonstrated. The direction and spatial distribution of the eccentricity of the generated optical patterns can be freely regulated. The restriction on the direction of a chiral optical pattern by the single phase gradient (±Δl) of an annular subzone vortex is solved. A chiral optical pattern is modulated by using the light-field superposition and phase shift, which is experimentally achieved by a spatial light modulator. The numerical and experimental results indicate that the controllable degrees of freedom are as many as five, namely, shape, size, direction, eccentricity, and number of arrays. Furthermore, we analyze the spatial propagation of the controllable chiral optical patterns in turbulence. The generation of chiral optical patterns has great potential application in optical tweezers, optical micromechanics, chiral microstructure fabrication, rotating microscopic illumination, and so on.

Quantum metrology aims to enhance the precision of various measurement tasks by taking advantages of quantum properties. In many scenarios, precision is not the sole target; the acquired information must be protected once it is generated in the sensing process. Considering a remote sensing scenario where a local site performs cooperative sensing with a remote site to collect sensitive information at the remote site, the loss of sensing data inevitably causes sensitive information to be revealed. Quantum key distribution is known to be a reliable solution for secure data transmission; however, it fails if an eavesdropper accesses the sensing data generated at a remote site. In this study we demonstrate that, by sharing entanglement between local and remote sites, secure quantum remote sensing can be realized, and the secure level is characterized by asymmetric Fisher information gain. Concretely, only the local site can acquire the estimated parameter accurately with Fisher information approaching 1. In contrast, the accessible Fisher information for an eavesdropper is nearly zero even if he or she obtains the raw sensing data at the remote site. This achievement is primarily due to the nonlocal calibration and control of the probe state at the remote site. Our results explore one significant advantage of “quantumness” and extend the notion of quantum metrology to the security realm.

Near-perfect anomalous reflection and refraction have been demonstrated using passive lossless metasurfaces and metagratings operating at microwave, infrared, and visible frequencies, while related studies at terahertz frequencies are lacking. Here we propose low-index (with a refractive index of 1.57) 3D-printed dielectric metagratings for efficient wide-band diffraction engineering at low terahertz frequencies. A simplified analytical model reveals that the number of propagating waveguide modes inside the grating is a key factor in diffraction engineering, and is insufficient in a low-index design regardless of the detailed dimensions in a period. Additional waveguide modes are introduced in asymmetric bilayer and trilayer metagratings, providing sufficient degrees of freedom for efficient large-angle anomalous refraction and beam splitting. These metagratings are inherently wide-band, benefiting from low dispersion of the waveguide modes. Three metagratings are designed, 3D printed, and tested experimentally at 0.14 THz for ${70}^{\circ }$ refraction, ${80}^{\circ }$ refraction, and $\pm {70}^{\circ }$ beam splitting. The measured efficiency shows good agreement with the design. The proposed metagratings, with simple structures and large feature sizes, can be easily scaled to applications at higher terahertz frequencies.

The topologically protected one-way edge modes for light show a way to manipulate the interaction between a single photon and a quantum emitter. We systematically study single-photon quantum optics in a topological waveguide supporting the frequency-dependent one-way edge mode at the boundary of a dynamically modulated photonic system, and compare the results of the single-photon interacting dynamics with the quantum emitter in a unidirectional waveguide. As a major difference, a photon in a topological waveguide propagates around the resonant site coupled to the quantum emitter due to the topological protection. Our results show that, despite different subtleties, photon-emitter interactions for one photon at the edge mode and for one photon propagating in a unidirectional waveguide hold similar features. In addition, we consider the interaction between the photon and a $\mathrm{\Lambda }$-type three-level quantum emitter in the topological waveguide, where we show the nonlocal correlated photon-state generation as well as the single-photon reflection with the frequency conversion. Our work provides insight for understanding the interaction between the topologically protected unidirectional-propagating photon and the quantum emitter, which holds promise for potential applications in quantum-information processing.

The electric field control of the exchange bias has considerable application scope in modern spintronic devices such as magnetic random-access memories. Herein, we utilize an all-solid-state $\mathrm{Li}$-ion redox capacitor with an antiferromagnetic electrode for reversible control of the exchange bias in a $\mathrm{Co}/{\mathrm{Co}}_3{\mathrm{O}}_4$ heterostructure. The controlled intercalation and deintercalation of Li ions in the antiferromagnetic ${\mathrm{Co}}_3{\mathrm{O}}_4$ anode via galvanostatic charge and discharge enables reversible and nonvolatile control of the exchange bias. The mechanism of exchange-bias control in the $\mathrm{Co}/{\mathrm{Co}}_3{\mathrm{O}}_4$ heterostructure is studied using various characterization techniques. This work establishes an approach to control the exchange bias in all-solid-state magnetoionics, which is of great practical importance for the development of modern spintronic devices.

We present amplitude and phase characterization of short optical pulses using dual-comb spectroscopy, where one comb is a well-characterized stable-frequency comb and the second is the pulse source to be measured. We demonstrate that the technique is suitable for truly arbitrary repetition rates of the two combs. Moreover, the system is highly sensitive and is shown to enable measurements of low-intensity pulses. In one experiment we use relatively long (80-ps) pulses generated by an actively mode-locked semiconductor laser at a repetition rate that is slightly different from that of the reference laser. In a second experiment, we employ 2-ps pulses generated by a fiber laser whose repetition rate differs significantly from that of the reference laser. We benchmark the method by comparison with conventional methods of pulse characterization. Furthermore, limitations due to fluctuations of the carrier-envelope offset and the repetition rate, are addressed and overcome by implementing computational nonuniform sampling of measured interferograms.

Resonant radio-frequency cavities enable exquisite time-energy control of electron beams when synchronized with laser-driven photoemission. We present a lossless monochromator design that exploits this fine control in the one-electron-per-pulse regime. The theoretically achievable maximum beam current on target is orders of magnitude greater than state-of-the-art monochromators for the same space-time-energy resolution. This improvement is the result of monochromating in the time domain, unconstrained by the transverse brightness of the electron source. We show analytically and confirm numerically that cavity parameters chosen to minimize energy spread perform the additional function of undoing the appreciable effect of chromatic aberration in the upstream optics. We argue that our design has significant applications in ultrafast microscopy, as well as potential for use in nontime resolved microscopy, provided future photoelectron sources of sufficiently small size and laser sources of sufficiently high repetition rate. Our design achieves in simulations more than two orders of magnitude reduction in beam energy spread, down to single digit milli-electron volts. Overcoming the minimum probe-size limit that chromatic aberration imposes, our design clears a path for high-current, high-resolution electron-beam applications at primary energies from single to hundreds of kilo-electron volts.

As the advances in quantum hardware bring us into the noisy intermediate-scale quantum (NISQ) era, one possible task we can perform without quantum error correction using NISQ machines is the variational quantum eigensolver (VQE) due to its shallow depth. A specific problem that we can tackle is the strongly interacting Fermi-Hubbard model, which is classically intractable and has practical implications in areas like superconductivity. In this paper, we outline the details about the gate sequence, the measurement scheme, and the relevant error-mitigation techniques for the implementation of the Hubbard VQE on a NISQ platform. We perform resource estimation for both silicon spin qubits and superconducting qubits for a 50-qubit simulation, which cannot be solved exactly via classical means, and find similar results. The number of two-qubit gates required is on the order of 20 000. Hence, to suppress the mean circuit-error count to a level such that we can obtain meaningful results with the aid of error mitigation, we need to achieve a two-qubit gate error rate of approximately ${10}^{-4}$. When searching for the ground state, we need a few days for one gradient-descent iteration, which is impractical. This can be reduced to around 10 min if we distribute our task among hundreds of quantum-processing units. Hence, implementing a 50-qubit Hubbard model VQE on a NISQ machine can be on the brink of being feasible in near term, but further optimization of our simulation scheme, improvements in the gate fidelity, improvements in the optimization scheme and advances in the error-mitigation techniques are needed to overcome the remaining obstacles. The scalability of the hardware platform is also essential to overcome the runtime issue via parallelization, which can be done on one single silicon multicore processor or across multiple superconducting processors.

This work concerns the steady-state dynamics of eccentric-rotor harvesters under the excitation of a forced pendulum, selected as a simplified representation of the swinging motion of the human arm during locomotion, to better understand their use for wrist-worn energy-harvesting applications. A linearized model predicts the behavior of nonresonant eccentric-rotor devices well and provides insight into the relationship between the rotor natural frequency and transducer-imposed electrical damping. Approximate analytical solutions are obtained via perturbation methods that show that the eccentric rotor shares many characteristics of a Duffing oscillator with softening spring nonlinearity. Finally, an interesting property of the eccentric rotor’s dynamics, namely, invariance of power output to changes in forcing frequency and amplitude over certain ranges of design parameters, is observed, forming the basis for a proposed resonant eccentric-rotor harvester with a wideband power output response.

We study high-order harmonic generation (HHG) with an annularly shaped high-power, high-repetition-rate laser. We investigate how the spatial separation (or divergence) of the incident laser and the generated high-order harmonics depends on the gas pressure when the laser intensity is in the overdriven regime. It is shown that, when the cone size of the incident annular beam is relatively large, the resulting harmonics are highly collimated along the propagation axis and are spatially separated from the incident beam. Under the overdriven condition, dynamic phase matching favors HHG emissions from short-trajectory electrons that are efficiently built up in the second half of the gas medium. We verify that using an annular beam is a simple and effective method for separating the intense incident infrared laser from the generated high harmonics. Such high-flux and low-divergence light pulses are expected to be powerful sources in the laboratory for ultrafast probing of electron dynamics.

The thermal properties of two-dimensional (2D) materials are of fundamental interest for energy recovery and electron cooling at the nanoscale. Exploring thermal transport in 2D systems is a challenging issue particularly in the substrate-supported architecture, which is the most relevant for applications. In this configuration, heat diffusion is strongly affected by thermal losses through the substrate and highly depends on interfacial properties. Here we investigate thermal transport of supported few-layer graphene nanowires by the Joule self-heating method in the high-temperature range (400 K $<T<$ 500 K). This method is valuable for the substrate-supported sample configuration. By using a thick and rough oxide layer, we find that thermal losses through the substrate are reduced by almost 1 order of magnitude with respect to the expected theoretical values. Most importantly, we unveil an effective reduction of the thermal conductivity, with values as low as 40 W ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$, comparable to reported values for supported graphene nanoribbons. Such a reduction can be induced by increased phonon scattering with the substrate and impurities in the system, which is inherent to the device fabrication. Our findings should stimulate research on alternative solutions for energy conversion at the nanoscale using graphene and other 2D materials when deposited on substrates.

Nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond are among the best-studied quantum-based sensors for a wide range of applications. Here, we demonstrate an optical polarization-based method for full vector magnetometry using ensembles of $\mathrm{N}$-$V$ centers. The anisotropy of the electric dipole moments of the $\mathrm{N}$-$V$ centers enables the assignment of the different magnetic field split transitions in the optically detected magnetic resonance spectra by appropriately setting the optical polarization in the excitation and/or detection of the $\mathrm{N}$-$V$ photoluminescence signal. By further employing the optical-selection rules for the driving microwave field, we show how the full vector of the investigated magnetic field can be restored without the need to apply external bias fields. We discuss different $\mathrm{N}$-$V$-center orientation implementations and limitations toward common approximations used in the $\mathrm{N}$-$V$ magnetometry literature for our approach.

A strong trend for quantum-based technologies and applications follows the avenue of combining different platforms to exploit their complementary technological and functional advantages. Micro and nanomechanical devices are particularly suitable for hybrid integration due to the ease of fabrication at multiscales and their pervasive coupling with electrons and photons. Here, we report on a nanomechanical technological platform where a silicon chip is combined with an aluminum nitride layer. Exploiting the $\mathrm{Al}\mathrm{N}$ piezoelectricity, surface acoustic waves (SAWs) are injected in the $\mathrm{Si}$ layer where the material has been locally patterned and etched to form a suspended nanostring. Characterizing the nanostring vertical displacement induced by the SAW, we find an external excitation peak efficiency in excess of 500 pm/V at 1-GHz mechanical frequency. Exploiting the long-term expertise in silicon photonic and electronic devices as well as the SAW robustness and versatility, our technological platform represents a candidate for hybrid quantum systems.

A multifunctional sensor based on a surface-acoustic-wave (SAW) device is fabricated. It allows independent measurements of temperature and applied magnetic field. Optimization of the multilayered device structure leads to a temperature-coefficient frequency of the Love-wave resonance reduced to zero. The sensitivity to an applied magnetic field is obtained through magnetostriction of a $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ layer. By use of shape anisotropy, the variation of intrinsic magnetic anisotropy with temperature is strongly reduced. On one hand, interrogating the device at the Love-wave-resonance frequency allows us to extract the applied magnetic field independently of the temperature in a [130–370 K] range. On the other hand, the Rayleigh or Leaky waves are less or not sensitive to applied fields but have a high temperature coefficient of frequency. So interrogating the device at the Rayleigh resonance frequency allows us to extract the temperature independently of the magnetic field. In addition, the used resonator geometry offers the possibility for future batteryless and wireless interrogation.

We demonstrate multiphoton interference using a resource-efficient frequency multiplexing scheme, suitable for quantum information applications that demand multiple indistinguishable and pure single photons. In our source, frequency-correlated photon pairs are generated over a wide range of frequencies by pulsed parametric down conversion. Indistinguishable single photons of a predetermined frequency are prepared using frequency-resolved detection of one photon to control an electro-optic frequency shift applied to its partner. Measured photon statistics show multiplexing increases the probability of delivering a single photon, without a corresponding increase to multiphoton events. Interference of consecutive outputs is used to bound the modal purity and demonstrate the nonclassical nature of the emitted light.

High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.

We report a study on the nonlinear optical properties of cobalt- ($\mathrm{Co}$) doped $\mathrm{Zn}\mathrm{O}$ nanorods (NRs) using the single-beam $Z$-scan technique. In the open-aperture measurements, the undoped sample shows two-photon absorption (TPA) characteristics, whereas the doped samples exhibit a saturable absorption (SA) signature. In the closed-aperture measurements, all the samples show a self-defocusing behavior. We correlate the nonlinear absorption coefficient with the exponential reduction of the optical band gap and explain it in terms of electronic-band-structure modification because of doping. Density-functional-theory calculation is also used to understand the observed nonlinear absorption and band-gap variation. The present study can help to understand the origin of SA in $\mathrm{Zn}\mathrm{O}$. Doped $\mathrm{Zn}\mathrm{O}$ nanorods can illustrate high SA and nonlinear refraction by appropriate $\mathrm{Co}$ doping, which can drive the technology for $Q$ switching and mode locking in solid-state and fiber lasers.