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.

We report high-resolution photocurrent (PC) spectroscopy of a single self-assembled $\mathrm{InAs}$/$\mathrm{GaAs}$ quantum dot embedded in an $n$-$i$ Schottky device in an applied vector magnetic field. The PC spectra of positively charged excitons ($X^{+}$) and neutral excitons ($X^0$) are obtained by two-color resonant excitation. With an applied magnetic field in the Voigt geometry, the double-$\mathrm{\Lambda }$ energy-level structure of $X^{+}$ and the dark states of $X^0$ are observed clearly in the PC spectra. In the Faraday geometry, the PC amplitude of $X^{+}$ decreases and is then quenched with increasing magnetic field, which provides a new way to determine the relative sign of the electron and hole $g$-factors. With an applied vector magnetic field, the electron and hole $g$-factor tensors of $X^{+}$ and $X^0$ are obtained. The anisotropy of the hole $g$-factors of both $X^{+}$ and $X^0$ is larger than that of the electron $g$-factors.

Cubic ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ perovskite is considered to be an ideal lead-free and air-stable alternative to traditional $A\mathrm{Pb}{X}_3$ [A = $\mathrm{Cs}$, $\mathrm{K}$, ${\mathrm{CH}}_3{\mathrm{NH}}_3$, $\mathrm{CH}({\mathrm{NH}}_2{)}_2$] perovskites for optoelectronic applications, such as solar cells and photodetectors. Herein, both experiments and theoretical calculations are carried out to study the lattice dynamics and temperature dependences of band gaps. Using Raman spectroscopy, we show that the obtained ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film maintains a long-range crystalline structure of the cubic phase within 77–300 K, which is further confirmed via lattice dynamics calculations based on density functional theory (DFT). An ultrasmall net blueshift of 0.035 eV deduced from its temperature-dependent photoluminescence (PL) spectrum is observed as the temperature increases from 103 to 300 K, making it an ideal material for optoelectronic devices. Notably, this blueshift of the ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film is significantly smaller than that of the DFT-calculated band-gap shift (∼0.085 eV) based on thermal expansion theory. This deviation is attributed to the strong phonon-electron (ph-e) interaction inherently occurring in ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ material, which counterbalances the thermal expansion effects. We further evaluate the magnitude of the ph-e interaction by fitting the full width at half maximum (FWHM) of the PL spectrum. The fitted Huang-Rhys factor (S) of 4.9 is consistent with reduced electronic dimensionality and increased vibrational degree of freedom, indicating stronger ph-e coupling strength in the ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ film compared with regular ${A\mathrm{Pb}X}_3$-type perovskites (S < 3.3). Adopting both thermal expansion and ph-e coupling effects, the ultrasmall band-gap shift is theoretically calculated and is in good agreement with the experimental blueshift of the band gap. In summary, our work illustrates the solid structural stability of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ in both lattice and electronic structures, paving the way for the next generation of optoelectronic devices.

In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena, such as weak localization (WL) and universal conductance fluctuations (UCFs). Here we use multipass Landau-Zener transitions at the avoided crossing of a highly coherent superconducting qubit to emulate these phenomena. The average and standard deviations of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCFs, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to the earlier work by Gustavsson et al. [Phys. Rev. Lett. 110, 016603 (2013)], who successfully emulated UCFs, but did not observe WL. This demonstration illustrates the use of nonadiabatic control to implement quantum emulation with superconducting qubits.

Interferometric spectroscopy with undetected photons (ISUP) utilizing quantum mechanically path-entangled photon pairs has received considerable attention as an optical-measurement platform. Recently, we carried out a proof-of-concept experiment to show that ISUP can be used to measure the transmission spectrum of a Fabry-Perot resonator. Here, we demonstrate that ISUP with dual stimulated parametric down-conversion (StPDC) processes, which allows us to perform infrared rovibrational spectroscopy with visible photons. In our ISUP method, quantum coherence between two independent signal photons from each StPDC crystal is induced by the indistinguishability of conjugate idler fields and results in high visibility of the signal single-photon interferometry at the nondegenerate wavelength. If the seed-beam intensity is imbalanced due to the sample absorption, the corresponding envelope modulation of the interference fringe as a function of the seed-beam frequency reveals the absorption spectrum of the optical sample. As a proof-of-principle experiment, the full rovibrational transmission spectrum at 1550 nm of the hydrogen cyanide (${\mathrm{H}}^{13}{\mathrm{C}}^{14}\mathrm{N}$) molecules in a gas cell is measured from the single-photon interference fringe of the signal fields at 807 nm. We thus anticipate that the single-photon ISUP technique with dual StPDC crystals will find broad use for the development of high-resolution atomic and molecular spectroscopy with undetected photons.

To build a global quantum-communication network, low-transmission, fiber-based communication channels can be supplemented by using a free-space channel between a satellite and a ground station on Earth. We construct a system that generates hyperentangled photonic “ququarts” and measures them to execute multiple quantum-communication protocols of interest. We successfully execute and characterize superdense teleportation, a modified remote-state preparation protocol that transfers more quantum information than standard teleportation, for the same classical information cost, and moreover, is in principle deterministic. Our measurements show an average fidelity of $0.94\pm 0.02$, with a phase resolution of approximately $7^{\circ }$, allowing reliable transmission of $>{10}^5$ distinguishable quantum states. Additionally, we demonstrate the ability to compensate for the Doppler shift, which would otherwise prevent sending time-bin encoded states from a rapidly moving satellite, thus allowing the low-error execution of phase-sensitive protocols during an orbital pass. Finally, we show that the estimated number of received coincidence counts in a realistic implementation is sufficient to enable faithful reconstruction of the received state in a single pass.

Stimulated adiabatic passage, as an efficient energy transfer in correlated systems, was intensively studied in condensed matter physics and quantum optics. Recently, a paradigm shift has been made to bring up quantum-classical analogs, giving rise to the acoustic version of stimulated Raman adiabatic passage and others. In this work, we focus on the adiabatic propagation of sound under topological protection, i.e., topological-pumping-based acoustic adiabatic propagation. We first study the topological pumping of sound in a two-state multicavity chain lattice, including the Su-Schrieffer-Heeger model, in which the acoustic energy is adiabatically transferred between the two topological end states under specific coupling interactions. Then, we consider the adiabatic propagation of sound via a dark mode in a three-state system. The three states correspond to two end states and one interface state in a heterostructured multicavity chain lattice, which is topologically protected. In a finite system, the three states strongly hybridize with the threefold energy degeneracy lifted. The hybridized states reside in the band gap of the systematic Hamiltonian spectrum in the topological pumping process, leading to a strong robustness against nonadiabatic perturbation, which allows for a faster and more efficient topological adiabatic propagation of acoustic waves.

Recent demonstrations of ultracoherent nanomechanical resonators introduce the prospect of developing protocols for solid-state sensing applications. Here, we propose to use two coupled ultracoherent resonator modes on a $\mathrm{Si}$${}_3$$\mathrm{N}$${}_4$ membrane for the detection of small nuclear spin ensembles. To this end, we employ parametric frequency conversion between nondegenerate modes. The nondegenerate modes result from coupled degenerate resonators, and the parametric conversion is mediated by periodic inversions of the nuclear spins in the presence of a magnetic scanning tip. We analyze potential noise sources and derive the achievable signal-to-noise ratio with typical experimental parameter values. Our proposal reconciles the geometric constraints of optomechanical systems with the requirements of scanning force microscopy and brings forth a promising platform for spin-phonon interaction and spin imaging.

Coupled micro and nanomechanical resonators are of significant interest within a number of areas of research, ranging from synchronization, nonlinear dynamics and chaos, to quantum sensing and transduction. Building upon our work on soft-clamped membrane resonators, here we present a study on phononic dimers, consisting of two defects embedded in a phononic crystal membrane. These devices exhibit widely tunable (2–100 kHz) interdefect coupling strengths, leading to delocalized hybrid modes with mechanical $Qf$ products $>{10}^{14}\mathrm{Hz}$ at room temperature, ensuring low thermomechanical force noise. The mode splitting exhibits a strong dependence on the dimer orientation within the crystal lattice, as well as the spatial separation between the two defects. Given the importance of dynamic range for sensing applications, we characterize the relevant mechanical nonlinearities, specifically the self- and cross-Duffing parameters, as well as self- and cross-nonlinear dampings. This work establishes soft-clamped resonators with engineered spatial and spectral multimode structure as a versatile mechanical platform both in the classical and quantum regimes. Applications in microwave-to-optical transduction and magnetic resonance force microscopy are particularly attractive prospects.

We perform an experimental study of dc-current-induced microwave emission in a magnetic tunnel junction (MTJ) consisting of three active magnetic layers. In this trilayer structure, in addition to a conventional bilayer orthogonal MTJ containing a perpendicular free layer and an in-plane fixed layer, a second perpendicular layer is introduced. We find that the microwave emission frequency induced by spin-transfer torque can reach as high as 6 GHz in the absence of any applied magnetic field. Moreover, microwave emission is observed for both current polarities, where a redshift is seen with an increase in the magnitude of the current. We discuss the spin dynamics of the observed bidirectional high-frequency emission and the physical origin of the redshift. The distinct microwave emission properties exhibited by this trilayer MTJ structure could potentially be useful for future applications in nanoscale spintronic devices in fields such as microwave communication and neuromorphic computing.

Acoustic equalization is the process of adjusting the frequency response of a broadband sound signal, which is widely used in communication-system and audio acoustics. Here we introduce the concept of an acoustic meta-equalizer, viz., a passive acoustic metamaterial-based filter that has a tunable frequency response within an ultrabroadband range (200 Hz–20 000 Hz) and is capable of generating a frequency equalization on the input sound signal. By using two kinds of acoustic resonant elements, viz., Helmholtz and Fabry-Perot resonators, we realize and demonstrate ultrabroadband and tunable filters within an integrated meta-structure. In analogy with the conventional equalizer concept in signal processing, we numerically, analytically, and experimentally demonstrate functional filters, signal reproductions, and sound-effect controls by the conceived acoustic meta-equalizer.

The performances of many spintronic devices are governed by the damping constant and magnetic anisotropies of constituent materials. Spin-orbit coupling (SOC) plays a key role and is at the origins of these material parameters. Electric field control of magnetic anisotropy is in high demand for developing energy-efficient nanoscale spintronics devices. Although electric field control of interfacial magnetic anisotropies is well studied and understood, the damping constant, on the other hand, is conventionally controlled by current-induced spin-orbit torque. Here, we use an alternative approach to demonstrate nonlinear control of the damping constant in ultrathin ferromagnetic films by an electric field. We explicitly show that the presence of the Rashba SOC at a ferromagnet-insulator interface and the electric field dependence of the Rashba coefficient may account for the observed nonlinear behavior. Furthermore, we show that engineering of the underlying and oxide material properties, i.e., bulk SOC and Rashba SOC, to tune the spin angular momentum relaxation pathways, can possibly increase the device functionality significantly.

Self-injection locking is a dynamic phenomenon representing stabilization of the emission frequency of an oscillator with a passive cavity enabling frequency-filtered coherent feedback to the oscillator cavity. For instance, self-injection locking of a semiconductor laser to a high-quality-factor (high-$Q$) whispering-gallery-mode microresonator can result in reduction of the laser linewidth by multiple orders of magnitude. The phenomenon was broadly studied in experiments, but there was no detailed theoretical model allowing improvement of the stabilization performance. In this paper we develop such a theory. We introduce five parameters identifying the efficiency of the self-injection locking in an experiment, comprising backscattering efficiency, phase delay between the laser and the high-$Q$ cavities, frequency detuning between the laser and the high-$Q$ cavities, the pump coupling efficiency, and the optical path length between the laser and the microresonator. Our calculations show that the laser linewidth can be reduced by 2 orders of magnitude compared with the case of nonoptimal self-injection locking. We present recommendations for experimental realization of the optimal self-injection-locking regime. The theoretical model provides deeper understanding of self-injection locking and benefits multiple practical applications of self-injection-locked oscillators.

Interference between coherent and dissipative coupling results in nonreciprocity in cavity magnonic devices, in which the isolation ratio can theoretically be infinite when the magnon-mode frequency exactly matches a zero-damping condition (ZDC). In order to obtain a nonreciprocal device possessing both a high isolation ratio and a broad operating bandwidth, we tune the side ZDCs by integrating a movable loop antenna, which can effectively control the radiation damping of the magnon mode. As a consequence, the effective operating bandwidth of the giant nonreciprocity can be broadened to a few hundred times the magnon linewidth. Our study offers a feasible method to design a broadband nonreciprocal device with a high isolation ratio by engineering photon-magnon coupling, which may be of benefit for future quantum and coherent information processing.

We report density-functional calculations that clarify the atomic and electronic structures of the oxygen vacancy $V_{\mathrm{O}}$ in amorphous $({\mathrm{Al}}_2{\mathrm{O}}_3{)}_{1-x}({\mathrm{Si}\mathrm{O}}_2{)}_x$ mixed oxides, which are promising candidates for the gate insulator in $\mathrm{Ga}\mathrm{N}$ technology. We construct microscopic models of the amorphous structure by the melt-quench scheme and then examine all the possible oxygen vacancies and determine the total-energy-minimized $V_{\mathrm{O}}$ structures. We find a clear tendency for $V_{\mathrm{O}}$’s formed at oxygen sites surrounded by fewer $\mathrm{Al}$ atoms to have a lower formation energy. More importantly, we find that a $V_{\mathrm{O}}$ surrounded by $\mathrm{Si}$ atoms alone does not induce deep levels in the energy gap of $\mathrm{Ga}\mathrm{N}$, whereas $V_{\mathrm{O}}$’s surrounded by some $\mathrm{Al}$ atoms induce a deep level. This theoretical finding strongly implies that the majority of $V_{\mathrm{O}}$’s in amorphous $({\mathrm{Al}}_2{\mathrm{O}}_3{)}_{1-x}({\mathrm{Si}\mathrm{O}}_2{)}_x$ are electrically inactive and not very harmful to device operation. Further, we explore the possibilities for structural transformation from the electrically active $V_{\mathrm{O}}$ to the electrically inactive $V_{\mathrm{O}}$. We identify reaction pathways for such transformations and obtain the corresponding energy barriers. The calculated occurrence rate for these transformations is high enough to ensure that thermal annealing at typical temperatures and times causes conversion of electrically active $V_{\mathrm{O}}$’s to inactive $V_{\mathrm{O}}$’s, providing a further advantage of $({\mathrm{Al}}_2{\mathrm{O}}_3{)}_{1-x}({\mathrm{Si}\mathrm{O}}_2{)}_x$ as a gate insulator over other oxides such as ${\mathrm{Si}\mathrm{O}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$.