The Hong-Ou-Mandel interference between independent photons plays a pivotal role in large-scale quantum networks involving distant nodes. Photons need to work in a pure state for indistinguishability to reach high-quality interference. Also, they need to have a sufficiently long coherence time to reduce the time-synchronization requirements in practical applications. In this paper, we discuss a scheme for generating a pure-state photon-pair source with a long coherence time in periodically poled potassium titanyl phosphate crystals. By selecting the appropriate pump laser and filter, we can simultaneously eliminate the frequency correlation of the parametric photons while achieving a long coherence time. We experimentally develop this pure-state photon-pair source of 780 nm on potassium titanyl phosphate crystals pumped by a 390-nm pulsed laser. The source provides a coherence time of tens of picoseconds, and it is shown to have the potential to be applied in long-distance quantum interference. Furthermore, we experimentally demonstrate the Hong-Ou-Mandel interference between two photon sources with visibility exceeding the classical limit.

Hybrid quantum systems based on magnetic platforms have witnessed the birth and fast development of quantum magnonics. Until now, most studies have relied on magnetic excitations in low-damping magnetic insulators, particularly yttrium iron garnet, while a large class of magnetic systems is ruled out in this interdisciplinary field. Here we propose the generation of a magnon bundle in a hybrid magnet-qubit system, where two or more magnons with the same energy and angular momentum are emitted simultaneously. By tuning the driving frequency of the qubit to match the detuning between magnon and qubit mode, one can effectively generate a magnon bundle via super-Rabi oscillations. In contrast with general wisdom, magnetic dissipation plays an enabling role in generating the magnon bundle, where the relaxation time of magnons determines the typical time delay between two successive magnons. The maximal damping that allows an antibunched magnon bundle can reach the order of 0.1, which may break the monopoly of low-dissipation magnetic insulators in quantum magnonics and enables a large class of magnetic materials for their applications in quantum manipulation. Further, our findings may provide a scalable and generic platform to study multimagnon physics and benefit the design of magnonic networks for quantum information processing.

Multistability has been so far exploited to modulate stiffness in mechanical metamaterials under either tension or compression. In this work, we unveil a multistable topological change with emerging zero modes delivering an outstanding reprogrammability of flexural stiffness. Theoretical analysis, experiments, and computations elucidate how the topological change transforms the initially rigid metamaterial into an extremely soft state. Blurring the boundary between structures and mechanisms, this work opens up an avenue for designing reprogrammable matter under flexure.

The implementation of a universal quantum processor still poses fundamental issues related to error mitigation and correction, which demand investigation of also platforms and computing schemes alternative to the main stream. A possibility is offered by employing multilevel logical units (qudits), naturally provided by molecular spins. Here we present the blueprint of a molecular spin quantum processor consisting of single molecular nanomagnets, acting as qudits, placed within superconducting resonators adapted to the size and interactions of these molecules to achieve a strong single spin-to-photon coupling. We show how to implement a universal set of gates in such a platform and to readout the final qudit state. Single-qudit unitaries (potentially embedding multiple qubits) are implemented by fast classical drives, while an alternative scheme is introduced to obtain two-qubit gates via resonant photon exchange. The latter is compared to the dispersive approach, finding in general a significant improvement. The performance of the platform is assessed by realistic numerical simulations of gate sequences, such as Deutsch-Josza and quantum simulation algorithms. The very good results demonstrate the feasibility of the molecular route towards a universal quantum processor.

Quantum sensing of low-frequency magnetic fields using nitrogen-vacancy– ($\mathrm{N}$-$V$) center ensembles has been demonstrated in multiple experiments with sensitivities as low as approximately $1\mathrm{pT}/\sqrt{\mathrm{Hz}}$. To date, however, demonstrations of high-frequency magnetometry in the gigahertz regime with $\mathrm{N}$-$V$ diamond are orders of magnitude less sensitive, above the $\mathrm{nT}/\sqrt{\mathrm{Hz}}$ level. Here, we adapt, for microwave frequencies, techniques that have enabled high-performance low-frequency quantum sensors. Using a custom-grown $\mathrm{N}$-$V$–enriched diamond combined with a noise-cancellation scheme designed for high-frequency sensing, we demonstrate a Rabi-sequence-based magnetometer able to detect microwave fields near 2.87 GHz with a record sensitivity of $3.4\mathrm{pT}/\sqrt{\mathrm{Hz}}$. We demonstrate both amplitude and phase sensing and project tunability over a 300-MHz frequency range. This result increases the viability of $\mathrm{N}$-$V$ ensembles to serve as microwave circuitry imagers and near-field probes of antennas.

Neutral shallow donors ($D^0$) in $\mathrm{Zn}\mathrm{O}$ have emerged as a promising candidate for solid-state spin qubits. Here we report on the formation of $D^0$ in $\mathrm{Zn}\mathrm{O}$ via implantation of $\mathrm{In}$ and subsequent annealing. The implanted $\mathrm{In}$ donors exhibit optical and spin properties on par with those of in situ–doped donors. The inhomogeneous linewidth of the donor-bound-exciton transition is less than 10 GHz, comparable to the optical linewidth of in situ $\mathrm{In}$. Longitudinal spin relaxation times ($T_1$) exceed reported values for in situ $\mathrm{Ga}$ donors, indicating that residual $\mathrm{In}$-implantation damage does not degrade $T_1$. Two-laser Raman spectroscopy of the donor spin reveals the hyperfine interaction of the donor electron with the spin-9/2 $\mathrm{In}$ nuclei. This work is an important step toward the deterministic formation of $\mathrm{In}$-donor qubits in $\mathrm{Zn}\mathrm{O}$ with optical access to a long-lived nuclear-spin memory.

Gate-defined quantum dots (QDs) have appealing attributes as a quantum computing platform. However, near-term devices possess a range of possible imperfections that need to be accounted for during the tuning and operation of QD devices. One such problem is the capacitive crosstalk between the metallic gates that define and control QD qubits. A way to compensate for the capacitive crosstalk and enable targeted control of specific QDs independent of coupling is by the use of virtual gates. Here, we demonstrate a reliable automated capacitive coupling identification method that combines machine learning with traditional fitting to take advantage of the desirable properties of each. We also show how the cross-capacitance measurement may be used for the identification of spurious QDs sometimes formed during tuning experimental devices. Our systems can autonomously flag devices with spurious dots near the operating regime, which is crucial information for reliable tuning to a regime suitable for qubit operations.

In practical applications of inkjet printing the nozzles in a printhead have intermittent idle periods, during which ink can evaporate from the nozzle exit. Inks are usually multicomponent where each component has its own characteristic evaporation rate resulting in concentration gradients within the ink. These gradients may directly and indirectly (via Marangoni flows) alter the jetting process and thereby its reproducibility and the resulting print quality. In the present work we study selective evaporation from an inkjet nozzle for water-glycerol mixtures. Through experiments, analytical modeling, and numerical simulations, we investigate changes in mixture composition with drying time. By monitoring the acoustics within the printhead, and subsequently modeling the system as a mass-spring-damper system, the composition of the mixture can be obtained as a function of drying time. The results from the analytical model are validated using numerical simulations of the full fluid mechanical equations governing the printhead flows and pressure fields. Furthermore, the numerical simulations reveal that the time-independent concentration gradient we observe in the experiments is due to the steady state of water flux through the printhead. Finally, we measure the number of drop formation events required in this system before the mixture concentration within the nozzle attains the initial (predrying) value, and find a stronger than exponential trend in the number of drop formations required. These results shed light on the complex physiochemical hydrodynamics associated with the drying of ink at a printhead nozzle, and help in increasing the stability and reproducibility of inkjet printing.

Metal thio(seleno)phosphates are a large family of homologous layered compounds hosting a rich variety of physical properties and potential applications. Besides the extensively explored optoelectronics, ferroelectric, and magnetic orders, mobile ions recently arise as an additional degree of freedom for creating emergent functionalities. Herein, we report the active manipulation of the electronic transport characteristics in a quaternary selenophosphate, ${\mathrm{Ag}\mathrm{Bi}\mathrm{P}}_2{\mathrm{Se}}_6$. By electrically driven accumulation or removal of mobile ${\mathrm{Ag}}^{+}$ ions at the metal-semiconductor interface, the Schottky barrier can be effectively modulated to produce switchable diodelike behavior with tunable rectification ratios as explicitly revealed by correlated elemental, spatial photocurrent, and surface potential microscopies. Furthermore, the superior photoelectric response of ${\mathrm{Ag}\mathrm{Bi}\mathrm{P}}_2{\mathrm{Se}}_6$ is demonstrated and can be incorporated to turn the induced rectifying transport off and on. The combined optoelectronic and ionotronic activities greatly expand the versatility of ${\mathrm{Ag}\mathrm{Bi}\mathrm{P}}_2{\mathrm{Se}}_6$ and related selenophosphates in next-generation electronic applications.

Nonradiative recombination in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ ($\mathrm{CIGS}$) solar cells has long been attributed to the antisite defects $M_{\mathrm{Cu}}$ (M = $\mathrm{In}$, $\mathrm{Ga}$), but the underlying mechanism is still elusive. Using rigorous first-principles calculations, we demonstrate that the antisites themselves cannot capture holes and hence cannot lead to efficient carrier recombination. Instead, internal conversion in the neutral charge state to the distorted DX center configuration opens an efficient hole-capture pathway. After hole capture, the positive charge state returns to the normal antisite configuration without any barrier to complete the entire recombination cycle. Our results show that the DX center is thermally accessible in ${\mathrm{Cu}\mathrm{Ga}\mathrm{Se}}_2$, but not in ${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_2$, due to its rather low conduction-band minimum; thus, the recombination rate in $\mathrm{CIGS}$ is composition dependent. These insights clarify the nonradiative recombination mechanism in ternary chalcopyrites and provide a guideline for composition engineering to enable the optimal performance of $\mathrm{CIGS}$ solar cells.

Lithium niobate (${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$, LN) devices play critical roles in quantum information processing. However, for special applications like quantum key distribution (QKD), the characteristics of materials and devices and their impact on practical systems must be intensively explored. For the first time, we reveal that the photorefractive effect in LN can be utilized as a potential loophole to carry out malicious attacks by the eavesdroppers. We take a commercial LN-based variable optical attenuator as an example to demonstrate the method we name the induced-photorefraction attack (IPA) and propose two techniques to enable controllable attacks. Our results show that eavesdroppers can fulfill an efficient source-side attack by injecting an optimized irradiation beam with only several nanowatts, which is realistic when accessing commercial fiber channels. These measures and techniques can be employed for all individual and on-chip LN devices. Our work opens a new window for the security research of real-life QKD and contributes to designing QKD systems with higher security.

Magnetically levitated superconducting microparticles offer a promising path to quantum experiments with picogram to microgram objects. In this work, we levitate a 700 $\mathrm{ng}\sim {10}^{17}\mathrm{amu}$ superconducting microsphere in a magnetic chip trap in which detection is integrated. We measure the center-of-mass motion of the particle using a dc superconducting quantum interference device magnetometer. The trap frequencies are continuously tunable between 30 and 160 Hz and the particle remains stably trapped over days in a dilution-refrigerator environment. We characterize the motional-amplitude-dependent frequency shifts, which arise from trap anharmonicities, namely, Duffing nonlinearities and mode couplings. We explain this nonlinear behavior using finite-element modeling of the chip-based trap potential. This work may constitute a first step toward quantum experiments and ultrasensitive inertial sensors with magnetically levitated superconducting microparticles.

We introduce a hybrid plasmonic-photonic device for on-chip all-optical switching and reading of ferrimagnet bits with perpendicular magnetic anisotropy in a racetrack spintronic memory, coupled onto an indium phosphide waveguide. The device comprises V-shaped gold plasmonic nanoantennas coupled with a photonic crystal cavity, which enables switching and reading of the magnetic state of nanoscale bits by enhancing the absorbed energy density and polar magneto-optical Kerr effect locally. Using a finite-difference time-domain method, we show that our device can switch and read targeted bits down to 100 nm in the presence of oppositely magnetized background regions in the racetrack with widths up to 120 nm, clearly outperforming a bare photonic waveguide. Our hybrid device provides the missing link between integrated photonics and nanoscale spintronics by tackling the challenges of nonlinear absorption in the waveguide, weak magneto-optics, and size mismatch, leading to the development of ultrafast and energy-efficient advanced on-chip applications.

Reversible logic can provide lower switching-energy costs relative to all irreversible logic, including those developed by industry in semiconductor circuits; however, more research is needed to understand what is possible. Superconducting logic, an exemplary platform for both irreversible and reversible logic, uses flux quanta to represent bits and the reversible implementation may switch state with low energy dissipation relative to the energy of a flux quantum. Here, we simulate reversible shift register gates that are ballistic: their operation is powered by the input bits alone. A storage loop is added relative to previous gates as a key innovation, which bestows an asynchronous property to the gate such that input bits can arrive at different times as long as their order is clearly preserved. The shift register represents bit states by flux polarity, both in the stored bit as well as the ballistic input and output bits. Its operation consists of the elastic swapping of flux between the stored and the moving bit. This is related to a famous irreversible shift register, developed prior to the advent of superconducting flux quanta logic (which used irreversible gates). In the base design of our ballistic shift register (BSR), there is one input port and one output port but we find that we can make other asynchronous ballistic gates by extension. For example, we show that two BSRs operate in sequence without power added, giving a 2-bit sequential memory. We also show that a BSR with two input and two output ports allows a bit state to be set from one input port and then conveyed out on either output port. The gate constitutes the first asynchronous reversible 2-input gate. Finally, for a better insight into the dynamics, we introduce a collective-coordinate model. We find that the gate can be described as motion in two coordinates subject to a potential determined by the input bit and initial stored flux quantum. Aside from the favorable asynchronous feature, the gate is considered practical in the context of energy efficiency, parameter margins, logical depth, and speed.

Longer-range interactions, or couplings beyond the second nearest neighbors, have long been overlooked in topological models for either topological insulators or semimetals. For natural (electronic) materials, such ignorance is reasonable as near-range couplings are always dominant. However, artificially constructed metamaterials can break such a limitation, allowing the longer-range couplings comparable with the nearer ones, achieving alternative properties unattainable in natural materials. Here, we report a one-dimensional acoustic metamaterial designed in terms of an extended Su-Schrieffer-Heeger (SSH) model, containing the nearest, and in particular the third-nearest couplings. In contrast to the conventional SSH model with the topological phases of the winding number one, the extended SSH model can host topological phases of winding number two, besides one, exhibiting amazing rotonlike dispersions. The larger winding number implies more end states emerging. All these predictions are confirmed by the experiments with the fabricated acoustic metamaterials. Such longer-range couplings can also be extended to two- or three-dimensional lattices, to further achieve the topological states exceptionally in metamaterials.

We demonstrate a flip-chip-based approach to microwave measurements of Andreev bound states (ABSs) in a gate-tunable planar Josephson junction (JJ) using inductively coupled superconducting low-loss resonators. By means of electrostatic gating, we present control of both the density and transmission of ABSs. Phase biasing of the device shifted the resonator frequency, consistent with the modulation of supercurrent in the junction. Two-tone spectroscopy measurements revealed an isolated ABS consistent with an average induced superconducting gap of $184\text{μ}\mathrm{eV}$ and a gate-tunable transmission approaching $0.98$. Our results represent the feasibility of using the flip-chip technique to address and study ABSs in planar JJs, and they constitute a promising path towards microwave applications with superconductor-semiconductor two-dimensional materials.

Sensing with undetected photons enables the measurement of absorption and phase shifts at wavelengths different from those detected. Here, we experimentally map the balance and loss parameter space in a nondegenerate nonlinear interferometer, showing the recovery of sensitivity despite internal losses at the detection wavelength. We further explore an interaction-free operation mode with a detector-to-sample incident optical power ratio of over 200. This allows changes in attowatt levels of power at $3.4\text{μ}\text{m}$ wavelength to be detected at 1550 nm, immune to the level of thermal black-body background. This reveals an ultrasensitive infrared imaging methodology capable of probing samples effectively “in the dark.”

The dynamics of charge carriers in disordered organic semiconductors is inherently difficult to probe by spectroscopic methods. Thermally stimulated luminescence (TSL) is an approach that detects the luminescence resulting from the recombination of spatially-well-separated geminate charge pairs, usually at low temperature. In this way, the density of states (DOS) for charges can be determined. In this study, we demonstrate that TSL can also be used for probing an occupied density of states formed by a low-temperature energetic relaxation of photogenerated charges. Another approach used to gain an insight into the charge-relaxation process is kinetic Monte Carlo (KMC) simulations. Here, we use both techniques to determine the energetic distribution of charges at low temperatures. We find that the charge dynamics is frustrated, yet this frustration can be overcome in TSL by using an infrared (IR) push pulse, and in KMC simulations by a long simulation time that allows for long-range tunneling. Applying the IR-push TSL to pristine amorphous films of 18 commonly used low-molecular-weight organic light-emitting diode materials, we find that the width of the occupied DOS amounts to about 2/3 of the available DOS. The same result is obtained in KMC simulations that consider spatial correlations between site energies. Without the explicit consideration of energetic correlations, the experimental values cannot be reproduced, which testifies to the importance of spatial correlations for charges.

Superconducting nanowire single-photon detectors (SNSPDs) are the highest-performing photon-counting technology in the near-infrared region. Because of delay-line effects, large-area SNSPDs typically trade off timing resolution and detection efficiency. This unavoidable fundamental constraint might limit their future deployment in demanding scientific applications. Here we introduce a detector design based on transmission-line engineering and differential readout for device-level signal conditioning, enabling a high system detection efficiency and a low detector jitter simultaneously. To make our differential detectors compatible with single-ended time taggers, we also engineer analog differential-to-single-ended readout electronics, with minimal impact on the system timing resolution. Our best niobium nitride differential SNSPD achieves a system detection efficiency of $(83.3\pm 4.3)\mathrm{\%}$ at $1550\mathrm{nm}$ and $(78\pm 5)\mathrm{\%}$ at $775\mathrm{nm}$. The lowest system jitter is $13.0\pm 0.4\mathrm{ps}$ at $1550\mathrm{nm}$ and $9.7\pm 0.4\mathrm{ps}$ at $775\mathrm{nm}$, limited by intrinsic contributions. These detectors also achieve sub-100-ps timing response at 1/100 of the maximum level, $30.7\pm 0.4\mathrm{ps}$ at 775 nm and $47.6\pm 0.4\mathrm{ps}$ at $1550\mathrm{nm}$, enabling time-correlated single-photon counting with high-dynamic-range response functions. Furthermore, because of the differential impedance-matched design, our detectors exhibit delay-line imaging capabilities and photon-number resolution. The properties and high-performance metrics achieved by our system make it a versatile photon-detection solution for quantum computing, quantum communication, and many other scientific applications.

Waveguiding in general and acoustic waveguiding in particular are possible under the condition of a transverse “discontinuity” or modulation in the refractive index. We propose a radically different approach that relies on imposing spinning on a column of air, leading to highly modified acoustic refractive indices for specific azimuthal modes. This result may be leveraged to realize not only the airborne acoustic counterpart of an optical fiber, i.e., an acoustic spinning fiber, but also a nonreciprocal unidirectional waveguiding mechanism, reminiscent of an “acoustic Zeeman effect.” The concept is demonstrated in the realm of acoustics, but may be applicable to other wave systems, e.g., in elastodynamics.