High-dimensional entanglement promises to increase the information capacity of photons and is now routinely generated, exploiting spatiotemporal degrees of freedom of single photons. A curious feature of these systems is the possibility of certifying entanglement despite strong noise in the data. We show that it is also possible to exploit this noisy high-dimensional entanglement for quantum key distribution by introducing a protocol that uses multiple subspaces of the high-dimensional system simultaneously. Our protocol can be used to establish a secret key even in extremely noisy experimental conditions, where qubit protocols fail. To show that, we analyze the performance of our protocol for noise models that apply to the two most commonly used sources of high-dimensional entanglement: time bins and spatial modes.

Frequency-comb-based optical two-way time and frequency transfer (O-TWTFT) can support future ultra-precise clock networks over free-space links. However, demonstrations have thus far only operated at one corner of a complex parameter space, which balances received optical power, timing performance, and update rate. Here, we analyze the performance of O-TWTFT across this parameter space, with a specific focus on extending the link distance at constant launch power and telescope aperture. We perform experiments across a three-node network spanning 28 km of turbulent air, and successfully demonstrate a more than 2000× reduction in the required received optical power corresponding to a potential 45× increase in distance. This distance increase does come with an associated reduction in timing precision, with the uncertainty increasing from 60 as to 20 fs at 10 s averaging times. However, this level of precision is still more than adequate for most applications. In addition, it comes with a reduction in sampling rate, which potentially limits this approach to static links. Interestingly, because this system optimization does not require any hardware modifications, O-TWTFT links could be dynamically tuned to support future long-distance optical clock networks over a range of conditions and applications.

We present a theoretical study of the collective quasiparticle excitations responsible for the electromagnetic response of ultrathin plane-parallel homogeneous periodic single-wall carbon-nanotube arrays and weakly inhomogeneous single-wall carbon-nanotube films. We show that in addition to varying film composition, the collective response can be controlled by varying the film thickness. For single-type nanotube arrays, the real part of the dielectric response shows a broad negative refraction band near a quantum interband transition of the constituent nanotube, whereby the system behaves as a hyperbolic metamaterial at higher frequencies than those classical plasma oscillations have to offer. By decreasing nanotube diameters it is possible to push this negative refraction into the visible region, and using weakly inhomogeneous multitype nanotube films broadens its bandwidth.

The interactions of a moving crack with self-emitted acoustic waves are studied in ion-implanted circular plates of crystalline silicon, where complex reproducible surface patterns made of local roughness variations are observed. A simple geometrical model, considering the sole propagation of $A_0$ Lamb waves inside the assembly, allows full prediction of all of these pattern shapes and their dependence on system parameters (crack velocity, elastic properties). Acoustic waves propagating along and behind the crack front are shown to play a central role in fracture-pattern formation. When the crack front is curved, surface patterns originate independently of any edge reflection or frequency preselection from acoustic waves propagating along and behind the crack. As in case of Kelvin wake patterns, fracture patterns emerge from geometrically induced coherence.

Space-time (ST) wave packets are propagation-invariant pulsed optical beams whose group velocity can be tuned in free space by tailoring their spatiotemporal spectral structure. To date, efforts on synthesizing ST wave packets have striven to maintain their propagation invariance. Here, we demonstrate that one degree of freedom of a ST wave packet—its on-axis spectrum—can be isolated and purposefully controlled independently of the others. Judicious spatiotemporal spectral amplitude and phase modulation yields ST wave packets with programmable spectral changes along the propagation axis, including red-shifting or blue-shifting spectra, or more sophisticated axial spectral encoding including bidirectional spectral shifts and accelerating spectra. In all cases, the spectral shift can be larger than the initial on-axis bandwidth, while preserving the propagation-invariance of the other degrees of freedom, including the wave packet spatiotemporal profile. These results may be useful in range finding in microscopy or remote sensing via spectral stamping.

In the present work we investigate the fundamental properties of a proton beam’s electromagnetic signal as a candidate for range verification in proton therapy. We compute the electric and magnetic fields from a pulsed proton beam in biological tissues in a cylindrical geometry via a dedicated analytical solution of the Maxwell equations. The tissues are specified through their dielectric response and stopping power, where we consider a straight trajectory with a range that corresponds to protons with an initial energy of 150 MeV. We analyze the characteristics of the pulse and the impact of the pulse shape. The effect of the tissue’s conductivity on the deposited charges is specifically taken into account. In addition, we calculate and analyze the radiative pulse through the Liénard-Wiechert approach. We find that the conductivity charge relaxation has a big influence on the electric field strength, even in the presence of boundaries. It strongly depends on its environment, i.e., the permittivity, conductivity, and boundaries of the surrounding tissues. The electric field that originates from the primary protons vanishes in a time scale of ns. Conversely, the magnetic field is independent of its environment and constant in strength during a rectangular pulse ($10\mu \mathrm{s}$). For $\mu \mathrm{A}$ peak beam currents it lies in the pT range, 10 cm away from the beam axis, and is therefore detectable through, e.g., optical magnetometry. Its spatial profile, however, does not exhibit a distinct feature with respect to the range, but decreases rather smoothly and characteristically along the beam axis. The typical velocity dependence of the magnetic field strength is suppressed, due to the increasing charge density downstream. Finally, the primary protons give rise to an electromagnetic pulse in the radio spectrum (it peaks at approximately 9 GHz). It originates spherically around the range and consists partially of bremsstrahlung and Cherenkov radiation. Yet, a broader comparison to other radiation sources, e.g., secondary electron bremsstrahlung, is required. Based on our findings, we conclude that the magnetic field signal has favorable properties that motivate its detection experimentally for range-verification purposes.

The fields of cavity quantum electrodynamics and magnetism have recently merged into cavity spintronics, investigating a quasiparticle that emerges from the strong coupling between standing electromagnetic waves confined in a microwave cavity resonator and the quanta of spin waves, magnons. This phenomenon is now expected to be employed in a variety of devices for applications ranging from quantum communication to dark matter detection. To be successful, most of these applications require a vast control of the coupling strength, resulting in intensive efforts to understanding coupling by a variety of different approaches. Here, the electromagnetic properties of both resonator and magnetic samples are investigated to provide a comprehensive understanding of the coupling between these two systems. Because the coupling is a consequence of the excitation vector fields, which directly interact with magnetization dynamics, a highly accurate electromagnetic perturbation theory is employed that predicts the resonant hybrid mode frequencies for any field configuration within the cavity resonator. The coupling is shown to be strongly dependent not only on the excitation vector fields and sample’s magnetic properties but also on the sample’s shape. These findings are illustrated by applying the theoretical framework to two distinct experiments: a magnetic sphere placed in a three-dimensional resonator and a rectangular, magnetic prism placed in a two-dimensional resonator. The theory provides comprehensive understanding of the overall behavior of strongly coupled systems and it can be easily modified for a variety of other systems.

Developing compact, low-dissipation, cryogenic-compatible microwave electronics is essential for scaling up low-temperature quantum computing systems. In this paper, we demonstrate an ultracompact microwave directional forward coupler based on high-impedance slow-wave superconducting-nanowire transmission lines. The coupling section of the fabricated device has a footprint of $416\mu {\mathrm{m}}^2$. At 4.753 GHz, the input signal couples equally to the through port and forward-coupling port (50:50) at $-6.7\mathrm{dB}$ with $-13.5\mathrm{dB}$ isolation. The coupling ratio can be controlled with dc bias current or temperature by exploiting the dependence of the kinetic inductance on these quantities. The material and fabrication processes are suitable for direct integration with superconducting circuits, providing a practical solution to the signal distribution bottlenecks in developing large-scale quantum computers.

Spin-orbit-torque (SOT) devices are promising candidates for the future magnetic memory landscape, as they promise high endurance, low read disturbance, and low read error, in comparison with spin-transfer torque devices. However, SOT memories are area intensive due to the requirement for two access transistors per bit. Here, we report a multibit SOT cell that has a single write channel shared among multiple bits, which enables an area-efficient memory design by reducing the number of access transistors. All combinations of digital information can be written in the multibit devices with a single current pulse. This functionality is facilitated by the electric field modulation of SOT polarity by tuning the heavy metal-ferromagnet interfacial oxidation state. Centered on the multibit devices, a shared-write-channel (SWC) memory design provides double the device density of current SOT magnetic random-access memory (MRAM). This improvement makes SOT MRAM appealing for its adoption over a wide range of memory hierarchies.

This paper presents the comprehensive performance analysis of thermionic power generation when the interelectrode vacuum gap shrinks to the submicron range. Although reducing the vacuum gap has been suggested as an effective approach to mitigate space-charge accumulation in thermionic-energy-conversion (TEC) devices, previous theoretical works have predicted the optimal gap distance in the single-digit micrometer range. However, we demonstrate that nanoscale charge and thermal interactions between thermionic electrodes, such as Schottky barrier lowering due to image charge perturbation and near-field enhanced radiative heat transfer, significantly affects the TEC performance within the submicron vacuum gap. Carefully conducted energy-balance analysis reveals that submicron-gap TEC at $d\approx 700$ nm can produce an approximately fourfold increase in power output with a higher energy conversion efficiency than micron-gap TEC under the same operating condition. In addition, significant thermionic and near-field radiative heating of the collector in the submicron-gap TEC system can be beneficially used to further enhance the power output and efficiency by combining with a bottom-cycle heat engine. We believe that the present work provides a theoretical framework for submicron-gap thermionic power generation as a promising energy recycling scheme for high-quality heat sources.

Ultralow-dissipation mechanical systems play an important role in metrology and exploring macroscopic quantum phenomena. Here we report a diamagnetic levitated micromechanical oscillator operating at 3 K with measured dissipation down to $0.59\mu \mathrm{Hz}$ and a quality factor up to $2\times {10}^7$. This mechanical system achieves the lowest dissipation among the state-of-the-art microscale and nanoscale mechanical systems reported to date, with orders-of-magnitude reduction over other systems based on different principles. The cryogenic diamagnetic levitated oscillator is applicable on a wide range of mass, making it a good candidate system for ultrasensitive measurements of both force and acceleration. By virtue of the strong magnetic gradient, this system is potentially capable of studying quantum spin mechanics.

Atmospheric free space and fiber have been widely exploited as the channels for quantum communication, and have enabled intercontinent and intercity applications. An air-sea free-space channel, being capable of linking the satellite-based quantum resource and underwater vehicle, has now become the last piece of the puzzle in building a global quantum communication network. Here, we present an experimental demonstration of air-water decoy-state quantum key distribution against high loss, meanwhile keeping a low quantum bit error rate less than 2.5% for different distances. We are able to reach a long-distance quantum key distribution in water channel that can enable real-life air-sea quantum-communication tasks. The demonstrated distance, even in coastal water of Jerlov type III, is up to 30 m, about one-order improvement over the proof-in-principle demonstrations in the previous experiments, and the channel loss is equivalent to 345-m-long clean seawater of Jerlov type I, representing a key step forward to practical air-sea secure communication.

Spin-orbit torque can drive electrical switching of magnetic layers. Here, we report that, at least for micrometer-sized samples, there is no simple correlation between the efficiency of dampinglike spin-orbit torque $({\xi }_{\mathrm{DL}}^j)$ and the critical switching current density of perpendicularly magnetized spin-current generator-ferromagnet heterostructures. We find that the values of ${\xi }_{\mathrm{DL}}^j$ based on switching current densities can either under- or overestimate ${\xi }_{\mathrm{DL}}^j$ by up to 10 times in a domain-wall depinning analysis, while, in the macrospin analysis based on the switching current density, ${\xi }_{\mathrm{DL}}^j$ can be overestimated by up to 1000 times. When comparing the relative strengths of ${\xi }_{\mathrm{DL}}^j$ of spin-current generators, the critical switching current densities by themselves are a poor predictor.

We introduce a theoretical concept of logic gates for sequenced data that contain a large number of data bits to be computed using a nonlinear nanomechanical resonator. Resonantly driven nanomechanical modes display the bistability of forced vibrations. Near the cusp point on the bifurcation curve, even a small change in driving force can lead to switching between the vibrational states with significantly different amplitudes. This enables a highly sensitive implementation of logic gates, including and, or, and xor gates. The underlying principles are simple and generic, suggesting that our method is applicable not only to nanomechanical systems but also to a wide variety of bistable physical, chemical, and biological systems.

The control of spin-dependent properties by voltage, not involving magnetization switching, has significant advantages for low-power spintronics. Here, we predict that the interfacial crystal Hall effect (ICHE) can serve this purpose. We show that the ICHE can occur in heterostructures composed of compensated antiferromagnetic metals and nonmagnetic insulators due to reduced symmetry at the interface, and it can be made reversible if the antiferromagnet is layered symmetrically between two identical ferroelectric layers. We explicitly demonstrate this phenomenon using density-functional-theory calculations for three material systems: ${\mathrm{Mn}\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{Ge}\mathrm{I}}_2$ and topological ${\mathrm{In}}_2{\mathrm{Te}}_3/{\mathrm{Mn}\mathrm{Bi}}_2{\mathrm{Te}}_4/{\mathrm{In}}_2{\mathrm{Te}}_3$ van der Waals heterostructures and a $\mathrm{Ge}\mathrm{Te}/{\mathrm{Ru}}_2\mathrm{Mn}\mathrm{Ge}/\mathrm{Ge}\mathrm{Te}$ heterostructure composed of three-dimensional materials. We show that all three systems reveal a sizable ICHE, while the last two exhibit a quantum ICHE and ICHE, respectively, that are reversible with ferroelectric polarization. Our proposal provides an alternative direction for voltage-controlled spintronics and offers as-yet unexplored possibilities for functional devices by heterostructure design.

We study the phase-controlled transmission properties in a compound system consisting of a three-dimensional copper cavity and an yttrium-iron-garnet (YIG) sphere. By tuning the relative phase of the magnon pumping and cavity-probe tones, constructive and destructive interferences occur periodically, which strongly modify both the cavity-field transmission spectra and the group delay of light. Moreover, the tunable amplitude ratio between pump-probe tones allows us to further improve the signal absorption or amplification, accompanied by either significantly enhanced optical advance or delay. Both the phase and amplitude ratio can be used to realize in situ tunable and switchable fast-slow light. The tunable phase and amplitude ratio lead to the zero reflection of the transmitted light and an abrupt fast-slow light transition. Our results confirm that direct magnon pumping through the coupling loops provides a versatile route to achieve controllable signal transmission, storage, and communication, which can be further expanded to the quantum regime, realizing coherent-state processing or quantum-limited precise measurements.

The present work proposes a $\mathrm{Ga}\mathrm{As}$-nanowire-based vertical metal-oxide-semiconductor (MOS) solar cell of quantum scale to achieve very high efficiency beyond the Shockley-Queisser (SQ) limit. Photogeneration and carrier transport in such devices are analytically modeled by adopting nonequilibrium Green’s function formalism based on second quantization field operators for the incident photons and generated photocarriers. The study suggests that the utilization of photogenerated light and heavy holes to harvest solar energy is capable of providing significantly higher power conversion efficiency above the SQ limit. Such superior efficiency is achieved due to the resonance of incident photon modes with the energy gap between three-dimensional-quantized electron states and two-dimensional-quantized hole subbands. The power conversion efficiency, along with other relevant solar-cell-performance parameters, such as open-circuit voltage, short-circuit current, fill factor, external quantum efficiency, and responsivity, is observed to depend significantly on the nanowire diameter and top-oxide thickness, which, in turn, controls the quantization effect in such MOS devices. The results show that the power conversion efficiency of 50% and above can be achieved in the present tosylate-modified poly(3,4-ethylenedioxythiophene) (PEDOT-Tos)/${\mathrm{Si}\mathrm{O}}_2/\mathrm{Ga}\mathrm{As}$-nanowire MOS solar cell for a combination of nanowire diameter and oxide thickness in the range of 18–14 nm and 4–2 nm, respectively. Thus, the proposed device scheme offers an alternative design route for next-generation solar cells with superior efficiency by engineering the quantization effect.

The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling-gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of such a fast entangling-gate operation presents a big technical challenge, as such the required laser source is not available off the shelf. For this, we engineer an ultrafast entangling-gate source based on a frequency comb. The source generates bursts of several hundred mode-locked pulses with pulse energy approximately 800 pJ at 5-GHz repetition rate at 393.3 nm and complies with all requirements for implementing a fast two-qubit gate operation. Using a single, chirped ultraviolet pulse, we demonstrate a rapid adiabatic passage in a ${\mathrm{Ca}}^{+}$ ion. To verify the applicability and projected performance of the laser system for inducing entangling gates we run simulations based on our source parameters. The gate time can be faster than a trap period with an error approaching ${10}^{-4}$.