Since the discovery of the Weyl fermion in the material $\mathrm{Ta}\mathrm{As}$, searching for new types of (quasi)particles in the universe of condensed matter and materials, beyond those captured in the real universe, has aroused tremendous interest. More recently, an exotic excitation, of linear dispersion and up to sixfold degeneracy, has been predicted theoretically in a three-dimensional material, which harbors hinge arc states on the boundary thus manifesting a nontrivial higher-order topology. The sixfold excitations, with the maximal degeneracy or degree of freedom of bosons, may open up an opportunity for exploring the extraordinary properties of topological matter such as three-dimensional robust one-way transport. However, such states have not been explored experimentally in any classical systems. In this work, we report the experimental realization of the sixfold excitation in a three-dimensional phononic crystal. We reveal that the linear sixfold excitation can be decomposed as two overlapping threefold spin-1 Weyl points with opposite topological charges ±2. We further demonstrate the hinge arc states associated with the excitation, which are featured with the longest hinge arc, revealing the higher-order topological nature of the material.

We observe that under illumination and at cryogenic temperatures, plateaus emerge in the current-voltage characteristic of non-photocurrent-matched interband cascade infrared photodetectors (ICIPs) with identical thickness of individual superlattice absorbers. We attribute these plateaus to the formation of internal electric potentials on the order of multiple millivolts originating from a gradually decreasing photocurrent in optically deeper stages caused by light attenuation. At elevated temperatures, the plateaus vanish once the dark current exceeds the maximum photocurrent. Contrarily, in a photocurrent-matched ICIP architecture, with individual absorber thicknesses increasing towards optically deeper stages, no additional internal potentials are formed under illumination. The plateaus are found to show a strong wavelength and bias dependence related to the dispersion of the absorption coefficient of the type-II-$\mathrm{In}\mathrm{As}/\mathrm{Ga}\mathrm{Sb}$-superlattice absorbers used. This could potentially be implemented in a two-color-detector approach that allows separation of different spectral bands by biasing of the ICIP.

Integrated microresonator facilitates the realization of quantum frequency comb (QFC), which provides a large number of discrete frequency modes with broadband spectral range and narrow linewidth. However, all previous demonstrations have focused on the generation of energy-time or time-bin entangled photons from QFC. Realizing polarization-entangled quantum frequency comb, which is a useful resource for the fundamental study of quantum mechanics and quantum information applications, remains challenging. Here, we demonstrate a broadband polarization-entangled quantum frequency comb by combining an integrated silicon-nitride microresonator with a Sagnac interferometer. With a free spectral range of about 99 GHz and a narrow linewidth of about 190 MHz, our source provides 22 polarization-entangled photon pairs with frequency covering the whole telecom C-band. The entanglement fidelities for all 22 pairs are above 81%, including 17 pairs with fidelities higher than 90%. Our demonstration paves the way for employing the polarization-entangled quantum frequency comb in quantum network using CMOS technology as well as standard dense wavelength division-multiplexing technology.

Decoy-state methods are essential to perform quantum key distribution (QKD) at large distances in the absence of single-photon sources. However, the standard techniques apply only if laser pulses are used that are independent and identically distributed. Moreover, they require that the laser pulses are fully phase randomized. However, realistic high-speed QKD setups do not meet these stringent requirements. In this work, we generalize decoy-state analysis to accommodate laser sources that emit imperfectly phase-randomized states. We also develop theoretical tools to prove the security of protocols with lasers that emit pulses that are independent, but not identically distributed. These tools can be used with recent work [G. Currás-Lorenzo, S. Nahar, N. Lütkenhaus, K. Tamaki, and M. Curty, Quantum Sci. Technol. (2023)] to prove the security of laser sources with correlated phase distributions as well. We quantitatively demonstrate the effect of imperfect phase randomization on key rates by computing the key rates for a simple implementation of the three-state protocol.

We introduce a semianalytical optimization method that can be applied to find the optimal magnetization for any permanent-magnet system regarding the desired field or objective. Our approach is based on the Landau-Lifshitz equation, which is normally employed to simulate micromagnetic systems. Instead, we recognize that the same mathematical formalism is also applicable to the optimization of macroscopic magnetic assemblies. This point of view has the advantage of being easy to intuitively visualize and thus is also easy to implement. Our approach can also be seen as a special case of the adjoint method, which is a gradient-based constrained optimization method. However, most studies applying the adjoint method to magnet design adopt the point of topology optimization, where the magnetic permeability tensor field depends on the design variables, making the problem highly nonlinear. Instead, we restrict to the less general problem of optimizing the direction of the magnetization in any point of a given design region. Our strategy leads to a more robust and computationally efficient algorithm since the underlying problem is intrinsically simpler. Additionally, since the integration of the underlying magnetostatic equations is performed numerically, the method is still very flexible and versatile. Previously, an equivalent method has been considered only for solving the magnetic inverse problem. In the present work, we fully demonstrate its effectiveness by presenting several prototypical optimization problems and we discuss the strengths and limitations of the approach in relation to the existing optimization methods.

Rydberg atoms have shown remarkable performance in sensing microwave fields. The sensitivity of such an electrometer based on optical readout of the atomic ensemble has been demonstrated to approach the photon-shot-noise limit. However, the sensitivity cannot be promoted infinitely by increasing the power of the probe light due to the increased collision rates and power broadening. Compared with classical light, the use of quantum light may lead to a better sensitivity with a lower number of photons. In this paper, we exploit entanglement in a microwave-dressed Rydberg electrometer to suppress the fluctuation of noise. The results show a sensitivity enhancement that beats the shot-noise limit in both cold- and hot-atom schemes. Through optimizing the transmission of the optical readout, our quantum advantage can be maintained with differing absorptive indexes of the atomic vapor, which makes it possible to apply a quantum light source in the absorptive electrometer.

Dynamical decoupling (DD) is perhaps the simplest and least resource-intensive error-suppression strategy for improving quantum computer performance. Here we report on a large-scale survey of the performance of 60 different DD sequences from ten families, including basic as well as advanced sequences with high-order error cancelation properties and built-in robustness. The survey is performed using three different superconducting-qubit IBMQ devices, with the goal of assessing the relative performance of the different sequences in the setting of arbitrary quantum state preservation. We find that the high-order universally robust (UR) and quadratic DD (QDD) sequences generally outperform all other sequences across devices and pulse-interval settings. Surprisingly, we find that DD performance for basic sequences such as the Carr-Purcell-Meiboom-Gill and XY4 sequences can be made to nearly match that of UR and QDD sequences by optimizing the pulse interval, with the optimal interval being substantially larger than the minimum interval possible on each device.

We propose a broadband acoustic metalaser that can effectively concentrate incident acoustic energy on the central axis for a long distance. The acoustic metalaser is axisymmetric and consists of a series of concentric-ring-like structures. By adjustment of the geometric parameters of the structure, the transmitted phase can be effectively controlled, and an energy-concentrated beam with a central pressure maximum along the beam axis can be generated. Because of the similar phase tendency of the metastructure at different frequencies, the same structure could achieve broadband acoustic energy convergence without the need for changes. We examine the broadband performance of the proposed metalaser using continuous sine waves of different frequencies from 2.5 to 18 kHz numerically and experimentally. In addition, the propagation of pulse waves with relative bandwidths of 53% and 117% through the metalaser is also investigated in the transient domain, where the effect of the acoustic metalaser on a broadband signal containing multiple frequencies is examined simultaneously. The results prove the ability of the metalaser for broadband acoustic energy convergence while the signal envelope is kept almost unchanged. Our work should find applications in many fields where the convergence of broadband or pulsed acoustic signals is desired.

We investigate the performance of the variational quantum eigensolver (VQE) for the problem of optimal flight-gate assignment. This is a combinatorial-optimization problem that aims at finding an optimal assignment of flights to the gates of an airport, in order to minimize the passenger travel time. To study the problem, we adopt a qubit-efficient binary encoding with a cyclic mapping, which is suitable for a digital quantum computer. Using this encoding in conjunction with the conditional value at risk (CVaR) as an aggregation function, we systematically explore the performance of the approach by classically simulating the CVaR VQE. Our results indicate that the method allows for finding a good solution with high probability and that it significantly outperforms the naive VQE approach. We examine the role of entanglement for the performance and find that ansätze with entangling gates allow for better results than pure product states. Studying the problem for various sizes, our numerical data show that the scaling of the number of cost-function calls for obtaining a good solution is not exponential for the regimes that we investigate in this work.

While there are several two-dimensional thermometry techniques that provide excellent spatial, temporal, and time resolution, there is a lack of three-dimensional (3D) thermometry techniques that work for a wide range of materials and offer good resolution in time, space, and temperature. We investigate electron paramagnetic resonance (EPR) of n-type silicon and germanium as a possible means of 3D thermometry. While in germanium the EPR linewidths are too broad for thermometry, EPR linewidths in silicon are reasonably narrow and exhibit a strong temperature dependence. The temperature dependence of the spin-lattice relaxation rate $(1/T_1)$ of conduction electrons in n-type $\mathrm{Si}$ have been extensively studied for low dopant concentrations and follows a $T^3$ law due to phonon broadening. For heavily doped $\mathrm{Si}$, which is desirable for good SNR for application in thermometry, impurity scattering is expected to decrease the temperature dependence of $1/T_1$. Our results show that, in heavily doped n-type $\mathrm{Si}$, spin-lattice relaxation induced by impurity scattering does not drastically decrease the temperature dependence of EPR linewidths. In P-doped $\mathrm{Si}$ with donor concentration of 7 × ${10}^{18}/{\mathrm{cm}}^3$, the EPR linewidth has a $T^{5/2}$ temperature dependence; the temperature dependence decreases to $T^{3/2}$ when the donor concentration is 7 × ${10}^{19}/{\mathrm{cm}}^3$. While the temperature dependence of linewidth decreases for heavier doping, EPR linewidth is still a sensitive thermometer. We define a figure of merit for SNR for thermometry from EPR linewidths of n-type $\mathrm{Si}$ and observe that increasing the doping results in a better SNR for thermometry. Using effective medium theory, we show that EPR linewidth can be a sensitive thermometer for application in 3D thermometry with systems embedding microparticles of heavily doped n-type $\mathrm{Si}$.

We investigated the polarization behaviors of the trion states in monolayer ${\mathrm{WS}}_2$ via helicity-resolved magnetophotoluminescence spectroscopy under linearly polarized laser excitation. With nondegenerate electron densities, the magnetic field dependence of the polarization degree of the triplet trions $(P_{X_T^{-}})$ is positive and that of the singlet trions $(P_{X_S^{-}})$ is negative. With weak degenerate electron densities, both dependencies are positive. A positive (negative) dependence on the magnetic field indicates that the relative intensity of the low-energy (high-energy) component, i.e., circularly polarized emission from the K (K′) valley, increases under a positive magnetic field. The magnetic-field and electron-density dependencies of $P_{X_T^{-}}$ and $P_{X_S^{-}}$ were well explained by using the scenario where a trion is a bound state of an exciton and an electron and taking into account the different valley polarizations of the excitons and the electrons due to the opposite energy shifts of the K and K′ valleys in a magnetic field. We propose a model to quantitatively fit the magnetic field–dependent $P_{X_T^{-}}$ and $P_{X_S^{-}}$ with different electron densities by considering the nonhomogeneity of the electron distribution. Our findings provide insights into the underlying physics of trion formation.

Radio-frequency identification (RFID) is a widely used technology for wireless data transfer between tags and readers. Passive ultrahigh-frequency (uhf) RFID architecture is a compromise between cost and performance in numerous retail applications, in which multiple goods must be labeled and simultaneously interrogated from a distance. Furthermore, for robust operation, passive tags must be visible from any direction and for any polarization to compensate for their accidental misalignments with respect to the reader’s antenna. Obtaining long-range omnidirectional operation with miniaturized tags remains a challenge, which limits the scope of emerging applications, including the Internet of small things. Here we develop the concept of resonance cascading and demonstrate a new architecture based on a high-index ceramic resonator. Taking advantage of frequency hopping between communication channels, we design several mutually orthogonal spectrally separated dipolar resonances to enable omnidirectional operation inside an RFID frequency band, instead of using traditional single-band quasi-isotropic antennas. As a result, we experimentally demonstrate a compact 28.5 × 27.5 × 27 mm$^{3}$ device, which can be omnidirectionally interrogated from a distance of over 10 m, which is further than has been previously achieved in the field of long-range omnidirectional uhf RFID tags. The concept of resonance cascading and spectral sharing can be further employed in a variety of wireless communication applications.

Magnetic skyrmions are topologically stabilized quasiparticles and are promising candidates for energy-efficient applications, such as storage but also logic and sensing devices. Here we present a concept for a multiturn counter-sensor device based on skyrmions, where the number of sensed rotations is encoded in the number of nucleated skyrmions. The skyrmion-boundary force in the confined geometry of the device in combination with the topology-dependent dynamics leads to the effect of automotion for certain geometries. For our case, we describe and investigate this effect with micromagnetic simulations and the Thiele equation in a triangular geometry with an attached reservoir as part of the counter-sensor device. We explore the device functionality, taking into account the influence of thermal diffusion. Additionally, we analyze a readout mechanism for the skyrmion storage, capable of quantifying skyrmion numbers and working effectively even in the presence of thermal diffusion. Finally, our investigation studies the deterministic nucleation of skyrmions.

Mechanical nonlinearities dominate the motion of nanoresonators already at relatively small oscillation amplitudes. Although single and coupled two-degree-of-freedom models have been used to account for experimentally observed nonlinear effects, it is shown that these models quickly deviate from experimental findings when multiple modes influence the nonlinear response. Here, we present a nonlinear reduced-order modeling methodology based on finite-element method simulations for capturing the global nonlinear dynamics of nanomechanical resonators. Our physics-based approach obtains the quadratic and cubic nonlinearities of resonators over a wide frequency range that spans 70 MHz. To qualitatively validate our approach, we perform experiments on a graphene nanodrum driven optothermally and show that the model can replicate diverse ranges of nonlinear phenomena, including multistability, parametric resonance, and different internal resonances without considering any empirical nonlinear fitting parameters. By providing a direct link between microscopic geometry, material parameters, and nonlinear dynamic response, we clarify the physical significance of nonlinear parameters that are obtained from fitting the dynamics of nanomechanical systems, and provide a route for designing devices with desired nonlinear behavior.

Carbon nanotubes are excellent nanoelectromechanical systems, combining high resonance frequency, low mass, and large zero-point motion. At cryogenic temperatures they display high mechanical quality factors. Equally they are outstanding single-electron devices with well-known quantum levels and have been proposed for the implementation of charge or spin qubits. However, the integration of these devices into microwave optomechanical circuits is hindered by a mismatch of scales between typical microwave wavelengths, nanotube segment lengths, and nanotube deflections. As experimentally demonstrated recently by Blien et al. [Nat. Comm. 11, 1363 (2020)], coupling enhancement via the quantum capacitance allows this restriction to be circumvented. Here we extend the discussion of this experiment. We present the subsystems of the device and their interactions in detail. An alternative approach to the optomechanical coupling is presented, allowing the mechanical zero-point motion scale to be estimated. Further, the mechanical damping is discussed, hinting at hitherto unknown interaction mechanisms.

The introduction of structural defects in otherwise periodic media is well known to grant space control and localization of waves in several fields of physics, from electromagnetism to acoustics and elasticity. Despite the variety of designs proposed so far to achieve mode localization, most of the approaches derive from contextual modifications that do not translate into a design paradigm due to the lack of a general theory. Few exceptions include designs endowed with topological dispersion bands, which, however, require changes over substantial portions of the structure. To overcome these limitations, here we introduce a rationale inspired by fractional electronic charges, to achieve topologically protected localized modes in continuous elastic media. We theoretically predict and experimentally observe the spectral flow of a localized mode across a bulk frequency gap by modulating a single structural parameter at any chosen location of architectured elastic media. The simplicity and generality of this approach may open avenues in designing elastic-wave-based devices for energy localization and harvesting.

CUPID is a next-generation neutrinoless double-$\beta$ decay experiment that will require cryogenic light detectors to improve background suppression, via the simultaneous readout of heat and light channels from its scintillating crystals. In this work, we showcase light detectors based on an alternative Ir-Pt bilayer transition-edge sensor. We have performed a systematic study to improve the thermal coupling between the photon absorber and the sensor, and thereby its responsivity. Our first devices meet CUPID’s baseline noise requirement of $<100$ eV rms. Our detectors have risetimes of approximately $180\text{μ}\mathrm{s}$ and measured timing jitter of $<20\text{μ}\mathrm{s}$ for the expected signal to noise at the $Q$ value of the decay, which achieves the CUPID’s criterion of rejecting two-neutrino double-$\beta$ decay pile-up events. The current work will inform the fabrication of future devices, culminating in the final TES design and a scaleable readout scheme for CUPID.

Simple, precise, and robust control is demanded for qubit operations on large quantum information processors. However, existing routes to high-fidelity quantum control heavily rely on arbitrary waveform generators that are difficult to scale up. Here, we show that the arbitrary control of a quantum system can be achieved by simply turning the control fields on and off in a proper sequence. The switching instances can be designed using conventional quantum optimal control algorithms, while the required computational resources for matrix exponentials can be substantially reduced. We demonstrate the flexibility and robustness of the resulting control protocol, and illustrate it in the context of superconducting quantum circuits. We expect this proposal to be achievable with current semiconductor and superconductor technologies, which offers a significant step towards scalable quantum computing.