Kinetic inductance detectors (KIDs) are superconducting energy-resolving detectors, sensitive to single photons from the near-infrared to ultraviolet. We study a hybrid KID design consisting of a $\beta$-phase tantalum ($\beta$-$\mathrm{Ta}$) inductor and a $\mathrm{Nb}\text{−}\mathrm{Ti}\text{−}\mathrm{N}$ interdigitated capacitor. The devices show an average intrinsic quality factor $Q_i$ of $4.3\times {10}^5\pm 1.3\times {10}^5$. To increase the power captured by the light-sensitive inductor, we 3D print an array of $150\times 150\text{μ}\mathrm{m}$ resin microlenses on the backside of the sapphire substrate. The shape deviation between design and printed lenses is smaller than $1\text{μ}\mathrm{m}$, and the alignment accuracy of this process is ${\delta }_x=+5.8\pm 0.5\text{μ}\mathrm{m}$ and ${\delta }_y=+8.3\pm 3.3\text{μ}\mathrm{m}$. We measure a resolving power for 1545–402 nm that is limited to 4.9 by saturation in the KID’s phase response. We can model the saturation in the phase response with the evolution of the number of quasiparticles generated by a photon event. An alternative coordinate system that has a linear response raises the resolving power to 5.9 at 402 nm. We verify the measured resolving power with a two-line measurement using a laser source and a monochromator. We discuss several improvements that can be made to the devices on a route towards KID arrays with high resolving powers.

Paramagnetic defects and nuclear spins are the major sources of magnetic-field-dependent spin relaxation in point-defect quantum bits. The detection of related optical signals has led to the development of advanced relaxometry applications with high spatial resolution. The nearly degenerate quartet ground state of the silicon-vacancy qubit in silicon carbide ($\mathrm{SiC}$) is of special interest in this respect, as it gives rise to relaxation-rate extrema at vanishing magnetic field values and emits in the first near-infrared transmission window of biological tissues, providing an opportunity for the development of sensing applications for medicine and biology. However, the relaxation dynamics of the silicon-vacancy center in $\mathrm{SiC}$ have not yet been fully explored. In this paper, we present results from a comprehensive theoretical investigation of the dipolar spin relaxation of the quartet spin states in various local spin environments. We discuss the underlying physics and quantify the magnetic field and spin-bath-dependent relaxation time $T_1$. Using these findings, we demonstrate that the silicon-vacancy qubit in $\mathrm{SiC}$ can implement microwave-free low-magnetic-field quantum sensors of great potential.

Propagation-based neutron phase-contrast tomography is demonstrated using the ISIS pulsed spallation source. The proof-of-concept tomogram with a Paganin-type phase-retrieval filter applied exhibits an effective net boost of $23\pm 1$ in the signal-to-noise ratio as compared with an attenuation-based tomogram, implying a boost in the effective degree of neutron brilliance of over 2 orders of magnitude. This comparison is for phase retrieval versus conventional absorption with no additional collimation in place. We provide expressions for the optimal phase-contrast geometry as well as conditions for the validity of the method. The underpinning theory is derived under the assumption of the sample being composed of a single material. The effective boost in brilliance may be employed to give reduced acquisition time, or may instead be used to keep exposure times fixed while improving the measured contrast.

The structural phase transition of the high-symmetry cubic phase of antiperovskite $\mathrm{Na}$${}_3$$\mathrm{O}\mathrm{Cl}$ is investigated by computing the phonon band structures of 14 different polymorphs with distinct types of $\mathrm{O}\mathrm{Na}$${}_6$ octahedral tilting. The resulting $P$-$T$ phase diagram shows that, at high temperature and low pressure, the high-symmetry cubic structure with $Pm\overline{3}m$ symmetry is the most stable phase. At low temperature and high pressure, on the other hand, the monoclinic structure with $P{2}_1/m$ symmetry becomes the most stable phase. In between those two, there is a region in the phase diagram where the orthorhombic structure with $Bmmb$ symmetry is the most stable phase. To improve upon the quasiharmonic results, we do additional calculations in the framework of the self-consistent phonon (SCP) theory, including lattice anharmonicity by using cubic and quartic interatomic force constants (IFCs). This is particularly important for the high-symmetry cubic phase. We find that by decreasing the temperature, the frequency of the soft phonon at the $M$ and $R$ symmetry points gradually shifts to lower values. From these results, we can infer that a phase transition occurs around 166–195 K upon soft-mode condensation. Due to the proximity of the soft-mode frequencies at both symmetry points $R$ and $M$, we expect a cubic-to-orthorhombic phase transition to be realized via simultaneous condensation of the two octahedral tilting modes.

We demonstrate a spectrum demodulation technique allowing for rapid imaging in scanning nitrogen-vacancy center magnetometry. Our method relies on a periodic excitation of the electron spin resonance by wide-band frequency sweeps at a kilohertz rate combined with a phase-locked detection of the photoluminescence signal. The technique is robust against changes in spectrum shape and photoluminescence intensity, and is readily extended by a frequency feedback to enable real-time tracking of the spin resonance. Fast scanning magnetometry is especially useful for samples where the signal dynamic range is large, of order millitesla, such as for ferromagnets or ferrimagnets. We demonstrate our method by mapping stray fields above the model antiferromagnet $\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$ (hematite) at pixel rates of up to $100\mathrm{Hz}$ and an image resolution exceeding one megapixel.

In a van der Waals heterostructure of two-dimensional materials, electronic properties are tunable by means of stacking orders. Here, we study the spin-dependent quantum transport in the bilayer of ferromagnetic zigzag-edged graphene/hexagonal-$\mathrm{BN}$ nanoribbons (ZGr-BNNRs) using density-functional theory combined with the Keldysh nonequilibrium Green’s-function method. We reveal a strong odd-even effect of transport across the ZGr-BNNRs and a giant magnetoresistance value observed only in even-width ZGr-BNNRs. More interestingly, this value can be optimized by engineering stacking orders, yielding the perfect spin polarization efficiency of 100% and the magnetoresistance value of over ${10}^4$ in even-width ZGr-BNNRs. Our results provide a route to design and fabricate high-performance spin filters and magnetic storage devices.

Trapped ions are promising candidates for nodes of a scalable quantum network due to their long-lived qubit coherence times and high-fidelity single- and two-qubit gates. Future quantum networks based on trapped ions will require a scalable way to route photons between different nodes. Photonic integrated circuits from fabrication foundries provide a compact solution to this problem. However, these circuits typically operate at telecommunication wavelengths that are incompatible with the strong dipole emissions of trapped ions. In this work, we demonstrate the routing of single photons from a trapped ion using a photonic integrated circuit. We employ quantum frequency conversion to match the emission of the ion to the operating wavelength of a foundry-fabricated silicon nitride photonic integrated circuit, achieving a total transmission of 31.0% ± 0.9% through the device. Using programmable phase shifters, we switch the single photons between the output channels of the circuit and demonstrate a 50:50 beam splitting condition. These results constitute an important step towards programmable routing and entanglement distribution in large-scale quantum networks and distributed quantum computers.

Spin-orbit-torque (SOT) switching is a potentially energy-efficient mechanism by which electrical current can control magnetization. We demonstrate room-temperature SOT switching in nanometer-thick ${\mathrm{Co}\mathrm{Fe}\mathrm{B}/\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_3$ bilayers despite the amorphous nature of CoFeB and the interface. This behavior is attributed to the large spin Hall angle in ${\mathrm{Sr}\mathrm{Ir}\mathrm{O}}_3$ as well as high spin transmission at the interface that contribute to the SOT efficiency ${\xi }_{\mathrm{SOT}}$. Our bilayers exhibit large ${\xi }_{\mathrm{SOT}}$ of up to 1.4 and effective spin Hall conductivity of $0.9\times {10}^5$ ($\hslash /$2e)${\mathrm{\Omega }}^{-1}{\mathrm{m}}^{-1}$. In our bilayers, we observe unidirectional magnetoresistance and current-induced magnetization switching with a low critical current density of $1.38\times {10}^{10}{\mathrm{A}/\mathrm{m}}^2$ at room temperature. Our results are promising for heterostructures combining technologically relevant CoFeB with high spin-orbit-coupling oxides.

We propose the realization of perfect valley precession by use of strain, which gives rise to an effective vector potential and moves two valleys in opposite directions in momentum space. It is shown that the spatial precession period is independent of both the Fermi energy and the transverse wave vector. This is crucial to perfect precession in a practical device with multiple transverse wave vectors or modes. To theoretically demonstrate the perfect valley precession, we investigate the conductivity of a Kekulé graphene superlattice/graphene/Kekulé graphene superlattice junction that acts as a valley transistor. Due to the valley precession, the conductivity exhibits perfect oscillations with the strain and the conductivity minima are very close to zero. Our finding provides a strategy to design ideal valleytronics and straintronics devices.

We present a method to project time-bin qutrits stored in an echo-based quantum memory using several successive partial readouts of the memory. We demonstrate how this scheme can be used to implement projections onto a full set of mutually unbiased bases and, therefore, enables the characterization of arbitrary quantum states. Further, we study the integration of this protocol for the case of atomic frequency comb spin-wave storage by simulating the full storage process and performing a storage experiment with bright time-bin pulses in ${}^{151}{\mathrm{Eu}}^{3+}$:${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$. In this context, a compound pulse for implementing partial readouts in quick succession is introduced and characterized.

We study the magnon propagation in a micrometer-size ferromagnetic waveguide affected by the proximity to a superconductor that hosts a vortex formation. By solving the time-dependent Ginzburg-Landau equations of superconductivity, we obtain the equilibrium state of the vortex configuration and the associated stray fields that act on the magnetic dynamics. Spin wave dynamics in the presence of the superconductors are inferred from the Landau-Lifschitz equation. For a quantitative comparison, the simulations are performed under the experimental conditions as reported by Dobrovolskiy et al. (Nature Physics, 15, 477 (2019)). We found that the presence of the vortex lattice leads to the formation of a Bloch-like band structure in the magnon spectrum. The width and the number of allowed bands are found to depend mainly on the in-plane component of the vortex field along the waveguide. A shift to lower energies of the lowest-order-allowed bands occurs due to the confinement of the magnonic modes above the locations of the superconductor vortices. We also studied the role of the films thickness. At least two GHz-frequency backward-volume magnetostatic spin waves modes are observed at the top and bottom surfaces (in addition to these magnetostatic surface modes, the vertical confinement can also give rise to longer-wavelength and low-frequency perpendicular standing spin waves). Effects of the vortex fields are found to depend strongly on the type of the magnon modes. It is found that the effect of the superconducting vortices of the spin waves can be emulated well by the stray fields of air-separated magnetic nanocubes of appropriate dimensions.

We study the tunneling effect between quantum valley Hall edge states under an applied electric field in bilayer graphene containing gaps. A topological transistor based on this tunneling effect is proposed. In the on state of the transistor, the quantum valley Hall edge state along one bilayer-graphene zigzag edge can tunnel to the other edge under an electric field. In the off state without the electric field, the edge state cannot tunnel to the other edge and should be reflected by the top armchair edge and retrace the route along the same zigzag edge. The performance of the transistor is shown to be robust against long-range impurities. Compared with the previous transistor based on quantum spin Hall edge states [Xu et al. Phys. Rev. Lett. 123, 206801 (2019)], this transistor based on quantum valley Hall edge states has the advantages that it does not need a ferromagnetic gate to bridge the gap in the edge states and the conductance is robust against both nonmagnetic and magnetic impurities.

In this work, we propose and explore a double-stage phononic crystal (PnC) consisting of a top suspended $\mathrm{ZnO}$ layer, an array of sandwiched $\mathrm{Si}$ pillars, and a bottom $\mathrm{ZnO}$ layer on a $\mathrm{Si}$ substrate. The proposed double-stage PnC exhibits interesting behavior of routing the incident surface acoustic waves (SAWs) based on polarization and frequency, so that it can be considered as an efficient building block for alternative multistage SAW components. Here, we design the double-stage PnC to behave as an efficient vertical SAW splitter in a Mach-Zehnder interferometer, using the excitation of appropriate surface-coupled modes through the periodic locally resonating pillars. Benefiting from the acoustoelectric effect in $\mathrm{ZnO}$, the exposed top $\mathrm{ZnO}$ layer serves as the sensing branch, and the bottom $\mathrm{ZnO}$ layer serves as the reference branch in the presented vertical Mach-Zehnder interferometer. The vertical configuration of the proposed design allows exposure of the top $\mathrm{ZnO}$ layer to external stimulations, such as exposure to ultraviolet illumination or a hydrogen environment, while protecting the underlying parts from being exposed. This structural aspect simplifies the experimental implementation of our design in comparison with the conventional planar Mach-Zehnder interferometers by avoiding the necessity of in-plane focusing of external stimulation on the sensing branch. Our optimized Mach-Zehnder interferometer shows an output transmission signal of −8 dB and a high extinction ratio of 23 dB at 8.6 GHz when the conductivity of the top $\mathrm{ZnO}$ layer is increased from about 1 S/m to 100 S/m via external stimulation. Here, we propose a highly sensitive miniature vertical surface acoustic Mach-Zehnder interferometer, which may open up alternative horizons for future multistage SAW components.

The ability to transmit quantum states over long distances is a fundamental requirement of the quantum internet and is reliant upon quantum repeaters. Quantum repeaters involve entangled photon sources that emit and deliver photonic entangled states at high rates and quantum memories that can temporarily store quantum states. Improvement of the entanglement distribution rate is essential for quantum repeaters, and multiplexing is expected to be a breakthrough. However, limited studies exist on multiplexed photon sources and their coupling with a multiplexed quantum memory. Here, we demonstrate the storing of a frequency-multiplexed two-photon source at telecommunication wavelengths in a quantum memory accepting visible wavelengths via wavelength conversion after 10-km distribution. To achieve this, quantum systems are connected via wavelength conversion with a frequency-stabilization system and a noise-reduction system. The developed system is stably operated for more than 42 h. Therefore, it can be applied to quantum repeater systems comprising various physical systems requiring long-term system stability.

We demonstrate the manipulation of perpendicular magnetic anisotropy (PMA) in magnetic tunnel junctions (MTJs) by varying the layer stacks and temperature. PMA is tuned to compensate the shape anisotropy, giving a nonhysteretic magnetic response, a noteworthy sensitivity enhancement, and a field detectability of $1.8\mathrm{nT}/\sqrt{\mathrm{Hz}}$ at 100 kHz. Such a method is further exemplified in multiple MTJs, providing a solution to obtain desired sensitivities and operating temperatures. Additionally, the electronic noise of this MTJ is revealed as a random telegraph noise (RTN) due to a generation-recombination process. The observed voltage-dependent RTN could potentially be applied to true random number generators.

We report the realization of a strong coupling between a $\mathrm{Gd}$${}^{3+}$ spin ensemble hosted in a scheelite ($\mathrm{CaWO}$${}_4$) single crystal and the resonant mode of a coplanar-strip-line superconducting cavity leading to a large separation of spin-photon states of 146 MHz. The interaction is well described by the Dicke model and the crystal-field Hamiltonian of the multilevel spin system. We observe a change of the crystal-field parameters due to the presence of photons in the cavity, which generates a significant perturbation of the crystal ground state. Using finite-element calculations, we numerically estimate the cavity sensing volume as well as the average spin-photon coupling strength of $g_0/2\pi \approx 620\mathrm{Hz}$. Lastly, the dynamics of the spin-cavity states are explored via pulsed measurements by recording the cavity ring-down signal as a function of the pulse length and the amplitude. The results indicate a potential method to initialize this multilevel system in its ground state via an active cooling process.

In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting quantum interference device, which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically $\pm 3.5\mathrm{GHz}$ around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain $T_1\sim 8\text{μ}\mathrm{s}$. The echo dephasing times are limited by flux noise even at optimal points and reach $T_{2E}\sim 4\text{μ}\mathrm{s}$, almost an order of magnitude longer than state of the art.

Memristive devices are promising elements for energy-efficient neuromorphic computing and future artificial-intelligence systems. For diffusive memristors, resistive switching occurs because of the sequential formation and rupture of conduction filaments between device electrodes due to drift and diffusion of silver nanoparticles in the dielectric matrix. This process is governed by the applied electric voltage. Here, both in experiment and in simulations we demonstrate that varying temperature offers an efficient control of memristor states and charges transport in the device. By raising and lowering the device temperature it is shown that the memristive state can be reset, even if it cannot be done by varying the applied voltage. In addition, a change in the spiking regime is observed when the spiking is generated in the memristive circuit at a constant applied voltage, but different device temperatures. Our simulations demonstrate a good qualitative agreement with the experiments, and help to explain the effects reported.