Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms, allowing for quantum processors and memories with reduced error rates and more modes. The complex surface chemistry of niobium, however, makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown—in particular, niobium hydride evolution—on single-photon coherence. We demonstrate cavities with quality factors $Q_{\mathrm{int}}\gtrsim 1.4\times {10}^9$ in the single-photon regime, a 15-fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a two- to fourfold reduction in the two-level system loss tangent and a three- to fivefold improvement in the residual resistance over traditional buffered chemical polishing etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of 11.3 ms. The accessibility of our method, which can be easily replicated in academic laboratory settings, together with the demonstration of its performance, mark an advancement in three-dimensional cQED.

Ultrafast dynamics of magnetic properties in the high gigahertz (GHz) or even terahertz (THz) frequency range may be exploited in next-generation data storage and processing devices, and therefore suitable methods for detecting ultrafast dynamics of magnetization need to be developed. In this work, we investigate the impact of optically induced ultrafast precession of magnetization on the anomalous Hall conductivity in a ferromagnetic $\mathrm{Co}\text{−}\mathrm{Pt}$ multilayer structure using THz-based pump-probe measurements. From the measured Faraday rotation, the time-dependent anomalous Hall conductivity ${\sigma }_{xy}$ is extracted, and our data reveals an initial ultrafast quench due to laser-induced demagnetization and a subsequent oscillation with a frequency that increases with the strength of the applied external magnetic field ${\mathbf{\text{H}}}_{\mathrm{ext}}$. for $H_{\mathrm{ext}}$= 30 kOe, the oscillation frequency reaches approximately 100 GHz. The magnetic-field dependence of the oscillation frequency is in good agreement with the prediction from the ferromagnetic resonance model, confirming that the observed oscillation is caused by coherent magnetization precession. These findings are considered to be useful for studies that aim to integrate spintronics and THz electronics.

The long-wavelength infrared band exhibits significant utility in thermal signature acquisition, molecular spectral analysis, and other applications. The up-conversion detection technique enables effective signal transduction into the detection bandwidth of silicon-based photodetectors, facilitating high-sensitivity photodetection. We demonstrate high-resolution up-conversion imaging for incoherent thermal targets in the $10$-$\text{μ}\mathrm{m}$ spectral regime. Furthermore, this work presents a derivation of analytical models characterizing depth of field and astigmatic aberration in up-conversion imaging systems, which show good agreement between theoretical and experimental results. The results demonstrate generalization to various up-conversion imaging systems, providing critical insights for their design and optimization.

The Gravity Recovery And Climate Experiment (GRACE) Follow-On mission was launched on May 22, 2018, to continue monitoring changes in the gravity field of the Earth by measuring distance variations between two spacecraft that fly 200 km apart in a low-Earth polar orbit. The laser ranging interferometer (LRI), a technology demonstrator onboard GRACE Follow-On, is the first of its kind to perform interspacecraft ranging measurements and has shown noise levels of 1 nm/$\sqrt{\mathrm{Hz}}$ at 100 mHz and 200 pm/$\sqrt{\mathrm{Hz}}$ at 5 Hz. Its development was shared between parties in Germany and the United States. A key optical component for the LRI’s success is the triple mirror assembly (TMA), which acts as a corner-cube retroreflector and enables the laser link between the two spacecraft. This paper presents the TMA design and characterization from the unit level to measurements in orbit. The in-orbit measurements furthermore provide the far-field intensity distribution of the Gaussian beams exchanged between the spacecraft after traveling 200 km. We address lessons learned that have influenced the design of the next generations of the LRI.

In this work, we derive the relationship between the states of strain and stress in pseudomorphic epitaxial layers on a substrate with differing equilibrium lattice parameters under symmetry-conserving conditions for the case of monoclinic crystal symmetry. We compare surface vectors across the template-epitaxial layer interface and impose conditions of a stress-free epitaxial layer. As a result, we demonstrate the theoretical existence of up to three possible unit cells that can establish on a given template. We demonstrate this approach for the special case of symmetry-conserving strain in a class of templates with $(h0l)$ surface orientations. We use $\beta$-$({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ on $(h0l)\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ as an example. We discuss the effects of the composition $x$ and surface orientation on the formation of three elastically stable unit cells, their strain and stress tensors, unit-cell axes, unit-cell volumes, lattice spacings, elastic potential energies, and stress-free directions. The previous paradigm for epitaxial layer growth, where the only stress-free direction is perpendicular to the growing surface, is not generally valid for low-symmetry materials. In the example here, we find that, in addition to the stress-free growth direction for crystallographic planes with low Miller indices, two possible competing domains can exist with stress-free directions oblique to the surface of the template for almost all planes $(h0l)$. We further demonstrate the effect of the orientation-dependent differently strained domains and calculate the strain-shifted anisotropic band-to-band transitions for $\beta$-$({\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}{)}_2{\mathrm{O}}_3$ on $(h0l)\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$. We also provide examples of x-ray diffraction analyses, which may help guide the search for the occurrence of such differently strained unit cells in epitaxial layers grown by experiment, here taking the $(\overline{2}01)$ surface orientation as case for demonstration.

We present a superconducting cavity-coupled double quantum dot (DQD) photodiode that achieves a maximum photon-to-electron conversion efficiency of 25% in the microwave domain. With a higher-quality-factor cavity and improved device design to prevent photon leakages through unwanted pathways, our device measures microwave signals down to the 100-aW power level and achieves sensitivity to probe microwave signals with one photon at a time in the cavity. We analyze the photodiode operation using the Jaynes-Cummings input-output theory, identifying the key improvements of stronger cavity-DQD coupling needed to achieve near-unity photodetection efficiency. The results presented in this work represent a crucial advancement toward near-unity microwave photodetection efficiency with single-cavity-photon sensitivity.

Acoustically soft boundaries are of substantial potential in acoustical designs relevant particularly to directional radiation of sound waves. Here an alternative strategy is proposed for designing a much thin metasurface capable of emulating an airborne soft boundary, by utilizing the unusual resonance that relies on excitation of evanescent waves on subwavelength scale. The metasurface is structured by a two-dimensional periodic array of oblate cavities that are open on the same plane to air ambience through apertures of negligible thickness. An analytical theory is developed on the reflection of plane and spherical sound waves incident on the proposed metasurface, and the trade-off between the thickness of the metasurface and working bandwidth are analyzed for adapting the metasurface to different design requirements. As a proof-of-concept demonstration, an airborne metasurface is designed and fabricated in our laboratory, with its thickness roughly 3 times thinner than the thinnest ever reported. The theoretical predictions are well confirmed both by the full-wave numerical simulations and the experimental measurements.

Vanadium dioxide (${\mathrm{VO}}_2$) exhibits an insulator-to-metal transition (IMT) above room temperature, which makes it an ideal candidate for resistive-switching- (RS) based artificial neural networks (ANNs). To enhance the viability of RS-based ANNs, it is beneficial to operate ${\mathrm{VO}}_2$ devices close to the transition temperature, minimizing the switching power required to induce the IMT. Therefore, to avoid spontaneous switching and enable energy-efficient operation, it is important to develop a thorough understanding of thermal effects and heat propagation through these systems. In this work, we investigate the RS behavior of electronically and thermally coupled ${\mathrm{VO}}_2$ multibridge devices grown on substrates with distinct thermal and crystalline properties. Multiple switching states are observed independently of the substrate material. IR-emissivity mapping is used to study switching events and locally probe the temperature. Our results show that the substrate with lower thermal conductivity enhances thermal crosstalk, which decreases the power needed to induce the IMT in thermally coupled neighboring devices. The present study shows that the substrate choice can play an important role in the energy efficiency of IMT-based devices for ANN applications.

Establishing quantum entanglement between pairs of distant quantum memories (QMs) is essential for numerous applications in quantum networks. In this paper, we propose some heralded schemes for entanglement generation between multiple pairs of QMs placed in long-distance separated nodes, where the qubits are encoded on QMs coupled to a one-dimensional waveguide. By distributing a pair of spatial-polarization hyperentangled photons to two distant nodes, where each photon scatters with QMs, we can generate entanglement between multiple pairs of distant QMs. In our schemes, the faulty scattering events can be filtered out and discarded via detecting the polarization states of scattered photon components within each module QM. Moreover, the responses from single-photon detectors placed at the end of each node enable us to obtain direct entangled states between QMs in a heralded manner. Eventually, we theoretically extend the proposed schemes by demonstrating specific regularities that arise when entangling more pairs of QMs. With recent advancements in hyperentanglement generation, our schemes become increasingly feasible for practical implementation.

A driven linear oscillator and a feedback mechanism are two necessary elements of any classical periodic clock. Here, we introduce a fully quantum clock using a driven oscillator in the quantum regime and coherent quantum feedback. We show that if we treat the model semiclassically, this system supports limit cycles, or self-sustained oscillations, as needed for a periodic clock. We then analyze the noise of the system quantum mechanically and prove that the accuracy of this clock is higher compared with the clock implemented with the classical measurement feedback. We experimentally implement the model using two superconducting cavities with incorporated Josephson junctions and microwave circulators for the realization of the quantum feedback. We confirm the appearance of the limit cycle and study the clock accuracy both in frequency and time domains. Under specific conditions of noisy driving, we observe that the clock oscillations are more coherent than the drive, pointing towards the implementation of a quantum autonomous clock.

The ability to tune quantum tunneling is key for achieving selectivity in manipulation of individual particles in quantum technology applications. In this work we count electron escape events out of a time-dependent confinement potential, realized as a dynamic quantum dot in a GaAs/AlGaAs heterostructure. A universal scaling relation of the escape probability as a function of potential barrier rise time and depth is established and developed as a method to probe tunneling rates over many orders of magnitude reaching the limit of shallow anharmonic confinement. Crossover to thermally activated transport is used to estimate the single time-energy scale of the universal model. In application to metrological single-electron sources, in situ calibrated control signals greatly extend the accessible dynamical range for probing the quantization mechanism. Validation of the cubic potential approximation sets a foundation for microscopic modeling of quantum tunneling devices in the shallow confinement regime.

Josephson traveling-wave parametric amplifiers with heterogeneously spaced junctions are theoretically investigated. We consider unit cells with three junctions and characterize their interaction by spatially displaced fields defined by node fluxes. To solve this problem we define the system action and apply the variational principle to obtain the static action, which determines the equations of motion. This work shows that gain and bandwidth can be increased by modifying the relative distance between junctions within the same unit cell, thus increasing the effective nonlinear interaction. We find an optimal sub-unit-cell size ratio, which maximizes both gain and bandwidth, while equally spaced junctions offer minimum performance. Even though this method does not rely on phase matching, the same device can operate at different pump frequencies, it requires a very large number of JJs to achieve gains above 20 dB. We show that this method can be combined with resonant-phase matching and predicts an ideal gain above 29 dB on a 4-GHz bandwidth with 1998 junctions. Despite possible challenges fabricating devices at the optimal sub-unit-cell size with current technologies, substantial gain increase can still be achieved at suboptimal sizes.

Nanomechanical resonators promise diverse applications from mass spectrometry to quantum information processing, requiring long phonon lifetimes and frequency stability. Although two-level-system (TLS) defects govern dissipation at millikelvin temperatures, the nature of frequency fluctuations remains poorly understood. In nanoscale devices, where acoustic fields are confined to subwavelength volumes, strong coupling to individual TLSs should dominate over defect ensemble effects. In this work, we monitor fast spectral diffusion of phononic crystal nanomechanical resonators while varying the temperature (10 mK–1 K), drive power (${10}^2$–${10}^5$ phonons), and phononic band structure. We consistently observe random telegraph signals (RTSs), which we attribute to state transitions of individual TLSs. The spectral diffusion is well explained by mechanical coupling to individual far-off-resonant TLSs, which are either thermally excited or strongly coupled to thermal fluctuators. Understanding this fundamental decoherence process, particularly its RTS structure, opens a clear path toward noise suppression for quantum and sensing applications.

We present a two-dimensional grating magneto-optical trap (2D gMOT) for strontium, which produces a slow atomic beam in a compact form factor. The strontium 2D gMOT, using permanent magnets with an atomic dispenser, cools and traps atoms along two dimensions in the diffraction plane of a grating chip using the ${}^1{S}_0\to {}^1{P}_1$ (blue) transition. The system is configured in such a way that the atoms emitted from the dispenser source do not degrade the diffraction efficiency of the grating for long-term stable use. The performance of the 2D gMOT is analyzed with plug and probe beams, where the atomic source achieves a flux of $2.1\times {10}^8$ atoms/s and mean velocity of 6.4 m/s for an input laser power of 75 mW. The velocity measurement allows for the precise characterization of atom-photon scattering properties, showing that the compact 2D gMOT is suitable for portable quantum devices such as atomic clocks, quantum sensors, and precision spectroscopy.

The $M{A}_2{Z}_4$ family of materials exhibits excellent thermal and mechanical stability, unique electronic properties, and ultrahigh carrier mobility, rendering them highly promising for low-dimensional nanodevice applications. Using atomic substitution modeling, we computationally investigate the structural configurations of ${\alpha }_1$- and ${\alpha }_2$-phase in three monolayers: ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_2{\mathrm{P}}_2$, $\mathrm{Mo}(\mathrm{Si}\mathrm{NP}{)}_2\text{−}\mathrm{A}$, and $\mathrm{Mo}(\mathrm{Si}\mathrm{NP}{)}_2$-$\mathrm{B}$. Our first-principles analysis reveals that these monolayers possess robust structural and mechanical stability alongside high carrier mobility, making them ideal candidates for next-generation nanoelectronics. Specifically, the simulations of pn-junction diodes based on Janus ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_2{\mathrm{P}}_2$ monolayers demonstrate excellent rectification effects, while ${\alpha }_1$-phase ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_2{\mathrm{P}}_2$ pn-junction diodes exhibit negative differential resistance at finite bias. Furthermore, the pin-junction field-effect transistors show distinct behavior under positive and negative gate voltages, and the associated phototransistors display robust photovoltaic responses in the violet and ultraviolet regions, underscoring their significant optoelectronic potential. These results demonstrate the potential of Janus $M{A}_2{Z}_4$-based monolayers for future high-performance electronic and optoelectronic applications.

Quantum emitters embedded in photonic integrated circuit (PIC) cavities offer a powerful and scalable platform for label-free refractive-index sensing at the nanoscale. We propose and theoretically analyze a sensing mechanism based on Purcell-enhanced modulation of the emitter’s spontaneous-emission lifetime, enabling detection of refractive-index changes via time-correlated single-photon counting (TCSPC). In contrast to traditional resonance-shift sensors, our approach exploits the lifetime sensitivity to variations in the local density of optical states, providing a sensing modality that is intensity independent, spectrally unresolvable, and potentially compatible with complementary metal-oxide-semiconductor technology. We derive analytical expressions linking refractive-index perturbations to relative lifetime shifts and identify an optimal off-resonance operation regime where the lifetime response becomes linear and maximally sensitive to small perturbations. As a PIC material, silicon offers seamless integration of single-photon avalanche detectors suitable for TCSPC. We therefore illustrate the applicability of the proposed mechanism for quantum emitters in silicon. Parametric evaluation of the analytical expressions shows that, for moderate-quality-factor photonic cavities ($Q={10}^5$–${10}^7$), this method enables refractive-index detection limits as low as ${10}^{-9}$ refractive index units—competitive with or even outperforming state-of-the-art plasmonic and microresonator sensors yet with significantly simpler instrumentation. Furthermore, long-lived emitters such as T centers in silicon offer a unique advantage, allowing subnanosecond lifetime shifts to be resolved with standard TCSPC systems. Although room-temperature operation of quantum emitters in silicon has yet to be demonstrated, our results lay the theoretical foundation for scalable, room-temperature, quantum-enabled refractive-index sensing—eliminating the need for spectral resolution and cryogenic infrastructure. Given the generic nature of the proposed approach, and with ongoing advances in PIC technologies such as diamond-on-silicon, hybrid silicon-silicon nitride, and silicon-silicon carbide platforms, the method can be readily extended to materials in which room-temperature operation of quantum emitters has already been experimentally demonstrated.

This paper presents an approach that uses a simple metamaterial topology that incorporates gas discharge tubes for high-power microwave protection. The design features two split-ring resonators positioned side by side with their splits oriented orthogonally. When exposed to low-power microwaves, each split-ring resonator induces a resonance that interacts to create a passband within a broad stop band that can be used for communication, facilitated by a phenomenon known as “electromagnetically induced transparency” (EIT). At high power levels, the gas in the integrated gas discharge tubes becomes ionized, forming plasma that acts as a switch to eliminate the EIT window, thereby reinstating the stop band for protection. Several prototypes have been developed for S-band operation based on this concept. There is good agreement among the analytical, numerical, and experimental results. The proposed device demonstrates superior protection with lower insertion loss in the off mode and higher isolation in the on mode. Its strong ability to handle high-power microwaves is achieved with the use of plasma-based switches instead of diodes, providing a reasonable response time and a straightforward design that enables rapid prototyping. In addition, the device demonstrates the ability to tune the frequency and power threshold, highlighting its versatility as a microwave protection device.