We study by means of full quantum simulations an asymmetric double-barrier semiconductor heterostructure refrigerator combining resonant tunneling filtering and thermionic emission. By varying the quantum well thickness, we first investigate the influence of the activation energy $W$ on the coefficient of performance (COP) and cooling power. We show that the best performances are obtained when $W$ equals the polar optical phonon energy of the material. However, we also emphasize that cooling power and the COP are severely limited by tunneling current at high bias. We then propose an original structure with a tilted potential barrier to reduce this degrading effect. Quantum simulations demonstrate that cooling properties of such tilted barrier devices are significantly improved in the out-of-equilibrium regime, where the thermionic cooling concept offers its best efficiency.

In a superconducting quantum processor with nearest-neighbor coupling, the dispersive interaction between adjacent qubits can result in an effective next-nearest-neighbor coupling whose strength depends on the state of the intermediary qubit. Here, we theoretically explore the possibility of engineering this next-nearest-neighbor coupling to implement controlled two-qubit operations where the intermediary qubit controls the operation on the next-nearest-neighbor pair of qubits. In particular, in a system comprising two types of superconducting qubits with anharmonicities of opposite sign arranged in an -$A$-$B$-$A$- pattern, where the unwanted static $ZZ$ coupling between adjacent qubits could be heavily suppressed, a switchable coupling between the next-nearest-neighbor qubits can be achieved via the intermediary qubit, the qubit state of which functions as an on-off switch for this coupling. Therefore, depending on the adopted activating scheme, various controlled two-qubit operations such as the controlled-$i$swap gate can be realized, potentially enabling circuit depth reductions as to a standard decomposition approach for implementing generic quantum algorithms.

We demonstrate that $\mathrm{Bi}$ thin films emit terahertz radiation when irradiated by circularly polarized femtosecond laser pulses. Terahertz emission that is polarized in a direction perpendicular to the plane of incidence is significantly enhanced when the pump pulse is circularly polarized compared with when it is linearly polarized. The enhanced terahertz emission reverses its polarity with the reversal of pump helicity. This helicity dependent terahertz emission is absent in the horizontal direction along the plane of incidence, and is odd with respect to the incident angle. Combined with the layer thickness dependence measurement, the dominant origin of terahertz emission is attributed to the photoinduced inverse spin Hall effect (ISHE) in $\mathrm{Bi}$. The temporal dynamics of the ultrafast photocurrents are extracted and the spin and charge dynamics are discussed. The observation of terahertz emission from the photoinduced ISHE in a thin film consisting of a single material indicates the unique character of $\mathrm{Bi}$, i.e., the long spin diffusion length and the large spin Hall angle, demonstrating the potential ability of the material to be applied to terahertz emitters and terahertz spintronics.

For most wood-based product uses it is essential to remove a large part of the water content from wet or green (fresh-cut) wood, to reduce further dimensional variations under varying humidity conditions, improve its mechanical characteristics, and protect it from biological attacks. However, the internal mechanisms of drying are not fully described. Here we observe drying at different scales using macroscopic measurements (weighing), nuclear magnetic resonance measurements allowing to distinguish bound- and free-water contents, and x-ray computed tomography images of air-liquid interfaces at the smallest pore scale (wood lumens). We show that during wood drying, even well above the fiber saturation point, bound-water diffusion in cell walls (instead of capillary effects) ensures the extraction of liquid water from pores and its transport towards the surface of evaporation, and thus controls the drying rate. The distribution of bound-water content (uniform or heterogeneous) along the main sample axis and the drying-rate evolution depend on the competition between the external conditions and a characteristic rate of transport due to bound-water diffusion. For sufficiently slow drying this distribution remains homogenous until free water is fully extracted. An original physical phenomenon is thus at work, which plays a major role in regulating water extraction, in that it maintains a constant drying rate and a homogeneous distribution of the (mean) water content throughout the material. These results provide sound concepts for modeling and controlling drying properties of wood materials. They open the way to the understanding or control of the properties of many other materials containing two water types in food or civil-engineering applications. Our results complete recent observations that bound-water diffusion also controls imbibition in hardwood and finally show that transfers between bound and free water play a major role in the interaction of plantlike systems with water.

In magnetometry using optically detected magnetic resonance of nitrogen vacancy (N-$V^{-}$) centers, we demonstrate speedup of more than 1 order of magnitude with a sequential Bayesian experiment design as compared with conventional frequency-swept measurements. The N-$V^{-}$ center is an excellent platform for magnetometry, with potential spatial resolution down to a few nanometers and demonstrated single-defect sensitivity down to nanoteslas per square root hertz. The N-$V^{-}$ center is a quantum defect with spin S = 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of the N-$V^{-}$ energy levels allows detection of the magnetic field via photoluminescence. We compare conventional N-$V^{-}$-center photoluminescence measurements that use predetermined sweeps of the microwave frequency with measurements using a Bayesian-inference method. In sequential Bayesian experiment design, the settings with maximum utility are chosen for each measurement in real time on the basis of the accumulated experimental data. Using this method, we observe an order of magnitude decrease in the N-$V^{-}$-center-magnetometry measurement time necessary to achieve a set precision.

We use the quasienergy structure emerging in a periodically driven fluxonium superconducting circuit to encode quantum information with dynamically induced flux-insensitive sweet spots. The framework of Floquet theory provides an intuitive description of these high-coherence working points located away from the half-flux symmetry point of the undriven qubit. This approach offers flexibility in choosing the flux bias point and the energy of the logical qubit states as shown in Huang et al. [arXiv:2004.12458 (2020)]. We characterize the response of the system to noise in the modulation amplitude and dc flux bias, and experimentally demonstrate an optimal working point that is simultaneously insensitive against fluctuations in both. We observe a 40-fold enhancement of the qubit coherence times measured with Ramsey-type interferometry at the dynamical sweet spot compared with static operation at the same bias point.

Superinductors have a characteristic impedance exceeding the resistance quantum $R_Q\approx 6.45\mathrm{k}\mathrm{\Omega }$, which leads to a suppression of ground-state charge fluctuations. Applications include the realization of hardware-protected qubits for fault-tolerant quantum computing, improved coupling to small-dipole-moment objects, and the definition of a new quantum-metrology standard for the ampere. In this work, we refute the widespread notion that superinductors can only be implemented based on kinetic inductance, i.e., using disordered superconductors or Josephson-junction arrays. We present the modeling, fabrication, and characterization of 104 planar aluminum-coil resonators with a characteristic impedance up to 30.9 $\mathrm{k}\mathrm{\Omega }$ at 5.6 GHz and a capacitance down to $\le 1$ fF, with low loss and a power handling reaching ${10}^8$ intracavity photons. Geometric superinductors are free of uncontrolled tunneling events and offer high reproducibility, linearity, and the ability to couple magnetically—properties that significantly broaden the scope of future quantum circuits.

The interactions of ferroelectric (FE) perovskite oxides $(AB{\mathrm{O}}_3)$ with light are increasingly being studied for different applications, such as photovoltaics and optoelectronics. The combination of different cations at the A and B sites to form solid solutions allows tuning of the material’s properties and, most importantly, the band gap ($E_g$), which sets the wavelength range of light absorption. Classic FE perovskite oxides, such as ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$, ${\mathrm{K}\mathrm{Nb}\mathrm{O}}_3,$ and ${\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3,$ exhibit $E_g$ > 3 eV, which limits their implementation in visible-light-absorbing devices. Furthermore, the tuning of their $E_g$ via a solid solution strategy to a lower $E_g$ range is limited by the requirement for the presence of a $d^0$ metal at the B site, which is necessary for the FE distortion, but leads to a larger $E_g$. This gives rise to the challenge of decreasing $E_g$, while maintaining FE distortion. Here, we use first-principles calculations to explore the FE and optical properties of the $({\mathrm{K}\mathrm{Nb}\mathrm{O}}_3{)}_x({\mathrm{K}\mathrm{Ti}}_{1/2}{\mathrm{Mo}}_{1/2}{\mathrm{O}}_3{)}_{1-x}$ (KNTM) perovskite oxide solid solution. The introduction of ${\mathrm{Ti}}^{4+}$ and ${\mathrm{Mo}}^{6+}$ into the parent ${\mathrm{K}\mathrm{Nb}\mathrm{O}}_3$ decreases the $E_g$ to about 2.2 eV for x = 0.9, while preserving or enhancing polarization. Experimental fabrication and characterization show that the obtained KNTM material at x = 0.9 has an orthorhombic structure at room temperature and a direct gap of <2.2 eV, confirming first-principles-based predictions. These properties make KNTM a promising candidate for further studies and applications as a visible-light-absorbing FE material.

As self-sustained oscillators, lasers possess the unusual ability to spontaneously synchronize. These nonlinear dynamics are the basis for a simple yet powerful stabilization technique known as injection locking, in which a laser’s frequency and phase can be controlled by an injected signal. Because of its inherent simplicity and favorable noise characteristics, injection locking has become a workhorse for coherent amplification and high-fidelity signal synthesis in applications ranging from precision atomic spectroscopy to distributed sensing. Within integrated photonics, however, these injection-locking dynamics remain relatively untapped—despite significant potential for technological and scientific impact. Here, we demonstrate injection locking in a silicon photonic Brillouin laser. Injection locking of this monolithic device is remarkably robust, allowing us to tune the laser emission by a significant fraction of the Brillouin gain bandwidth. Harnessing these dynamics, we demonstrate amplification of small signals by more than 23 dB. Moreover, we demonstrate that the injection-locking dynamics of this system are inherently nonreciprocal, yielding unidirectional control and backscatter immunity in an all-silicon system. This device physics opens the door to strategies for phase-noise reduction, low-noise amplification, and backscatter immunity in silicon photonics.

A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity, and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multiqubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits that are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single strong microwave tone to one of the qubits, inducing a coupling between the two-qubit $|f,g⟩$ and $|g,e⟩$ states, with $|g⟩$, $|e⟩$, and $|f⟩$ denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of $97.5\pm 0.3\mathrm{\%}$ at a gate duration of 126 ns, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it requires neither additional drive lines nor tunable couplers.

The ability to throw objects is important to hominid evolution and has been central to many technological innovations in human history. The conditions for optimal throwing performance, however, are in general unknown. Here we present ejection experiments of projectiles composed of a rigid and a soft layer. The optimal performance is achieved when the soft layer of the projectiles matches the acceleration dynamics of the throwing engine. Compared with a fully rigid projectile, a bilayered projectile with a small soft elastic layer can achieve a more than 300% increase in translational kinetic energy. Our study emphasizes the importance of not only designing projectiles to optimize the stability in flight, the aerodynamic profile, and the spin, but also to carefully match the mechanical impedance of the projectiles and their thrower.

Detectors of propagating microwave photons have recently been realized using superconducting circuits. However, a number-resolved photocounter is still missing. In this article, we demonstrate a single-shot counter for propagating microwave photons that can resolve up to three photons. It is based on a pumped Josephson ring modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement-based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of $0.96\pm 0.04$, and a dark-count probability of $0.030\pm 0.002$ for an average dead time of $4.5\mu \mathrm{s}$. To maximize its performance, the device is first used as an in situ waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from $54\pm 2\mathrm{ to }99\mathrm{\%}$.

Recent advances in metasurfaces have shown that efficient wavefront transformation can be obtained by carefully designing the bianisotropic response of the metasurfaces. However, as will be shown in this paper, applying such a scheme for complicated wavefront transformations will lead to nonreciprocal local response, which is difficult to realize physically, if the field is not designed properly. In this paper, we identify the local power conservation requirement and introduce surface waves into the scattered field to meet such requirements. Based on such a scheme, we develop a bianisotropic metasurface design approach for near-perfect arbitrary beam splitting and anomalous reflection. Such a passive and reciprocal design approach enables experimental demonstration of surface-wave-enhanced wavefront transformation.

Single-photon detectors and bolometers represent the bridge between different topics in science, such as quantum computation, astronomy, particle physics, and biology. Nowadays, superconducting bolometers and calorimeters are the most-sensitive detectors in the terahertz and subterahertz bands. Here, we propose and demonstrate a Josephson escape sensor (JES) that could find natural application in astrophysics. The JES is composed of a fully superconducting one-dimensional Josephson junction, whose resistance-versus-temperature characteristics can be precisely controlled by a bias current. Therefore, differently from traditional superconducting detectors, the JES sensitivity and working temperature can be in situ simply and finely tuned depending on the application requirements. A JES bolometer is expected to show an intrinsic thermal-fluctuation-noise noise-equivalent power on the order of ${10}^{-25}{\mathrm{W/Hz}}^{1/2}$, while a JES calorimeter could provide a frequency resolution of about 2 GHz, as deduced from the experimental data. In addition, the sensor can operate at the critical temperature (i.e., working as a conventional transition-edge sensor), with a noise-equivalent power of approximately $6\times {10}^{-20}{\mathrm{W/Hz}}^{1/2}$ and a frequency resolution of approximately $100$ GHz.

Atom-light interactions in micro- and nanoscale systems hold great promise for alternative technologies based on integrated emitters and optical modes. We present the design architecture, construction method, and characterization of an all-glass alkali-metal vapor cell with nanometer-scale internal structure. Our cell has a glue-free design that allows versatile optical access, in particular with high numerical aperture optics, and incorporates a compact integrated heating system in the form of an external deposited indium tin oxide layer. By performing spectroscopy in different illumination and detection schemes, we investigate atomic densities and velocity distributions in various nanoscopic landscapes. We apply a two-photon excitation scheme to atoms confined in one dimension within our cells, achieving resonance line widths more than an order of magnitude smaller than the Doppler width. We also demonstrate sub-Doppler line widths for atoms confined in two dimensions to micron-sized channels. Furthermore, we illustrate control over vapor density within our cells through nanoscale confinement alone, which could offer a scalable route towards room-temperature devices with single atoms within an interaction volume. Our design offers a robust platform for miniaturized devices that could easily be combined with integrated photonic circuits.

Low-frequency noise in ${\mathrm{Y}}_2{\mathrm{O}}_3$-based resistive random-access memory devices with analog switching is studied at intermediate resistive states and as a function of dc cycling. A universal $1/f^{\alpha }$-type behavior is found, with a frequency exponent of $\alpha \approx 1.2$ that is independent of the applied reset voltage or the device resistance and is attributed to the intrinsic abundance of oxygen vacancies unique to the structure of yttria. Remarkably, the noise magnitude in the high resistive state systematically decreases through dc training. This effect is attributed to the stabilization of the conductive filament via the consumption of oxygen vacancies, thus reducing the number of active fluctuators in the vicinity of the filament.

We leverage aperiodic temporal modulations and nonreciprocity to realize PT-symmetric phenomena without the need for gain and loss provided by external mechanisms. We develop our approach using a general coupled-mode framework, and verify our results via detailed simulations in a conservative electronic circuit, demonstrating wave phenomena such as scattering PT-phase transition and anisotropic transmission resonances in a Hermitian system. Our concept opens the possibility of implementing PT-symmetric phenomena without gain, with applications in sensing, communications, and a broad range of quantum physics.

Ferromagnetic rings have been proposed as an attractive geometry for spintronic storage devices, for instance by storing information in the chirality of the vortex state. Asymmetric rings in particular offer a simple approach to control the vortex-state chirality, and the asymmetry yields additional degrees of freedom for manipulating the states such as by the choice of the field angle with respect to the axes of the system. By using time-resolved scanning electron microscopy with polarization analysis, we directly demonstrate here the influence of the initial magnetization configuration and the field orientation with respect to the ring symmetry axis on the switching mode. The results of experimental imaging are explained by micromagnetic simulations. In particular, for a given field strength we observe switching from the vortex state to the onion state if the symmetry axis of the asymmetric ring is perpendicular to the field orientation. If the symmetry axis deviates slightly from the field orientation, the dynamics change to onion-to-reverse-onion switching with different possible switching pathways. This provides a mechanism for selectively setting different magnetic states by an appropriate choice of the amplitude and orientation of the magnetic field. Furthermore, the angular dependence of the switching is shown to vary with the vortex-state chirality, which could be of use for chirality-dependent logic.

Circuit quantum electrodynamics (QED) employs superconducting microwave resonators as quantum buses. In circuit QED with semiconductor quantum-dot-based qubits, increasing the resonator impedance is desirable as it enhances the coupling to the typically small charge dipole moment of these qubits. However, the gate electrodes necessary to form quantum dots in the vicinity of a resonator inadvertently lead to a parasitic port through which microwave photons can leak, thereby reducing the quality factor of the resonator. This is particularly the case for high-impedance resonators, as the ratio of their total capacitance over the parasitic port capacitance is smaller, leading to larger microwave leakage than for 50-$\mathrm{\Omega }$ resonators. Here, we introduce an implementation of on-chip filters to suppress the microwave leakage. The filters comprise a high-kinetic-inductance nanowire inductor and a thin-film capacitor. The filter has a small footprint and can be placed close to the resonator, confining microwaves to a small area of the chip. The inductance and capacitance of the filter elements can be varied over a wider range of values than their typical spiral inductor and interdigitated capacitor counterparts. We demonstrate that the total linewidth of a 6.4 GHz and approximately 3-$\mathrm{k}\mathrm{\Omega }$ resonator can be improved down to 540 kHz using these filters.

Conventional medical ultrasound systems normally implement ray-based imaging algorithms, such as delay-and-sum beamforming, whose severest limitation derives from the implicit assumption of constant-velocity media. As a result, in the case of two or more tissues with different velocities, the image of the underlying targets appears strongly degraded both in placement and in resolution. The proposed ultrasound-imaging strategy, a value-added application of concepts developed in the context of seismic prospecting, avoids this restriction by relying on the undulatory description of the physical process and not on the geometric one. Echoes are sensed in the synthetic aperture configuration by the transducer, whose elements sequentially emit a nearly spherical wave front that covers the whole region of interest. Given a macrovelocity model, the recorded echoes become the initial condition for the downward propagator of the ultrasound wavefield. The processing is in three steps: time-reverse propagation of the sensed echoes, forward simulation of the wavefield emitted by the source element, and partial imaging by computing the zero-lag temporal correlation of the two wavefields to detect the scattering structures. The final reconstruction is obtained by stacking all the partial results on a single image. Laboratory tests, performed on experimental data acquired on a physical phantom, with and without an aberrant layer, prove the effectiveness of the proposed method even in the case of vertical and lateral velocity variations, with images of impressive spatial resolution and highly accurate target placement.