The construction of a photon-photon quantum phase gate based on photonic nonlinearity has long been a fundamental issue, and is vital for deterministic and scalable photonic quantum-information processing. It requires not only strong nonlinear interaction at the single-photon level but also suppressed phase noise and spectral entanglement for high gate fidelity. In this paper, we propose that a high-quality-factor microcavity with strong ${\chi }^{(2)}$ nonlinearity can be quantized to anharmonic energy levels and can be effectively treated as an artificial atom. Such an artificial atom has a size much larger than the photon wavelength, which enables passive and active ultrastrong coupling to traveling photons. A high-fidelity quantum control phase gate is realized by our mediating the phase between photons with an intermediate artificial atom in a photonic molecule structure. The scheme avoids two-photon emission and thus eliminates spectral entanglement and quantum phase noise. Experimental realization of the artificial atom can be envisioned on an integrated photonic chip and holds great potential for single-emitter-free, room-temperature quantum-information processing.

The current practice of manually tuning quantum dots (QDs) for qubit operation is a relatively time-consuming procedure that is inherently impractical for scaling up and applications. In this work, we report on the in situ implementation of a recently proposed autotuning protocol that combines machine learning (ML) with an optimization routine to navigate the parameter space. In particular, we show that a ML algorithm trained using exclusively simulated data to quantitatively classify the state of a double-QD device can be used to replace human heuristics in the tuning of gate voltages in real devices. We demonstrate active feedback of a functional double-dot device operated at millikelvin temperatures and discuss success rates as a function of the initial conditions and the device performance. Modifications to the training network, fitness function, and optimizer are discussed as a path toward further improvement in the success rate when starting both near and far detuned from the target double-dot range.

Receiver protectors (RPs) shield sensitive electronics from high-power incoming signals that might damage them. Typical RP schemes range from simple fusing and PIN diodes to superconducting circuits and plasma cells—each having a variety of drawbacks ranging from unacceptable system downtime and self-destruction to significant insertion losses and power consumption. Here, we theoretically propose and experimentally demonstrate a unique self-shielding RP based on a coupled-resonator microwave waveguide, with a topological defect being inductively coupled to a diode. This RP utilizes a charge-conjugation- ($\mathcal{C}$) symmetric resonant-defect mode that is robust against disorder and demonstrates high transmittance at low incident powers. When the incident power exceeds a critical value, a self-induced resonant trapping effect occurs, leading to a dramatic suppression of transmittance and a simultaneous increase of the reflectance close to unity. The proposed RP device is self-protected from overheating and electrical breakdown and can be utilized in radars, reflection altimeters, and a broad range of communication systems.

Despite two decades of research, drag reduction with superhydrophobic (SHPo) surfaces has never been experimentally confirmed under the most coveted condition, i.e., high Reynolds number flows in open water, thus casting doubt on its ultimate impact. Here, we present large drag reductions (approximately 30%) by SHPo surfaces tested on the sea at ${\mathrm{Re}}_x$ estimated to be as high as $6.5\times {10}^6$. To achieve success, we prepare $4\times 7{\mathrm{cm}}^2$ silicon samples of re-entrant microtrenches for maximum plastron retention and test them as a portion of hull surface underneath a motorboat using a custom-developed comparative shear sensor. The state of plastron on the sample surfaces are observed throughout the tests via a custom-made miniature underwater camera. The successful results attest the importance of microscopic nuances of SHPo surfaces for plastron retention and drag reduction, guiding directions for future applications.

Hamiltonian-based quantum computation is a class of quantum algorithms in which the problem is encoded in a Hamiltonian and the evolution is performed by a continuous transformation of the Hamiltonian. Universal adiabatic quantum computing, quantum simulation, and quantum annealing are examples of such algorithms. Up to now, all implementations of this approach have been limited to qubits coupled via a single degree of freedom. This gives rise to a stoquastic Hamiltonian that has no sign problem in quantum Monte Carlo simulations. In this paper, we report implementation and measurements of two superconducting flux qubits coupled via two canonically conjugate degrees of freedom—charge and flux—to achieve a nonstoquastic Hamiltonian. We perform microwave spectroscopy to extract circuit parameters and show that the charge coupling manifests itself as a ${\sigma }^y{\sigma }^y$ interaction in the computational basis. We observe destructive interference in quantum coherent oscillations between the computational-basis states of the two-qubit system. Finally, we show that the extracted Hamiltonian is nonstoquastic over a wide range of parameters.

The manipulation and coupling of molecule gears is the first step toward realizing molecular-scale mechanical machines. Here, we theoretically investigate the behavior of such gears using molecular-dynamics simulations. Within a nearly rigid-body approximation, we reduce the dynamics of the gears to the rotational motion around the orientation vector. This allows us to study their behavior based on a few collective variables. Specifically, for a single hexa(4-tert-butylphenyl)benzene molecule, we show that the rotational-angle dynamics correspond to those of a Brownian rotor. For two such coupled gears, we extract the effective interaction potential and find that it is strongly dependent on the center-of-mass distance. Finally, we study the collective motion of a train of gears. We demonstrate the existence of three different regimes, depending on the magnitude of the driving torque of the first gear: underdriving, driving, and overdriving, which correspond, respectively, to no collective rotation, collective rotation, and only single-gear rotation. This behavior can be understood in terms of a simplified interaction potential.

Superparamagnetic tunnel junctions (SMTJs) have emerged as a competitive, realistic nanotechnology to support novel forms of stochastic computation in CMOS-compatible platforms. One of their applications is to generate random bitstreams suitable for use in stochastic computing implementations. We describe a method for digitally programmable bitstream generation based on precharge sense amplifiers. This generator is significantly more energy efficient than SMTJ-based bitstream generators that tune probabilities with spin currents and a factor of 2 more efficient than related CMOS-based implementations. The true randomness of this bitstream generator allows us to use them as the fundamental units of a novel neural network architecture. To take advantage of the potential savings, we codesign the algorithm with the circuit, rather than directly transcribing a classical neural network into hardware. The flexibility of the neural network mathematics allows us to adapt the network to the explicitly energy-efficient choices we make at the device level. The result is a convolutional neural network design operating at approximately $150$ nJ per inference with $97\mathrm{\%}$ performance on the MNIST data set—a factor of 1.4 to 7.7 improvement in energy efficiency over comparable proposals in the recent literature.

The best spatial resolution so far achieved in thermal imaging is several nanometers, much coarser than routinely achieved for other physical properties. Here we propose a method to map electronic temperature variations in operating nanoscale conductors by relying solely upon electrical tunneling current measurements. The proposed measurement scheme involves two scanning probe operations to measure the conductance and thermopower, respectively. These two measurements are shown to determine the local temperature with high accuracy in nanoscale conductors, where the Wiedemann-Franz law holds quite generally. The proposed scanning tunneling thermometer, owing to its operation in the tunneling regime, would be capable of mapping temperature variations with subnanometer resolution, thereby enhancing the resolution of scanning thermometry by some 2 orders of magnitude.

Metallic apertures remain one of the most intensively investigated optical structures due to their rich behavior. This encompasses physical phenomena such as Bragg scattering, surface plasmons, or localized modes, offering great potential for many applications in the growing fields of nanophotonics and plasmonics. Here, we show that isolated metallic nanoholes can additionally support edge plasmons in the form of whispering-gallery modes and we demonstrate that, although high-order modes remain dark at normal incidence, they can be excited at oblique illumination. As revealed by simulations, the existence of these modes in nanohole arrays is fundamental to understanding complex resonant phenomena and to explaining previous unexpected experimental results found in metamaterials. Notably, the symmetries of the employed lattice determine the excited mode orders, which, together with a strong dependence on the film thickness, yields an easily and highly tailorable optical response. Furthermore, some modes display unusually narrow line widths (approximately 7 nm) and a high sensitivity to the environment, resulting in one of the highest sensing figures of merit (approximately 80) reported for a localized plasmon. These singular features may find application in the construction of special polarizers and filters, single-layer zero-transmittance absorbers, and high-performance nanosensors.

Quantum communication schemes such as quantum key distribution (QKD) and superdense teleportation provide unique opportunities to communicate information securely. Increasingly, optical communication is being extended to free-space channels, but atmospheric turbulence in free-space channels requires optical receivers and measurement infrastructure to support many spatial modes. Here, we present a multimode Michelson-type time-delay interferometer using a field-widened design for the measurement of phase-encoded states in free-space communication schemes. The interferometer is constructed using glass beam paths to provide thermal stability, a field-widened angular tolerance, and a compact footprint. The performance of the interferometer is highlighted by measured visibilities of $99.02\pm 0.05\mathrm{\%}$ and $98.38\pm 0.01\mathrm{\%}$ for single- and multimode inputs, respectively. Additionally, high-quality multimode interference is demonstrated for arbitrary spatial-mode structures and for temperature changes of $\pm 1.0{}^{\circ }\mathrm{C}$. The interferometer has a measured optical-path-length drift of $130\mathrm{nm}/{}^{\circ }\mathrm{C}$ near room temperature. With this setup, we demonstrate the measurement of a two-peaked multimode single-photon state used in time-phase QKD with a visibility of $97.37\pm 0.01\mathrm{\%}$.

We study a magnetogranular chain composed of stainless-steel beads which are placed inside an appropriately designed periodic magnetic field. The latter provides attractive forces between the particles, leading to a stable structure, free of mechanical boundaries. Using a scanning laser-based probe at the individual-particle level, we observe experimentally the propagation of longitudinal as well as transverse-rotational waves. In addition, we obtain the dispersion band diagram. In the linear regime, these observations are well supported by a mass-spring model that takes into account both a normal and a shear mechanical coupling between the beads, considering translational and rotational degrees of freedom. In the weakly nonlinear regime, we present experimental results including beating in the amplitude of the second harmonic for the longitudinal waves and propagation under oblique driving excitation. A theoretical model that takes into account the Hertzian contact mechanics, dissipation, and the finite size of the system captures well the results for second-harmonic generation with longitudinal waves. This magnetosensitive system offers great freedom to design complex waveguide geometries where the interplay between geometry, wave polarization, and nonlinearity may pave the way toward the development of advanced signal-processing elastic devices.

Entangling operations are among the most important primitive gates employed in quantum computing, and it is crucial to ensure high-fidelity implementations as systems are scaled up. We experimentally realize and characterize a simple scheme to minimize errors in entangling operations related to the residual excitation of mediating bosonic oscillator modes that both improves gate robustness and provides scaling benefits in larger systems. The technique employs discrete phase shifts in the control field driving the gate operation, determined either analytically or numerically, to ensure all modes are de-excited at arbitrary user-defined times. We demonstrate an average gate fidelity of 99.4(2)% across a wide range of parameters in a system of ${{}^{171}\mathrm{Yb}}^{+}$ trapped ion qubits, and observe a reduction of gate error in the presence of common experimental error sources. Our approach provides a unified framework to achieve robustness against both static and time-varying laser amplitude and frequency detuning errors. We verify these capabilities through system-identification experiments revealing improvements in error susceptibility achieved in phase-modulated gates.

A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (N-$V$) center, the initialization fidelity is limited by fluctuations in the defect’s charge state during optical pumping. Here we use real-time control to deterministically initialize the N-$V$ center’s charge state at room temperature. We demonstrate a maximum charge initialization fidelity of $(99.4\pm 0.1)\mathrm{\%}$ and present a quantitative model of the initialization process that allows system-level optimization of the spin-readout signal-to-noise ratio. Even when the overhead associated with the initialization sequence is accounted for, increasing the charge initialization fidelity from the steady-state value of 75% to nearly 100% allows a factor-of-2 speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an estimated ac magnetic sensitivity of $1.3\mathrm{nT}/{\mathrm{Hz}}^{1/2}$ for our single-N-$V$-center spin. The real-time control method is immediately beneficial for quantum-sensing applications with N-$V$ centers as well as for probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multiqubit systems.

We establish angle-resolved thermal emission spectroscopy as an alternative platform to characterize the intrinsic eigenmode properties of non-Hermitian systems. This method can directly map the dispersion of metacrystals within the light cone with high angular resolution. To illustrate its usefulness, we demonstrate the existence of bound states in the continuum (BICs) and non-Hermitian Fermi arcs in a planar corrugated metacrystal by measuring its angle-resolved thermal emission spectra. We show that a change in the thickness of the metacrystal can induce a band inversion between a BIC and a radiative state, and a pair of exceptional points emerge when the band inversion occurs. With this approach, the band mapping of non-Hermitian photonic systems can become a relatively straightforward task.

We study ballistic and dissipative quantum transport of electrons in realistic semiconductor devices. Ballistic electron transport is modeled by solving self-consistently the Schrödinger and Poisson equations in the two-dimensional plane of the device using the effective-mass approximation. The quantum transmitting boundary method (QTBM) is used to enforce open boundary conditions. Regarding dissipative transport, contrary to the widely used quantum simulations based on the nonequilibrium Green’s function (NEGF) or the Wigner-function method, we model scattering using the Pauli master equation. This density-matrix-based formalism allows us to describe efficiently electronic transport—also accounting for realistic inelastic scattering processes—in semiconductor devices of sizes comparable to the electron wavelength. To this end, we analyze theoretically the impact of electron-phonon and surface-roughness scattering in silicon ultrathin-body (UTB) double-gate FETs. Our simulations show that electron transport is mainly dissipative even in devices with such small active regions. Of particular note is the predominant impact of surface-roughness scattering in UTB devices. Our calculations are consistent with the observation that scattering with surface roughness is the major factor limiting the scaling of CMOS devices.

We demonstrate an all-optical method for time-domain characterization of ultrashort optical pulses using nonionizing lasers. A moderately intense laser pulse couples the doubly excited states of helium and induces the ac Stark effect. A synchronized weak signal field perturbs the coupling process and alters the quasienergies of the dressed atom, upon which the carrier oscillation of the signal field is encoded. The nonionizing lasers used in current approach minimize plasma-induced pulse distortion and provide a complementary detection scheme for effective and reliable optical oscilloscope.

We investigate the dynamic behavior and topology of quasiperiodic resonant metastructures. We show that the quasiperiodic arrangement of resonators introduces frequency band gaps in addition to the locally resonant band gap defined by the natural frequency of the resonators. The concept is illustrated on a beam with an array of mechanical resonators. Numerical evaluation of the spectrum as a function of the quasiperiodic arrangement of resonators reveals a structure reminiscent of a Hofstadter butterfly and allows the study of key topological properties. The results illustrate the occurrence of additional band gaps that are topologically nontrivial and that host edge-localized modes in finite structures. The occurrence of these gaps and of the associated edge states is demonstrated experimentally by measuring the frequency response of the beam and by evaluating the spatial distribution of selected operational deflection shapes. The results unveil the potential of deterministic quasiperiodic structural designs to induce wave localization and attenuation over multiple frequency bands, which may find applications in vibration isolation and energy harvesting, among others.

Stability and toxicity issues with the hybrid lead iodide perovskite ${\mathrm{MAPbI}}_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites ${\mathrm{Cs}}_2{\mathrm{Au}}_2{X}_6$ ($X=\mathrm{I},\mathrm{Br},\mathrm{Cl}$). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with ${\mathrm{MAPbI}}_3$. However, careful investigation of defect physics reveals the possibility of deep-level defects (such as $V_X$, $V_{\mathrm{Cs}}$, $X_{\mathrm{Au}}$, $X_{\mathrm{Cs}}$, ${\mathrm{Au}}_i$, and ${\mathrm{Au}}_X$, $X=\mathrm{I},\mathrm{Br}$), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance-limiting defects and pave the way for future studies on this class of materials.

The distribution of electric current on an electrode surface in an electrolyte varies with time due to charge accumulation at a capacitive interface, as well as due to the electrode kinetics and concentration polarization in the medium. Initially, the potential at the electrode-electrolyte interface is uniform, resulting in a nonuniform current distribution due to the uneven ohmic drop of the potential in the medium. Over time, however, the nonuniform current density causes a spatially varying rate of the charge accumulation at the interface, breaking down its equipotentiality. We develop an analytical model to describe such a transition at a capacitive interface when the current is below the mass-transfer limitation and demonstrate that the steady distribution of the current is achieved when the current density is proportional to the capacitance per unit area, which leads to a linear voltage ramp at the electrode. More specific results regarding the dynamics of this transition are provided for a disk electrode, along with an experimental validation of the theoretical result. These findings may be of importance for many electrochemical applications and, in particular, for proper design of the electroneural interfaces.

The ability to shape ultrasound fields is important for particle manipulation, medical therapeutics, and imaging applications. If the amplitude and/or phase is spatially varied across the wave front, then it is possible to project “acoustic images.” When attempting to form an arbitrary desired static sound field, acoustic holograms are superior to phased arrays due to their significantly higher phase fidelity. However, they lack the dynamic flexibility of phased arrays. Here, we demonstrate how to combine the high-fidelity advantages of acoustic holograms with the dynamic control of phased arrays in the ultrasonic frequency range. Holograms are used with a 64-element phased array, driven with continuous excitation. Movement of the position of the projected hologram via phase delays that steer the output beam is demonstrated experimentally. This allows the creation of a much more tightly focused point than with the phased array alone, while still being reconfigurable. It also allows the complex movement at a water-air interface of a “phase surfer” along a phase track or the manipulation of a more arbitrarily shaped particle via amplitude traps. Furthermore, a particle manipulation device with two emitters and a single split hologram is demonstrated that allows the positioning of a “phase surfer” along a one-dimensional axis. This paper opens the door for new applications with complex manipulation of ultrasound while minimizing the complexity and cost of the apparatus.