The Bell basis, a set of maximally entangled biphoton states, is a critical prerequisite towards quantum information processing, and many quantum applications have highlighted the requirement for manipulation of the high-dimensional Bell basis. While Bell states can be created by using ingenious single-photon quantum gates, their implementation complexity in higher dimensions is significantly increased. Here we present an elaborate approach to show that the adaptive pump modulation enables the efficient preparation of the Bell basis in arbitrary-dimensional Hilbert space. A complete set of four-dimensional orbital angular momentum Bell states is experimentally created, yielding high fidelities for certifying the entanglement dimensionality. Our strategy can be simply generalized to prepare more complex forms of quantum states, even exploiting other physical degrees of freedom. Also, it can facilitate the use of high-dimensional entanglement in a variety of quantum protocols, in particular those requiring quantum dense coding.

Almost four decades ago, Bergman and Milton independently showed that the isotropic effective electric permittivity of a two-phase composite material with a given volume fraction is constrained to lie within lens-shaped regions in the complex plane that are bounded by two circular arcs. An implication of particular significance is a set of limits to the maximum and minimum absorption of an isotropic composite material at a given frequency. Here, after giving a short summary of the underlying theory, we show that the bound corresponding to one of the circular arcs is at least almost optimal by introducing a certain class of hierarchical laminates. In regard to the second arc, we show that a tighter bound can be derived using variational methods. This tighter bound is optimal as it corresponds to assemblages of doubly coated spheres, which can be easily approximated by more realistic microstructures. We briefly discuss the implications for related problems, including bounds on the complex polarizability.

The Talbot effect has found profound applications in a wide variety of research fields, facilitating the development of sensing, imaging, and lithography, among others. Taking advantage of the flexibilities offered by the bidimensional Talbot effect, we propose and experimentally demonstrate a 2-bit coding acoustic metasurface lens. It is capable of arranging the far-field wave-energy distribution and thus tailoring the passive amplification of wave pattern at positions away from the source. By simply adjusting two different types of embedded helicoid units and their layout in the metasurface lens to change the coding sequences, the intensity amplification factor and period of the self-image can be manipulated simultaneously within the same imaging plane. The proposed Talbot lens allows flexible far-field sound redistribution and enhancement. Along with the nonresonant nature, it may open possibilities in applications such as acoustic communication, sonography, and signal processing.

Quantum information can be protected from noise and decoherence by encoding it into a decoherence-free subspace of the full Hilbert space. In this approach, it is important to devise implementations of quantum logic gates acting on the encoded logical qubits. Here, we experimentally investigate the implementation of an entangling linear optical controlled-$Z$ gate on two logical qubits encoded into pairs of physical qubits, which protects the quantum information from collective dephasing. This decoherence-resilient two-qubit gate on logical qubits is realized using a suitable four-qubit quantum gate on the physical qubits, which may help to protect the encoded quantum information from systematic errors arising due to imperfect operations on single logical qubits. We characterize the gate operation on input superposition states and demonstrate its ability to protect quantum coherence even under complete collective dephasing.

Plasmonic nanopores are the subject of extensive investigations as a potential platform to enable efficient optical readout in translocation experiments with biomolecules such as DNA and proteins. They allow for the engineering of electromagnetic fields at the nanoscale, which are typically used to enhance the emission efficiency of fluorescent molecules. Their features make them suitable for detection strategies based on the energy transfer between translocating molecules and the nanopore itself. Here, we carry out an optical experiment to show that a handful of diffusing dyes acting as donors can exchange energy via Förster-resonance energy transfer (FRET) with a gold nanopore functionalized with dyes behaving as acceptors. The FRET pair is composed of ATTORho6G (donor) and Alexa610 (acceptor). To perform this proof-of-concept experiment, we use a gold nanopore with a diameter of 80 nm, prepared on a ${\text{Si}}_3{\text{N}}_4$ membrane. We observe that the presence of the acceptors on the walls of the nanopore reduces the lifetime of the diffusing donors. In addition, we observe that the presence of the acceptors reduces the fluorescence signal on the donor detection channel and increases the fluorescence signal on the acceptor detection channel. The combination of these three effects gives us enough evidence to claim that the diffusing donors exchange energy with the functionalized nanopore via FRET, despite the relatively large size of the nanopore. The FRET efficiency of the process is found to be of the order of $30\mathrm{\%}$, which is in fairly good agreement with a theoretical value obtained using a simplified model.

$G{e}_{1-x}{\mathrm{Sn}}_x$ quantum wells can be incorporated into $\mathrm{Si}\text{−}\mathrm{Ge}$–based structures with low-carrier effective masses, high mobilities, and the possibility of direct band-gap devices with x ∼ 0.1. However, the electrical properties of p-type $G{e}_{1-x}{\mathrm{Sn}}_x$ devices are dominated by a thermally activated mobility and metallic behavior. At 30 mK the transport measurements indicate localization with a mobility of $380{\mathrm{cm}}^2$/Vs, which is thermally activated with a temperature-independent carrier density of 4 × ${10}^{11}{\mathrm{cm}}^{-2}$. This weakly disordered system with conductivity, σ ∼ $e^2$/h, where e is the fundamental charge and h is Planck’s constant, is a result of negatively charged “Sn-vacancy” complex states in the barrier layers that act as hole traps. A measured hole effective mass of 0.090 ± $0.005m_e$ from the Shubnikov-de Haas effect, where $m_e$ is the free electron mass shows that the valence band is heavy hole dominated and is similar to p-type $\mathrm{Ge}$ with the compressive strain playing the role of quenching the spin-orbit coupling and shifting the unoccupied light-hole states to higher hole energies. The $G{e}_{1-x}{\mathrm{Sn}}_x$ devices have a high quantum mobility of approximately 36 000 ${\mathrm{cm}}^2$/Vs that is not thermally activated. The ratio of transport-to-quantum mobility of approximately 0.01 in $G{e}_{1-x}{\mathrm{Sn}}_x$ devices is unusual and points to several competing scattering mechanisms in the different experimental regimes.

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation.

Quantum computation with quantum gates induced by geometric phases is regarded as a promising strategy in fault-tolerant quantum computation, owing to its robustness against operational noise. However, because of the parametric restrictions in previous schemes, the main robust advantage of holonomic quantum gates is reduced. Here, we experimentally demonstrate a solution scheme, obtaining nonadiabatic holonomic single-qubit quantum gates with optimal control in a trapped ${}^{171}{\mathrm{Yb}}^{+}$ ion based on a three-level system with resonant driving, which also has the advantages of rapid evolution and convenient implementation. Compared with previous geometric gates and conventional dynamical gates, the superiority of our scheme is that it is more robust against control amplitude errors, which is confirmed by the gate infidelity as measured by both quantum-process tomography and random benchmarking methods. In addition, we outline how nontrivial two-qubit holonomic gates can also be realized using currently available experimental technology. Thus, our experiment confirms the feasibility of this robust and fast holonomic quantum-computation strategy.

Unidirectional wave propagation in nonreciprocal structures enables exciting opportunities to control and enhance wave-matter interactions in extreme ways. Within this context, here we investigate the possibility of using terminated unidirectional plasmonic waveguides to enhance typically weak nonlinear effects by orders of magnitude. We theoretically demonstrate remarkable levels of electric field enhancement and confinement (field hot spots) when the unidirectional waveguiding structure is terminated with a suitable boundary that fully stops the one-way mode. Such a large field enhancement, originating from a nonresonant effect, is fundamentally different from the narrow-band field concentration effects in resonant plasmonic structures. Instead, it is analogous to the broadband response of plasmonic tapers, but without the need for any adiabatic impedance matching. We show that this effect can indeed lead to a substantial boosting of nonlinear light-matter interactions, exemplified by an improvement of several orders of magnitude in the third-harmonic-generation efficiency, which is of large significance for several applications. More broadly, our findings show the potential of extreme nonreciprocal configurations for enhanced wave-matter interactions.

We investigate the demonstration and quantification of the strong coupling between excitons and guided photons in a $\mathrm{Ga}\mathrm{N}$ slab waveguide. The dispersions of waveguide polaritons are measured from $T=6$ to 300 K through gratings. They are carefully analyzed within four models based on different assumptions, in order to assess the strong-coupling regime. We prove that the guided photons and excitons are strongly coupled at all investigated temperatures, with a small $(11\mathrm{\%})$ dependence on the temperature. However, the values of the Rabi splitting strongly vary among the four models: the “coupled oscillator” model overestimates the coupling; the analytical “Elliott-Tanguy” model precisely describes the dielectric susceptibility of $\mathrm{Ga}\mathrm{N}$ near the excitonic transition, leading to a Rabi splitting equal to $82\pm 10$ meV for fundamental transverse-electric mode; the experimental ellipsometry-based model leads to smaller values of $55\pm 6$ meV. We evidence that, for waveguides including active layers with large oscillator strengths, as required for room-temperature polaritonic devices, a strong bending of the modes’ dispersion is not necessarily the signature of the strong coupling, which requires for its reliable assessment a precise analysis of the material dielectric susceptibility.

The integration, scale-up, and multiplexing arrays of superconducting qubits in quantum circuits are the main challenges of superconducting quantum technology. Here we experimentally investigate the solid-state qubit multiplexing readout scheme, containing coplanar quarter-wavelength resonators coupled with a planar Xmon-type qubit, connected to a common coplanar transmission line. We find that the qubit energy spectrum is modified in the presence of an additional exciting signal at the fundamental frequency of the neighboring resonators. We attribute the origin of this effect to the electromagnetic field propagating through the common ground plane, which changes the qubit’s characteristics. Our finding may be useful for the development of scalable superconducting quantum integrated circuits with arrays of multiplexed or coupled qubits for applications in superconducting quantum processing and computing.

Exploring controllable interactions lies at the heart of quantum science. Neutral Rydberg atoms provide a versatile route toward flexible interactions between single quanta. Previous efforts mainly focused on the excitation annihilation (EA) effect of the Rydberg blockade due to its robustness against interaction fluctuation. We study another effect of the Rydberg blockade, namely transition slow-down (TSD). In TSD, a ground-Rydberg cycling in one atom slows down a Rydberg-involved state transition of a nearby atom, which is in contrast to EA that annihilates a presumed state transition. TSD can lead to an accurate controlled-not (cnot) gate with a submicrosecond duration of about $2\pi /\mathrm{\Omega }+ϵ$ by two pulses, where $ϵ$ is a negligible transient time to implement a phase change in the pulse and $\mathrm{\Omega }$ is the Rydberg Rabi frequency. The speedy and accurate TSD-based cnot gate makes neutral atoms comparable (superior) to superconducting (ion-trap) systems.

Effective quantum information processing is tantamount in part to minimization of the quantum resources needed by quantum logic gates. Here, we propose an optimization of an $n$-controlled-qubit Fredkin gate with a maximum of $2n+1$ two-qubit gates and $2n$ single-qudit gates by exploiting auxiliary Hilbert spaces. The number of logic gates required improves on earlier results on simulating arbitrary $n$-qubit Fredkin gates. In particular, the optimal result for a one-controlled-qubit Fredkin gate (which requires three qutrit-qubit partial-swap gates) breaks the theoretical nonconstructive lower bound of five two-qubit gates. Furthermore, using an additional spatial-mode degree of freedom, we design a possible architecture to implement a polarization-encoded Fredkin gate with linear optical elements.

We report a large enhancement of the spin-orbit-torque (SOT) efficiency achieved by inserting an ultrathin $\mathrm{Co}$ layer between a single-crystal $\mathrm{Pt}$ layer and an epitaxial ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layer. We attribute this to the strong interfacial orbital hybridization between $\mathrm{Pt}$ and $\mathrm{Co}$. Another effect of this hybridization is the spin-rotation effect based on the out-of-plane component of the magnetic moment, which explains the observation of out-of-plane torques. Our results offer an effective approach to engineer the SOT in a conventional ferromagnet-heavy metal system, which might provide opportunities for spintronic application.

Mirror-symmetric acoustic metascreens producing perfect absorption independently of the incidence side are theoretically and experimentally reported in this work. The mirror-symmetric resonant building blocks of the metascreen support symmetric and antisymmetric resonances that can be tuned to be at the same frequency (degenerate resonances). The geometry of the building blocks is optimized to critically couple both the symmetric and the antisymmetric resonances at the same frequency, allowing perfect absorption of sound from both sides of the metascreen. A hybrid analytical model based on the transfer-matrix method and the modal decomposition of the exterior acoustic field is developed to analyze the scattering properties of the metascreen. The resulting geometry is three-dimensionally printed and experimentally tested in an impedance tube. The experimental results agree well with the theoretical predictions, proving the efficiency of these metascreens for the perfect absorption of sound in ventilation problems.

We present a method relying on shortcuts to adiabaticity to achieve quantum detection of high-frequency signals at the nanoscale in a robust manner. More specifically, our protocol delivers tailored amplitudes and frequencies for control fields that, firstly, enable the coupling of the sensor with high-frequency signals and, secondly, minimize errors that would otherwise spoil the detection process. To exemplify the method, we particularize to detection of signals emitted by fast-rotating nuclear spins with nitrogen-vacancy-center quantum sensors. However, our protocol is straightforwardly applicable to other quantum devices such as silicon-vacancy centers, germanium-vacancy centers, or divacancies in silicon carbide.

Low-energy muon-spin-rotation spectroscopy (LE-$\mu \mathrm{SR}$) is employed to study silicon and carbon vacancies in proton-irradiated $4H$-$\mathrm{Si}\mathrm{C}$. We show that the implanted muon is quickly attracted to the negative $\mathrm{Si}$ vacancy ($V_{\mathrm{Si}}$), where it forms a paramagnetic muonium (${\mathrm{Mu}}^0$) state, resulting in a reduction of the diamagnetic fraction. In samples with predominantly $\mathrm{C}$ vacancies ($V_{\mathrm{C}}$), on the other hand, the formation of ${\mathrm{Mu}}^0$ is very short lived and the muon quickly captures a second electron to form a diamagnetic ${\mathrm{Mu}}^{-}$ state. The results are corroborated by density-functional calculations, where significant differences in the relaxation mechanism of the nearest-neighbor dangling bonds of the vacancies are discussed. We propose that the LE-$\mu \mathrm{SR}$ technique is capable of differentiating between high-spin and negative-$U$ behavior in semiconducting materials. Finally, our findings emphasize the large potential of LE-$\mu \mathrm{SR}$ to probe near-surface semiconductor defects, a capability that is crucial for further development of many electronic and quantum technology applications.

Nonclassical optomechanical correlations enable optical control of mechanical motion beyond the limitations of classical driving. Here we investigate the feasibility of using pulsed cavity optomechanics to create and verify nonclassical phase-sensitive correlations between light and the motion of a levitated nanoparticle in a realistic scenario. We show that optomechanical two-mode squeezing can persist even at the elevated temperatures of state-of-the-art experimental setups. We introduce a detection scheme based on optical homodyning that allows the revealing of nonclassical correlations without full optomechanical state tomography. We provide an analytical treatment using the rotating-wave approximation (RWA) in the resolved-sideband regime and prove its validity with a full numerical solution of the Lyapunov equation beyond the RWA. We build on parameters of current experiments for our analysis and conclude that the observation of nonclassical correlations, which are essential for quantum sensing, quantum engines, and quantum simulations with levitated nanoparticles, is possible with state-of-the-art capabilities. The general treatment can be applied to other optomechanical platforms.

Subcooled flow boiling heat transfer of carbon dioxide (${\mathrm{CO}}_2$) at the microscale near the critical pressure is experimentally studied. The onset of nucleate boiling and the local heat-transfer coefficient are obtained under 26 experimental conditions, consisting of 17 sample points each, yielding 442 data points. Excess temperatures of about 1 K at the onset of nucleate boiling and heat-transfer coefficients of up to $200\mathrm{kW}/{\mathrm{m}}^2\mathrm{K}$ are recorded. The heat-transfer coefficient and the onset of nucleate boiling models and correlations developed for low reduced pressures are compared to current experimental results. Modifications to these models are introduced to account for conditions pertinent to reduced pressures around unity. Overall, the correlations do not fully capture current measurements, suggesting that the physics of flow boiling heat transfer near the critical conditions exhibits some unique characteristics. Furthermore, it is observed that the experimentally obtained heat-transfer coefficient is better predicted by a model developed for flow boiling of ${\mathrm{CO}}_2$. However, with increasing pressure, deviations between experiments and predictions increase.