We study the transmission through a subwavelength metallic slit loaded with a single nanoresonator. To gain physical insight into the problem a theoretical model combining the dipole approximation and image theory is developed. The model shows that the coupling between the single nanoresonator and the slit's cavity modes serves as a localized analog to an infinite nanoresonator array. This virtual array supports a surface image-lattice resonance due to the coherent self-scattering of the single nanoresonator. Thereby, it may lead to the ability to mimic many recently reported intriguing physical phenomena of real surface-lattice resonances in nanoresonator arrays, by a single-subwavelength system. We specifically show that it leads to enhanced light-matter interaction, and to the appearance of an extraordinary transmission window. The theoretical results are in good agreement with full-wave numerical simulations.

An efficient biphoton emission process in semiconductors can enable the realization of highly tunable lasers, squeezed light sources, and entangled photon pair sources on an integrated platform. We propose a general single-photon recycling scheme to improve the overall efficiency of the typically weak biphoton emission process in a broad class of semiconductor materials. Using a rate-equation-based analysis, we first frame the general design principles and subsequently design a one-dimensional photonic crystal cavity to reach the ideal photon recycling limit for the spontaneously emitted single photons. The cavity is designed with realistic constituent materials to achieve high biphoton output efficiency in the absence of nonradiative recombination channels.

Exceptional points (EPs) are the singular branch points of the non-Hermitian Hamiltonian spectrum. They lead to many exotic phenomena of non-Hermitian systems. However, it is hard to realize parity-time $\left(\mathcal{PT}\right)$ symmetry breaking at EPs by increasing the optical gain due to the limitation of the optical gain materials. Here we propose a scheme to realize gain-induced $\mathcal{PT}$ symmetry breaking in two coupled passive optical resonators with a Stokes gain in the optomechanical resonator. As the Stokes gain results from radiation pressure coupling, it depends on the strength of the pump field. For this reason, $\mathcal{PT}$ symmetry breaking can be triggered by increasing the strength of the pump field. Moreover, Stokes gain also relies on the effective detuning of optomechanics, which is linear to the photon number. Therefore, $\mathcal{PT}$ symmetry breaking can also be induced by increasing the coupling strength between two coupled resonators and thus decreasing the photon number of the optomechanical resonator. In addition, this work opens up a method to control hidden $\mathcal{PT}$ symmetry breaking and nonlinear dynamics with radiation pressure coupling.

Spin qubits in semiconductor quantum dots are a promising candidate for scalable quantum computation. A key step in quantum error correction with spin qubits is the capability to perform high-fidelity parity measurement much faster than the coherence time and to generate entanglement deterministically. Here we propose a parity measurement scheme for two spin qubits by modulated pulses, using a superconducting resonator as a detector. We show that the distinction between spin-parity states can be achieved with a high fidelity within a short measurement time. The combination of parity measurement and feedback control allows the deterministic preparation of various spin entangled states. In addition, the method can be directly applied to parity measurements and surface codes of multiple spin qubits. These results pave the way for fault-tolerant quantum computation with spin qubits.

We study light scattering off a two-dimensional array of atoms driven to Rydberg levels. We show that the problem can be mapped to a generalized model of waveguide QED, consisting of multiple one-dimensional photonic channels (transverse modes), each of which is directionally coupled to a corresponding Rydberg surface mode of the array. In the Rydberg blockade regime, collective excitations of different surface modes block each other, leading to multichannel correlated photonic states. Using an analytical approach, we characterize interchannel quantum correlations, and elucidate the role of collective two-photon resonances of the array. Our results open new possibilities for multimode many-body physics and quantum information with photons in a free-space platform.

Macrorealism (MR) is the worldview that certain quantities may take definite values at all times irrespective of past or future measurements and may be experimentally falsified via the Leggett-Garg (LG) inequalities. We put this worldview to the test for systems described by a continuous variable $x$ by seeking LG violations for measurements of a dichotomic variable $Q=\text{sign}\left(x\right)$, in the case of Gaussian initial states in a quantum harmonic oscillator. Extending our earlier analysis [C. Mawby and J. J. Halliwell, Phys. Rev. A 105, 022221 (2022)], we find analytic expressions for the temporal correlators. An exploration of parameter space reveals significant regimes in which the two-time LG inequalities are violated, and likewise at three and four times. To obtain a physical picture of the LG violations, we exploit the continuous nature of the underlying position variable and analyze the relevant quantum-mechanical currents, Bohm trajectories, and Wigner function. We also show that larger violations are possible using the Wigner LG inequalities. Further, we extend the analysis to LG tests using coherent state projectors, thermal coherent states, and squeezed states.

Incompatibility of quantum devices is one of the cornerstones of quantum theory, and the incompatibility of quantum measurements and channels has been linked to quantum advantage in certain information-theoretic tasks. In this work, we focus on the less well explored question of the incompatibility of quantum instruments, that is, devices that describe the measurement process in its entirety, accounting for both the classical measurement outcome and the quantum postmeasurement state. In particular, we focus on the recently introduced notion of parallel compatibility of instruments, which has been argued to be a natural notion of instrument compatibility. We introduce, in a manner similar to the case of measurements and channels, the incompatibility robustness of quantum instruments and derive universal bounds on it. We then prove that postprocessing of quantum instruments is a free operation for parallel compatibility. Last, we provide families of instruments for which our bounds are tight and families of compatible indecomposable instruments.

The Eastin-Knill theorem is a central result of quantum error correction theory and states that a quantum code cannot correct errors exactly, possess continuous symmetries, and implement a universal set of gates transversely. As a way to circumvent this result, there are several approaches in which one gives up on either exact error correction or continuous symmetries. In this context, it is common to employ a complementary measure of fidelity as a way to quantify quantum state distinguishability and benchmark approximations in error correction. Despite having useful properties, evaluating fidelity measures stands as a challenging task for quantum states with a large number of entangled qubits. With that in mind, we address two distance measures based on the sub- and superfidelities as a way to bound error approximations, which in turn require a lower computational cost. We model the lack of exact error correction to be equivalent to the action of a single dephasing channel, evaluate the proposed fidelity-based distances both analytically and numerically, and obtain a closed-form expression for a general $N$-qubit quantum state. We illustrate our bounds with two paradigmatic examples, an $N$-qubit mixed GHZ state and an $N$-qubit mixed $W$ state.

We have made an absolute, reference-free measurement of the $1s2s{}^{3}S_1\to 1s^2{}^{1}S_0$ relativistic magnetic dipole transition in He-like sulfur. The highly charged sulfur ions were provided by an electron-cyclotron resonance ion source, and the x rays were analyzed with a high-precision double-crystal spectrometer. A transition energy of 2430.3685(97) eV was obtained and is compared to most advanced bound-state quantum electrodynamics calculations, providing an important test of two-electron QED effects and precision atomic structure methods in medium-$Z$ species. Thanks to the extremely narrow natural linewidth of this transition and to the large dispersion of the spectrometer at this energy, a complementary study is also performed to evaluate the impact of different silicon crystal atomic form-factor models in the transition-energy analysis. We find no significant dependence on the model used to determine the transition energy.

A full six-dimensional Born-Oppenheimer singlet potential-energy surface is constructed for the reaction CaF $+$ CaF $\to {\mathrm{CaF}}_2 +$ Ca using a multireference configuration-interaction electronic structure calculation. The ab initio data thus calculated are interpolated by Gaussian process regression. The four-body potential-energy surface features one $D_{2h}$ global minimum and one $C_s$ local minimum, connected by a barrierless transition state that lends insight to the reaction mechanism. This surface is intended to be of use in understanding ultracold chemistry of CaF molecules.

We report the experimental determination of the collision-energy dependence of the muon transfer rate from the ground state of muonic hydrogen to oxygen at near-thermal energies. A sharp increase by nearly an order of magnitude in the energy range 0–70 meV was found that is not observed in other gases. The results set a reliable reference for quantum-mechanical calculations of low-energy processes with exotic atoms and provide firm ground for the measurement of the hyperfine splitting in muonic hydrogen and the determination of the Zemach radius of the proton by the FAMU collaboration.

We investigate a fermionic superfluid with Raman-induced spin-orbit coupling immersed in a Bose-Einstein condensate. By minimizing the total free energy, we find that, with moderate repulsive interspecies interaction, a phase separation occurs where the otherwise nontopological uniform phase is divided into two parts: a purely fermionic one and a Bose-Fermi mix characterized by nontrivial topology with the winding number $W=1$. We verify that Majorana zero modes emerge at the phase interfaces by numerical simulations of the coupled Bogoliubov–de Gennes and Gross–Pitaevskii equations in real space. The tunability of the phase interfaces enables a direct manipulation of the predicted Majorana zero modes.

It is well known that gravitational waves distort equilibrium matter globally, making them amenable to detection with laser interferometers. Less well known is the fact that gravitational waves create local nonequilibrium stresses inside matter, which could conceivably lead to alternative detection methods. The gravitational wave-to-matter coupling $\kappa$ is a transport coefficient depending on the material and is poorly known for most substances. In the present work, we calculate $\kappa$ for a superfluid Fermi gas near unitarity using large-$N$ techniques, finding $\kappa =\frac{n}{12m}$, with $n$ the number density and $m$ the mass of the fermion, matching the result for free Dirac fermions at zero temperature. Our prediction is amenable to nonperturbative theoretical as well as experimental tests.

We studied nonlinear Landau-Zener tunneling under the effect of higher-order dispersion, transformed the Gross-Pitaevskii (GP) equation with higher-order dispersion terms into a nonlinear two-energy level form using a two-mode approximation, and comprehensively analyzed the loop structure of the lowest-energy band and the nonlinear Landau-Zener tunneling. The results show that, when third-order dispersion coefficient is not zero, there is a spatial inversion symmetry breaking of the energy band structure and unbalanced Landau-Zener tunneling. By analyzing the fixed point's nature of the classical Hamiltonian, we obtain a law for the variation of the loop structure and adiabatic tunneling probability with the dispersion coefficient, consistent with our numerical analysis results. In addition, we analyzed the real-time evolution of solitons with transverse bias and observed the tunneling phenomenon. Consistent with our analysis, the adjustment of the dispersion term can effectively control optical tunneling, which provides an alternative idea for optical switching.

We apply the support vector machine (SVM) algorithm to derive a set of entanglement witnesses (EW) to identify entanglement patterns in families of four-qubit states. The effectiveness of SVM for practical EW implementations stems from the coarse-grained description of families of equivalent entangled quantum states. The equivalence criteria in our work is based on the stochastic local operations and classical communication classification and the description of the four-qubit entangled Werner states. We numerically verify that the SVM approach provides an effective tool to address the entanglement witness problem when the coarse-grained description of a given family state is available. We also discuss and demonstrate the efficiency of nonlinear kernel SVM methods as applied to four-qubit entangled state classification.

In recent years, quantum state engineering and quantum information processing using microwave fields and photons have received increasing attention. In addition, multiqubit gates play an important role in quantum information processing. In this work, we propose to encode a photonic qubit via two arbitrary orthogonal eigenstates (with eigenvalues $\pm 1$, respectively) of the photon-number parity operator. With such encoding, we then present a single-step method to realize a multi-target-qubit controlled-phase gate with one photonic qubit simultaneously controlling $n-1$ target photonic qubits, by employing $n$ microwave cavities coupled to one superconducting flux qutrit. This proposal can be applied not only to implement nonhybrid multi-target-qubit controlled-phase gates using photonic qubits with various encodings, but also to realize hybrid multi-target-qubit controlled-phase gates using photonic qubits with different encodings. The gate realization requires only a single-step operation. The gate operation time does not increase with the number of target qubits. Because the qutrit remains in the ground state during the entire operation, decoherence from the qutrit is greatly suppressed. As an application, we show how to apply this gate to generate a multicavity Greenberger-Horne-Zeilinger (GHZ) entangled state with general expression. Depending on the specific encodings, we further discuss the preparation of several nonhybrid and hybrid GHZ entangled states of multiple cavities. We numerically investigate the circuit-QED experimental feasibility of creating a three-cavity spin-coherent hybrid GHZ state. This proposal can be extended to accomplish the same tasks in a wide range of physical systems, such as multiple microwave or optical cavities coupled to a three-level natural or artificial atom.