Topological insulators that exhibit localized states at the edges or interference regions have been extensively investigated. Among these, waterborne acoustic topological interface states have significant potential in applications such as particle manipulation. However, the large impedance of water can no longer be ignored and sets difficulty in the construction of the waveguide in observing the topological states. In this paper, we propose an effective strategy to circumvent this issue and achieve the direct observation of the topological interface state in a waterborne acoustic waveguide as well as the associated nonzero Zak phases. Moreover, the robustness of the interface state against disorder is validated, suggesting a viable method for studying underwater topological acoustic fields.

Transmission and reflection of sound across a dense layer of long rectangular quasi-two-dimensional bubbles in a polymeric matrix are measured over two frequency ranges, 150–260 and 670–1130 kHz. Contrary to what is expected, considering the high acoustic impedance contrast between air and the matrix, there is high transmission across the layer, especially in the lower-frequency range. These results are explained by a monopolar Minnaert-like resonance of bubbles, which radiates sound in both directions of the bubble layer. The Minnaert resonance is computed for two-dimensional bubbles, revealing a resonance frequency within the lowest-frequency range used. The transmission and reflection coefficients are theoretically computed by considering the bubble array as a homogeneous layer with a resonant acoustic impedance.

We present a field-deployed quantum key distribution (QKD) link based on energy-time entanglement. Three nodes are connected, bridging the cities of Nice and Sophia Antipolis, by means of commercial grade optical fibers, spanning over a total distance of 50 km. We have built a packaged and resilient source of energy-time entangled photon pairs and implemented remote interferometer stabilization. Moreover, the clocks of the two end users are synchronized exploiting photon-counting techniques only, therefore requiring no dedicated channel. The system runs over the ITU 100-GHz standard telecom grid, through which a secure key rate of 7.0 kbit/s is extracted for one pair of channels. A homemade software performs all the necessary postprocessing procedures enabling to establish secret keys in real time. We present the first fully operational entanglement-based metropolitan QKD system being totally self-sufficient, i.e., requiring no additional overheads to be operated on a conventional metropolitan fiber network.

Quantum key distribution (QKD) enables private communications with information-theoretic security. To guarantee the practical security of QKD, it is essential that QKD systems are implemented in accordance with theoretical requirements and are robust with regard to side-channel attacks. Here we study a prominent attack on QKD transmitters known as the laser-seeding attack (LSA). It consists in injecting photons into the laser of the transmitter in an attempt to modify the outgoing light in some way that is beneficial to the eavesdropper. In this work we measure the response of a QKD transmitter to the LSA as a function of the optical power injected, allowing us to quantify the level of optical attenuation required to mitigate the attack. Further, we use a laser-rate-equation model to numerically simulate the effects of the LSA on a gain-switched laser. With this model we are able to reproduce previous experimental results, as well as generate new insight into the LSA by examining the effects of the LSA when the QKD transmitter is operated with different laser current driving parameters.

Recent progress on metasurfaces has revealed the nonlocal features of this kind of multiunit artificial structure. It has been shown that there is always intercoupling between the constituent units, which must be considered in their design. Such an intercoupling makes the design process difficult, especially when the number of constituent units becomes large. We show in this paper that a metasurface can be generally described by a multiport structural model, based on which an efficient design method can be established. To show the main idea of the model and the adaptability of the method based on it, nonlocal metastructures with various target fields, including an anomalous reflector, mode convertor in waveguide, wave focuser, and structure for a bound state in a continuum are designed. The successfully designed metastructures show that the suggested method is generally valid for structures with various target fields and is also efficient enough for structures with a large number of constituent units. The model gives not only an efficient method for designing metasurfaces, but also a general understanding of metastructures, based on which nonlocal metastructures for field control in complex environments, such as in organic tissue and architectural structures, can be expected.

Two-dimensional (2D) semiconductors are a promising solution for the miniaturization of electronic devices and for the exploration of novel physics. However, practical applications and demonstrations of physical phenomena are hindered by high Schottky barriers at the contacts to 2D semiconductors. While the process of image-force barrier lowering (IFBL) can considerably decrease the Schottky barrier, IFBL is not fully understood for the majority of prevalent contact geometries. We introduce an alternative technique to determine the IFBL potential energy with application spanning far beyond that of any existing method. We do so by solving Poisson’s equation with the boundary conditions of two metal surfaces separated by an angle $\mathrm{\Omega }$. We then prove that our result can also be obtained with the method of images provided a non-Euclidean, cone manifold space is used. The resulting IFBL is used to calculate the expected contact resistance of the most prevalent contact geometries. Finally, we investigate contact resistance and show how stronger IFBL counteracts the effect of larger depletion width with increasing contact angle. We find that top contacts experience lower contact resistance than edge contacts. Remarkably, our results identify tunable parameters for reducing Schottky barriers and likewise contact resistance to edge-contacted 2D materials, enhancing potential applications.

Ghost imaging can be implemented with illumination light with pronounced quantum characters. In this work, we address the inverse problem of ghost imaging, i.e., the tomography of the quantum states of illumination light based on ghost imaging. We showcase the characterization of multimode Gaussian state light represented by its covariance matrix. With the information of spatial modes, we reconstruct the quantum states of light using the variation of SNR of the ghost images formed by the input light under a given set of optical operations. We demonstrate that ghost imaging can also be used to characterize the non-Gaussian light by its deviation from Gaussian states, which can serve as a criterion for the non-Gaussianity of the input light. The spatial resolving power of the proposed method can be advantageous for quantum information applications involving structured light, such as multiplexing with spatial modes.

We present the results of studying the parameters of the magnetic $M_X$ resonance in an all-optical sensor built according to the two-beam Bell-Bloom scheme in nonzero ultraweak magnetic fields in which the effects of spin-exchange broadening suppression are partially manifested. We report on the features of the resonance under these conditions. We also optimize the resonance parameters to achieve maximum sensitivity in magnetoencephalographic sensors. We demonstrate an improvement in the ultimate achievable sensitivity of an all-optical $M_X$ sensor by a factor of 4 or more, which in our experiment corresponds to a decrease from 13 to $3{\text{fT/Hz}}^{1/2}$ in a volume of $0.13{\mathrm{cm}}^3$. We also report the effect of incomplete suppression of spin-exchange broadening under conditions of strong transverse modulated optical pumping, and propose a semiempirical model to describe it.

Antineutrinos from a nuclear reactor comprise an unshieldable signal, which carries information about the core. A gadolinium-doped, water-based Cherenkov detector has been investigated for detection of reactor antineutrinos for midfield remote reactor monitoring for nonproliferation applications. Two independent reconstruction and analysis pathways have been developed and applied to a number of representative reactor signals to evaluate the sensitivity of a kiloton-scale, gadolinium-doped Cherenkov detector as a remote monitor prototype. The sensitivity of four detector configurations to nine reactor signal combinations was evaluated for a detector situated in Boulby Mine, close to the Boulby Underground Laboratory in the UK. It was found that a 22-m detector with a gadolinium-doped, water-based liquid scintillator fill is sensitive to an approximately 3-${\mathrm{GW}}_{\mathrm{th}}$ reactor at a standoff of approximately $150\mathrm{km}$ within 2 years in the current reactor landscape. A larger detector would be required to achieve a more timely detection or to monitor smaller or more distant reactors.

Controlling the flow of quantum information is a fundamental task for quantum computers, which is unfeasible to realize on classical devices. Coherent devices, which can process quantum states are thus required to route the quantum states that encode information. In this paper we demonstrate experimentally the smallest quantum transistor with a superconducting quantum processor, which is composed of a collector qubit, an emitter qubit, and a coupler (transistor gate). The interaction strength between the collector and emitter qubits is controlled by the frequency and state of the coupler, effectively implementing a quantum switch. Through the coupler-state-dependent Heisenberg (inherent) interaction between the qubits, a single-step (native) conditional $i$swap operation can be applied. To this end, we find that it is useful to take into consideration the higher-energy level for achieving a native and high-fidelity transistor operation. By reconstructing the quantum process tomography, we obtain an operation fidelity of $92.36\mathrm{\%}$ when the transistor gate is open ($i$swap implementation) and $95.23\mathrm{\%}$ in the case of closed gate (identity gate implementation). The architecture has strong potential in quantum information processing applications with superconducting qubits.

A discrete vortex, formed by a one-dimensional (1D) ring array of lasers, has high output power as compared with a conventional continuous vortex, and therefore has attracted considerable interest due to widespread applications in various fields. We present a method for probing the magnitude and sign of the topological charge (l) of an unknown discrete vortex, by analyzing the interference pattern of a 1D ring array of lasers. The interference pattern of an unknown discrete vortex with $l\ne 0$ is averaged with the interference pattern corresponding to $l=0$, which gives rise to a variation in the fringe visibility as a function of the laser number ($j$) in a 1D ring array. The number of dips observed in the fringe-visibility curve is found to be proportional to the magnitude of the topological charge of a discrete vortex. After determination of the magnitude, the sign of $l\ne 0$ is determined by averaging its interference pattern with the interference pattern corresponding to $l=1$. The number of dips in the fringe-visibility curve decreases by 1 for positive l values and increases by 1 for negative l values. Further, we verify our method against the phase disorder, and it is found that the phase disorder does not influence accurate determination of the topological charge of a discrete vortex. The working principle as well as numerical and experimental results are presented for discrete vortices with topological charges from small to large values. Excellent agreement between the experimental results and the numerical simulations is found. Our method can be useful in applications of discrete vortices especially where conventional continuous vortices have power limitations.

The switching of a magnetic tunnel junction (MTJ) using femtosecond laser pulses enables a possible path for nonvolatile, ultrafast, and low-dissipative memories. In this work, we demonstrate successful field-free 50-fs single-laser-pulse-driven magnetization reversal of a [$\mathrm{Tb}$/$\mathrm{Co}$]-based storage layer in a perpendicular MTJ with a estimated absorbed energy of 68.6 fJ/bit. The nanofabricated MTJ devices have an optimized bottom reference electrode and show tunnel-magnetoresistance-ratio (TMR) values up to 74% after patterning down to sub-100-nm lateral dimensions. Experiments on continuous films reveal peculiar reversal patterns of concentric rings with opposite magnetic directions, above a certain threshold fluence. These rings have been correlated with the patterned-device switching probability as a function of the applied laser fluence. Moreover, the magnetization reversal is independent of the duration of the laser pulse. According to our macrospin model, the underlying magnetization-reversal mechanism can be attributed to an in-plane reorientation of the magnetization due to a fast reduction of the out-of-plane uniaxial anisotropy and a magnetization precession around the local effective-anisotropy axis. These aspects are of great interest both for the physical understanding of the switching phenomenon and their consequences for all-optical-switching memory devices, since they allow for a large fluence operation window with high resilience to pulse-length variability.

Measurement-device-independent (MDI) quantum key distribution (QKD) closes all the loopholes in the detection side, which greatly improves the practical security of QKD. Spontaneous parametric down-conversion (SPDC) sources play a significant role in MDI QKD by using the heralding strategies to achieve unique features, such as the passive-decoy method, which reduces side-channel information leakage from the source. However, photon pairs from realistic SPDC sources are often spectrally entangled. This lowers the Hong-Ou-Mandel (HOM) interference visibility, and thus increases the QKD error rate. Unfortunately, research efforts so far have not considered this effect quantitatively. This leads to an overestimation in the secure key rate; therefore their results cannot be applied to practical sources. In this article, we close this gap by decomposing the spectral entangled state into Schmidt eigenstates, then find their contributions to the secure key rate. In addition, our work allows an accurate optimization of pumping and filtering conditions to maximize the final key rate. We note that our model is general and can be applied to other quantum information processes involving HOM interference, such as quantum teleportation, entanglement swapping, and quantum computation.

Interference with a nonlinear cavity mode undergoing a self-sustained limit cycle can lead to superradiant amplification of incident waves. This effect, which results from nonlinear wave-mode coupling, is theoretically predicted and experimentally confirmed in this work. On the theoretical side, we present a nonlinear coupled-mode framework to derive an analytical expression for the scattering matrix. These predictions are then validated against experiments on a superradiant acoustic meta-atom, which uses deep-cavity whistling to create a limit cycle. The proposed all-acoustic meta-atom requires no active elements to manipulate the sound field, but achieves superradiant amplification by nonreciprocal coupling between the incident wave and the synchronized whistling tone (limit cycle). We demonstrate with this setup that the nonlinear phenomena we present can be exploited to quasipassively tune the scattering matrix of a resonant cavity. As a proof of concept, we convert a lossy, weakly reflective scatterer into a reflection amplifier. We explain how the synchronization-based realization of superradiance reported in this work may be exploited in the future to design lossless, nonacoustic scatterers for metamaterial applications.

Currently available quantum computing hardware based on superconducting transmon architectures realizes networks of hundreds of qubits with the possibility of controlled nearest-neighbor interactions. However, the inherent noise and decoherence effects of such quantum chips considerably alter basic gate operations and lead to imperfect outputs of the targeted quantum computations. In this work, we focus on the characterization of crosstalk effects, which manifest themselves in correlations between simultaneously executed quantum gates on neighboring qubits. After a short explanation of the physical origin of such correlations, we show how to efficiently and systematically characterize the magnitude of such crosstalk effects on an entire quantum chip using the randomized benchmarking protocol. We demonstrate the introduced protocol by running it on real quantum hardware provided by IBM and observe significant alterations in gate fidelities due to crosstalk. Lastly, based on the gained information, we propose more accurate means to simulate noisy quantum hardware by devising a crosstalk-aware noise model.

Motivated by recent advances in the inverse design of electromagnetic materials, we develop two methods for manipulating flexural waves on thin elastic plates. First, we derive a technique for determining plate pinning or mass loading of a thin plate, designing structures that focus elastic energy or isolate a region of space from vibrations. Taking inspiration from the adjoint method in electromagnetism, we show how to design graded plates to act as lenses or perform mode shaping. Both of the methods presented are simple, versatile, and straightforward to implement, making them useful for designing a wide range of devices for sensing, energy harvesting, and vibration isolation.

We propose a rotation sensing scheme based on the Sagnac interferometer for a single trapped particle system. In this scheme, the spin and vibrational states of the particle, prepared in a coherent state, are translated into a Schrödinger cat state by applying a strong magnetic field gradient to create the spin-dependent force. By reflecting the phase related to the rotational rate accumulated by the two arms of the interferometer to the spin state, we can obtain the rotational rate of the apparatus by performing spin measurement. The scheme can also be applied to the situation with the vibration state in the thermal state. The sensing sensitivity can reach $S=4.47\times {10}^{-7}\mathrm{rad}/\mathrm{s}\sqrt{\mathrm{Hz}}$ for a suspended diamond nanoparticle and $S=5.65\times {10}^{-6}\mathrm{rad}/\mathrm{s}\sqrt{\mathrm{Hz}}$ for a trapped ${}^{40}{\mathrm{Ca}}^{+}$. Combining with the experimental parameters, our scheme is applied to realize the gyroscope in a trapped single nanoparticle and atomic ion.