In parametric down-conversion, a nonlinear crystal is pumped by a laser and spontaneous emission takes place in signal and idler modes according to the phase-matching conditions. A seed laser can stimulate the emission in the signal beam if there is mode overlap between them. This also enhances the emission in the idler beam, affecting its coherence properties. While the degree of coherence of the idler field as a function of the seed power has already been studied, the transverse coherence length has not yet been properly investigated. The transverse coherence length is a key parameter of optical beams that determines the beam divergence, for example. Here, we present a theoretical and experimental investigation of the transverse coherence length in stimulated down-conversion. In addition, we make a connection between stimulated down-conversion and partially coherent sources like the Gaussian-Schell model beams, and show that in general the idler field cannot be described with this model.

The design of easy-to-operate high-fidelity two-qubit gates remains an area of ongoing research. Many of the common schemes require dedicated control lines, while others are vulnerable to issues of frequency crowding. Here, we propose a scheme for coupling a chain of transmons acting as logical qubits via a quantum bus of auxiliary qubits. The auxiliary array is made of floating transmons and through the use of mediated interactions we generate effective capacitance between them beyond the nearest neighbor. Logical qubits are not directly coupled to each other but they can be coupled by bringing them closer in frequency to the far-detuned auxiliary arrays. This allows for tunable coupling between non-neighboring logical qubits and for the application of entangling gates to three or more qubits at once.

The commercial and industrial demand for the solution of hard combinatorial optimization problems pushes forward the development of efficient solvers. One of them is the Ising machine, which can solve combinatorial problems mapped to Ising Hamiltonians. In particular, spintronic hardware implementations of Ising machines can be very efficient in terms of area and performance, and are relatively low cost considering the potential to create hybrid CMOS-spintronic technology. Here, we perform a comparison of oscillator-based and probabilistic paradigms of Ising machines on several hard max-cut instances, analyzing their scalability and performance at software level. We show that probabilistic Ising machines outperform oscillator-based Ising machines in terms of the number of iterations required to achieve the problem’s solution. Nevertheless, high-frequency spintronic oscillators with subnanosecond synchronization times could be very promising as ultrafast Ising machines. In addition, considering that a oscillator-based Ising machine acts better for max-cut problems because of the absence of the linear term in the Ising Hamiltonian, we introduce a procedure to encode max-3SAT to max cut. We foresee potential synergic interplays between the two paradigms.

The generation, propagation, and control of twisted light in a photonic chip has developed to meet the demands of modern optics, contributing to different research fields such as classical and quantum information technologies and integrated microfluidics. Here, we show how a single Gaussian illumination can excite different twisted-light modes into a buried annular waveguide, including that carrying orbital angular momentum (OAM) or superpositions of its constituents. We show that is possible to encode one-by-multiple information of independent OAM modes, unveiling the dynamics of the generation of OAM modes in a photonic chip. Owing to the degree of freedom of OAM light, OAM-circuit photonics can be explored to enhance high-capacity data processing for quantum or classical networking.

A wideband vibration energy harvester with multiple nonlinear forces is investigated. The nonlinearities are due to repulsive magnets and hardening springs, which gives rise to multistabilities between a number of energy branches. Not all branches are accessible by a simple up or down sweep of the driving frequency and in particular the highest energy branch is often hidden, requiring a suitable frequency schedule to be accessed. Detailed theoretical understanding of the energy branch structure along with robust experimental methods are essential for characterizing each of the energy branches to enhance the energy output from such a vibration energy harvesting system. We introduce a graphical representation in the form of eye diagrams based on time-resolved measurements of acceleration and output voltage to study the dynamical features of the different branches. This generic approach allows us to optimize the design, which results in 1.3 mW of power generated at 1 g over the 44-Hz frequency bandwidth while maintaining a small footprint of $1.23{\mathrm{cm}}^3$. The energy conversion ratio of the energy harvester at 120-Hz drive frequency is 0.52 for the high-energy branch.

This work investigates nanosecond superparamagnetic switching in 50-nm-diameter in-plane magnetized magnetic tunnel junctions (MTJs). Due to the small in-plane uniaxial anisotropy, dwell times below $10\text{ns}$ and autocorrelation times down to $5\text{ns}$ are measured for circular superparamagnetic tunnel junctions (SMTJs). SMTJs exhibit probabilistic switching of the magnetic free layer, which can be used for the generation of true random numbers. The quality of the random bit streams generated by our SMTJ is evaluated with a statistical test suite (NIST STS, sp 800-22) and shows true randomness after three exclusive or (xor) operations of four random SMTJ bit streams. A low-footprint CMOS circuit is proposed for fast and energy-efficient random-number generation. We demonstrate that the probability of a 1 or 0 can be tuned by spin-transfer torque (STT), while the average bit-generation rate is mainly affected by the current density via Joule heating. Although both effects are always present in MTJs, most often Joule heating is neglected. However, with a resistance-area (RA) product of $15\mathrm{\Omega }\text{μ}{\mathrm{m}}^2$ and current densities of the order of $1{\text{MA/cm}}^2$, an increasing temperature at the tunneling site results in a significant increase in the switching rate. As Joule heating and STT scale differently with current density, the device design can be optimized based on our findings.

We present the concept and the results of an investigation of an all-optical vector-magnetic-field sensor scheme developed for biological applications such as nonzero-field magnetoencephalography and magnetocardiography. The scheme differs from the classical two-beam Bell-Bloom scheme in that the detecting laser beam is split into two beams, which are introduced into the cell in orthogonal directions, and the ratio of the amplitudes of the magnetic resonance signals in these beams and their phase difference are measured; strong optical pumping from the lower hyperfine level of the ground state ensures the resonance line narrowing, and detection in two beams is carried out in balanced schemes by measuring the beam-polarization rotation. The proposed sensor is compact, resistant to variations of parameters of laser radiation and highly sensitive to the angle of deflection of the magnetic field vector—with an estimated scalar sensitivity of the order of $16{\mathrm{fT}/\mathrm{Hz}}^{1/2}$ in the $8\times 8\times 8{\mathrm{mm}}^3$ cell, an angular sensitivity of $4\times {10}^{-7}$ rad, or 0.08”, was demonstrated.

Single-shot all-optical helicity-independent switching is established in $\mathrm{Co}/\mathrm{Ho}$ multilayers extending the number of material combinations showing this fascinating property. This ability is studied in wedge-shaped $\mathrm{Co}/\mathrm{Ho}$ multilayers by varying thicknesses and repetition numbers. Surprisingly, even though the spin-orbit coupling is larger than in $\mathrm{Tb}$ and $\mathrm{Dy}$, which should increase the dissipation of angular momentum to the lattice, the pulse duration versus fluence state diagram is close to the one of the $\mathrm{Gd}$-based system.

Laser cooling of a semiconductor material, in which heat is extracted by the emission of photons, requires near-perfect external radiative efficiency. In this theoretical work, we propose a cooling system based on carrier extraction in a large-band-gap reservoir. The electron-hole pairs generated in the material to be cooled are extracted in such a reservoir by absorption of phonons, then carrying a heat flux. With an analytical detailed-balance model, we show that this concept is applicable even in materials with moderate external radiative efficiency. Moreover, by adjustment of the band gap of the reservoir to the laser power, this system can either reach high efficiency or transfer high power with lower efficiency.

Topological insulators are promising candidates for dissipationless electronics and spintronics. For the design and application of small-sized topological transistors, it is vital to suppress the off-state leakage and keep utilizing the edge or surface states to carry the on-state current. Using the nonequilibrium Green’s function method, the transport properties of clean and disordered silicene were studied in the nanoscale regime. The results revealed the following. (1) The low-energy electron transport across the scattering region included two types of paths, namely, helical edge states and interstate tunnels. The choice of electrons for the two transmission paths was related to the length of the scattering region. When there was a band gap in the $k_y$ direction, electrons tended to tunnel between armchair-edge states along the $x$ axis. It was only when the length met $N_x=3n+1$ that the electrons mainly propagated through the helical edge states. (2) The weak electric field could significantly enhance the wave-function overlap between armchair-edge states and can be used to switch electron-transport paths. (3) The origin of the leakage current in the nanoscale transistors was interstate quantum tunneling; this was promoted—instead of suppressed—by weak or strong disorders. (4) The effect of a strong electric field on the electron transport was opposite to that of the weak field. After reaching a critical value of ${\lambda }_E>2{\lambda }_{\mathrm{SO}}$, the vertical electric field decreased the interstate tunneling probability and increasing the staggered potential was an effective method to suppress the off-state leakage.

Superconducting radio-frequency (SRF) cavities currently rely on niobium (Nb), and could benefit from a higher-$T_c$ surface, which would enable a higher operating temperature, lower surface resistance, and higher maximum fields. Surface zirconium (Zr) doping is one option for improvement, which has not previously been explored, likely because bulk alloy experiments showed only mild $T_c$ enhancements of 1–2 K relative to Nb. Our ab initio results reveal a more nuanced picture: an ideal bcc Nb-Zr alloy would have $T_c$ over twice that of niobium, but displacements of atoms away from the high-symmetry bcc positions due to the Jahn-Teller-Peierls effect almost completely eliminates this enhancement in typical disordered alloy structures. Ordered Nb-Zr alloy structures, in contrast, are able to avoid these atomic displacements and achieve higher calculated $T_c$ up to a theoretical limit of 17.7 K. Encouraged by this, we tested two deposition methods: a physical-vapor Zr deposition method, which produced Nb-Zr surfaces with $T_c$ values of 13.5 K, and an electrochemical deposition method, which produced surfaces with a possible 16-K $T_c$. An rf test of the highest-$T_c$ surface showed a mild reduction in BCS surface resistance relative to Nb, demonstrating the potential value of this material for RF devices. Finally, our Ginzburg-Landau theory calculations show that realistic surface doping profiles should be able to reach the maximum rf fields necessary for next-generation applications, such as the ground-breaking LCLS-II accelerator. Considering the advantages of Nb-Zr compared to other candidate materials such as ${\mathrm{Nb}}_3\mathrm{Sn}$ and Nb-Ti-N, including a simple phase diagram with relatively little sensitivity to composition, and a stable, insulating ${\mathrm{Zr}\mathrm{O}}_2$ native oxide, we conclude that Nb-Zr alloy is an excellent candidate for next-generation, high-quality-factor superconducting rf devices.

A piezotronic field-effect transistor device based on a topological insulator (TI) is proposed based on wurtzite/zincblende $\mathrm{InAs}$ quantum wells (QWs). The subthreshold swing can reach 5 mV/decade. The subthreshold swing of FETs requires at least a 60 mV gate voltage due to the thermal voltage. Polarization in these QWs can drive a large-gap TI with a bulk band gap of approximately 50 meV. The TI maintains a large band gap characteristic under a compressive stress of 7 GPa.

Many quantum sensing suggestions rely on temporal correlations found in photon pairs generated by parametric down-conversion. In this work, we show that the temporal correlations in light with a thermal photon statistics can be equally useful for such applications. Using a subthreshold laser diode as an ultrabright source of thermal light, we demonstrate optical range finding to a distance of up to 1.8 km.

Resonator measurements are a simple but powerful tool to characterize a material’s microwave response. The losses of a resonant mode are quantified by its internal quality factor $Q_{\mathrm{i}}$, which can be extracted from the scattering coefficient in a microwave reflection or transmission measurement. Here we show that a systematic error on $Q_{\mathrm{i}}$ arises from Fano interference of the signal with a background path. Limited knowledge of the interfering paths in a given setup translates into a range of uncertainty for $Q_{\mathrm{i}}$, which increases with the coupling coefficient. We experimentally illustrate the relevance of Fano interference in typical microwave resonator measurements and the associated pitfalls encountered in extracting $Q_{\mathrm{i}}$. On the other hand, we also show how to characterize and utilize the Fano interference to eliminate the systematic error.

Near-infrared color centers in silicon are emerging candidates for on-chip integrated quantum emitters, optical-access quantum memories, and sensing. We access ensemble G-color-center formation dynamics and radiation-induced atomic disorder in silicon for a series of megaelectronvolt proton-flux conditions. The photoluminescence results reveal that the G centers are formed more efficiently by pulsed-proton irradiation than by continuous-wave proton irradiation. The enhanced transient excitations and dynamic annealing within nanoseconds allows optimization of the ratio of G-center formation to nonradiative defect accumulation. The G centers preserve narrow line widths of about 0.1 nm when they are generated by moderate pulsed-proton fluences, while the line width broadens significantly as the pulsed-proton fluence increases. This implies vacancy or interstitial clustering by overlapping collision cascades. The tracking of G-center properties for a series of irradiation conditions enables sensitive probing of atomic disorder, serving as a complementary analytical method for sensing damage accumulation. Aided by ab initio electronic structure calculations, we provide insight into the atomic disorder induced inhomogeneous broadening by introducing vacancies, silicon interstitials, and oriented strain fields in the vicinity of a G center. A vacancy leads to a tensile strain and can result in either a red shift or a blue shift of the G-center emission, depending on its position relative to the G center. Meanwhile, $\mathrm{Si}$ interstitials lead to compressive strain, which results in a monotonic red shift. High-flux and tunable ion pulses enable the exploration of the fundamental dynamics of radiation-induced defects as well as methods for the optimization of G-center formation and qubit synthesis for quantum information processing.