Random number generators underpin the security of current and future cryptographic systems and are therefore a likely target for attackers. Quantum random number generators have been hailed as the ultimate sources of randomness. However, as shown in this work, the susceptibility of the sensitive electronics required to implement such devices poses a serious threat to their security. We present an out-of-band electromagnetic injection attack on a photonic quantum random number generator through which an adversary can gain full control of the output. In our first experiment, the adversary forces the binary output of the generator to become an alternating string of 1s and 0s, with near 100% success. This attack may be spotted by a vigilant user performing statistical tests on their output strings. We therefore envisage a second more subtle attack in which the adversary forces the output to be a random pattern known to them, thus rendering any protection based on statistical tests ineffective.

Adiabatic operations are powerful tools for robust quantum control in numerous fields of physics, chemistry, and quantum information science. The inherent robustness due to adiabaticity can, however, be impaired in applications requiring short evolution times. We present a single versatile gradient-based optimization protocol that combines adiabatic control with effective Hamiltonian engineering in order to design adiabatic operations tailored to the specific imperfections and resources of an experimental setup. The practicality of the protocol is demonstrated by engineering a fast, 2.3 Rabi cycle-long adiabatic inversion pulse for magnetic resonance with built-in robustness to Rabi field inhomogeneities and resonance offsets. The performance and robustness of the pulse is validated in a nanoscale force-detected magnetic resonance experiment on a solid-state sample, indicating an ensemble-averaged inversion accuracy of 99.997%. We further showcase the utility of our protocol by providing examples of adiabatic pulses robust to spin-spin interactions, parameter-selective operations, and operations connecting arbitrary states, each motivated by experiments.

Tunable synthetic spin-orbit coupling (SSOC) is one of the key challenges in various quantum systems, such as ultracold atomic gases, topological superconductors, and semiconductor quantum dots. Here we experimentally demonstrate controlling the SSOC by investigating the anisotropy of spin-valley resonance in a silicon quantum dot. As we rotate the applied magnetic field in plane, we find a striking nonsinusoidal behavior of resonance amplitude that distinguishes SSOC from the intrinsic spin-orbit coupling (ISOC), and associate this behavior with the previously overlooked in-plane transverse magnetic field gradient. Moreover, by theoretically analyzing the experimentally measured SSOC field, we predict the quality factor of the spin qubit could be optimized if the orientation of the in-plane magnetic field is rotated away from the traditional working point.

Topological matter and topological optics have been studied in many systems, with promising applications in materials science and photonics technology. These advances motivate the study of the interaction between topological matter and light, as well as topological protection in light-matter interactions. In this work, we study a waveguide-interfaced topological atom array. The light-matter interaction is nontrivially modified by topology, yielding optical phenomena. We find topology-enhanced photon absorption from the waveguide for a large Purcell factor, i.e., $\mathrm{\Gamma }/{\mathrm{\Gamma }}_0\gg 1$, where $\mathrm{\Gamma }$ and ${\mathrm{\Gamma }}_0$ are the atomic decays to the waveguide and environment, respectively. To understand this unconventional photon absorption, we propose a multichannel scattering approach and study the interaction spectra for edge- and bulk-state channels. We find that, by breaking inversion and time-reversal symmetries, optical anisotropy is enabled for the reflection process, but the transmission is isotropic. Through a perturbation analysis of the edge-state channel, we show that the anisotropy in the reflection process originates from the waveguide-mediated non-Hermitian interaction. However, the inversion symmetry in the non-Hermitian interaction makes the transmission isotropic. At a topology-protected atomic spacing, the subradiant edge state exhibits huge anisotropy. Because of the interplay between edge- and bulk-state channels, a large topological bandgap enhances nonreciprocal reflection of photons in the waveguide for weakly broken time-reversal symmetry, i.e., ${\mathrm{\Gamma }}_0/\mathrm{\Gamma }\ll 1$, producing complete photon absorption. We show that our proposal can be implemented in superconducting quantum circuits. The topology-enhanced photon absorption is useful for quantum detection. This work shows the potential to manipulate light with topological quantum matter.

Laser-wakefield acceleration has been demonstrated to be a promising technique for compact electron accelerators. However, it is still challenging to achieve high-quality 100-MeV electron bunches of subfemtosecond duration with current techniques. Here, we present and numerically demonstrate an efficient scheme to produce such high-energy tunable ultrashort electron bunches, which is achieved by the use of a nonlinear wakefield driven by a terawatt few-cycle laser pulse in a tailored plasma channel. With the driving laser pulse energy of 25 mJ only, the resulting electron bunch can reach 101 MeV with 7.8-pC charge, approximately 4 mrad divergence, approximately 3% energy spread, energy efficiency above 3%, and a duration of hundreds of attoseconds. As such laser pulses may be obtained with a kilohertz repetition rate and high stability, our scheme could be interesting for ultrafast science and accelerator community.

Imaging the (sub)micron scale over large areas with high temporal resolution becomes increasingly necessary for the development and investigation of novel materials under realistic operation conditions. Small angle x-ray scattering imaging methods provide micro- and nanoscale structural information of materials. A fundamental shortcoming of such methods is the long acquisition time required to investigate macroscopic objects. In this work, we propose a single shot imaging method that allows reciprocal space sensitivity at a local level while maintaining spatial resolution for imaging macroscopic objects. We use an instrument that is sensitive to the ultrasmall angle x-ray scattering range and utilize it to image unstable polydisperse particle systems. This allows us to observe in real time the evolution of the local average particle diameter due to the stratification of the microparticles.

Existing acoustical tweezers (ATs) modulate source geometries to achieve monochromatic focusing, the wide frequency space allowed by sources is neglected. Here an analogous approach is proposed to synthesize AT fields in temporal frequency instead of wave vector. The base element is a standing wave, its counterpart in beam ATs is the field of a pitch. After field periodization, radiation-force theory is extended to nonmonochromatic waves having arbitrary in-period force based on discrete-frequency Fourier analysis. Fourier acoustical tweezers (FATs) achieve a minimal spacing of $\lambda /4$ for independent trapping. System complexity is reduced to two sources per dimension.

We show that doped cubic iron pyrite, which is a diamagnetic semiconductor, becomes ferromagnetic when $p$-type doped. We furthermore find that this material can exhibit high spin polarization both for tunneling and transport devices. These results are based on first-principles electronic structure and transport calculations. This illustrates the use of $p$-type doping without magnetic impurities as a strategy for obtaining ferromagnetic semiconducting behavior, with implications for spintronic applications that require both magnetic ordering and good mobility. This is a combination that has been difficult to achieve by doping semiconductors with magnetic impurities. We show that phosphorus and arsenic may be effective dopants for achieving this behavior.

Broadband orbital-angular-momentum (OAM) multiplexing is of great interest due to its ability to increase channel capacity in wireless communications. However, broadband OAM-mode-reconfigurable transmitarrays have yet to be developed. Here, a broadband 1-bit electronically reconfigurable transmitarray for OAM-mode-reconfigurable beam generation is proposed, fabricated, and tested. The OAM transmitarray includes $20\times 20$ elements with a 1-bit phase resolution, each element is controlled by two $p$-$i$-$n$ diodes with a minimum insertion loss of 0.9 dB and a 3-dB transmission bandwidth of 17.1%. A nearly perfect 1-bit phase resolution is obtained by reversing the current. The OAM mode can be switched between $l=+1$ and $l=+2$ based on a steering-logic board. Both simulation and experiment results show the transmitarray can generate a broadband OAM-mode-reconfigurable beam from 9.5 to 10.5 GHz. The proposed transmitarray may have potential applications in broadband OAM-multiplexing communications.

Acoustic radiation force is a net force experienced by an object under the action of an acoustic wave. Most theoretical models require the acoustic wave to be periodic, if not purely monofrequency, and are therefore irrelevant for the study of acoustic radiation force due to acoustic pulses. Here, we introduce the concept of finite-duration pulses, which is the most general condition to derive the acoustic radiation force. In the case of small spheres, we extend the Gor’kov formula to unsteady acoustic fields such as traveling pulses and interfering wave packets. In the latter case, our study suggests that the concept of acoustic contrast is also relevant to express the acoustic radiation force. For negative acoustic contrast particles, the acoustic trapping region narrows with shorter pulses, whereas positive contrast particles (such as biological cells) can fall in secondary traps when the pulse width deviates from an optimal value. This theoretical insight may help to improve the selectivity of pulsed acoustic tweezers.

Silicon-quantum-dot qubits must contend with low-lying valley excited states that are sensitive functions of the quantum-well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated $\mathrm{Si}$/$\mathrm{Si}$-$\mathrm{Ge}$ double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited-state energies spanning multiple wafers, quantum dots, and orbital states, which are crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for increasing valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley-splitting measurements for guiding the development of $\mathrm{Si}$ qubits.

High-density electron-bunch train–based tunable coherent terahertz radiation with a narrow spectral bandwidth is highly essential for many scientific applications. Here we propose a scheme to produce ultrahigh-peak-current (more than 10 kA) electron-bunch trains with tunable picosecond spacing for the generation of such terahertz sources. Passing through a plasma section and interacting with the self-excited nonlinear plasma wake, the beam gains a “sawtooth” energy modulation. By means of magnetic optics, it is then effectively converted into a beam density modulation forming microbunches with a high charge of up to a few nanocoulombs and a bunching factor as high as approximately $0.8$. Tunable narrowband terahertz radiation with energy on the order of 10 mJ can be generated, which we show in both theoretical analyses and simulations via this process. This scheme can be realized in accelerator facilities with repetition rates on the order of kilohertz or even submegahertz, which provides great potential in modern science and related technologies.

While coherently driven Kerr microcavities have rapidly matured as a platform for frequency-comb formation, such microresonators generally possess weak Kerr coefficients; consequently, triggering comb generation requires millions of photons to be circulating inside the cavity. This suppresses the role of quantum fluctuations in the dynamics of the comb. In this paper, we realize a minimal version of coherently driven Kerr-mediated microwave-frequency combs in the circuit quantum electrodynamics (cQED) architecture, where the fluctuations of the quantum vacuum are the primary limitation on comb coherence. We achieve a comb phase coherence of up to 35 $\mu \mathrm{s}$, approaching the theoretical device quantum limit of 55 $\mu \mathrm{s}$ and vastly longer than the inherent lifetimes of the modes, of 13 ns. The ability within cQED to engineer stronger nonlinearities than optical microresonators, together with operation at cryogenic temperatures, and the excellent agreement of comb dynamics with quantum theory indicates a promising platform for the study of complex dynamics of quantum nonlinear systems.

Absorption measurement is an exceptionally versatile tool for many applications in science and engineering. For absorption measurements using laser beams of light, the sensitivity is theoretically limited by the shot noise due to the fundamental Poisson distribution of the photon number in laser radiation. In practice, the shot-noise limit can only be achieved when all other sources of noise are eliminated. Here, we use seeded squeezed light to demonstrate that direct-absorption measurements can be performed with a sensitivity beyond the shot-noise limit. We present a practically realizable scheme, where intensity-squeezed beams are generated by a seeded four-wave mixing process in an atomic rubidium-vapor cell. More than 1.2 dB quantum advantage for the measurement sensitivity is obtained at faint absorption levels ($\le 10\mathrm{\%}$). We also present a detailed theoretical analysis to show that the observed quantum advantage when corrected for optical loss would be equivalent to 3 dB. Our experiment demonstrates a direct sub-shot-noise measurement of absorption that requires neither homodyne or lock-in nor logic coincidence-detection schemes. It is therefore very applicable in many circumstances where sub-shot-noise-level absorption measurements are highly desirable.

The diffusion of particles has wide repercussions, ranging from particle-based soft-matter systems to solid-state systems with particular electronic properties. Recently, in the field of magnetism, the diffusion of magnetic skyrmions, topologically stabilized quasiparticles, has been demonstrated. Here, we show that, by applying a magnetic in-plane field, and therefore, breaking the symmetry of the system, skyrmion diffusion becomes anisotropic, with faster diffusion parallel to the field axis and slower diffusion perpendicular to it. We furthermore show that the absolute value of the applied magnetic in-plane field controls the absolute values of the diffusion coefficients, so that one can thereby tune both the orientation of the diffusion and its strength. Based on the stochastic Thiele equation, we can explain the observed anisotropic diffusion as a result of the elliptical deformation of the skyrmions by the application of the in-plane field.

The effect of multiaxial mechanical stress on angle-dependent magnetization reversals is studied on a single 10-nm ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film, a single 10-nm ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ film, and a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$(10 nm)/${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$(10 nm) bilayer. These films are grown on flexible $\mathrm{Al}$ “kitchenlike” foils. These flexible foils are bent on two convex optical lenses to apply multiaxial mechanical stresses of different magnitudes. In order to understand the mechanisms driving the physical properties of the flexible systems, the magnetic films are also grown on hard native-oxide-covered $\mathrm{Si}$(100) substrates. By probing the angular dependence of the coercive field and the remanent magnetization, we show that the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ single layer and the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ bilayer grown on $\mathrm{Si}$ present a uniaxial anisotropy. Along their easy axis, their domain structure at the coercive field exhibits large domains with sawtooth domain walls. However, the ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ layer grown on $\mathrm{Si}$ shows a cubic anisotropy and sharp domain walls with a right-angle geometry along its easy axis. All layers grown on $\mathrm{Al}$ foils show development or enhancement of a uniaxial anisotropy arising from the $\mathrm{Al}$ surface morphology. Multiaxial mechanical stress applied to the flexible $\mathrm{Al}$ foils results in very different nonreversible effects as a function of the composition of the layered system. For ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$, multiaxial mechanical stress has no effect, as expected for a nonmagnetostrictive system. For ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$, it results in a full in-plane randomization of its magnetization-reversal properties. For the bilayer, multiaxial mechanical stress does not fully randomize the in-plane magnetization-reversal properties: the bilayer retains a uniaxial character.

We study the implementation of quantum-key-distribution (QKD) systems over quantum-repeater infrastructures. We particularly consider quantum repeaters with encoding and compare them with probabilistic quantum repeaters. To that end, we propose two decoder structures for encoded repeaters that not only aid system performance but also make the implementation aspects easier by removing two-qubit gates from the QKD decoder. By developing several scalable numerical and analytical techniques, we then identify the resilience of the setup to various sources of error in gates, measurement modules, and initialization of the setup. We apply our techniques to three- and five-qubit repetition codes and obtain the normalized secret key generation rate per memory per second for encoded and probabilistic quantum repeaters. We quantify the regimes of operation, where one class of repeater outperforms the other, and find that there are feasible regimes of operation where encoded repeaters—based on simple three-qubit repetition codes—could offer practical advantages.

Cavity resonators are promising resources for quantum technology, while native nonlinear interactions for cavities are typically too weak to provide the level of quantum control required to deliver complex targeted operations. Here we investigate a scheme to engineer a target Hamiltonian for photonic cavities using ancilla qubits. By off resonantly driving dispersively coupled ancilla qubits, we develop an optimized approach to engineering an arbitrary photon-number-dependent Hamiltonian for the cavities while minimizing the operation errors. The engineered Hamiltonian admits various applications including canceling unwanted cavity self-Kerr interactions, creating higher-order nonlinearities for quantum simulations, and designing quantum gates resilient to noise. Our scheme can be implemented with coupled microwave cavities and transmon qubits in superconducting circuit systems.