The development of gain-driven polaritons offers a fresh approach to enhancing solid-state microwave sources and amplifiers, apart from the traditional concept of the maser. Despite their potential, these polaritons are governed by intricate dynamics that remain largely unexplored, impeding practical applications. The authors employ time-domain measurements to investigate the transient response of gain-driven polaritons, revealing distinct dynamical behaviors: damping, zero damping, and antidamping. This insight could impact the engineering of ultrafast polaritonic devices tailored for coherent microwave and optical applications.

Spin pumping can generate spin current by driving a magnetic system into resonance, a phenomenon that can be electrically detected via the inverse spin Hall effect. However, the efficiency of spin current generation has remained limited, compared to that of spin-to-charge conversion. This work systematically demonstrates an innovative technique for electrically probing propagating spin waves and creating spin current with high efficiency, mediated by flat-band magnons excited by broad wave vectors. These findings lay a promising foundation for further experiments with propagating spin waves.

The ultrafast all-optical control of magnetization without relying on heat is promising for magnetic recording technology. While the magnetization switching between two stable bit states does not require control over light polarization, photomagnetic toggling of magnetization (equivalent to the XOR logic operation) can be achieved. This study probes the efficiency of a back-switching scenario between two stable bit states, using a pair of femtosecond laser pulses with either the same or orthogonal orientations of linear polarization. Such a nonthermal toggle regime not only can provide recording at rates up to 50 GHz, but also can perform basic logic operations.

Microwave-free magnetometry with N-$V$ centers has emerged as a complementary method to traditional techniques when the use of microwaves is impractical, particularly in applications involving metals and biological samples. Integration of this method with imaging capabilities offers the potential for nondestructive probing in a 2D spatial plane, while also capturing temporal dynamics within existing technological constraints. It is evident that the limits of sensitivity have not been fully realized, and improvements may be achieved via faster specialized cameras and advanced color-center fabrication.

Machines based on coupled bistable oscillators can rapidly produce high-quality solutions to difficult problems in combinatorial optimization. While the dynamics of such systems can be derived, exactly why these dynamics are so good for optimization is unclear. This study presents a complete mathematical equivalence between coupled-oscillator machines and the primal-dual method of Lagrange multipliers, elucidating the precise mathematical role of each hardware component and enabling the principled design of more sophisticated optimization machines. Simulations show that such a circuit consumes extremely low amounts of power and energy per optimization, even for many variables.

While public key cryptography enables the Internet to carry secure communications for billions of people every day, this critical technology is not infallible. It is well known that the Internet needs to transition to new “post-quantum” cryptosystems, to protect against the possibility of future cryptanalysis using quantum computers. This Perspective suggests that there is a broader scope for using quantum technologies to enhance cybersecurity, including fundamentally different approaches to constructing hardware security modules, to protect the secret keys that are the linchpin of cryptography on the Internet.

Dynamic beamforming is critical in applications such as radar detection, holographic imaging, and reconfigurable intelligent surfaces (RIS). This Perspective reviews a revolutionary and economical technique to achieve dynamic beamforming, utilizing the moiré pattern formed by twisted stacked metasurfaces. Research here faces challenges such as far-field calculations and the inverse design of specific radiation patterns, due to our limited understanding of the complex mode coupling between the moiré pattern and the metallic back plate. The authors outline potential solutions and project the future applications and research directions for the reflective moiré metasurface.

Spin waves and their quanta magnons are the collective excitations of a spin systems of a magnetic material, which offer the potential for higher efficiency and lower energy consumption in solving specific issues in data processing. This Perspective discusses the current challenges in realizing magnonic circuits based on the building blocks developed to date, and further looks at the application of magnons in neuromorphic networks and stochastic, reservoir, and quantum computing, and discusses their advantages over conventional electronics in these areas.

Reconstructing the effective equation of motion for the time evolution of a subset of degrees of freedom of a larger system remains a problem of interest in quantum physics. Many methods have been developed, but they either rely on an ad hoc ansatz, demand data that is not experimentally accessible, or lack physical interpretability. The authors employ machine-learning methods to infer the effective dynamical generator from a noisy, finite set of local measurements. Their method yields interpretable results that may be used to infer noise models on quantum simulators, or to study thermalization dynamics in quantum many-body systems.

While haloscope experiments searching for axion dark matter with cylindrical microwave cavity resonators are the most sensitive to date, that sensitivity is degraded at high frequencies, due to geometric scaling. The authors demonstrate a prototype thin-shell cavity resonator that decouples volume from resonant frequency, and thus avoids such degradation. As the resonator comprises two mechanically isolated pieces, a protocol is developed for robust, automated precision alignment, which enables a wide tuning range for the resonator’s axion-sensitive TM${}_{010}$ mode. A discussion of the instrument’s feasibility for high-frequency probes of the post-inflationary scenario is also offered.

Accurate measurement of a material’s coefficient of thermal expansion is a potentially powerful probe for investigating phase transitions. Unfortunately, familiar techniques are either low-resolution or put stringent requirements on sample preparation. This study implements ultrastable optical-fiber interferometry for contactless optical measurement with picometer spatial resolution, and can measure fragile samples less than 100 µm thick with millikelvin-level temperature resolution. Using their method on BaFe${}_2$As${}_2$, the authors discover hysteresis at the first-order phase transition between its antiferromagnetic and paramagnetic phases, with a boundary moving at about 188 µm/s.

The development of gain-driven polaritons offers a fresh approach to enhancing solid-state microwave sources and amplifiers, apart from the traditional concept of the maser. Despite their potential, these polaritons are governed by intricate dynamics that remain largely unexplored, impeding practical applications. The authors employ time-domain measurements to investigate the transient response of gain-driven polaritons, revealing distinct dynamical behaviors: damping, zero damping, and antidamping. This insight could impact the engineering of ultrafast polaritonic devices tailored for coherent microwave and optical applications.

Spin pumping can generate spin current by driving a magnetic system into resonance, a phenomenon that can be electrically detected via the inverse spin Hall effect. However, the efficiency of spin current generation has remained limited, compared to that of spin-to-charge conversion. This work systematically demonstrates an innovative technique for electrically probing propagating spin waves and creating spin current with high efficiency, mediated by flat-band magnons excited by broad wave vectors. These findings lay a promising foundation for further experiments with propagating spin waves.

The ultrafast all-optical control of magnetization without relying on heat is promising for magnetic recording technology. While the magnetization switching between two stable bit states does not require control over light polarization, photomagnetic toggling of magnetization (equivalent to the XOR logic operation) can be achieved. This study probes the efficiency of a back-switching scenario between two stable bit states, using a pair of femtosecond laser pulses with either the same or orthogonal orientations of linear polarization. Such a nonthermal toggle regime not only can provide recording at rates up to 50 GHz, but also can perform basic logic operations.

Microwave-free magnetometry with N-$V$ centers has emerged as a complementary method to traditional techniques when the use of microwaves is impractical, particularly in applications involving metals and biological samples. Integration of this method with imaging capabilities offers the potential for nondestructive probing in a 2D spatial plane, while also capturing temporal dynamics within existing technological constraints. It is evident that the limits of sensitivity have not been fully realized, and improvements may be achieved via faster specialized cameras and advanced color-center fabrication.

Machines based on coupled bistable oscillators can rapidly produce high-quality solutions to difficult problems in combinatorial optimization. While the dynamics of such systems can be derived, exactly why these dynamics are so good for optimization is unclear. This study presents a complete mathematical equivalence between coupled-oscillator machines and the primal-dual method of Lagrange multipliers, elucidating the precise mathematical role of each hardware component and enabling the principled design of more sophisticated optimization machines. Simulations show that such a circuit consumes extremely low amounts of power and energy per optimization, even for many variables.