We investigate a system composed of an ensemble of room-temperature rare-earth ions embedded in a bulk crystal, intrinsically coupled to internal strain via their sensitivity to the surrounding crystal field. We evidence the generation of a mechanical response under resonant atomic excitation. We find this motion to be the sum of two fundamental, resonant optomechanical backaction processes: a conservative, piezo-orbital mechanism, resulting from the modification of the crystal field associated with the promotion of the ions to their excited state, and a dissipative, nonradiative photothermal process related to the phonons generated throughout the atomic population relaxation. Our work expands the horizons of research in hybrid optomechanics, and unveils unexplored interactions that may be key for understanding the dephasing dynamics of ultracoherent rare-earth ions.

Brownian motion imposes a hard limit on the overall precision of a nanomechanical measurement. Here, we present a combined experimental and theoretical study of the Brownian dynamics of a quintessential nanomechanical system, a doubly clamped nanomechanical beam resonator, in a viscous fluid. Our theoretical approach is based on the fluctuation-dissipation theorem of statistical mechanics: we determine the dissipation from fluid dynamics; we incorporate this dissipation into the proper elastic equation to obtain the equation of motion; and the fluctuation-dissipation theorem then directly provides an analytical expression for the position-dependent power spectral density (PSD) of the displacement fluctuations of the beam. We compare our theory to experiments on nanomechanical beams immersed in air and water and obtain excellent agreement. Within our experimental parameter range, the Brownian-force noise driving the nanomechanical beam has a colored PSD due to the “memory” of the fluid; the force noise remains mode independent and uncorrelated in space. These conclusions are not only of interest for nanomechanical sensing but also provide insight into the fluctuations of elastic systems at any length scale.

The linear magnetoelectric effect (ME) is the phenomenon by which an electric field produces a magnetization. Its observation requires both time-reversal and space-inversion symmetries to be broken, as in multiferroics. While the ME effect has only been studied in insulating materials, it can actually exist in noncentrosymmetric conductors such as two-dimensional electron gases (2DEGs) with Rashba spin-orbit coupling. It is then coined the Edelstein effect (EE), by which a bias voltage—generating a charge current—produces a transverse spin density, i.e., a magnetization. Interestingly, 2D systems are sensitive to voltage gating, which provides an extra handle to control the EE. Here, we show that the sign of the EE in a ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ 2DEG can be controlled by a gate voltage. We propose various logic devices harnessing the dual control of the spin density by current and gate voltages and discuss the potential of our findings for gate-tunable nonreciprocal electronics.

The technique of laser ultrasonics perfectly meets the need for noncontact, noninvasive, nondestructive mechanical probing of nanometer- to millimeter-size samples. However, this technique is limited to the excitation of low-amplitude strains, below the threshold for optical damage of the sample. In the context of strain engineering of materials, alternative optical techniques enabling the excitation of high-amplitude strains in a nondestructive optical regime are needed. We introduce here a nondestructive method for laser-shock wave generation based on additive superposition of multiple laser-excited strain waves. This technique enables strain generation up to mechanical failure of a sample at pump laser fluences below optical ablation or melting thresholds. We demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in Nb-${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ substrates, with associated strains in the percent range and pressures up to 3 GPa at 1 kHz repetition rate and close to 10 GPa for several hundred shocks. This study paves the way for the investigation of a host of high-strain SAW-induced phenomena, including phase transitions in conventional and quantum materials, plasticity and a myriad of material failure modes, chemistry and other effects in bulk samples, thin layers, and two-dimensional materials.

The maximum clique problem (MCP) is a typical nondeterministic polynomial-time complete (NP-complete) problem that is considered to be intractable for classical computers. Photons are expected to tackle these computationally hard problems as they are intrinsically suitable for parallel computing. Here, we propose and experimentally demonstrate a scalable and programmable three-dimensional photonic processor that can efficiently solve the MCP with photons in a parallel manner. By designing three-dimensional waveguide networks, we map the MCP into a photonic chip fabricated by femtosecond laser direct writing. The prototype demonstrates the superiority of photonic parallel computing based on non-von Neumann structures. Furthermore, we show that our photonic processor can be applied to search for the optimal molecular docking configurations, a major issue in pharmaceutical medication development. We manage to map different graphs into a single three-dimensional waveguide network by encoding injection conditions of photons, which build an alternative paradigm to indirectly program three-dimensional photonic chips. The combination of parallel and fully programmable capabilities of photonic computing paves the way for wider applications.

The dynamical properties of skyrmions can be exploited to build devices with new functionalities. Here, we first investigate a skyrmion-based ring-shaped device by means of micromagnetic simulations and Thiele’s equation. We subsequently show three application scenarios: (1) a clock with tunable frequency that is biased with an electrical current having a radial spatial distribution, (2) an alternator, where the skyrmion circular motion driven by an engineered anisotropy gradient is converted into an electrical signal, and (3) an energy harvester, where the skyrmion motion driven by a thermal gradient is converted into an electrical signal, thus providing a heat recovery operation. We also show how to precisely tune the frequency and amplitude of the output electrical signals by varying material parameters, geometrical parameters, and number and velocity of skyrmions, and we further prove the correct device functionality under realistic conditions of room temperature and internal material defects. Our results open an alternative route for the realization of energy efficient nanoscale clocks, generators, and energy harvesters.

A permanent magnet can be levitated simply by placing it in the vicinity of another permanent magnet that rotates in the order of 200 Hz. This surprising effect can be easily reproduced in the laboratory with off-the-shelf components. Here, we have investigated this novel type of magnetic levitation experimentally and clarified the underlying physics. Using a 19-mm-diameter spherical Nd-Fe-B magnet as the rotor magnet, we have captured the detailed motion of levitating spherical $\mathrm{Nd}$-$\mathrm{Fe}$-$\mathrm{B}$ magnets, denoted floater magnets, as well as the influence of the rotation speed and magnet size on the levitation. We have found that as levitation occurs, the floater-magnet frequency locks with the rotor magnet and, noticeably, that the magnetization of the floater is oriented close to the axis of rotation and toward the like pole of the rotor magnet. This is in contrast to what might be expected by the laws of magnetostatics, as the floater is observed to align its magnetization essentially perpendicular to the magnetic field of the rotor. Moreover, we have found that the size of the floater has a clear influence on the levitation: the smaller the floater, the higher the rotor speed that is necessary to achieve levitation and the further away the levitation point shifts. Despite the unexpected magnetic configuration during levitation, we have verified that magnetostatic interactions between the rotating magnets are responsible for creating the equilibrium position of the floater. Hence, this type of magnetic levitation does not rely on gravity as a balancing force to achieve an equilibrium position. Based on theoretical arguments and a numerical model, we show that a constant vertical field and eddy-current-enhanced damping are sufficient to produce levitation from rest. This enables a gyroscopically stabilized counterintuitive steady-state moment orientation and the resulting magnetostatically stable midair equilibrium point. The numerical model displays the same trends with respect to the rotation speed and the floater-magnet size as seen in the experiments.

${}^{171}{\mathrm{Yb}}^{3+}$ in ${\mathrm{YVO}}_4$ is a promising candidate for building quantum networks with good optical addressability, excellent spin properties and a secondary nuclear-spin quantum register. However, the associated long optical lifetime necessitates coupling to optical resonators for faster emission of single photons and to facilitate control of single ${}^{171}\mathrm{Yb}$ ions. Previously, single ${}^{171}\mathrm{Yb}$ ions were addressed by coupling them to monolithic photonic crystal cavities fabricated via lengthy focused ion beam milling. Here, we design and fabricate a hybrid platform based on ions coupled to the evanescently decaying field of a GaAs photonic crystal cavity. For the most strongly coupled ion close to the GaAs-YVO interface, we find a 64-fold reduction in lifetime corresponding to a Purcell enhancement of 179. For an ion with a Purcell enhancement of 21, we experimentally detect and demonstrate coherent optical control. The results show a promising route toward a quantum network with ${}^{171}\mathrm{Yb}$-${\mathrm{YVO}}_4$ using a highly scalable platform that can readily be applied to other quantum emitters in the near-infrared.

Classical artificial neural networks have witnessed widespread successes in machine-learning applications. Here, we propose fermion neural networks (FNNs) whose physical properties, such as local density of states or conditional conductance, serve as outputs, once the inputs are incorporated as an initial layer. Comparable to back propagation, we establish an efficient optimization, which entitles FNNs to competitive performance on challenging machine-learning benchmarks. FNNs also directly apply to quantum systems, including hard ones with interactions, and offer in situ analysis without preprocessing or presumption. Following machine learning, FNNs precisely determine topological phases and emergent charge orders. Their quantum nature also brings various advantages: quantum correlation entitles more general network connectivity and insight into the vanishing gradient problem, quantum entanglement opens up alternative avenues for interpretable machine learning, etc.

Quantum computers based on gate-defined quantum dots (QDs) are expected to scale. However, as the number of qubits increases, the burden of manually calibrating these systems becomes unreasonable and autonomous tuning must be used. There has been a range of recent demonstrations of automated tuning of various QD parameters such as coarse gate ranges, global state topology (e.g., single QD, double QD), charge, and tunnel coupling with a variety of methods. Here, we demonstrate an intuitive, reliable, and data-efficient set of tools for an automated global state and charge tuning in a framework deemed physics-informed tuning (PIT). The first module of PIT is an action-based algorithm that combines a machine learning classifier with physics knowledge to navigate to a target global state. The second module uses a series of one-dimensional measurements to tune to a target charge state by first emptying the QDs of charge, followed by calibrating capacitive couplings and navigating to the target charge state. The success rate for the action-based tuning consistently surpasses $95\mathrm{\%}$ on both simulated and experimental data suitable for off-line testing. The success rate for charge setting is comparable when testing with simulated data, at $95.5(5.4)\mathrm{\%}$, and only slightly worse for off-line experimental tests, with an average of $89.7(17.4)\mathrm{\%}$ (median $97.5\mathrm{\%}$). It is noteworthy that the high performance is demonstrated both on data from samples fabricated in an academic cleanroom as well as on an industrial 300-mm process line, further underlining the device agnosticism of PIT. Together, these tests on a range of simulated and experimental devices demonstrate the effectiveness and robustness of PIT.

Future large-scale quantum computing devices based on semiconductor spin qubits require multiplexed microwave control signals for manipulation and readout of the qubits. However, interfacing these control signals between room temperature and cryogenic temperatures where the qubits operate is a complex technical challenge. Here, we show a microwave modulator based on a nanoengineered semiconductor quantum dot that is designed to operate at millikelvin temperatures alongside the qubits. We operate the modulator as a mixer and a frequency multiplier over a bandwidth of up to 25 GHz. We estimate the power dissipation when driving quantum gates using frequency-up-converted signals under realistic experimental conditions to be 4 pW, highly compatible with the cooling powers available in current commercial dilution refrigerators. The device is fabricated in silicon using atomic precision lithography, providing a pathway toward combining qubits and classical control functionality on the same integrated chip.

Long-lived singlet states (LLSs) of nuclear spin pairs have been extensively studied and utilized in the isotropic phase via liquid-state nuclear magnetic resonance. However, there are hardly any reports of LLSs in the anisotropic phase that allows contribution from the dipolar coupling in addition to the scalar coupling, thereby opening many exciting possibilities. Here we report observing LLS in a pair of nuclear spins partially oriented in the nematic phase of a liquid crystal solvent. The spins are strongly interacting via the residual dipole-dipole coupling. We observe LLS in the oriented phase living up to 3 times longer than the usual spin-lattice relaxation time constant ($T_1$). Upon heating, the system undergoes a phase transition from nematic into isotropic phase, wherein the LLS is up to 5 times longer lived than the corresponding $T_1$. Interestingly, the LLS prepared in the oriented phase can survive the transition from the nematic to the isotropic phase. As an application of LLS in the oriented phase, we utilize its longer life to measure the small translational diffusion coefficient of solute molecules in the liquid crystal solvent. Finally, we propose utilizing the phase transition to lock or unlock access to LLS.

The long-lived and optically addressable spin states of silicon vacancies ($V_{\mathrm{Si}}$) in $4H$-SiC make them promising qubits for quantum communication and sensing. These color centers can be created in both the hexagonal ($V1$) and in the cubic ($V2$) local crystallographic environments of the $4H$-SiC host. While the spin of the $V2$ center can be efficiently manipulated by optically detected magnetic resonance at room temperature, spin control of the $V1$ center above cryogenic temperatures has so far remained elusive. Here, we show that the dynamic strain of surface acoustic waves can overcome this limitation and efficiently excite magnetic resonances of $V1$ centers up to room temperature. Based on the width and temperature dependence of the acoustically induced spin resonances of the $V1$ centers, we attribute them to transitions between spin sublevels in the excited state. The acoustic spin control of both kinds of $V_{\mathrm{Si}}$ centers in their excited states opens alternative ways for applications in quantum technologies based on spin optomechanics.

In spin-transfer-torque (STT) magnetic random-access memory (MRAM), both switching current and switching time depend on the damping of the free layer. It is known that the magnetic damping constant (α) varies with temperature; because STT MRAM cells operate well above room temperature, there is an urgent need to determine and understand damping properties in STT MRAM free layers at elevated temperatures. This paper reports on comparative studies of high-temperature frequency-dependent ferromagnetic resonance (FMR) in two STT MRAM free layers: (i) a low-α free layer with split $\mathrm{W}$ layers, and (ii) a conventional free layer with a single $\mathrm{W}$ layer. Comprehensive FMR measurements and analyses show that (1) the damping constant of the low-α free layer is always smaller than that in the conventional free layer, as the temperature increases from 300 to 520 K; (2) the damping constant increases monotonically with temperature in both the free layers; and (3) the damping increase is relatively modest in the low-α free layer, from about 0.0035 to about 0.0079, but is more pronounced in the conventional free layer, from about 0.0092 to about 0.028. These results can be interpreted in terms of the temperature dependence of the damping produced by spin-flip magnon-electron scattering and the difference of the Curie temperatures in the two free layers. They clearly establish the technological advantage of the low-α free layer over the conventional free layer from the perspective of the STT MRAM application. In addition, the high-temperature FMR data also yield important information about the temperature dependence of the effective perpendicular anisotropy field and the origin of the inhomogeneous line broadening in the free layers.

Single-electron sources are an essential component of modern quantum nanoelectronic devices. Owing to their high accuracy and stability, they have been successfully employed for metrology applications, studying fundamental matter interactions and more recently for electron quantum optics. They are traditionally driven by state-of-the-art arbitrary wave-form generators that are capable of producing single-electron pulses on the sub-100-ps time scale. In this work, we use an alternative approach for generating ultrashort electron wave packets. By combining several harmonics provided by a frequency comb, we synthesize Lorentzian voltage pulses and then use them to generate electron wave packets. Through this technique, we report on the generation and detection of an electron wave packet with a temporal duration of 27 ps generated on top of the Fermi sea of a two-dimensional electron gas. Electron pulses this short enable studies on elusive ultrafast fundamental quantum dynamics in nanoelectronic systems and may pave the way to implementing flying-electron qubits by means of levitons.

Hot-carrier solar cells (HCSCs) are optoelectronic heat engines that could, in principle, achieve efficient solar energy conversion with a high detailed-balance limit of $85\mathrm{\%}$ at full solar concentration. Approaching this limit requires negligible carrier energy loss to the lattice and reversible carrier extraction via narrow, highly conductive energy windows. Practical implementations of HCSCs with nonideal contacts such as two-dimensional (2D) contacts and thermionic barriers have excess heat flows resulting in irreversible extraction. We examine the electronic heat flows in such schemes to explain where power losses originate and show that they determine carrier temperatures and device performance, providing insight into optimal characteristics of an HCSC. A compromise between maximizing current extraction and minimizing the associated heat flow determines the optimal extraction energy. This optimization for a thermionic barrier contact shows that remarkably low carrier temperatures ($\sim 500$ K) are required for optimal operation at 45% efficiency for a 1-eV-band-gap absorber under the particle conservation model, compared to $\sim 1000$ K at 60% efficiency for a black-body absorber under the impact ionization model, even in the absence of carrier-phonon interactions. Furthermore, we quantify the irreversible component of the heat flow and show that it is minimal for highly conductive 2D contacts that approach the Carnot limit. The results suggest that heat flows in HCSCs can be probed by measuring carrier temperature trends by varying applied bias and contact parameters, which may present clearer evidence of HCSC operation.

We present a study of the piezostrain-tunable gyrotropic dynamics in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ vortex microstructures fabricated on a $0.7\mathrm{Pb}[{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}]{\mathrm{O}}_3$-${0.3\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_3$ single-crystal substrate. Using field-modulated-spin-rectification measurements, we demonstrate large frequency tunability (up to 45%) in individual microdisks accessed locally with low surface voltages, and magnetoresistive readout. With increased voltage applied to the substrate, we observe a gradual decrease of the vortex-core gyrotropic frequency associated with the contribution of the strain-induced magnetoelastic energy. The frequency tunability strongly depends on the disk size, with increased frequency downshift for disks with larger diameter. Micromagnetic simulations suggest that the observed size effects originate from the joint action of the strain-induced magnetoelastic and demagnetizing energies in large magnetic disks. These results enable a selective energy-efficient tuning of the vortex gyrotropic frequency in individual vortex-based oscillators with all-electrical operation.

To achieve subpicometer sensitivities in the millihertz band, laser interferometric inertial sensors rely on some form of reduction of the laser-frequency noise, typically by locking the laser to a stable frequency reference, such as the narrow-line-width resonance of an ultrastable optical cavity or an atomic or molecular transition. In this paper, we report on a compact laser-frequency stabilization technique based on an unequal-arm Mach-Zehnder interferometer that is subnanometer stable at $10\text{μ}\mathrm{Hz}$, subpicometer at $0.5$ mHz, and reaches a noise floor of $7\mathrm{fm}/\sqrt{\mathrm{Hz}}$ at 1 Hz. The interferometer is used in conjunction with a dc servo to stabilize the frequency of a laser down to a fractional instability below $4\times {10}^{-13}$ at averaging times from 0.1 to 100 s. The technique offers a wide operating range, does not rely on complex lock-acquisition procedures, and can be readily integrated as part of the optical bench in future gravity missions.

The local oscillator in a practical continuous-variable quantum key distribution system fluctuates at any time during the key distribution process, which may open security loopholes for the eavesdropper to hide her eavesdropping behaviors. Based on this, we investigate a more stealthy quantum attack where the eavesdroppers simulate random fluctuations of local oscillator intensity in a practical discrete-modulated continuous-variable quantum key distribution system. Theoretical simulations show that both communicating parties will misestimate channel parameters and overestimate the secret key rate due to the modified attack model, even though they have monitored the mean local oscillator intensity and shot noise as commonly used. Specifically, the eavesdropper’s manipulation of random fluctuations in LO intensity disturbs the parameter estimation in a realistic discrete-modulated continuous-variable quantum key distribution system, where the experimental parameters are always used for constraints of the semidefinite program modeling. The modified attack introduced by random fluctuations of local oscillator can only be eliminated by monitoring the local oscillator intensity in real time, which places a higher demand on the accuracy of monitoring technology. Moreover, similar quantum hacking will also occur in a practical local local oscillator system by manipulating the random fluctuations in pilot intensity, which shows the strong adaptability and the role of the proposed attack.

The challenge of pattern recognition is to invoke a strategy that can accurately extract features of a dataset and classify its samples. In realistic scenarios, this dataset may be a physical system from which we want to retrieve information, such as in the readout of optical classical memories. The theoretical and experimental development of quantum reading has demonstrated that the readout of optical memories can be significantly enhanced through the use of quantum resources (namely, entangled input states) over that of the best classical strategies. However, the practicality of this quantum advantage hinges upon the scalability of quantum reading, and up to now its experimental demonstration has been limited to individual cells. In this work, we demonstrate quantum advantage at a fixed resource, namely, at fixed mean probe energy, in the multicell problem of pattern recognition. Through experimental realizations of digits from the Modified National Institute of Standards and Technology (MNIST) handwritten digit dataset, and the application of advanced classical postprocessing, we report the use of entangled probe states and photon counting to achieve quantum advantage in classification error over that achieved with classical resources, confirming that the advantage gained through quantum sensors can be sustained throughout pattern recognition and complex postprocessing. This motivates future developments of quantum-enhanced pattern recognition of bosonic loss within complex domains.