We experimentally demonstrate a programmable parametric mode sorter of high-dimensional signals in a composite spatiotemporal Hilbert space through mode-selective quantum frequency up-conversion. As a concrete example and with quantum communication applications in mind, we consider the Laguerre-Gaussian and Hermite-Gaussian modes as the spatial and temporal state basis for the signals, respectively. By modulating the spatiotemporal profiles of the up-conversion pump, we demonstrate the faithful selection of signal photons in those modes and their superposition modes. Our results show an improvement in the quantum mode-sorting performance by coupling the up-converted light into a single-mode fiber and/or operating the up-conversion at the edge of phase matching. Optimizing pump temporal profiles allows us to achieve more than 12-dB extinction for mutually unbiased basis (MUB) sets of the spatiotemporal modes. This fully programmable and efficient system could be a viable resource for quantum communications, quantum computation, and quantum metrology.

We analyze electric fields in ion sources generated by quasicontinuous photoionization of cold Rb atoms trapped in the focal spot of a near-concentric, in-vacuum cavity for 1064-nm laser light. Ion streams are extracted with an external electric field, $\mathbf{F}$. Stark effects of Rb $57F$ and of nearby high-angular-momentum Rydberg levels, which exhibit large, linear Stark shifts, are employed to study the net electric-field probability distribution within the ion-source region over an extraction-field range of $0<F<0.35\text{V/cm}$. For $F=0$, we also investigate ion-field-induced Stark spectra of the $60P_{1/2}$ state, which exhibits a (lesser) quadratic electric-field response that affords a simplified electric-field analysis. Experimental Rydberg spectra are compared with theoretical Stark spectra, which are weighted with net electric-field distributions obtained from classical ion-trajectory simulations that include Coulomb interactions. Experiments and models are in good agreement. At small $F$ and high ion-source rates, the field approximately follows a Holtsmark distribution, and the ion streams are degraded by the Coulomb microfields. With increasing $F$ and at lower ion-source rates, the fields become narrowly distributed around $\mathbf{F}$, resulting in directional ion streams that are less degraded by microfields. Our results are of interest for monitoring cold-ion sources for focused-ion-beam applications, where Coulomb interactions are of concern, and for studies of electric fields in cold plasmas.

Quantum sensors offer unparalleled precision, accuracy, and sensitivity for a variety of measurement applications. We report a compact magnetometer based on a ferrimagnetic sensing element in an oscillator architecture that circumvents challenges common to other quantum sensing approaches such as limited dynamic range, limited bandwidth, and dependence on vacuum, cryogenic, or laser components. The device exhibits a fixed, calibration-free response governed by the electron gyromagnetic ratio. Exchange narrowing in the ferrimagnetic material produces submegahertz transition linewidths despite the high unpaired spin density (approximately ${10}^{22}{\mathrm{cm}}^{-3}$). The magnetometer achieves a minimum sensitivity of 100 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ to ac magnetic fields of unknown phase and a sensitivity below 200 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ over a $1$ MHz bandwidth. By encoding magnetic field in frequency rather than amplitude, the device provides a dynamic range in excess of 1 mT. The passive, thermal initialization of the sensor’s quantum state requires only a magnetic bias field, greatly reducing power requirements compared to laser-initialized quantum sensors. With additional development, this device promises to be a leading candidate for high-performance magnetometry outside the laboratory, and the oscillator architecture is expected to provide advantages across a wide range of sensing platforms.

It is predicted that magnetic skyrmions can be controllably moved on surfaces using a focused laser beam. Here an absorbed power of the order 1 mW, focused to a spot size of the order 1 µm, results in a local temperature increase of around 50 K, and a local perpendicular strain of the order ${10}^{-3}$ due to the thermoelastic effect. For positive magnetoelastic coupling this generates a strong attractive force on skyrmions due to the magnetoelastic effect. The resultant motion is dependent on forces due to (i) gradients in the local strain-induced magnetic anisotropy, (ii) temperature gradients resulting in magnetic parameter gradients due to their temperature dependences, especially local effective anisotropy gradients, and (iii) Magnus effect acting on objects with nonzero topological number. Using dynamical magnetothermoelastic modeling, it is predicted skyrmions can be moved with significant velocities (up to 80 m/s shown), both for ferromagnetic and antiferromagnetic skyrmions, even in the presence of surface roughness. This mechanism of controllably moving single skyrmions in any direction, as well as addressing multiple skyrmions in a lattice, offers an approach to constructing and studying skyrmionic devices with all-optical control.

Oscillatory devices have recently attracted significant interest as key components of computing systems based on biomimetic neuronal spiking. An understanding of the time scales underlying the spiking is essential for engineering fast, controllable, low-energy devices. However, we find that the intrinsic dynamics of these devices is difficult to properly characterize, as they can be heavily influenced by the external circuitry used to measure them. Here we demonstrate these challenges using a ${\mathrm{VO}}_2$ Mott oscillator with a sub-100-nm effective size, achieved using a nanogap cut in a metallic carbon nanotube electrode. Given the nanoscale thermal volume of this device, it would be expected to exhibit rapid oscillations. However, due to external parasitics present within commonly used current sources, we see orders-of-magnitude slower dynamics. We outline methods for determining when measurements are dominated by extrinsic factors and discuss the operating conditions under which intrinsic oscillation frequencies may be observed.

We demonstrate a tunable, chip-scale wavelength reference to greatly reduce the complexity and volume of cold-atom sensors. A 1-mm optical path length microfabricated cell provides an atomic wavelength reference, with dynamic frequency control enabled by Zeeman-shifting the atomic transition through the magnetic field generated by the printed-circuit-board coils. The dynamic range of the laser frequency stabilization system is evaluated and used in conjunction with an improved generation of chip-scale cold-atom platforms that traps 4 million ${}^{87}\mathrm{Rb}$ atoms. The scalability and component consolidation provide a key step forward in the miniaturization of cold-atom sensors.

Air-bridge thermophotovoltaic (ABTPV) devices demonstrate near-99% utilization of photons whose energy is less than the semiconductor band gap (known as out-of-band, OOB, photons). Here, we determine 55.5% is the thermodynamic limit at an emitter temperature of 1400 K for single junction ABTPVs based on detailed balance, which compares to a thus-far reported efficiency of 32%. A technology computer-aided design model for the ($\text{In,Ga})\mathrm{As}$ diode predicts that suppressing the surface-recombination velocity to <200 cm/s significantly improves the open-circuit voltage and the shunt resistance, resulting in 48.6% as the practical efficiency limit. We find that a 1% OOB loss due to free-carrier absorption in doped regions of the TPV can drastically decrease the efficiency at larger band gaps and lower emitter temperature. We also analyze the effects of shadowing by metal grids, which generally introduces only minor losses in TPV performance.

Dielectric loss is known to limit state-of-the-art superconducting qubit lifetimes. Recent experiments imply upper bounds on bulk dielectric loss tangents on the order of 100 parts per billion but because these inferences are drawn from fully fabricated devices with many loss channels, these experiments do not definitely implicate or exonerate the dielectric. To resolve this ambiguity, we devise a measurement method capable of separating and resolving bulk dielectric loss with a sensitivity at the level of $5\times {10}^{-9}$. The method, which we call the dielectric dipper, involves the in situ insertion of a dielectric sample into a high-quality microwave cavity mode. Smoothly varying the participation of the sample in the cavity mode enables a differential measurement of the dielectric loss tangent of the sample. The dielectric dipper can probe the low-power behavior of dielectrics at cryogenic temperatures and does so without the need for any lithographic process, enabling controlled comparisons of substrate materials and processing techniques. We demonstrate the method with measurements of sapphire grown by edge-defined film-fed growth (EFG) in comparison to high-grade sapphire grown by the heat-exchanger method (HEMEX). For EFG sapphire, we infer a bulk loss tangent of $63(8)\times {10}^{-9}$ and a substrate-air interface loss tangent of $15(3)\times {10}^{-4}$ (assuming a sample surface thickness of $3\mathrm{nm}$). For a typical transmon, this bulk loss tangent would limit device quality factors to $Q⪅20\times {10}^6$, suggesting that bulk loss is likely the dominant loss mechanism in the longest-lived transmons on sapphire. We also demonstrate this method on HEMEX sapphire and bound its bulk loss tangent to be less than $19(6)\times {10}^{-9}$. As this bound is about 3 times smaller than the bulk loss tangent of EFG sapphire, the use of HEMEX sapphire as a substrate would lift the bulk dielectric coherence limit of a typical transmon qubit to several milliseconds.

Defect-free atom arrays are a precursor for quantum information processing and quantum simulation with neutral atoms. Yet, large-scale defect-free atom arrays can be challenging to realize, due to the losses encountered when rearranging stochastically loaded atoms to achieve a desired target array. Here, we demonstrate a parallel rearrangement algorithm that uses multiple mobile tweezers to independently sort and compress atom arrays in a way that naturally avoids atom collisions. With a high degree of parallelism, our algorithm offers a reduced move complexity compared to both single-tweezer algorithms and existing multitweezer algorithms. We further determine the optimal degree of parallelism to be a balance between an algorithmic speedup and multitweezer inhomogeneity effects. The defect-free probability for a 225-atom array is demonstrated to be as high as 33(1)% in a room-temperature setup after multiple cycles of rearrangement. The algorithm presented here can be implemented for any target array geometry with an underlying periodic structure.

Modern electron microscopy and spectroscopy are key technologies for studying the structure and composition of quantum and biological materials in fundamental and applied sciences. High-resolution spectroscopic techniques and aberration-corrected microscopes are often limited by the relatively large energy distribution of currently available beam sources. This can be improved by a monochromator, with the significant drawback of losing most of the beam current. Here, we study the field-emission properties of a monocrystalline niobium-tip electron field emitter at 5.2 K, well below the superconducting transition temperature. The emitter fabrication process can generate two tip configurations, with or without a nanoprotrusion at the apex, strongly influencing the field-emission energy distribution. The geometry without the nanoprotrusion has nanoampere beam currents, long-term stability, and an energy width of around 100 meV. The beam current can be increased by 2 orders of magnitude by xenon-gas adsorption. We also study the emitter performance up to 82 K and demonstrate that the energy width of the beam can be below 40 meV with high emitter brightness even at liquid-nitrogen cooling temperatures when an apex nanoprotrusion is present. Furthermore, the spatial and temporal electron-electron correlations of the field emission are studied at normal and superconducting temperatures and the influence of Nottingham heating is discussed. This monochromatic source will allow exceptional accuracy and resolution in electron microscopy, spectroscopy, and high-coherence quantum applications.

Recent theoretical work has highlighted that quantizing a superconducting circuit in the presence of time-dependent flux $\mathrm{\Phi }(t)$ generally produces Hamiltonian terms proportional to $d\mathrm{\Phi }/dt$ unless a special allocation of the flux across inductive terms is chosen. Here, we present an experiment probing the effects of a fast flux ramp applied to a heavy-fluxonium circuit. The experiment confirms that naïve omission of the $d\mathrm{\Phi }/dt$ term leads to theoretical predictions inconsistent with experimental data. Experimental data are fully consistent with recent theory that includes the derivative term or equivalently uses “irrotational variables” that uniquely allocate the flux to properly eliminate the $d\mathrm{\Phi }/dt$ term.

The illicit trafficking of radioactive materials, especially weapon-grade uranium or plutonium, is a significant security threat. Nuclear forensics helps trace the illicit trafficking of radioactive materials. The present study develops the methods for the forensics of the possible origins of fuels irradiated in nuclear reactors, which are the most powerful sources producing radioactive materials, including plutonium. Three key factors are significant for irradiated fuel forensics, namely, initial ${}^{235}\mathrm{U}$ enrichment, burnup, and the type of irradiation nuclear reactors. The methods for the first two are determined based on experimental data of six nuclear-reactor technologies and are further verified using the neutron-transport-depletion coupling simulation of the two major commercial reactor technologies, a pressurized-water reactor (PWR) and a boiling-water reactor (BWR). In addition, three machine-learning techniques are applied to discriminate between a PWR and a BWR, which are quite similar in neutronic properties, with nice accuracy and generalization ability. In summary, the presently determined methods provide a reliable pathway to predict the origins of spent nuclear fuels.

A distributed quantum network would require quantum nodes capable of performing arbitrary quantum information protocols with high fidelity. So far the challenge has been in realizing such quantum nodes with features for scalable quantum computing. We show here that using the solid-state spins in 4H silicon carbide (4H $\mathrm{Si}\mathrm{C}$) such a goal could be realized, wherein a controlled generation of highly coherent qubit registers using nuclear spins is possible. Using a controlled isotope concentration and coherent control we perform here atomistic modeling of the central spin system formed by the electron spin of a silicon-vacancy color center ($V_{\mathrm{Si}}^{-}$ center) and the noninteracting nuclear spins. From this we lay out conditions for realizing a scalable nuclear-spin (${}^{13}\mathrm{C}$ or ${}^{29}\mathrm{Si}$) register, wherein independent control of the qubits alongside their mutual controlled operations using the central electron spin associated to the $V_{\mathrm{Si}}^{-}$ center in 4H $\mathrm{Si}\mathrm{C}$ are achieved. Further, the decoherence and entanglement analysis provided here could be used to evaluate the quantum volume of these nodes. Our results mark a clear route towards realizing scalable quantum memory nodes for applications in distributed quantum computing networks and further for quantum information protocols.

A single qutrit with transitions selectively driven by weakly coupled reservoirs can implement one of the world’s smallest refrigerators. We analyze the performance of $N$ such fridges that are collectively coupled to the reservoirs. We observe a quantum boost, made manifest in a quadratic scaling of the steady-state cooling current with $N$. As $N$ grows further, the scaling reduces to linear, since the transitions responsible for the quantum boost become energetically unfavorable. Fine-tuned interqutrit interactions may be used to maintain the quantum boost for all $N$ and also for not perfectly collective scenarios.

In this work, we improve the performance of an optically pumped spintronic terahertz emitter (STE) by a factor of up to 6 in field amplitude through an optimized photonic and thermal environment. Using high-energy pump pulses (energy 5 mJ, fluence >1 ${\mathrm{mJ}/\mathrm{cm}}^2$, wavelength 800 nm, duration 80 fs), we routinely generate terahertz pulses with focal peak electric fields above 1.5 MV/cm, fluences of the order of 1 ${\mathrm{mJ}/\mathrm{cm}}^2$, and a spectrum covering the range 0.1–11 THz. Remarkably, the field and fluence values are comparable to those obtained from a state-of-the-art terahertz table-top high-field source based on tilted-pulse-front optical rectification in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$. The optimized STE inherits all attractive features of the standard STE design, for example, ease of use and the straightforward rotation of the terahertz polarization plane, without the typical 75% power loss found in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ setups. It, thus, opens up a promising pathway to nonlinear terahertz spectroscopy. Using low-energy laser pulses (2 nJ, 0.2 ${\mathrm{mJ}/\mathrm{cm}}^2$, 800 nm, 10 fs), the emitted terahertz pulse has a focal peak electric field of 100 V/cm, which corresponds to a 2-fold improvement, and covers the spectrum 0.3–30 THz.

Donor dots realized with phosphorus atoms in silicon have proven to be excellent hosts for electron spin qubits as they provide a strong confining potential that results in small wave functions and well-isolated ground states. As a promising candidate for large-scale quantum computers, such qubits have demonstrated fast, high-fidelity single-shot readout (99.8%) and extremely long coherence times (seconds) with single-qubit gates exceeding 99.94% fidelity. However, high-fidelity two-qubit gates in this platform have been elusive, with charge noise being one of the key limiting factors. Charge noise causes unwanted fluctuations in the exchange coupling between electron spins resulting in logic gate errors, a process that could be minimized if we could engineer a large enough magnetic field difference between the qubits. In this work, we show that using the donor nuclear spins as nanomagnets we can engineer a large magnetic field gradient ($>800\mathrm{MHz}$) between the qubits thereby minimizing sensitivity to charge noise and reducing errors during two-qubit controlled-NOT (cnot) gate operation. We develop a comprehensive theoretical framework with realistic noise sources for performing cnot gates via controlled rotation (crot) using multidonor dot qubits. We show that by engineering the number and location of donors within the dots we can control the hyperfine couplings to maximize the energy difference between electron spin qubits. As a result, we show that the cnot gate error rates can be reduced by a factor of 4 when using multidonor dots as compared to single donors. Our results provide a theoretical roadmap to show how to achieve cnot fidelities as high as 99.98% by optimizing both the local magnetic environment and the operating parameters of multidonor dot qubits.

We report the narrow inhomogeneous distribution of the zero-phonon line from tin-vacancy ($\mathrm{Sn}$-V) centers in diamond and the overlap of spectra from multiple separated $\mathrm{Sn}$-V centers. Photoluminescence excitation spectroscopy measurements at a cryogenic temperature show that $\mathrm{Sn}$-V centers exhibit stable fluorescence and linewidths close to the Fourier-transform-limited linewidth. The inhomogeneous distribution is as low as about 4 GHz, which enables the observation of $\mathrm{Sn}$-isotope-dependent resonant frequencies. Owing to the narrow inhomogeneous distribution, we observe multiple $\mathrm{Sn}$-V centers showing almost identical photon frequencies and linewidths. Identical $\mathrm{Sn}$-V centers are also observed between two diamond samples, confirming the reliable fabrication of high-quality $\mathrm{Sn}$-V centers.

We present a multiscale simulation framework to compute the current versus voltage (I-V) characteristics of metal-oxide-metal structures building the core of conductive bridging random-access memory (CBRAM) cells and to shed light on their resistance switching properties. The approach relies on a finite-element model whose input material parameters are extracted either from ab initio or from machine-learned empirical calculations. The applied techniques range from molecular dynamics and nudged elastic band to electronic and thermal quantum transport. Such an approach drastically reduces the number of fitting parameters needed and makes the resulting modeling environment more accurate than traditional ones. The developed computational framework is then applied to the investigation of an $\mathrm{Ag}/a$-${\mathrm{Si}\mathrm{O}}_2/\mathrm{Pt}$ CBRAM, reproducing experimental data very well. Moreover, the relevance of Joule heating is assessed by considering various cell geometries. It is found that self-heating manifests itself in devices with thin conductive filaments with few-nanometer diameters and at current concentrations in the tens-microampere range. With the proposed methodology it is now possible to explore the potential of not-yet fabricated memory cells and to reliably optimize their design.

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells exceed power conversion efficiencies of 23%. However, the fill factor of these solar cells, with best values around 80%, is relatively low ($\mathrm{Si}$ reaches 84.9%) mostly due to diode factors greater than 1. Recently, we proposed metastable defects, a general feature of the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ alloy, to be the origin of the increased diode factor even in low injection. Here, we measure the diode factor of the bare absorber layers using excitation-dependent photoluminescence. The increased diode factor above 1 can be well described by the model of metastable defects, as well as a slight excitation dependence within the experimentally accessible range of excitation intensities. We discuss how the excitation dependence of the diode factor depends on the parameters of the metastable defects. Within the same model, we can additionally describe the experimental diode factors of n- and p-type epitaxial $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ films. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0% is obtained by $\mathrm{Ag}$ alloying, i.e., an $(\mathrm{Ag},\mathrm{Cu})(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$-based solar cells.

Optomechanical accelerometers promise quantum-limited readout, high detection bandwidth, self-calibration, and radiation-pressure stabilization. We present a simple, scalable platform that enables these benefits with nano-$g$ sensitivity at acoustic frequencies, based on a pair of vertically integrated ${\mathrm{Si}}_3{\mathrm{N}}_4$ membranes with different stiffnesses, forming an optical cavity. As a demonstration, we integrate an ultrahigh-$Q$ ($>{10}^7$), millimeter-scale ${\mathrm{Si}}_3{\mathrm{N}}_4$ trampoline membrane above an unpatterned membrane on the same $\mathrm{Si}$ chip, forming a finesse $\mathcal{F}\approx 2$ cavity. Using direct photodetection in transmission, we resolve the relative displacement of the membranes with a shot-noise-limited imprecision of $7\mathrm{fm}/\sqrt{\mathrm{Hz}}$, yielding a thermal-noise-limited acceleration sensitivity of $0.6\mu \mathrm{g}/\sqrt{\mathrm{Hz}}$ over a 1-kHz bandwidth centered on the fundamental trampoline resonance (40 kHz). To illustrate the advantage of radiation-pressure stabilization, we cold damp the trampoline to an effective temperature of 4 mK and leverage the reduced energy variance to resolve an applied stochastic acceleration of $50\mathrm{ng}/\sqrt{\mathrm{Hz}}$ in an integration time of minutes. In the future, we envision a small-scale array of these devices operating in a cryostat to search for fundamental weak forces such as dark matter.