Classical interferometers are indispensable tools for the precise determination of various physical quantities. Their accuracy is bound by the standard quantum limit. This limit can be overcome by using quantum states or nonlinear quantum elements. Here, we present the experimental study of a nonlinear Josephson interferometer operating in the microwave regime. Our quantum microwave parametric interferometer (QUMPI) is based on superconducting flux-driven Josephson parametric amplifiers combined with linear microwave elements. We perform a systematic analysis of the implemented QUMPI. We find that its interferometric power exceeds the shot-noise limit and observe sub-Poissonian photon statistics in the output modes. Furthermore, we identify a low-gain operation regime of the QUMPI that is essential for optimal quantum measurements in quantum illumination protocols.

Photon loss is the fundamental issue toward the development of quantum networks. To circumvent loss quantum repeaters were proposed, which need high-performance quantum memories. We present an alternative proposal to mitigate photon loss even at large distances and hence to create a global-scale quantum communication architecture without quantum repeaters. In this proposal, photons are sent directly through space, using a chain of co-moving low-earth orbit satellites. This satellite chain would bend the photons to move along the Earth’s curvature and control photon loss due to diffraction by effectively behaving like a set of lenses on an optical table. Numerical modeling of photon propagation through these “satellite lenses” shows that diffraction loss in entanglement distribution can be almost eliminated even at global distances of 20 000 km while considering beam truncation at each satellite and the effect of different errors (e.g., “satellite lens” focal-length fluctuation). In the absence of diffraction loss, the effect of other losses (especially reflection loss) becomes relevant and they are investigated in detail. The total loss is estimated to be less than 30 dB at 20 000 km if other losses are constrained to 2% at each satellite, with 120-km satellite separation and 60-cm diameter satellite telescopes eliminating diffraction loss. Such a low-loss satellite-based optical-relay protocol would enable robust, multimode global quantum communication and would not require either quantum memories or repeater protocol. The protocol can also be the least lossy in almost all distance ranges available (200–20 000 km). Recent advances in space technologies may soon enable an affordable launch facility for such a satellite-relay network. We further introduce the “qubit transmission” protocol, which has a plethora of advantages with both the photon source and the detector remaining on the ground. A specific lens setup was designed for the “qubit transmission” protocol, which performed well in simulation that included atmospheric turbulence in the satellite uplink.

Measuring the losses arising from different materials and interfaces is crucial to improving the coherence of superconducting quantum circuits. Although this has been of interest for a long time, current studies can either only provide bounds to those losses, or require several devices for a complete characterization. In this work, we introduce a method to measure the microwave losses of materials and interfaces with a single multimode superconducting resonator. We demonstrate a formalism for analyzing the loss sensitivity of multimode systems and discuss the design strategies of multimode resonators for material loss studies. We present two types of multimode superconducting resonators for the study of bulk superconductors: the forky whispering-gallery-mode resonator and the ellipsoidal cavity. We use these resonators to measure the surface dielectric, conductor, and seam losses of high-purity (5N5) aluminum and aluminum alloy (6061), as well as how they are affected by chemical etching, diamond turning, and thin-film coating. We find that chemical etching and diamond turning reduce both the surface dielectric and conductive losses of high-purity aluminum, but provide no appreciable improvement to the seam. Coating the surfaces of diamond-turned aluminum alloys with electron-beam-evaporated or sputtered aluminum thin films significantly reduces all three losses under study. In addition, we study the effect of chemical etching on the surface of high-purity aluminum using transmission electron microscopy and find that the chemical etching process creates a thinner and more uniform oxide layer, consistent with the observed improvement in the surface dielectric loss.

In the field of spintronics, there is a strong demand for barrier materials in magnetic tunnel junctions (MTJs) having high tunnel magnetoresistance (TMR) and low resistance area product (RA). However, the design of such barrier materials with atomically controlled composition, vacancies, and disordering of the constituent elements is still challenging due to the combinatorial explosion of potential candidates. Very recently, a quantum annealing (QA) method has been utilized for combinatorial optimization problems in materials science. In this paper, we perform a proof-of-concept study by applying the QA approach combining with first-principles calculations and a machine-learning factorization machine (FM) to MTJs with inverse-type spinel ${\mathrm{Mg}\mathrm{Ga}}_2{\mathrm{O}}_4$ tunnel barrier. We treat 252 combinations of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Ga}}^{3+}$ cation disordering in the barrier layer of the MTJs and discuss the effect of the cation disordering on the structural stability, TMR, and RA. Our method is superior to simulated annealing, Bayesian optimization, and random sampling in searching for the best MTJs for low total energy and high TMR, but not so for low RA. We also revealed physical origins of high TMR and low RA behind the cation disordering. The present work highlights the applicability and advantage of materials informatics using FM + QA with first-principles calculations in designing spintronic MTJ devices.

Coherent coupling between spin-wave excitations (magnons) and microwave photons in a cavity may disclose new paths to unconventional phenomena as well as novel applications. Here, we present a systematic investigation of yttrium iron garnets (YIG) films on top of coplanar waveguide resonators made of superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO). We first show that spin-wave excitations with frequency higher than the Kittel mode can be excited by putting in direct contact a 5-$\text{μ}\mathrm{m}$-thick YIG film with the YBCO coplanar resonator (cavity frequency ${\omega }_c/2\pi =8.65$ GHz). With this configuration, we obtain very large values of the collective coupling strength $\lambda /2\pi \approx 2$ GHz and cooperativity $C=5\times {10}^4$. Transmission spectra are analyzed by a modified Hopfield model for which we provide an exact solution that allows us to well reproduce spectra by introducing a limited number of free parameters. It turns out that the coupling of the dominant magnon mode with photons exceeds 0.2 times the cavity frequency, thus demonstrating the achievement of the ultrastrong-coupling regime with this architecture. Our analysis also shows a vanishing contribution of the diamagnetic term, which is a peculiarity of pure spin systems.

Diamond is a very promising material for various applications, and understanding its basic properties is key for the development of future devices. In particular, the low-field mobility of holes has never been measured below $\sim 80\mathrm{K}$ in ultrapure diamond. In order to measure this mobility, we developed an innovative time-of-flight electron-beam-induced-current technique. A 1 ns pulsed electron beam was made to impact the diamond semiconductor, inducing charge-carrier creation and motion through the 546-$\text{μ}\mathrm{m}$-thick bulk diamond, under the influence of an applied electric field. The resulting signal was analyzed using the transient-time technique. Thus, the velocity of electrons and holes was assessed as a function of the temperature from 13 to 300 K and as a function of the electric field with values ranging from 1.5 to 9200 V ${\mathrm{cm}}^{-1}$. A low-field mobility value of $1.03\pm 0.05\times {10}^6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ was measured for holes at 13 K, demonstrating that diamond is a suitable material to transport charge carriers in a ballistic regime at a scale of 10 $\text{μ}\mathrm{m}$.

We have demonstrated a precision atom-interferometry gravimeter based on matter-wave Bragg diffraction. By decreasing the atom temperature and increasing the atom number with Raman sideband cooling, it is shown that our Bragg atom gravimeter can achieve a short-term sensitivity of $2.2\text{μ}\mathrm{Gal}/{\mathrm{Hz}}^{1/2}(1\mathrm{Gal}=1{\mathrm{cm}/\mathrm{s}}^2$), improved by nearly a factor of 2 compared to that of a state-of-the-art atom gravimeter. The resolution of our atom gravimeter has reached $0.08\text{μ}\mathrm{Gal}$ after an integration time of 2000 s, comparable to a cryogenic superconducting gravimeter. Based on extremely sensitive and stable gravity measurements, a test of local Lorentz invariance in the gravity sector was performed, where the accuracy of the upper bound on the space-space component was improved by a factor of 4.

This paper presents an analytical framework for the study of scattering and diffraction phenomena in space-time-modulated metallic gratings. Using a Lorentz transformation, it is shown that a particular class of space-time-modulated gratings behave effectively as moving media. We take advantage of this property to derive a closed analytical solution for the wave scattering problem. In particular, using our formalism it is possible to avoid space-time Floquet-Bloch expansions, as the solution of the problem in the original laboratory frame (grating parameters are periodic in space and time) is directly linked to a comoving frame where the metallic grating is time invariant (grating parameters are periodic only in space). In this way, we identify a fundamental connection between moving metallic gratings and space-time-modulated metamaterials, and exploit this link to study the nonreciprocal response of the structure. Some limitations and difficulties of the alternative nonrelativistic Galilean approach are discussed and the benefits of the Lorentz approach are highlighted. Finally, some analytical results are presented in order to validate the formalism. The results include scenarios involving TM ($p$) and TE ($s$) normal and oblique incidence, even beyond the onset of the diffraction regime. Furthermore, we show how the synthetic Fresnel drag can tailor the Goos-Hänchen effect and create a specular point shifted towards the direction of the synthetic motion, independent of the sign of the incidence angle.

We use a pure spin current originating from the spin Hall effect to generate a spin-orbit torque strongly reducing the effective damping in an adjacent ferromagnet. Because of additional microwave excitation, large spin-wave amplitudes are achieved exceeding the threshold for four-magnon scattering, thus resulting in additional spin-wave signals at discrete frequencies. Two or more modes are generated below and above the directly pumped mode with equal frequency spacing. It is shown how this nonlinear process can be controlled in magnonic waveguides by the applied dc current and the microwave pumping power. The sudden onset of the nonlinear effect after exceeding the thresholds can be interpreted as a spiking phenomenon, which makes the effect potentially interesting for neuromorphic computing applications. Moreover, we investigated this effect under microwave frequency and external field variation. The appearance of the additional modes was investigated in the time domain, revealing a time delay between the directly excited and the simultaneously generated nonlinear modes. Furthermore, spatially resolved measurements show different spatial decay lengths of the directly pumped mode and nonlinear modes.

The magnetocaloric effect is the change in temperature of a magnetic material in response to a change in its magnetization, mediated by an external magnetic field. Since the early 20th century, its main application has been magnetic refrigeration, which has the potential to outperform conventional liquid-vapor refrigerators. In this paper, we combine the magnetocaloric effect with magneto-optical control of near-field radiative heat transfer to introduce an alternative thermal switch with two tuning knobs: a magnetocaloric one that can adjust the temperature gradient and a magneto-optical one that can adjust the conductance of the gap between two materials participating in radiative heat transfer. We consider two representative material systems at room temperature: a $\mathrm{Gd}$-$\mathrm{SiC}$ system with magnetocaloric tunability and a $\mathrm{Gd}$-$\mathrm{InAs}$ system with both magnetocaloric and magneto-optical tunability. Our model predicts several different operating modes of the thermal switch enabled by the magnetocaloric effect, including one where the flow of heat is reversed. Simultaneously, the magneto-optical response of $\mathrm{InAs}$ enables fine-tuning of the spectral heat flux via control of the electromagnetic surface modes. Our work recontextualizes near-field radiative heat transfer between magnetic materials and serves as an example of combining an i-caloric effect (the thermal response of a material due to a change in an applied field i, where i = magneto-, elasto-, electro-, etc.) with a complementary mode of heat transfer to enable new thermal technologies.

We experimentally study the mass partitioning of a fragmenting liquid sheet formed after the impact of a ns-laser pulse on a tin microdroplet, and its dependence on laser pulse energy and droplet size. We present the temporal evolution of individual liquid fractions: the sheet and its bounding rim, ligaments protruding from the rim, and droplets shed by the ligaments, applying machine learning to analyze subresolution fragments. Our results show that the temporal evolution of the mass partitioning between the sheet, rim, ligaments, and fragments is independent of the deformation Weber number—following Wang and Bourouiba [J. Fluid Mech. 935, A29 (2022)] for the analogous droplet-pillar impact case, extending the work to larger Weber numbers and to a system where the timescale of deformation is fully decoupled from impact. The full mass partitioning is accounted for by quantifying the further contributions unique to the laser-droplet impact case: that of a centrally located mass remnant, and the mass ablated by the laser pulse. These findings can be employed to optimize the mass utilization of the liquid tin that is used as target material in the production of extreme ultraviolet light for nanolithography.

A fundamental result of quantum mechanics is that the fluctuations of a bosonic field are given by its temperature $T$. An electromagnetic mode with frequency $\omega$ in the microwave band has a significant thermal photon occupation at room temperature according to the Bose-Einstein distribution $\overline{n}=k_{B}T/\hslash \omega$. The room-temperature thermal state of a $(2\pi \times 3)$-GHz mode, for example, is characterized by a mean photon number $\overline{n}\sim 2000$ and variance $\mathrm{\Delta }{n}^2\approx {\overline{n}}^2$. This thermal variance sets the measurement noise floor in applications ranging from wireless communications to positioning, navigation, and timing to magnetic resonance imaging. We overcome this barrier in continuously cooling a $(2\pi \times 2.87)$-GHz cavity mode by coupling it to an ensemble of optically spin-polarized nitrogen-vacancy (N-$V$) centers in a room-temperature diamond. The N-$V$ spins are pumped into a low entropy state via a green laser and act as a heat sink to the microwave mode through their collective interaction with microwave photons. Using a simple detection circuit, we report a peak noise reduction of $-2.3\pm 0.1\text{dB}$ and minimum cavity mode temperature of $150\pm 5\text{K}$. We also present a linearized model to identify the important features of the cooling, and demonstrate its validity through magnetically tuned, spectrally resolved measurements. The realization of efficient mode cooling at ambient temperature opens the door to applications in precision measurement and communication, with the potential to scale towards fundamental quantum limits.

In the field of cavity nano-optomechanics, the nanoresonator-in-the-middle approach consists in inserting a subwavelength-sized deformable resonator, here a nanowire, into the small mode volume of a fiber microcavity. Internal resonances in the nanowire enhance the light nanowire interaction that provide giant coupling strengthes—sufficient to enter the single-photon regime of cavity optomechanics—at the condition to precisely position the nanowire within the cavity field. Here we expose a theoretical description that combines an analytical formulation of the Mie scattering of the intracavity light by the nanowire and an input-output formalism describing the dynamics of the intracavity optical eigenmodes. We investigate both facets of the optomechanical interaction describing the position-dependent parametric and dissipative optomechanical coupling strengths, as well as the optomechanical force field experienced by the nanowire. We find quantitative agreement with a recent experimental realization. We discuss the specific phenomenology of the optomechanical interaction that acquires a vectorial character since the nanowire can identically vibrate along both transverse directions: the optomechanical force field presents a nonzero rotational, while anomalous positive cavity shifts are expected. Taking advantage of the large Kerr-like nonlinearity, this work opens perspectives in the field of quantum optics with nanoresonators with, for instance, broadband squeezing of the outgoing cavity fields close to the single-photon level.

We propose a hybrid classical-quantum digitized counterdiabatic algorithm to tackle the protein-folding problem on a tetrahedral lattice. Digitized counterdiabatic quantum computing is a paradigm developed to compress quantum algorithms via the digitization of the counterdiabatic acceleration of a given adiabatic quantum computation. Finding the lowest-energy configuration of the amino-acid sequence is an NP-hard optimization problem that plays a prominent role in chemistry, biology, and drug design. We outperform state-of-the-art quantum algorithms using problem-inspired and hardware-efficient variational quantum circuits. We apply our method to proteins with up to nine amino acids, using up to 17 qubits on quantum hardware. Specifically, we benchmark our quantum algorithm with Quantinuum’s trapped ions, and Google’s and IBM’s superconducting circuits, obtaining high success probabilities with low-depth circuits as required in the noisy intermediate-scale quantum era.

A search for high-speed and low-energy memory devices puts antiferromagnetic thin films at the forefront of spintronic research. Here, we develop a material model of a granular antiferromagnetic thin film with uniaxial anisotropy and provide fundamental insight into the interaction of antiferromagnetic domain walls with grain boundaries. This model is validated on thin films of the antiferromagnetic insulator ${\mathrm{Cr}}_2{\mathrm{O}}_3$, revealing complex mazelike domain patterns hosting localized nanoscale domains down to 50 nm. We show that the intergrain magnetic parameters can be estimated based on an analysis of high-resolution images of antiferromagnetic domain patterns examining the domain patterns’ self-similarity and the statistical distribution of domain sizes. Having a predictive material model and understanding of the pinning of domain walls on grain boundaries, we put forth design rules to realize granular antiferromagnetic recording media.

This study explores the possibility and presents a methodology to synthesize a classical mechanical analogue to the quantum mechanical one-dimensional (1D) Kitaev model. While being fundamentally different, we identify significant conceptual similarities between the two models that culminate in the occurrence, in the classical analogue system, of topologically nontrivial bound states that are akin to Majorana zero modes. By reformulating the Hamiltonian of the classical system in a form reminiscent of second quantization, we show that a 1D staggered classical mechanical chain can exhibit dynamic characteristics analogueous to Kitaev’s 1D superconducting model, as well as its characteristic bound states. The nontrivial topological nature of the bound states is further confirmed by the topological band-structure analysis and by the topological invariant. While the non-Abelian nature of these states remains an open question, these results allow envisioning the possibility to achieve topological braiding in classical mechanical systems.

Hybrid quantum systems that utilize controllable coupling between phonons and superconducting qubits could pave the way for a new generation of compact quantum memories and sensor technologies. However, the same electromechanical interaction mechanisms that enable coherent qubit-phonon coupling can also lead to decoherence due to the spontaneous phonon emission. In this paper we study the dynamics of a qubit coupled to an acoustic resonator by a piezoelectric film. By varying the surface topography of the resonator from rough to flat to shaped, we explore the crossover from fast decay of an excited qubit to quantum-coherent coupling between the qubit and an isolated phonon mode. The developed theoretical model allows us to establish and check quantitative links between the results obtained for the rough and shaped topographies. Discrepancy between the theory and the data for the flat resonator points to the imperfections of the studied device. Our experimental approach may be used for precision measurements of crystalline vibrations, the design of quantum memories, and the study of electromechanical contributions to dielectric loss.

Further miniaturization of electronic devices necessitates the introduction of new materials, including piezoelectric thin films, that exhibit electromechanical functionalities without significant degradation in response due to substrate-induced clamping. To identify material systems with superior piezoelectric properties as thin films, simplified and quantitative electromechanical characterization techniques are required. Here, single-beam, laser Doppler vibrometry is used to detect ac electric-field-induced surface displacement in the frequency range 1–100 kHz with low error (around 6% at 10 kHz) and resolution of 0.0003 nm. The technique is used to quantify both electrostriction and piezoelectric responses (surface displacement values <0.05 nm) of various thin films. Requirements for sample geometry and device structures are established and measurement accuracy and resolution are validated against measurements from the literature via synchrotron-based diffraction measurements. A general methodology to measure and extract the piezoelectric coefficients for thin-film samples using finite-element modeling is presented and applied to determine the $d_{33}$ coefficient and visualize the response in substrate-clamped 50–400-nm-thick ${\mathrm{Pb}\mathrm{Zr}}_{0.52}{\mathrm{Ti}}_{0.48}{\mathrm{O}}_3$ films, especially as compared to bulk versions with the same sample geometry.

The Hong-Ou-Mandel interference between independent photons plays a pivotal role in large-scale quantum networks involving distant nodes. Photons need to work in a pure state for indistinguishability to reach high-quality interference. Also, they need to have a sufficiently long coherence time to reduce the time-synchronization requirements in practical applications. In this paper, we discuss a scheme for generating a pure-state photon-pair source with a long coherence time in periodically poled potassium titanyl phosphate crystals. By selecting the appropriate pump laser and filter, we can simultaneously eliminate the frequency correlation of the parametric photons while achieving a long coherence time. We experimentally develop this pure-state photon-pair source of 780 nm on potassium titanyl phosphate crystals pumped by a 390-nm pulsed laser. The source provides a coherence time of tens of picoseconds, and it is shown to have the potential to be applied in long-distance quantum interference. Furthermore, we experimentally demonstrate the Hong-Ou-Mandel interference between two photon sources with visibility exceeding the classical limit.

Hybrid quantum systems based on magnetic platforms have witnessed the birth and fast development of quantum magnonics. Until now, most studies have relied on magnetic excitations in low-damping magnetic insulators, particularly yttrium iron garnet, while a large class of magnetic systems is ruled out in this interdisciplinary field. Here we propose the generation of a magnon bundle in a hybrid magnet-qubit system, where two or more magnons with the same energy and angular momentum are emitted simultaneously. By tuning the driving frequency of the qubit to match the detuning between magnon and qubit mode, one can effectively generate a magnon bundle via super-Rabi oscillations. In contrast with general wisdom, magnetic dissipation plays an enabling role in generating the magnon bundle, where the relaxation time of magnons determines the typical time delay between two successive magnons. The maximal damping that allows an antibunched magnon bundle can reach the order of 0.1, which may break the monopoly of low-dissipation magnetic insulators in quantum magnonics and enables a large class of magnetic materials for their applications in quantum manipulation. Further, our findings may provide a scalable and generic platform to study multimagnon physics and benefit the design of magnonic networks for quantum information processing.