Replacing the bulky cryoliquid-based cooling stages of cryoenabled instruments by chip-scale refrigeration is envisioned to disruptively reduce the system size similar to microprocessors did for computers. Electronic refrigerators based on superconducting tunnel junctions have been anticipated to provide a solution, but reaching the necessary above the 1-K operation temperature range has remained a goal out of reach for several decades. We show efficient electronic refrigeration by $\mathrm{Al}$-${\mathrm{Al}\mathrm{O}}_x$-$\mathrm{Nb}$ superconducting tunnel junctions starting from bath temperatures above 2 K. The junctions can deliver electronic cooling power up to approximately $\mathrm{mW}/{\mathrm{mm}}^2$, which enables us to demonstrate tunnel-current-driven electron temperature reduction from 2.4 K to below 1.6 K (34% relative cooling) against the phonon bath. Our work shows that the key material of integrated superconducting circuits—niobium—enables powerful cryogenic refrigerator technology. This result is a prerequisite for practical cryogenic chip-scale refrigerators and, at the same time, it introduces a new electrothermal tool for quantum heat-transport experiments.

Entangling microwave and optical photons is one of the promising ways to realize quantum transduction through quantum teleportation. This paper investigates the entanglement of microwave-optical photon pairs generated from an electro-optomechanical system driven by a blue-detuned pulsed Gaussian pump. The photon pairs are obtained through weak parametric-down-conversion, and their temporal correlation is revealed by the second-order correlation function. We then study the discrete variable entanglement encoded in the time-bin degree of freedom, where entanglement is identified by Bell-inequality violation. Furthermore, we estimate the laser-induced heating and show that the pulse-pumped system features lower heating effects while maintaining a reasonable coincidence photon-counting rate.

We have studied daisy-chained extraordinary magnetoresistance (EMR) devices based on high-quality monolayer graphene encapsulated in hexagonal boron nitride at room temperature. The largest magnetoresistance achieved in our devices is 4.6 × 107%, the record for EMR devices to date. The magnetic field sensitivity dR/dB reaches 104 kΩ/T, exceeding the previous record set by encapsulated graphene by more than 300%, and is comparable with state-of-the-art graphene Hall sensors at cryogenic temperatures (4.2 K). We demonstrate that daisy-chaining multiple EMR devices is an alternative way to reach arbitrarily high sensitivity and signal-to-noise ratio, and extremely small noise equivalent field for weak magnetic field detection. Finally, we show the evidence of metal contact–induced Fermi-level pinning in the sample and its influence on graphene properties, current distribution, and EMR performance. We highlight the EMR geometry as an interesting alternative to the Hall geometry for fundamental physics studies.

Physical-reservoir computing (PRC) is an information-processing framework where the nonlinear physics of a reservoir can be leveraged to compute a task. Since the readout consists of linear and static weights, most of the computation, requiring nonlinear transformation and memory of the inputs, is done by the natural dynamics of the reservoir. Furthermore, training those readout weights using linear regression can be computationally fast and lightweight to implement. In this study, we propose a method to utilize the hysteresis effect, traditionally considered a hindrance in computation, to generate nonlinearity and memory in PRC. Specifically, we developed a model that emulates hysteresis properties by introducing latency in the reservoir states, and we demonstrate its effectiveness through dynamical analysis. This approach allows us to relax the requirement for a recurrent weight matrix as implemented in traditional echo state networks. Furthermore, we validate the practical applicability of this hysteretic reservoir framework with a hardware implementation using a simple coil transformer. Our findings indicate that materials exhibiting hysteresis effects can be harnessed to unlock a new class of computational materials, heralding a novel framework for PRC.

We present an alternative approach to determine the optical properties of materials in the nanoscale range using optical tweezers (OTs). Fluorescent polymer-based nanostructures (pdots) are optically trapped in a Gaussian-beam OT and the trap stiffness is studied as a function of various parameters of interest. We explicitly show that properties such as the refractive index and the optical anisotropy of these nanostructures can be determined with high accuracy by comparing the experimental data to an optical-force model. In particular, we demonstrate that the effective optical properties of these pdots can be modulated by changing the light wavelength that excites the sample, opening the door for a fine tuning of their optical response, with possible applications in the development of alternative sensors and/or other optoelectronic devices.

In this paper, we introduce a horizontally oriented photophoretic “boat” trap that is capable of capturing and self-loading the largest reported (radius $\ge 1\text{μ}\mathrm{m}$) photophoreticaly trapped solid gold particles in air for more than 1 h. Once trapped, particles are held stably, without hopping, even as the trap is modified to scan axially or expanded to a larger size to increase the capture cross section. We theoretically present and experimentally demonstrate each of these affordances.

A metamaterial receiver, featuring a 19-ring facet structure and intended for usage with a lens, has been successfully designed, fabricated, and tested. The presented concept uses cold-electron bolometers (CEBs) as sensing elements, which enables the design of multiabsorber receivers maintaining a balance between power load and noise. The micrometric size of the CEBs eliminates the need for microwave feed lines, facilitating integration within antenna slots. Using a ${\mathrm{YB}\mathrm{a}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}(\mathrm{YB}\mathrm{CO})$ Josephson junction oscillator, it is demonstrated that the CEB metamaterial receiver has broadband response, approaching 400 GHz, in two frequency bands: 0.15–0.55 and 0.9–1.3 THz. Additionally, the photon-noise-limited operation of the CEB metamaterial, operating at 0.3 K, is shown.

${\mathrm{Zr}\mathrm{O}}_2$ and ${\mathrm{Hf}\mathrm{O}}_2$ have attracted significant attention in the past decade due to their unexpected ferroelectric properties at the nanoscale, enabling the miniaturization of lead-free ferroelectric devices. Elemental doping is commonly used to further enhance their ferroelectric properties. However, in contrast to ${\mathrm{Hf}\mathrm{O}}_2$, the role of dopants with varying valences in ${\mathrm{Zr}\mathrm{O}}_2$ remains incompletely understood at the atomic level. Herein, through first-principles simulations, we investigated the impact of 21 dopants on ${\mathrm{Zr}\mathrm{O}}_2$, revealing a volcano-type relationship between the coercive field and ionic radii of dopants. Our simulations demonstrated that $\mathrm{Al}$ and $\mathrm{Ga}$ doping in ${\mathrm{Zr}\mathrm{O}}_2$ not only enhance the stability of the polar orthorhombic phase but also reduce the coercive field by more than 50% relative to its pristine polar phase. Furthermore, we identified eight other dopants, such as $\mathrm{Fe}$, $\mathrm{Ti}$, $\mathrm{Cu}$, $\mathrm{Co}$, $\mathrm{Y}$, $\mathrm{La}$, $\mathrm{Ce}$, and $\mathrm{Sr}$, with the potential to improve the ferroelectric properties of ${\mathrm{Zr}\mathrm{O}}_2$. The phonon analysis further elucidated the origin of the different roles of these dopants in ${\mathrm{Zr}\mathrm{O}}_2$. We believe our results hold promise for guiding the design of low-voltage operational ferroelectric devices, thereby advancing the ferroelectric properties of ${\mathrm{Zr}\mathrm{O}}_2$-based materials.

The dispersive-readout scheme enables quantum nondemolition measurement of superconducting qubits. An increased readout power can shorten the readout time and reduce the state-discrimination error but can promote qubit transitions into higher noncomputational states. The ability to predict the onset of these measurement-induced state transitions can aid the optimization of qubit circuits and provide means for comparing the readout performance of different qubit types. Building upon the concept of dressed coherent states, we consider two straightforward metrics for determining the maximum number of photons that can be used for dispersive readout without causing state transitions. We focus on the fluxonium readout to demonstrate the independence of the metrics from any qubit-type-specific approximations. The dispersive readout of transmons and other superconducting qubits can be treated universally in the same fashion.

Reference-frame-independent quantum key distribution (RFI-QKD) has gained widespread attention due to its unique advantage for practical application, as it circumvents the need for active reference-frame alignment within the system. However, in comparison to the standard BB84 protocol, the original six-state RFI protocol requires a greater number of quantum states to be operated by Alice and Bob, which is an aspect that merits optimization. In this work, we propose a four-state RFI protocol and illustrate that Alice and Bob each require only four quantum states to perform channel estimation that remains independent of reference-frame deviation, which can proficiently reduce the system complexity. Furthermore, through numerical simulations taking the finite-key-size effect into consideration, we show that the four-state RFI protocol can achieve a secure key rate and transmission distance on par with those of the original six-state RFI protocol. Finally, an experiment over 200 km is conducted to evaluate the feasibility of our scheme. We believe that our protocol can streamline the implementation of RFI-QKD and thereby contribute to the practical advancement of RFI-QKD.

We provide a comprehensive analytical description of the effective coupling associated with an antiresonance within a hybrid system comprising a quasiclosed photonic cavity and a ferrimagnetic material. While so-called level attraction between a resonant system inside an open cavity is well understood, the physical underpinnings of this phenomenon within quasiclosed cavities have remained elusive. Leveraging input-output theory, we successfully differentiate between the repulsive and attractive aspects of this coupling. Our proposed model demonstrates that by understanding the phase jump at the resonances and the studied antiresonance, we can predict the nature of the effective coupling of the antiresonance for a given position of the ferrimagnet in the cavity.

We present a technique for integrating ultraclean carbon nanotubes into superconducting circuits, aiming to realize Josephson junctions based on one-dimensional elementary quantum conductors. This technique primarily involves depositing a nanotube in the final step, thus preserving it from the inherent contaminations of nanofabrication and maintaining contact solely with superconducting electrodes and a crystalline hexagonal boron nitride substrate. Through transport measurements performed in both the normal and superconducting states, we demonstrate that our method yields high-quality junctions with Josephson energies suitable for quantum device applications, such as carbon nanotube-based superconducting qubits.

Structured optical fields have led to several ground-breaking techniques in classical imaging and microscopy. At the same time, in the quantum domain, position-momentum entangled photon fields have been shown to have several unique features that can lead to beyond-classical imaging and microscopy capabilities. Therefore, it is natural to expect that position-momentum entangled two-photon fields that are structured can push the boundaries of quantum imaging and microscopy even further beyond. Nonetheless, the existing experimental schemes are able to produce either structured two-photon fields without position-momentum entanglement, or position-momentum entangled two-photon fields without structured correlations. In this article, by manipulating the phase-matching condition of the spontaneous parametric down-conversion process, we report experimental generation of two-photon fields with various structures in their spatial correlations. We experimentally measure the minimum bound on the entanglement of formation and thereby verify the position-momentum entanglement of the structured two-photon field. We expect this work to have important implications for quantum technologies related to imaging and sensing.

We present an experimental demonstration of a single-shot, nondestructive electron beam diagnostic based on the ionization of a low-density pulsed gas jet. In our study, 7 MeV electron bunches from a radio-frequency photoinjector, carrying up to 100 pC of charge, traversed a localized distribution of nitrogen gas (${\mathrm{N}}_2$). The interaction of the electron bunches with the ${\mathrm{N}}_2$ gas generated a correlated signature in the ionized particle distribution, which was spatially magnified using a series of electrostatic lenses and recorded with a microchannel-plate detector. Various modalities, including point-to-point imaging and velocity mapping, are investigated. A temporal trace of the detector current enabled the identification of single- and double-ionization events. The characteristics of the ionization distribution, dependence on gas density, total bunch charge, and other parameters, are described. Approaches to scaling to higher electron bunch density and energy are suggested. Additionally, the instrument proves useful for comprehensive studies of the ionization process itself.

Piezoelectric phononic crystal resonators (PCRs) are a promising platform for acoustic quantum processing, yet their performance is currently limited by coupling to an ensemble of saturable two-level system (TLS) defects within the resonator material. Motivated by its excellent bulk mechanical properties and high crystallinity, we address this by fabricating PCRs using thin-film quartz as the substrate. At single-phonon powers and millikelvin temperatures—requisite conditions for quantum phononic processing—we demonstrate large internal mechanical quality factors, $Q_i>160000$. This represents an order-of-magnitude improvement in single-phonon lifetimes for piezoelectric PCR. We characterize the loss channels in these devices and find that, although improved, the low-power response is still limited by coupling to a TLS bath and that a significant portion of the TLSs are associated with the aluminum coupling electrodes. To explore the high-power response, we perform ring-down measurements and demonstrate high-power quality-factor–frequency products $Q_i\cdot f=1.4\times {10}^{16}$ Hz.