In the circuit quantum electrodynamics architectures, to realize a long-range quantum network mediated by a flying photon, it is necessary to shape the temporal profile of emitted photons to achieve a high transfer efficiency between two quantum nodes. In this work, we demonstrate an alternative single-rail and dual-rail time-bin-shaped photon generator without additional flux-tunable elements, which can act as a quantum interface of a point-to-point quantum network. In our approach, we adopt a qubit-resonator-transmission-line configuration, and the effective coupling strength between the qubit and the resonator can be varied by parametrically modulating the qubit frequency. In this way, the coupling is directly proportional to the parametric modulation amplitude and covers a broad tunable range beyond 20 MHz for the sample we used. Additionally, when emitting shaped photons, we find that the spurious frequency shift (−0.4 MHz) due to parametric modulation is small and can be readily calibrated through chirping. We develop an efficient photon field measurement setup based on the data-stream processing of GPU. Utilizing this system, we perform photon temporal profile measurement, quantum state tomography of photon field and quantum process tomography of single-rail quantum state transfer based on a heterodyne-measurement scheme. The single-rail encoding state-transfer fidelity of shaped photon emission is $90.32\mathrm{\%}$, and that for unshaped photons is $97.20\mathrm{\%}$, respectively. We believe that the fidelity of shaped photon emission is mainly limited by the qubit coherence time. The results demonstrate that our method is hardware efficient, simple to implement, and scalable. It could become a viable tool in a high-quality quantum network utilizing both single-rail and dual-rail time-bin encoding.

The adiabatic elastocaloric effect relates changes in the strain that a material experiences to resulting changes in its temperature. While elastocaloric materials have been utilized for cooling in room-temperature applications, the use of such materials for cryogenic cooling remains relatively unexplored. Here, we use a strain load-unload technique at low temperatures, similar to those employed at room temperature, to demonstrate a large cooling effect in ${\mathrm{Tm}\mathrm{VO}}_4$. For strain changes of $1.8\times {10}^{-3}$, the inferred cooling reaches approximately 50% of the material’s starting temperature at 5 K, justifying the moniker “giant.” Beyond establishing the suitability of this class of material for cryogenic elastocaloric cooling, these measurements also provide additional insight into the entropy landscape in the material as a function of strain and temperature, including the behavior proximate to the quadrupolar phase transition.

Entanglement generation between remote qubit systems is the central tasks for quantum communication. Future quantum networks will have to be compatible with low-loss telecom bands and operate with large separation between qubit nodes. Single-click heralding schemes can be used to increase entanglement rates at the cost of needing an optically phase-synchronized architecture. In this paper we present such a phase synchronization scheme for a metropolitan quantum network, operating in the low-loss telecom L band. To overcome various challenges such as communication delays and optical power limitations, the scheme consists of multiple tasks that are individually stabilized. We characterize each task, identify the main noise sources, motivate the design choices, and describe the synchronization schemes. The performance of each of the tasks is quantified by a transfer-function measurement that investigates the frequency response and feedback bandwidth. Finally we investigate the resulting optical phase stability of the fully deployed system over a continuous period of 10 h, reporting a short-term stability standard deviation of $\sigma \approx {30}^{\circ }$ and a long-term stability of the average optical phase to within a few degrees. The scheme presented served as a key enabling technology for a nitrogen-vacancy-center-based metropolitan quantum link. This scheme is of interest for other quantum network platforms that benefit from an extendable and telecom-compatible phase-synchronization solution.

The analysis and optimization of solar energy conversion and light-emitting devices can greatly benefit from equivalent circuit models describing their response. However, a general model of electron-hole recombination in semiconductors is currently missing. This study presents equivalent circuit models of radiative and nonradiative electron-hole recombination processes in the small-signal regime based on their linearized analytical treatment. While resistors accurately describe radiative recombination, bipolar transistors represent the general model of nonradiative recombination, and they can be replaced by resistive elements only in special cases. The proposed recombination model is integrated into a transmission-line circuit representation of complete devices that is equivalent to the linearized drift-diffusion equations in one dimension. For mixed conducting devices, such as halide perovskite solar cells, appropriate simplifications of the complete model provide analytical solutions describing the bulk and interfacial polarization effects that influence the small-signal electrostatics, recombination currents, and overall impedance. The resulting analysis facilitates data interpretation and is relevant to a wide range of materials and devices used for solar energy conversion as well as other optoelectronic and photoelectrochemical applications.

This paper develops a protocol to synchronize clocks on board a network of satellites equipped with quantum resources. We show that, in such a constellation, satellites reinforce each other’s sync capabilities, forming a common clock that is more stable and precise than its constituents. We envision the resulting network as a master clock able to distribute time across the globe, providing the basis for a future quantum global navigation satellite system or a space-based quantum network. As an example of its capabilities, we show that a constellation of 50 satellites equipped with modest quantum resources, and distributed amongst five orbits at an altitude of 500 km, allows the synchronization of clocks spread across the globe at subnanosecond precision.

Low-loss deposited dielectrics are beneficial for the advancement of superconducting integrated circuits for astronomy. In the microwave band (approximately 1–10 GHz) the dielectric loss at cryogenic temperatures and low electric field strengths is dominated by two-level systems. However, the origin of the loss in the millimeter-submillimeter band (approximately 0.1–1 THz) is not understood. We measured the loss of hydrogenated-amorphous-$\mathrm{Si}\mathrm{C}$ films in the 0.27–100-THz range using superconducting-microstrip resonators and Fourier-transform spectroscopy. The agreement between the loss data and a Maxwell-Helmholtz-Drude dispersion model suggests that vibrational modes above 10 THz dominate the loss in hydrogenated amorphous $\mathrm{Si}\mathrm{C}$ above 200 GHz.

Single-wall carbon nanotubes (SWCNTs) are effectively narrow ribbons of two-dimensional (2D) materials with atomically precise edges. They are ideal systems to harness quantum transport straintronics (QTS), i.e., using mechanical strain to control quantum transport. Their large sub-band energy spacing (approximately $0.8$ eV) leads to transistors with a single quantum transport channel. We adapt an applied model to study QTS in uniaxially strained quasimetallic-SWCNT transistors. The realistic device parameters are based on an existing experimental platform, with channel lengths of $L=50$ nm, diameters $d\approx 1.5$ nm, and strains up to ${\epsilon }_{\mathrm{total}}\approx 7\mathrm{\%}$. We demonstrate that the charge carrier’s propagation angle (the helix angle with respect to the tube’s axis) $\mathrm{\Theta }$ is fully tunable with ${\epsilon }_{\mathrm{total}}$. When $\mathrm{\Theta }$ reaches ${90}^{\circ }$, the conductance $G$ is completely suppressed. A strain-generated band gap can be tuned up to approximately equal to $400$ meV. Mechanical strain adds both scalar ${\varphi }_{\epsilon }$ and vector $\mathbf{\text{A}}$ gauge potentials to the transistor’s Hamiltonian. These potentials create a rich spectrum of quantum interferences in $G$, which can be described as a mechanical Aharonov-Bohm effect. The charge carriers’ quantum phase can be controlled by purely mechanical means. For instance, a full $2\pi$ phase shift can be induced in a (12,9) tube by a 0.7 $\mathrm{\%}$ strain change. This work opens opportunities to add quantitative quantum transport strain effects to the toolbox of quantum technologies based on 2D materials and their nanotubes.

Quantum Hall resistance (QHR) standards play an essential role in the traceability route for the units ohm, farad, ampere, and kilogram in the revised International System of Units (SI). While the primary realization of the unit ohm is still mainly based on $\mathrm{Ga}\mathrm{As}$ QHR standards in extreme conditions, epitaxial graphene on $\mathrm{Si}\mathrm{C}$ is a promising alternative to develop QHR standards operating under relaxed conditions due to its wide Landau level splitting and strong Fermi level pinning. Here, we show that epitaxial graphene QHR standards can be operated to reach a high accuracy of less than 2 nΩ/Ω (two parts per billion) at a moderate magnetic flux density of B = 4.5 T, high current of I = 232.5 µA, and easier to access temperature of T = 4.2 K, simultaneously. Repeated measurements in these relaxed conditions over 2.5 years demonstrate the temporal stability and robustness of the QHR standard with an accuracy of 2 nΩ/Ω. Furthermore, the accuracy of the graphene QHR standards has been maintained without any indications of degradation, even after experiencing long-distance transport of more than 800 km between two metrology institutes. When implemented, graphene QHR standards may lead to broader dissemination of primary resistance standards beyond national metrology institutes, extending to calibration laboratories and industry on-site.

Bilayers of ${\mathrm{Nd}\mathrm{Co}}_5{/\mathrm{Ni}}_8{\mathrm{Fe}}_2$ can act as reconfigurable racetracks thanks to the parallel stripe-domain configuration present in the hard magnetic material with weak perpendicular anisotropy (${\mathrm{Nd}\mathrm{Co}}_5$), and its imprint on the soft magnetic layer (${\mathrm{Ni}}_8{\mathrm{Fe}}_2$). This pattern hosts domain walls with well-defined vortex-antivortex configurations and establishes paths for their deterministic propagation under the effect of pulsed currents. Magnetic transmission x-ray microscopy experiments show guided domain-wall propagation events within the ${\mathrm{Ni}}_8{\mathrm{Fe}}_2$ layer above a threshold current of $3\times {10}^{11}{\mathrm{A}/\mathrm{m}}^2$. Propagation direction is governed by the change of in-plane magnetization, with opposite displacements for head-to-head and tail-to-tail domain walls. A comparison between domain-wall propagation events at remanence and under an applied in-plane field reveals the presence of an exchange bias field that keeps memory of the last saturated state. The link between the magnetic history stored in the hard magnetic layer and the direction of domain-wall propagation in the soft magnetic layer is a magnetic spring at the ${\mathrm{Nd}\mathrm{Co}}_5{/\mathrm{Ni}}_8{\mathrm{Fe}}_2$ interface, as shown by micromagnetic simulations. This leads to a system, which behaves as a hard-soft magnetic composite with reconfigurable capabilities for the controlled propagation of magnetic spin textures.

Extracting information from quantum systems is crucial in quantum physics and information processing. Methods based on randomized measurements, such as shadow estimation, show advantages in effectively achieving such tasks. However, randomized measurements require the application of random unitary evolution, which unavoidably necessitates frequent adjustments of the experimental setup or circuit parameters, posing challenges for practical implementations. To address these limitations, positive operator–valued measurements (POVMs) have been integrated to realize real-time single-setting shadow estimation. In this work, we advance the POVM-based shadow estimation by reducing the controlled-not (cnot)-gate count for the implementation circuits of informationally complete POVMs (IC POVMs), in particular, symmetric IC POVMs (SIC POVMs), through the dimension-dilation framework. We show that any single-qubit minimal IC POVM can be implemented with the use of at most two cnot gates, while a SIC POVM can be implemented with only one cnot gate. In particular, we provide a concise form of the compilation circuit of any SIC POVM along with an efficient algorithm for the determination of gate parameters. Moreover, we apply the optimized compilation circuit to shadow estimation, showcasing its noise-resilient performance and highlighting the flexibility in compiling various SIC POVMs. Our work paves the way for the practical applications of qubit IC POVMs on quantum platforms.

We report the preparation of Bose-Einstein condensates (BECs) by integrating laser cooling with a grating magneto-optical trap and forced evaporation in a magnetic trap on a single chip. This approach allows us to reach BEC threshold with a BEC of $6\times {10}^4$ rubidium-87 atoms with a single laser cooling beam. Our results represent a significant advance in the robustness and reliability of cold-atom-based inertial sensors, especially for applications in demanding field environments.

On the quest to understand and reduce environmental noise in $\mathrm{Si}$ spin qubits, hyperfine interactions between electron and nuclear spins impose a major challenge. Silicon is a promising host material because one can increase the spin coherence time by removing spinful ${}^{29}\mathrm{Si}$ isotopes. As more experiments rely on isotopic purification of $\mathrm{Si}$, the role of other spinful atoms in the device should be clarified. This is not a straightforward task, as the hyperfine interactions with atoms in the barrier layers are poorly understood. We use density-functional theory to determine the hyperfine tensors of both $\mathrm{Si}$ and $\mathrm{Ge}$ in a crystalline epitaxial $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ quantum well as well as $\mathrm{Si}$ and $\mathrm{O}$ atoms in an amorphous $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$ (MOS) interface structure. On the basis of the results, we estimate the dephasing time $T_2^{\ast }$ due to magnetic noise from the spin bath and show that the coherence is limited by interactions with non-$\mathrm{Si}$ barrier atoms to a few microseconds in $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ (for nonpurified $\mathrm{Ge}$) and about 100 $\text{μ}\mathrm{s}$ in $\mathrm{Si}$-MOS. Expressing these numbers alternatively, we find in $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ the interactions with $\mathrm{Ge}$ dominate below a ${}^{29}\mathrm{Si}$ content of 1000 ppm, and, due to low natural concentration of the spinful oxygen isotopes, the interactions with oxygen in $\mathrm{Si}$-MOS become significant only below a ${}^{29}\mathrm{Si}$ content of 1 ppm.

The need for sensitively and reliably probing magnetization dynamics has been increasing in various contexts such as studying novel hybrid magnonic systems, in which the spin dynamics strongly and coherently couple to other excitations, including microwave photons, light photons, or phonons. Recent advances in quantum magnonics also highlight the need for employing the magnon phase as quantum state variable, which is to be detected and mapped out with high precision in on-chip micro- and nanoscale magnonic devices. Here, we demonstrate a facile optical technique that can directly perform concurrent spectroscopic and imaging functionalities with spatial and phase resolutions, using infrared strobe light operating at 1550-nm wavelength. To showcase the methodology, we spectroscopically studied the phase-resolved spin dynamics in a bilayer of Permalloy and yttrium iron garnet ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG), and spatially imaged the backward-volume spin-wave modes of YIG in the dipolar spin-wave regime. Using the strobe light probe, the detected precessional phase contrast can be directly used to construct the map of the spin wave’s wave front, in the continuous-wave regime of spin-wave propagation and in the stationary state, without needing any optical reference path. By selecting the applied field, frequency, and detection phase, the spin-wave images can be made sensitive to the precession amplitude and phase. Our results demonstrate that infrared optical strobe light can serve as a versatile platform for magneto-optical probing of magnetization dynamics, with potential implications in investigating hybrid magnonic systems.

Probing electrical and magnetic properties in aqueous environments remains a frontier challenge in nanoscale sensing. Our inability to do so with quantitative accuracy imposes severe limitations, for example, on our understanding of the ionic environments in a diverse array of systems, ranging from alternative materials to the living cell. The nitrogen-vacancy (N-$V$) center in fluorescent nanodiamonds (FNDs) has emerged as a good candidate to sense temperature, pH, and the concentration of paramagnetic species at the nanoscale, but comes with several hurdles such as particle-to-particle variation, which render calibrated measurements difficult, and the challenge to tightly confine and precisely position sensors in aqueous environment. To address this, we demonstrate robust N-$V$ spin relaxometry within optically trapped FNDs. In a proof-of-principle experiment, we show that optically trapped FNDs enable highly reproducible nanomolar sensitivity to the paramagnetic ion, ${\mathrm{Gd}}^{3+}$. We capture the three distinct phases of our experimental data by devising a model analogous to nanoscale Langmuir adsorption combined with spin coherence dynamics. Our work provides a basis for routes to sense free paramagnetic ions and molecules in biologically relevant conditions.

Single-photon detectors are “blind” after the detection of a photon, and thereafter display a characteristic recovery in efficiency, during which the number of undetected photons depends on the statistics of the incident light. We show how the efficiency recovery, photon statistics, and intensity have an interdependent relationship which suppresses a detector’s ability to count photons and measure correlations. We also demonstrate this effect with an experiment using $n$ such detectors to determine the $n\mathrm{th}$-order correlation function with pseudothermal light.

Owing to its repeaterlike rate-loss scaling, twin-field quantum key distribution (TF-QKD) has repeatedly exhibited in the laboratory its superiority for secure communication over record fiber lengths. Field trials pose a new set of challenges, however, which must be addressed before the technology’s roll-out into the real world. Here, we verify in the field the viability of using independent optical frequency combs, installed at sites separated by a straight-line distance of 300 km, to achieve a versatile TF-QKD setup that has no need for optical frequency dissemination and thus enables an open and network-friendly fiber configuration. Over 546- and 603-km symmetric links, we record a finite-size secure key rate (SKR) of 0.53 bit/s and an asymptotic SKR of 0.12 bit/s, respectively. Of practical importance, the setup is demonstrated to support 44-km fiber asymmetry in the 452-km link. Our work represents an important step toward incorporation of long-haul fiber links into large quantum networks.

Wide-field magnetic microscopy using nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond can yield high-quality magnetic images of dc and ac magnetic fields. The unique combination of micron-scale spatial resolution of scalar or vector fields at room temperature and parallel camera readout make this an appealing technique for applications in biology, geology, condensed-matter physics, and electronics. However, while $\mathrm{N}$-$V$ magnetic microscopy has achieved great success in these areas, historically the accessible frequency range has been limited. In this paper, we overcome this limitation by implementing the recently developed technique of quantum frequency mixing. With this approach, we generate wide-field magnetic images of test structures driven by alternating currents up to 70 MHz, well outside the reach of dc and Rabi magnetometry methods. With further improvements, this approach could find utility in hyperspectral imaging for electronics power-spectrum analysis, electronics diagnostics and troubleshooting, and quantum computing hardware validation.

Due to continued dimensional scaling, read sensors in hard disk drives face pressing challenges related to rising electrical resistance and thus impedance mismatch problems. All-metal, or at least low-resistance, magnetic field sensors are thus of high interest to displace current magnetic tunnel junction readers. Spin-accumulation sensors based on metallic nonlocal spin valves (NLSVs) are promising candidates but need further signal optimization, particularly in the ultrathin limit required for applications. Here, we report a wide-ranging study of spin signal optimization in thin $\mathrm{Al}/{\mathrm{Al}\mathrm{O}}_x/\mathrm{Co}$-$\mathrm{Fe}$-based NLSVs, using interfacial resistance-area-product tuning to mitigate spin back-diffusion. The Al-oxidation-controlled resistance-area product is tuned over 8 orders of magnitude, providing a complete picture of the evolution of the interfacial transport mechanism, spin signal, and nonlocal background. Signal improvements up to factors of almost ${10}^3$ are obtained over transparent-interface devices (to about 0.15 Ω, or ∼2 mΩ µ${\mathrm{m}}^2$), and the upper limit of feasible resistance-area product in NLSVs is established. Most significantly, the spin signals in such NLSVs at application-relevant resistances still lie substantially beneath theoretical limits, due to diminished tunneling spin polarization of unknown origin. Connecting tunneling devices to the transparent limit, a universal spin polarization versus interfacial resistance-area product relation is thus uncovered in ${\mathrm{Al}\mathrm{O}}_x$-based devices, highlighting the specific issue limiting technological progress in this area. Potential strategies to achieve further gains are discussed in detail.

In this work, we explore the formation of transmission gaps in wave spectra of a ferrite film covered by a thin vanadium dioxide (${\mathrm{VO}}_2$) stripe with a single groove on its surface. It was found that this waveguide does not manifest noticeable spin-wave reflections, but still demonstrates rejection bands in its transmission characteristic. This is due to a finite drop of spin-wave group velocity in a wide frequency range affected by the conducting ${\mathrm{VO}}_2$ layer with a smooth profile inhomogeneity. The main feature of the proposed waveguides is a short groove length required for the formation of spectral regions with enhanced spin-wave signal rejection. Namely, the utilization of a groove with a length of 66 µm results in a pronounced rejection band with a depth of 10 dB and a width of 19 MHz. To achieve these values in conventional magnonic crystals, several millimeters of a periodic array of reflectors are required. Therefore, we believe that the obtained findings pave a clear way forward for microminiaturizing spin-wave devices, where features of magnonic crystals are exploited.

DNA flexibility is a key determinant of biological function, from nucleosome positioning to transcriptional regulation, motivating a direct measurement of the bend-torque response of individual DNA molecules. In this work, DNA bending is detected with the use of a nanomechanical torque balance formed by the tethering of a ferromagnetic nanoparticle probe by an individual DNA molecule to a diamond magnetic field imager. The torque exerted by the DNA in response to bending caused by an applied magnetic torque is measured by wide-field imaging of quantum defects near the surface of the diamond. Qualitative measurements of differences in DNA biomechanical binding configuration are demonstrated, and as a proof of principle, a quantitative measurement of the bend response is made for individual DNA molecules. This quantum-enabled measurement approach could be applied to characterize the bend response of biophysically relevant short DNA molecules as well as the sequence dependence of DNA bending energy.