High-rate, high-fidelity entanglement distribution is essential to the creation of a quantum internet, but recent achievements in fiber (248 km at a 9-${\mathrm{s}}^{-1}$ rate) and satellite-based (1200 km at a 1.1-${\mathrm{s}}^{-1}$ rate) entanglement distribution fall far short of what is needed. Chen et al. [Phys. Rev. Appl. 19, 054209 (2023)] proposed a means for dramatically increasing entanglement-distribution rates via a scheme they called zero-added-loss multiplexing (ZALM). ZALM’s quantum transmitter employs a pair of Sagnac-configured spontaneous parametric down-converters (SPDCs), channelization via dense wavelength-division multiplexing (DWDM) filtering, and partial Bell-state measurements (BSMs) to realize a heralded source of frequency-multiplexed polarization-entangled biphotons. Each biphoton is transmitted to Alice and Bob along with a classical message identifying its frequency channel and whether a ${\psi }^{-}$ singlet or a ${\psi }^{+}$ triplet was heralded. Alice’s and Bob’s quantum receivers then use DWDM filtering and temporal-mode conversion to interface their received biphotons to intracavity color-center quantum memories. This paper delves deeply into ZALM’s SPDCs, partial BSMs, and Duan-Kimble loading of Alice’s and Bob’s quantum memories. Its principal results—the density operators for the SPDC sources and the quantum memories—allow heralding probability, heralding efficiency, and fidelity to be evaluated for both the polarization-entangled biphotons and the loaded quantum memories, thus enabling exploration of the parameter space for optimizing ZALM’s performance. Even without a comprehensive optimization analysis, the paper’s examples already demonstrate two critical features of the ZALM architecture: (1) the necessity of achieving a near-separable channelized biphoton wave function to ensure that the biphoton sent to Alice and Bob is of high purity; and (2) the premium placed on Alice’s and Bob’s temporal-mode converters enabling narrowband push-pull memory loading to ensure that the arriving biphoton’s state is faithfully transferred to the intracavity color centers.

We reexamine the well-known Duan-Kimble entanglement scheme, wherein the state of a single-photon qubit is entangled with a quantum memory consisting of a single-atom qubit in a strongly coupled optical cavity, providing the capability to load the photon’s state into the memory. We correct a common error appearing in some subsequent papers regarding the validity of the single-photon reflectivity function that characterizes the essential phase shift at the heart of the protocol. Using the validated analytical solution, we introduce an improved scheme—the push-pull configuration—where the photon and cavity are tuned at the midpoint between atomic resonances and show that it can outperform the original on-off configuration in which the photon and cavity are tuned exactly to one of the atomic resonances. The performance metric used is the final memory-state fidelity versus the heralding probability, which determines the memory-loading rate. The results should play a role in optimizing future quantum repeater schemes based on the Duan-Kimble protocol.

The new generation of strained-silicon-nitride resonators harbors great promise for scanning force microscopy, especially when combined with the extensive toolbox of cavity optomechanics. However, accessing a mechanical resonator inside an optical cavity with a scanning tip is challenging. Here we experimentally demonstrate a cavity-based scanning force microscope based on a silicon-nitride-membrane sensor. We overcome geometric constraints by making use of the extended nature of the mechanical resonator normal modes, which allows us to spatially separate the scanning and readout sites of the membrane. Our microscope is geared towards low-temperature applications in the zeptonewton regime, such as nanoscale nuclear-spin detection and imaging.

The dispersion relation of spin waves can vary monotonically about the center of the Brillouin zone, allowing zero-momentum wavepackets to flow unidirectionally, which is of interest for applications. Techniques such as propagating-spin-wave spectroscopy are inoperative in such cases because of the difficulty to identify the spin-wave wavevector at a particular frequency within a spectrum. Here we present a method to analyze this case and apply it to acoustic spin waves in a synthetic antiferromagnet in the scissors state, in which we confirm that propagation parallel to the applied fields is unidirectional. Interestingly, we find that in this unidirectional situation, the phase accumulated by the spin waves propagating between two antennas is not proportional to the antenna spacing. It is also a function of the two other lengths of the problem: the antenna width and the spin-wave decay length. Accounting for them is required to avoid wavevector errors in the dispersion relations.

We propose and demonstrate transmission-based dispersive readout of a superconducting qubit using an all-pass resonator, which preferentially emits readout photons toward the output. This is in contrast to typical readout schemes, which intentionally mismatch the feedline at one end so that the readout signal preferentially decays toward the output. We show that this intentional mismatch creates scaling challenges, including larger spread of effective resonator linewidths due to nonideal impedance environments and added infrastructure for impedance matching. A future architecture using multiplexed all-pass readout resonators would avoid the need for intentional mismatch and potentially improve the scaling prospects of quantum computers. As a proof-of-concept demonstration of “all-pass readout,” we design and fabricate an all-pass readout resonator that demonstrates insertion loss below 1.17 dB at the readout frequency and a maximum insertion loss of 1.53 dB across its full bandwidth for the lowest three states of a transmon qubit. We demonstrate qubit readout with an average single-shot fidelity of 98.1% in 600 ns; to assess the effect of larger dispersive shift, we implement a shelving protocol and achieve a fidelity of 99.0% in 300 ns.

This paper describes the light-shift laser-lock (LSLL) technique, an alternative method intended for laser-based compact atomic clocks. The technique greatly simplifies the laser setup by stabilizing the pumping-laser frequency to the same atoms involved in the clock operation, without the need of an external reference. By alternating two clock sequences, the method estimates and cancels out a controlled amount of induced light shift, acting on the laser frequency. The LSLL technique is compatible with state-of-the-art three-level clocks and was demonstrated with field-programmable-gate-array-based electronics on a pulsed-optically-pumped vapor-cell clock developed at INRIM. The results have shown that the LSLL technique operates robustly, having a capture range of gigahertz without significantly compromising clock stability. In our tests, the clock exhibited a white frequency noise of $3.2\times {10}^{-13}{\tau }^{-1/2}$ for averaging times $\tau$ up to 4000 s, reaching a floor below 1×${10}^{-14}$ up to 100 000 s. This level of performance meets the requirements of future global navigation satellite systems on-board clocks, adding the benefits of a reduced clock footprint, as well as increased reliability and robustness.

We propose and demonstrate a combined static- and oscillating-field alkali atom magnetometer and use it as a magnetic induction tomography sensor. The magnetometer realizes simultaneous measurements of the static and oscillating magnetic fields using two different Zeeman transitions of a single sensor. This approach dramatically enhances the long-term stability and sensitivity of detection of low-conductivity objects. Using our dual-frequency magnetometer, we detect small containers with salt water with conductivity as low as 0.55 S/m. We achieved a conductivity measurement uncertainty of 0.18 S/m with a 10-s integration time. This performance is sufficient to distinguish between healthy and malignant tissue in the human body. The dual-frequency magnetometer can also be used as a self-stabilized radio-frequency magnetometer.

Discrete-modulated continuous-variable quantum key distribution offers a pragmatic solution, greatly simplifying experimental procedures, while retaining robust integration with classical optical communication. Theoretical analyses have progressively validated the comprehensive security of this protocol, paving the way for practical experimentation. However, imperfect sources in practical implementations introduce noise. The traditional approach is to assume that eavesdroppers can control all of the source noise, which overestimates the ability of eavesdroppers and underestimates the secret-key rate. Some parts of source noise are intrinsic and cannot be manipulated by the eavesdropper, so they can be seen as trusted noise. We tailor a trusted-noise model specifically for the discrete-modulated protocol and upgrade the security analysis accordingly. Simulation results demonstrate that this approach successfully mitigates the negative impact of an imperfect source on system performance, while maintaining security of the protocol. Furthermore, our method can be used in conjunction with a trusted-detector-noise model, effectively reducing the influence of both source noise and detector noise in the experimental setup. This is a meaningful contribution to the practical deployment of discrete-modulated-continuous-variable-quantum-key-distribution systems.

A unique bistable transition has been identified in granular-colloidal gel composites, resulting from shear-induced phase separation of the gel phase into dense blobs. In energy applications, it is critical to understand how this transition influences electrical performance. Mixing conductive colloids with conductive inclusions, we find that the conductivity and elasticity move in concert, both decreasing in the collapsed phase-separated state. Surprisingly, with insulating inclusions, these properties can become decoupled, with the conductivity instead increasing despite the collapse of the gel structure.

Fully processed SiC power MOSFETs emit light during switching of the gate terminal, while the drain and source terminals are both grounded. The emitted photons are caused by defect-assisted recombination of electrons and holes at the $4H$-$\mathrm{([A-Z][a-z])([A-Z])}/{\mathrm{([A-Z][a-z])([A-Z])}}_2$ interface, and can be detected through the SiC substrate. Here we present time-gated spectroscopic characterization of these interfacial point defects. Unlike in previous studies, the devices were opened in such a way that the drain contact remained electrically active. A separate examination of the photons emitted at the rising and falling transitions of the gate-source voltage enabled the extraction of two different spectral components. One of these components consists of a single transition with phonon replicas of a local vibrational mode with an astonishingly high energy of 220 meV—well above the highest phonon modes in $4H$-SiC and ${\mathrm{([A-Z][a-z])([A-Z])}}_2$ of 120 and 137 meV, respectively. On the basis of a quantum mechanical model, we successfully fitted its emission spectrum and assigned it to donor-acceptor-pair recombination involving a carbon-cluster-like defect. Other transitions were assigned to ${\mathrm{EH}}_{6/7}$-assisted, ${\mathrm{EK}}_2$-D, and nitrogen-aluminum donor-acceptor-pair recombination. Because of the relevance of these defects in the operation of SiC MOSFETs, these insights will contribute to improved reliability and performance of these devices.

Characterizing temporally correlated noise and “non-Markovian” qubit dynamics is a key prerequisite for achieving noise-tailored error mitigation and optimal device performance. Quantum noise spectroscopy can provide quantitative estimation of the noise spectral features; however, in its current form it is highly vulnerable to implementation nonidealities, notably, state preparation and measurement (SPAM) errors. Further to that, existing protocols have been mostly developed for dephasing-dominated noise processes, with competing dephasing and relaxation effects being largely unaccounted for. We introduce quantum noise spectroscopy protocols inspired by spin-locking techniques that enable the characterization of arbitrary temporally correlated multiaxis noise on a qubit with fixed energy splitting, while remaining resilient to realistic static SPAM errors. By validating the performance of our protocol in both numerical simulation and on cloud-based IBM quantum processors, we demonstrate the successful separation and estimation of native noise spectrum components as well as SPAM error rates. We find that SPAM errors can significantly alter or mask important noise features, with spectra overestimated by up to 26.4% in a classical noise regime. Clear signatures of nonclassical noise are manifest in the reconstructed IBM-qubit dephasing spectra, once SPAM-error effects are compensated for. Our work provides a timely tool for benchmarking realistic sources of noise in qubit devices.

We report on the trapping of single rubidium atoms in large arrays of optical tweezers comprising up to 2088 sites in a cryogenic environment at $6\text{K}$. Our approach relies on the use of microscope objectives that are in vacuum but at room temperature, in combination with windowless thermal shields into which the objectives are protruding to ensure a cryogenic environment for the trapped atoms. To achieve enough optical power for efficient trapping, we combine two lasers at slightly different wavelengths. We discuss the performance and limitations of our design. Finally, we demonstrate atom-by-atom rearrangement of an 828-atom target array using moving optical tweezers controlled by a field-programmable gate array.

The experimental realization of a spin-wave reservoir chip employing ferromagnetic permalloy thin films is presented. The novel device facilitates the interference of three spherical wave-excited surface mode spin waves within a rectangular waveguide via strategically positioned slits, enabling the detection of electrical signals from surface mode spin waves across all four observation antennas. Through the experiments conducted, it is confirmed that the device functions as a one-input, four-output reservoir capable of estimating external magnetic fields. Notably, the results demonstrate the device’s capacity to retain memory up to one step prior in short-term memory tasks, while confirming the effectiveness of spin-wave interference induced by Huygens slits in enhancing nonlinearity, as observed in parity-check tasks. Furthermore, the inclusion of additional detection antennas contributes to improved learning accuracy, highlighting the significant progress achieved by the spin-wave reservoir chip. These findings underscore substantial progress toward practical implementation, with promising avenues for further development and refinement, showing its remarkable ability to process signals at high speeds, even with 0.8-ns pulse sequences.

High-quality, distributed quantum entanglement is the distinctive resource for quantum communication and forms the foundation for the unequaled level of security that can be assured in quantum key distribution. While the entanglement provider does not need to be trusted, the secure key rate drops to zero if the entanglement used is too noisy. In this paper, we show experimentally that QPA is able to increase the secure key rate achievable with QKD by improving the quality of distributed entanglement, thus increasing the quantum advantage in QKD. Beyond that, we show that QPA enables key generation at noise levels that previously prevented key generation. These remarkable results were only made possible by the efficient implementation exploiting hyperentanglement in the polarization and energy-time degrees of freedom. We provide a detailed characterization of the gain in secure key rate achieved in our proof-of-principle experiment at different noise levels. The results are paramount for the implementation of a global quantum network linking quantum processors and ensuring future-proof data security.

Interlayer exchange coupling has been intensively studied for over 30 years and has been successfully incorporated into almost all magnetic thin-film devices. In this study we find that antiferromagnetic interlayer exchange coupling can be achieved across, and improved by, spacer layers containing over 60 at.% of magnetic material. Measurements of sputtered $\mathrm{Fe}$-doped spacer layers with x-ray magnetic circular dichroism reveal that the magnetic atoms in such spacer layers retain a large magnetic moment. The addition of magnetic atoms to the spacer layer is shown to double the coupling strength, leading to the largest-ever-observed antiferromagnetic bilinear coupling strength in magnetic multilayers deposited by magnetron sputtering, which is the industrial technique of choice. Electronic structure calculations in this work predict an experimentally obtained dependence of interlayer exchange coupling on both the magnetic material concentration and the spacer-layer thickness, in contrast with the prevailing understanding of interlayer exchange coupling.

The negatively charged nitrogen-vacancy (N-$V^{-}$) center in diamond has shown great potential in nanoscale sensing and quantum information processing due to its rich spin physics. An efficient coupling with light, providing strong luminescence, is crucial for realizing these applications. Laser-written waveguides in diamond promote N-$V^{-}$ creation and improve their coupling to light but, at the same time, induce strain in the crystal. The induced strain contributes to light guiding but also affects the energy levels of N-$V^{-}$ centers. We probe N-$V^{-}$ spin states experimentally with the commonly used continuous-wave zero-field optically detected magnetic resonance (ODMR). In our waveguides, the ODMR spectra are shifted, split, and consistently asymmetric, which we attribute to the impact of local strain. To understand these features, we model ensemble ODMR signals in the presence of strain. By fitting the model results to the experimentally collected ODMR data, we determine the strain tensor components at different positions, thus determining the strain profile across the waveguide. This shows that zero-field ODMR spectroscopy can be used as a strain imaging tool. The resulting strain within the waveguide is dominated by a compressive axial component transverse to the waveguide structure, with a smaller contribution from vertical and shear strain components.

Despite great fundamental and technological importance, triferroic multiferroics are still substantially unexplored, especially their semiconducting characteristics. Here, we propose a design principle for achieving semiconducting triferroic multiferroicity in a bilayer lattice by utilizing van der Waals stacking as a perturbation. We further demonstrate this principle in a real material: bilayer T′-${\mathrm{Ti}\mathrm{Br}}_2$. Based on first-principles calculations, we show that bilayer T′-${\mathrm{Ti}\mathrm{Br}}_2$ exhibits semiconducting and antiferromagnetic properties. Additionally, its crystal symmetry gives rise to 120° ferroelasticity, and interlayer charge redistribution leads to an out-of-plane electric polarization, thereby realizing the intriguing semiconducting triferroicity. Furthermore, this system is predicted to possess several extraordinary properties, including ferroelastic control of the magnetization orientation and ferroelectric control of the absolute values of spin-polarization-density distributions.

Many laboratories routinely cool samples to 10 mK, but relatively few can cool condensed matter below 1 mK. Easy access to the microkelvin range would prove highly desirable in fields such as quantum sensors and quantum materials. Such temperatures are achieved with adiabatic nuclear demagnetization. Existing nuclear-demagnetization refrigerators (NDRs) are “single-shot,” and the recycling time is incompatible with some submillikelvin experiments. Furthermore, a high cooling power is required to overcome the excess heat load of nanowatt order on NDRs precooled by cryogen-free dilution refrigerators. We report the performance of an aluminum NDR designed for powerful cooling when part of a dual-stage continuous NDR (CNDR). Its thermal resistance is minimized to maximize the cycling rate of the CNDR and consequently its cooling power. At the same time, its susceptibility to eddy current heating is minimized. A CNDR based on two of the aluminum NDRs presented here would achieve a cooling power of approximately 40 nW at 560 $\text{μ}\mathrm{K}$ less than $6$ days after cooling from room temperature, with a small offset in electronic temperature that decreases as the time-dependent heat load decays.

Decreasing the number of cables that bring heat into the cryostat is a critical issue for all cryoelectronic devices. In particular, arrays of superconducting nanowire single-photon detectors (SNSPDs) could require more than ${10}^6$ readout lines. Performing signal-processing operations at low temperatures could be a solution. Nanocryotrons, superconducting nanowire three-terminal devices, are good candidates for integrating sensing and electronics on the same technological platform as SNSPDs in photon-counting applications. In this work, we demonstrate that it is possible to read out, process, encode, and store the output of SNSPDs using exclusively superconducting nanowires patterned on niobium nitride thin films. In particular, we present the design and development of a nanocryotron ripple counter that detects input voltage spikes and converts the number of pulses to an $N$-digit value. The counting base can be tuned from 2 to higher values, enabling higher maximum counts without enlarging the circuit. As a proof of principle, we first experimentally demonstrate the building block of the counter, an integer-$N$ frequency divider with $N$ ranging from 2 to 5. Then, we demonstrate photon-counting operations at 405 nm and 1550 nm by coupling an SNSPD with a two-digit nanocryotron counter partially integrated on chip. The two-digit counter can operate in either base 2 or base 3, with a bit-error rate lower than $2\times {10}^{-4}$ and a count rate of ${10}^7{\mathrm{s}}^{-1}$. We simulate circuit architectures for integrated readout of the counter state and we evaluate the capabilities of reading out an SNSPD megapixel array that would collect up to ${10}^{12}$ counts per second. The results of this work, combined with our recent publications on a nanocryotron shift register and logic gates, pave the way for the development of nanocryotron processors, from which multiple superconducting platforms may benefit.

Bulk-mode acoustic waves in a crystalline material exert lattice strain through the thickness of the sample, which couples to the spin Hamiltonian of defect-based qubits such as the nitrogen-vacancy (N-V) center defect in diamond. This mechanism has previously been harnessed for unconventional quantum spin control, spin decoherence protection, and quantum sensing. Bulk-mode acoustic wave devices are also important in the microelectronics industry as microwave filters. A key challenge in both applications is a lack of appropriate operando microscopy tools for quantifying and visualizing gigahertz-frequency dynamic strain. In this work, we directly image acoustic strain within N-V center-coupled diamond thin-film bulk acoustic wave resonators using stroboscopic scanning hard x-ray diffraction microscopy at the Advanced Photon Source. The far-field scattering patterns of the nanofocused x-ray diffraction encode strain information entirely through the illuminated thickness of the resonator. These patterns have a real-space spatial variation that is consistent with the bulk strain’s expected modal distribution and a momentum-space angular variation from which the strain amplitude can be quantitatively deduced. We also perform optical measurements of strain-driven Rabi precession of of the N-V center spin ensemble, providing an additional quantitative measurement of the strain amplitude. As a result, we directly measure one of the six N-V spin-stress coupling parameters, $b=2.73(2)$ MHz/GPa, by correlating these measurements at the same spatial position and applied microwave power. Our results demonstrate a unique technique for directly imaging ac lattice strain in micromechanical structures and provide a direct measurement of a fundamental constant for the N-V center defect spin Hamiltonian.