Sensing the magnetic field emitted by individual nuclear spins would allow important insights into the structures of proteins and nanoscale devices. Toward this goal, ultrasensitive silicon nitride resonators have recently emerged as scanning force sensors, but to achieve the sensitivity required for single-spin sensing, the readout noise of these sensors must be reduced. In this work, the authors demonstrate a scanning force microscope based on a silicon nitride membrane embedded in an optical cavity for low-noise readout. They find that laser phase noise crucially impacts the sensor’s usable bandwidth.

In 2004, L. M. Duan and H. J. Kimble proposed a way to load a single-photon polarization qubit into a quantum memory consisting of a four-state atom or color center that is strongly coupled to an optical cavity. That scheme has been widely studied and demonstrated, and is at the heart of a recent proposal (zero-added-loss multiplexing, ZALM) to increase the rate of entanglement distribution by multiplexing states of entangled photon pairs. The authors report an improved version of the Duan-Kimble scheme that simultaneously achieves high memory-loading fidelity and loading probability. They also rederive the underlying equations, to clear up a misconception in the literature.

Zero-added-loss multiplexing (ZALM) promises to vastly increase entanglement-distribution rates, a critical necessity for the coming quantum Internet. This work investigates ZALM’s heralded source of entangled photon pairs and the loading of their entangled states into pairs of intracavity color-center quantum memories; it exposes ZALM’s trade-offs between distribution rate, heralding probability, heralding efficiency, and entangled-state fidelity. Surprisingly, even with ideal equipment there is a nonzero probability that an incorrect entangled state will be heralded. Perfect transfer of the entangled photonic state to memory is possible when the state is sufficiently bandwidth-compressed.

Magnetic Weyl semimetals present particular promise for spintronic applications, as their band-structure topology can be tuned by a magnetic field. Systematic experimental investigation of magnetic tunnel junctions (MTJs) with electrodes of ferromagnetic Weyl semimetal is needed. This study develops fully epitaxial single-crystalline MTJs featuring Co${}_2$MnGa, a clean material system for investigating Weyl physics in the devices. Along the way, the authors establish the relationship between tunneling magnetoresistance and the degree of chemical and topological ordering of Co${}_2$MnGa.

Magnon-based hybrid quantum systems show potential for quantum information processing, but their scalability is questionable, due to the short-range nature of direct magnon coupling. The authors propose and analyze a practical design for scalable hybrid quantum devices that enable remote coupling of magnon arrays through microwave photons within a superconducting coplanar-waveguide resonator. By modulating the magnon frequencies, topological magnon chains are obtained, which enables tunable chiral interactions with solid-state spins. This work opens possibilities for quantum computing, quantum communication, and quantum sensing based on magnons and solid-state spins.

Trapped ions offer much promise for near-term implementations of quantum computing. To ensure that the ions remain coherent, laser-cooling schemes are required to stave off decoherence caused by motional heating, but effective schemes must satisfy trade-offs between motional and dephasing errors, and between cooling power and chain length. This work uses a mix of theoretical and computational techniques to establish best practices for laser cooling for long chains of trapped ions. It turns out that placing the coolant at the center of an ion chain is always optimal.

Robust interfaces between single-photon sources and quantum memories are pivotal for tomorrow’s quantum network architectures. The authors present a room-temperature ladder-type atomic memory for single photons at 895 nm. Comprehensive performance characterization reveals the capabilities of the memory for high bandwidth and low noise, and benchmarking shows its compatibility with state-of-the-art quantum dot single-photon sources. This work paves the way toward a heterogeneous on-demand interface between a single-photon source and an optical memory for buffering and synchronization in quantum network nodes.

In spintronics, conventional devices exploiting spin-orbit torque cannot simultaneously provide both low damping and strong torque, the two necessities for energy-efficient operation. The authors take a different approach to meet both criteria, using Fe-Ni alloy films with steep vertical gradients in composition. Intriguingly, they find that sizable spin-orbit torque emerges even in an alloy without any intentional compositional gradient, due to a gradient in growth-induced strain. Their results give perspective for materials engineering of spin-orbit-torque devices, leveraging asymmetry not only in chemical composition, but also in atomic-scale lattice structure.

Semiconductor spin qubits are a promising platform for spin-based quantum computing due to their high density and fast gate speeds. Most research to date has focused on linear quantum dot arrays with limited qubit connectivity. Scaling up these devices is desired, but the fabrication process has been an obstacle. Using a semiconductor manufacturing approach, the authors demonstrate a closely packed two-dimensional array of quantum dots with qubit fidelities exceeding 99.8%. The device architecture opens the door to fabricating larger two-dimensional quantum dot arrays with high connectivity.

Atomic clocks are crucial for fundamental science and practical technology, but their sensitivity is often constrained by the standard quantum limit. With a cold-atom coherent-population-trapping clock, the authors design and experimentally demonstrate an adaptive Bayesian quantum frequency estimation protocol that approaches the Heisenberg scaling with respect to total interrogation time. This atomic clock also yields improved fractional frequency stability and enhanced robustness against technical noises. This work provides a high-precision approach to atomic clock locking, and holds promising applications in various interferometry-based quantum sensors.

Spin defects embedded in a scalable material platform with bright telecom emission are promising candidates for quantum communication technologies. $V^{4+}$ in SiC fulfills these criteria, but its potential is limited by the lack of understanding of its spin relaxation mechanisms. This study employs all-optical measurements to reveal that the site-dependent spin $T_1$ values can exceed 20 seconds and identifies the mechanism of the spin relaxation processes. These insights lead to a proposal to enable qubit operations at higher temperatures, significantly reducing the infrastructure requirements and paving the way for practical realization of quantum technologies based on $V^{4+}$ in SiC.

Quantum data encoding is a crucial technique for tackling practical problems with quantum computers, but achieving accurate encoding has been hindered by noise in real-world quantum devices. In this study, the authors develop a large language model to generate quantum operations for data encoding. Their results show that some of these generated operations are more resilient to noise than traditional methods, offering new hope for the realization of practical computations such as quantum chemistry simulations and quantum machine learning.

Deep-learning-enabled design is an essential part of common metamaterial applications such as diverse metasurfaces, plasmonic nanostructures, and photonic crystals. Further progress in this field has become stultified, though, due to the lack of physical insight in “black box” design approaches. The authors propose a neural-network framework that includes an adversary channel based on the familiar Kramers-Kronig relations, yielding high-precision output that conforms to the internal physics even in cases of incomplete physical representations and equations. This framework is also widely applicable, beyond just metasurface design.

The non-Hermitian electro-optic (EO) effect in materials with Berry-curvature dipoles (BDs) is important for applications like terahertz lasing. This study shows that a stack of low-symmetry two-dimensional materials inside a cavity supports a nonreciprocal growing mode, which generates an unstable resonance that leads to self-sustained oscillations. It is also found that the chiral nature of the Berry-dipole gain can control the laser’s handedness, enhancing the design of chiral terahertz lasers.

This Letter addresses a canonical imaging problem: how to estimate a small transverse displacement of a laser beam as precisely as possible. This problem is central to many applications, including scanning-probe atomic force microscopy, free-space optical communication, and optomechanics. Based on the geometry of the imaging system, the center wavelength of the laser probe, and the return-path propagation length, the authors identify the optimal spatial mode in which to excite a laser probe, when paired with the most sensitive receiver allowed by quantum estimation theory. Several phase-sensitive, mode-resolving receivers that achieve the mentioned quantum precision limit are presented.

This study of phononic crystals in thin-film superfluid helium presents an avenue for applications in quantum information processing and advanced sensing technologies, as it could enable precise control of sound waves at the quantum level. Progress here has been hampered by our limited understanding of phonon behavior in quantum fluids, plus the technical challenges of manipulating these waves in very cold superfluid helium. This research tackles these issues by implementing strong confinement of mechanical modes in superfluid thin films, using a phononic crystal structure. The work establishes a promising platform for achieving mechanical nonlinearities on the single-phonon level.

Absolute air pressure measurement based on molecular mechanical collision with an optically levitated nanoparticle can provide high accuracy, but the dephasing processes complicate the extraction of the interactions in levitated oscillator systems. This study overcomes this challenge by performing second-order coherence analysis, enabling independent measurement of energy decoherence rates while eliminating dephasing errors. The nanoscale vacuum gauge achieves precise pressure measurements from atmospheric pressure down to 7 $\times$ 10${}^{-6}$ mbar. This demonstrates that mechanical nano-oscillators are promising candidates for precision pressure sensing applications.

Transition-edge sensors (TESs) have proven to be excellent single-photon detectors, but little is known about their capability to detect electrons. The authors investigate this by employing a ‘cold’ electron source, based on quantum field emission from carbon nanotubes; the emitted electrons had a kinetic energy of about 100 eV and were successfully detected by the TES. The resulting energy resolution for electrons that are fully absorbed in the TES active volume is of the order of 1 eV and found to be compatible with the photon energy resolution of the same device. This study opens possibilities for high-resolution low-energy electron detection.

Sensing the magnetic field emitted by individual nuclear spins would allow important insights into the structures of proteins and nanoscale devices. Toward this goal, ultrasensitive silicon nitride resonators have recently emerged as scanning force sensors, but to achieve the sensitivity required for single-spin sensing, the readout noise of these sensors must be reduced. In this work, the authors demonstrate a scanning force microscope based on a silicon nitride membrane embedded in an optical cavity for low-noise readout. They find that laser phase noise crucially impacts the sensor’s usable bandwidth.

In 2004, L. M. Duan and H. J. Kimble proposed a way to load a single-photon polarization qubit into a quantum memory consisting of a four-state atom or color center that is strongly coupled to an optical cavity. That scheme has been widely studied and demonstrated, and is at the heart of a recent proposal (zero-added-loss multiplexing, ZALM) to increase the rate of entanglement distribution by multiplexing states of entangled photon pairs. The authors report an improved version of the Duan-Kimble scheme that simultaneously achieves high memory-loading fidelity and loading probability. They also rederive the underlying equations, to clear up a misconception in the literature.

Zero-added-loss multiplexing (ZALM) promises to vastly increase entanglement-distribution rates, a critical necessity for the coming quantum Internet. This work investigates ZALM’s heralded source of entangled photon pairs and the loading of their entangled states into pairs of intracavity color-center quantum memories; it exposes ZALM’s trade-offs between distribution rate, heralding probability, heralding efficiency, and entangled-state fidelity. Surprisingly, even with ideal equipment there is a nonzero probability that an incorrect entangled state will be heralded. Perfect transfer of the entangled photonic state to memory is possible when the state is sufficiently bandwidth-compressed.

Magnetic Weyl semimetals present particular promise for spintronic applications, as their band-structure topology can be tuned by a magnetic field. Systematic experimental investigation of magnetic tunnel junctions (MTJs) with electrodes of ferromagnetic Weyl semimetal is needed. This study develops fully epitaxial single-crystalline MTJs featuring Co${}_2$MnGa, a clean material system for investigating Weyl physics in the devices. Along the way, the authors establish the relationship between tunneling magnetoresistance and the degree of chemical and topological ordering of Co${}_2$MnGa.

Magnon-based hybrid quantum systems show potential for quantum information processing, but their scalability is questionable, due to the short-range nature of direct magnon coupling. The authors propose and analyze a practical design for scalable hybrid quantum devices that enable remote coupling of magnon arrays through microwave photons within a superconducting coplanar-waveguide resonator. By modulating the magnon frequencies, topological magnon chains are obtained, which enables tunable chiral interactions with solid-state spins. This work opens possibilities for quantum computing, quantum communication, and quantum sensing based on magnons and solid-state spins.

Trapped ions offer much promise for near-term implementations of quantum computing. To ensure that the ions remain coherent, laser-cooling schemes are required to stave off decoherence caused by motional heating, but effective schemes must satisfy trade-offs between motional and dephasing errors, and between cooling power and chain length. This work uses a mix of theoretical and computational techniques to establish best practices for laser cooling for long chains of trapped ions. It turns out that placing the coolant at the center of an ion chain is always optimal.

Robust interfaces between single-photon sources and quantum memories are pivotal for tomorrow’s quantum network architectures. The authors present a room-temperature ladder-type atomic memory for single photons at 895 nm. Comprehensive performance characterization reveals the capabilities of the memory for high bandwidth and low noise, and benchmarking shows its compatibility with state-of-the-art quantum dot single-photon sources. This work paves the way toward a heterogeneous on-demand interface between a single-photon source and an optical memory for buffering and synchronization in quantum network nodes.

In spintronics, conventional devices exploiting spin-orbit torque cannot simultaneously provide both low damping and strong torque, the two necessities for energy-efficient operation. The authors take a different approach to meet both criteria, using Fe-Ni alloy films with steep vertical gradients in composition. Intriguingly, they find that sizable spin-orbit torque emerges even in an alloy without any intentional compositional gradient, due to a gradient in growth-induced strain. Their results give perspective for materials engineering of spin-orbit-torque devices, leveraging asymmetry not only in chemical composition, but also in atomic-scale lattice structure.

Semiconductor spin qubits are a promising platform for spin-based quantum computing due to their high density and fast gate speeds. Most research to date has focused on linear quantum dot arrays with limited qubit connectivity. Scaling up these devices is desired, but the fabrication process has been an obstacle. Using a semiconductor manufacturing approach, the authors demonstrate a closely packed two-dimensional array of quantum dots with qubit fidelities exceeding 99.8%. The device architecture opens the door to fabricating larger two-dimensional quantum dot arrays with high connectivity.

Atomic clocks are crucial for fundamental science and practical technology, but their sensitivity is often constrained by the standard quantum limit. With a cold-atom coherent-population-trapping clock, the authors design and experimentally demonstrate an adaptive Bayesian quantum frequency estimation protocol that approaches the Heisenberg scaling with respect to total interrogation time. This atomic clock also yields improved fractional frequency stability and enhanced robustness against technical noises. This work provides a high-precision approach to atomic clock locking, and holds promising applications in various interferometry-based quantum sensors.

Spin defects embedded in a scalable material platform with bright telecom emission are promising candidates for quantum communication technologies. $V^{4+}$ in SiC fulfills these criteria, but its potential is limited by the lack of understanding of its spin relaxation mechanisms. This study employs all-optical measurements to reveal that the site-dependent spin $T_1$ values can exceed 20 seconds and identifies the mechanism of the spin relaxation processes. These insights lead to a proposal to enable qubit operations at higher temperatures, significantly reducing the infrastructure requirements and paving the way for practical realization of quantum technologies based on $V^{4+}$ in SiC.