Long-distance quantum communication can usher in hack-proof communication, enable precision quantum sensing technologies, and ultimately yield a quantum Internet. Existing protocols for global-scale quantum communication demand high-performance quantum memories, which have limited communication distance. This study proposes a chain of satellites in low Earth orbit to directly transmit photonic qubits through space, using the satellites like optical lenses to counter diffraction loss. Simulations and analysis of different quantum communication protocols using this relay system show the feasibility of building a global quantum network of only satellites, without requiring quantum memories.

Diamond is hard: Carrier mobility in diamond is a key parameter for the development of future electronics and quantum devices, yet the low-field mobility of holes in ultrapure diamond is unknown, below 80 K or so. This study presents a time-of-flight technique using electron-beam-induced current to measure the velocities of electrons and holes as a function of temperature and electric field. A low-field mobility of (1.03$\pm$0.05)$\times$10${}^6$ cm${}^2$ V${}^{-1}$ s${}^{-1}$ is measured for holes at 13 K, demonstrating that diamond is a suitable material for ballistic transport of charge carriers at a length scale of greater than 10 μm.

Controlling magnon-photon coupling is one of the keys to enabling cavity magnonics in several emerging applications, where the realization of all-on-chip devices is crucial to integrating magnonic systems with microwave circuits. This study shows that ultrastrong coupling can be achieved with a ferrimagnetic film in direct contact with a superconducting resonator. Analysis shows that the diamagnetic coupling term is vanishingly small, suggesting a potential route to superradiant phase transitions. These results ought to be relevant for microwave technologies including memory devices, microwave-to-optical transducers, haloscopes for axion detection, and coherent microwave sources.

Materials informatics has boosted materials design, but the search for optimal atomic configurations in spintronic devices is challenging, due to many degrees of freedom and the need to design at the atomic level. Quantum annealing offers a breakthrough for such challenges in huge search spaces. The authors propose a combination of quantum annealing, machine learning, and first-principles calculations that is computationally cheaper than ordinary machine learning in designing atomically disordered spinel oxides (promising materials for magnetoresistive devices). Furthermore, the origins of physical properties of interest can be interpreted from the obtained Ising model Hamiltonian.

Understanding the loss mechanisms in materials is crucial to improving coherence in superconducting quantum circuits. The authors present a technique based on multimode superconducting resonators that distinguishes and quantifies all loss channels in relevant materials. Applying this technique reveals that both chemical etching and diamond turning reduce surface losses in high-purity aluminum, while coating diamond-turned surfaces with thin-film aluminum significantly improves joint quality. This method can be used to design on-chip superconducting devices to characterize microwave losses, as well as to quantify the effects of fabrication processes.

Interferometers are extremely powerful tools for precision measurements in a plethora of research fields and applications, such as the detection of gravitational waves. The authors present experimental realization of a nonlinear microwave interferometer based on superconducting quantum circuits. Useful properties of this device range from a signal-to-noise ratio that exceeds the shot-noise limit, to sub-Poissonian intensity fluctuations between its outputs. These intriguing findings will promote applications ranging from quantum illumination to the search for axionic dark matter.

Because any unitary operation is a rotation, in a sense measuring rotation is the most universal sort of measurement. In practice, precise rotation measurements are essential, from magnetometry to inertial navigation to fundamental tests of physics. The ultimate limits for simultaneously measuring all of the components of a rotation are dictated by quantum theory, and here are tested using light’s orbital angular momentum and multiplane light conversion. Rotated states are projected onto a set of coherent states to deduce the rotation parameters, using a method inspired by GPS. The results are near the ultimate limits of quantum precision.

Precise clock synchronization is important for quantum networking, enhanced position, navigation, timing, and other applications that require clock synchronization better than GPS, but remains an ongoing challenge. The authors propose a solution using hardware native to quantum networking. This quantum-time-transfer technique can quickly achieve picosecond-level clock synchronization despite relatively low-performance quantum-photon sources and detection equipment. Furthermore, it is robust against the high loss and high noise channel conditions representative of daytime space-Earth links, and could provide high-precision secure timing in GPS-denied environments.

This work demonstrates experimentally that the advantage gained in sensing using quantum photonic resources can be sustained, and even amplified, through complex classical post-processing aimed at extracting relevant features. Despite the very different architectures of the classical algorithms tested here, the quantum advantage in classification performance appears to be robust and qualitatively very consistent. Thus the results argue for widespread use of quantum sensing technologies, in any field that deals with pattern recognition in large datasets.

Know your enemy: In practical inline systems for continuous-variable quantum key distribution, the local oscillator (LO) is particularly vulnerable to being controlled by hackers. The authors propose a modified LO intensity attack with random fluctuations, which has the advantage of evading the commonly used monitoring technologies. Moreover, similar quantum hacking could also target the pilot intensity in a LO system, which indicates the strong adaptability and practicability of the proposed attack. This work could be of great significance to the practical security of continuous-variable quantum key distribution.

To reach subpicometer sensitivities in the millihertz frequency range, inertial sensors based on laser interferometry must reduce laser-frequency noise, typically by using ultrastable optical reference cavities or atomic or molecular references. This study presents a simpler solution: a quasimonolithic Mach-Zehnder interferometer with intentionally mismatched arm lengths, plus two photodiodes at balanced output ports. Here structural stability is transferred to laser frequency via amplification, integration, and feedback of the balanced-detection signal. Improvements to the thermal design and isolation of a compact setup allow stabilities similar to those of reference systems.

Power-saving solutions are sought for manipulating the dynamics of magnetic microelements in emerging spintronic rf devices. The authors show that local piezostrain is an efficient extrinsic means to control the gyrotropic motion of magnetic vortices, with low voltages and all-electrical operation. In addition, they find that the frequency-strain tuning ratio scales inversely with the size of the vortex-based microdevice. Their approach offers an extra dimension in the frequency tuning of vortex-based spintronic oscillators.

Control over the ferromagnetic resonance frequency is a key ingredient for applications of magnetic structures in the fields of spintronics and magnonics. In this study the authors uncover rich magnetization dynamics and high resonance frequencies in electronically interacting hybrid superconductor-ferromagnetic multilayers, where additional magnetic anisotropy and unconventional antiferromagneticlike interaction between ferromagnetic layers are realized via circulating superconducting currents. These phenomena point to wide applicability of such multilayers in magnonics, as well as integration into various superconducting hybrid systems.

The electron-hole liquid is an exotic state of matter in which a gas of excitons condenses to form electronic droplets. So far, its formation has been limited to materials with indirect band gaps at cryogenic temperatures. In this Letter researchers demonstrate the existence of an electron-hole liquid at room temperature, by utilizing the electron-phonon interaction in a film of coupled nanocrystals of direct-band-gap CsPbBrI${}_2$. The resulting liquid state shows remarkable stability under ambient conditions, and opens up possibilities for optoelectronic applications such as lasers and LEDs that do not require low temperatures.

This Letter proposes a versatile approach for using high-order exceptional points (EP) with enhanced sensitivity in LC-based wireless sensing systems. EPs are remarkably responsive to minute parameter variations, which suggests promising applications in sensors. Whereas previous schemes have relied on PT-symmetric circuits comprised of coupled gain-neutral-loss RLC resonators, here the authors investigate a broader pseudo-Hermitian system configuration involving gain-loss-loss elements. This generalized method for EP implementation in electronic circuits provides a more accurate model for practical wireless sensing systems, with significant implications for wireless communication.

Number factorization is instrumental in cybersecurity and machine-learning applications, for example. Although quantum protocols have enabled remarkable breakthroughs here, their limitations have spurred classical alternatives undergirded by the physics of superposition of coherent waves, but these are extremely difficult to implement under realistic noisy environments. The authors propose and demonstrate with random optical waves a factoring protocol that is free from this shortcoming. Their theory also exposes a fundamental link between statistical optics and number theory. This protocol may trigger advances in factoring with acoustic and matter waves as well.

Diamond is hard: Carrier mobility in diamond is a key parameter for the development of future electronics and quantum devices, yet the low-field mobility of holes in ultrapure diamond is unknown, below 80 K or so. This study presents a time-of-flight technique using electron-beam-induced current to measure the velocities of electrons and holes as a function of temperature and electric field. A low-field mobility of (1.03$\pm$0.05)$\times$10${}^6$ cm${}^2$ V${}^{-1}$ s${}^{-1}$ is measured for holes at 13 K, demonstrating that diamond is a suitable material for ballistic transport of charge carriers at a length scale of greater than 10 μm.

Controlling magnon-photon coupling is one of the keys to enabling cavity magnonics in several emerging applications, where the realization of all-on-chip devices is crucial to integrating magnonic systems with microwave circuits. This study shows that ultrastrong coupling can be achieved with a ferrimagnetic film in direct contact with a superconducting resonator. Analysis shows that the diamagnetic coupling term is vanishingly small, suggesting a potential route to superradiant phase transitions. These results ought to be relevant for microwave technologies including memory devices, microwave-to-optical transducers, haloscopes for axion detection, and coherent microwave sources.

Materials informatics has boosted materials design, but the search for optimal atomic configurations in spintronic devices is challenging, due to many degrees of freedom and the need to design at the atomic level. Quantum annealing offers a breakthrough for such challenges in huge search spaces. The authors propose a combination of quantum annealing, machine learning, and first-principles calculations that is computationally cheaper than ordinary machine learning in designing atomically disordered spinel oxides (promising materials for magnetoresistive devices). Furthermore, the origins of physical properties of interest can be interpreted from the obtained Ising model Hamiltonian.

Understanding the loss mechanisms in materials is crucial to improving coherence in superconducting quantum circuits. The authors present a technique based on multimode superconducting resonators that distinguishes and quantifies all loss channels in relevant materials. Applying this technique reveals that both chemical etching and diamond turning reduce surface losses in high-purity aluminum, while coating diamond-turned surfaces with thin-film aluminum significantly improves joint quality. This method can be used to design on-chip superconducting devices to characterize microwave losses, as well as to quantify the effects of fabrication processes.

Long-distance quantum communication can usher in hack-proof communication, enable precision quantum sensing technologies, and ultimately yield a quantum Internet. Existing protocols for global-scale quantum communication demand high-performance quantum memories, which have limited communication distance. This study proposes a chain of satellites in low Earth orbit to directly transmit photonic qubits through space, using the satellites like optical lenses to counter diffraction loss. Simulations and analysis of different quantum communication protocols using this relay system show the feasibility of building a global quantum network of only satellites, without requiring quantum memories.

Interferometers are extremely powerful tools for precision measurements in a plethora of research fields and applications, such as the detection of gravitational waves. The authors present experimental realization of a nonlinear microwave interferometer based on superconducting quantum circuits. Useful properties of this device range from a signal-to-noise ratio that exceeds the shot-noise limit, to sub-Poissonian intensity fluctuations between its outputs. These intriguing findings will promote applications ranging from quantum illumination to the search for axionic dark matter.

Because any unitary operation is a rotation, in a sense measuring rotation is the most universal sort of measurement. In practice, precise rotation measurements are essential, from magnetometry to inertial navigation to fundamental tests of physics. The ultimate limits for simultaneously measuring all of the components of a rotation are dictated by quantum theory, and here are tested using light’s orbital angular momentum and multiplane light conversion. Rotated states are projected onto a set of coherent states to deduce the rotation parameters, using a method inspired by GPS. The results are near the ultimate limits of quantum precision.

Precise clock synchronization is important for quantum networking, enhanced position, navigation, timing, and other applications that require clock synchronization better than GPS, but remains an ongoing challenge. The authors propose a solution using hardware native to quantum networking. This quantum-time-transfer technique can quickly achieve picosecond-level clock synchronization despite relatively low-performance quantum-photon sources and detection equipment. Furthermore, it is robust against the high loss and high noise channel conditions representative of daytime space-Earth links, and could provide high-precision secure timing in GPS-denied environments.

This work demonstrates experimentally that the advantage gained in sensing using quantum photonic resources can be sustained, and even amplified, through complex classical post-processing aimed at extracting relevant features. Despite the very different architectures of the classical algorithms tested here, the quantum advantage in classification performance appears to be robust and qualitatively very consistent. Thus the results argue for widespread use of quantum sensing technologies, in any field that deals with pattern recognition in large datasets.

Know your enemy: In practical inline systems for continuous-variable quantum key distribution, the local oscillator (LO) is particularly vulnerable to being controlled by hackers. The authors propose a modified LO intensity attack with random fluctuations, which has the advantage of evading the commonly used monitoring technologies. Moreover, similar quantum hacking could also target the pilot intensity in a LO system, which indicates the strong adaptability and practicability of the proposed attack. This work could be of great significance to the practical security of continuous-variable quantum key distribution.

To reach subpicometer sensitivities in the millihertz frequency range, inertial sensors based on laser interferometry must reduce laser-frequency noise, typically by using ultrastable optical reference cavities or atomic or molecular references. This study presents a simpler solution: a quasimonolithic Mach-Zehnder interferometer with intentionally mismatched arm lengths, plus two photodiodes at balanced output ports. Here structural stability is transferred to laser frequency via amplification, integration, and feedback of the balanced-detection signal. Improvements to the thermal design and isolation of a compact setup allow stabilities similar to those of reference systems.

Power-saving solutions are sought for manipulating the dynamics of magnetic microelements in emerging spintronic rf devices. The authors show that local piezostrain is an efficient extrinsic means to control the gyrotropic motion of magnetic vortices, with low voltages and all-electrical operation. In addition, they find that the frequency-strain tuning ratio scales inversely with the size of the vortex-based microdevice. Their approach offers an extra dimension in the frequency tuning of vortex-based spintronic oscillators.