The Gravity Recovery and Climate Experiment–Follow On (GRACE-FO) mission measures changes in the Earth’s gravitational field using intersatellite microwave and laser links. The received optical power at which these laser links operate robustly constrains mission parameters, such as spacecraft separation, optical beam divergence angle, and technical noise requirements. This work optimizes the phasemeter, a critical component of the link, for low-received optical power phase-tracking of the GRACE-FO flight spare laser. Experimental measurements and analytical modeling demonstrated that interferometric links using the GRACE-FO flight spare laser can be tracked down to 200 femtowatts (60 dB Hz), ten times lower than the 70-dB-Hz signal strength budgeted for the GRACE-FO laser link. Operating at this level affords an increase by a factor of 3.16 in the separation, divergence angle, or relaxation to technical noise requirements over current GRACE-FO design settings.

In spaceborne gravitational-wave (GW) detectors, each satellite employs an ultrastable oscillator (USO) as a clock reference to trigger the analog-to-digital converter (ADC) for digital processing. Achieving precise timekeeping is vital for attaining the required detection sensitivity. However, both ADC aperture jitter and sampling clock jitter can disrupt the sampling timing, leading to distortion in the digitized signal and significantly compromising measurement accuracy. To mitigate these issues, spaceborne GW detection typically involves two steps: first, applying pilot-tone correction to the digitized data to eliminate ADC aperture jitter noise, and second, using clock-noise-cancellation techniques to suppress clock jitter noise, ultimately satisfying the detection requirements for spaceborne GWs. Yet, currently, there is a lack of clear elucidation on the effective clock reference within the satellite after pilot-tone correction, which is also the target of the clock-noise-cancellation technique. Clarifying this point is crucial, as it not only elucidates the physical mechanism through which clock noise affects GW detection but also guides the formulation of parameter indicators for the frequency distribution system at the experimental level. In this paper, we theoretically demonstrate, based on the fundamental principles of phase measurement, that the pilot tone becomes the effective clock reference after pilot-tone correction. Furthermore, when applying typical clock-noise-cancellation techniques to suppress pilot frequency jitter, it is crucial to ensure a high degree of coherence between the modulation frequency and the pilot frequency. To further validate this clock reference issue, we conducted electrical experiments using square waves to synchronize various clock drive frequencies. The experimental results are consistent with the theoretical predictions, demonstrating that the additional noise is better than the level of $3\times {10}^{-6}{\mathrm{rad}/\mathrm{Hz}}^{1/2}$ in the 1 mHz to 1 Hz frequency bandwidth, which falls below the fundamental noise level of typical spaceborne GW detectors.

Rapid and accurate initialization of qubits, known as reset, is a crucial building block for various tasks in quantum information processing, such as quantum error correction and estimation of the statistics of noisy quantum devices with many qubits. We demonstrate unconditional reset of a frequency-tunable transmon qubit that simultaneously resets multiple excited states using a metamaterial waveguide engineered to provide a cold bath over a wide spectral range, while providing strong protection against Purcell decay of the qubit. We report a reset error below 0.13% (0.16%) when prepared in the first (second) excited state of the transmon within 88 ns. In addition, through the sharp roll-off in the density of states of the metamaterial waveguide, we implement a leakage-reduction unit that selectively resets the transmon’s second excited state to 0.285(3)% residual population within 44 ns while acting trivially in the computational subspace as an identity operation that preserves encoded information with an infidelity of 0.72(1)%.

The conventional formulation of quantum sensing has mostly been developed in the context of local estimation, where the unknown parameter is roughly known. In contrast, global sensing, where the prior information is incomplete and the unknown parameter is only known to lie within a broad interval, is practically more engaging but has received far less theoretical attention. Available formulations of global sensing rely on adaptive Bayesian strategies requiring on-the-fly change in measurement settings or minimizing average uncertainty yielding unsaturable bounds. Here, we provide an operationally motivated approach to global sensing for fixed but optimized settings. Our scheme yields a saturable precision bound optimizing the measurement as well as the probe preparation simultaneously. The formalism is general and computationally scalable for generic bosonic multimode Gaussian or many-particle free-fermionic quantum sensors. We illustrate the implications for Gaussian thermometry and Gaussian phase estimation by showing that the optimal measurement changes, either gradually or abruptly, from homodyne for local sensing, toward heterodyne for global sensing. In contrast, for fermionic transverse $XY$ probes, the optimal measurement basis stays fixed independent of the width.

Magnetic-resonance relaxation-time distributions are commonly used to characterize the pore spaces of fluid-saturated porous media such as concrete, soils, and petroleum-reservoir rocks. The pore size is a critical determinant of the relaxation behavior observed in a fluid-saturated porous medium. It is experimentally challenging to explore the pore size in isolation because changing the sample to change the pore size usually leads to a concurrent change in surface relaxivity. To explore the effect of changing the pore size in isolation, we employ bead packs of different bead sizes. In this way, the matrix and therefore surface relaxivity should be constant as the pore size changes. The measured relaxation lifetimes vary systematically in paramagnetic and diamagnetic glass-bead systems. Direct proportionality between the relaxation lifetime and pore size is observed across the fast-exchange and intermediate regimes of Brownstein-Tarr theory. Although the relaxation-diffusion regime is often incorrectly assigned, the proportionality between lifetime and pore size ensures that the use of the $T_2$ distribution as a proxy for pore size in applied studies is sound.

In this paper, we present a method for security analysis of Trojan-horse attacks (THAs) launched on a practical fiber-based quantum key distribution system across a wide spectral range. To achieve this, we use optical time-domain reflectometry (OTDR) for spectral reflectance analysis in the near-infrared range λ = 1100–1800 nm with centimeter-level resolution and a noise floor down to −80 dB. A total theoretical security analysis of the THA side channel considering the spectral reflectance and transmittance data over this spectral range is conducted. To the best of our knowledge, our OTDR setup and the corresponding results are the first of their kind to be obtained across a wide spectral range.

This letter presents a dual-quadrature phasemeter, an implementation of a phase-locked loop designed to track the phase of homodyne and heterodyne intersatellite laser links. Dual-quadrature phasemeters use dual-quadrature optical detection to enable an alternate phase readout scheme that operates with wider bandwidths and on signals with carrier-frequency differences down to dc. Analytical modeling demonstrates that dual-quadrature phasemeters overcome the bandwidth limitations of conventional phasemeters. Numerical simulations of noise linearity tests found that the phase-tracking error of the dual-quadrature phasemeter is less than 10 $\mathrm{microcycle}/\sqrt{\mathrm{Hz}}$ in the presence of nonlinear cyclic errors. The dual-quadrature phasemeter enables exploration of an optical configuration for retroreflector-based space geodesy missions with architecture similar to that implemented in the Gravity Recovery and Climate Experiment Follow-On mission. One such alternate configuration is proposed that is capable of tracking satellite separation without requiring a frequency offset between local and incoming light, eliminating the need for optical frequency shifters.

Quantum random-number generators (QRNGs), which exploit quantum mechanical effects to produce truly random numbers, are extensively used in information security and cryptography. Despite their inherent randomness, practical implementations of QRNGs, particularly those based on CMOS sensors, face security challenges due to device imperfections and external interference. This paper investigates the security vulnerabilities of QRNG systems against source-signal-injection attacks, specifically focusing on electrical signal fluctuations that modulate the intensity of the light source. This modulation induces subtle classical fluctuations in the photon-detection process, which are misinterpreted by legitimate users as quantum noise. This leads to an overestimation of secure entropy, compromising the security of the system. We demonstrate the effectiveness of the attack through experimental design and implementation, simulating practical-attack scenarios and assessing the impact on the entropy of generated random numbers. Our experiments reveal that while the altered random numbers can still pass National Institute of Standards and Technology (NIST) tests, the attacker can extract additional entropy without significantly affecting the statistical properties of the random numbers. To mitigate these risks, we introduce a defense mechanism that sets an entropy-threshold range and continuously monitors entropy variations, effectively preventing classical fluctuations from being misinterpreted as secure entropy. These findings hopefully provide critical insights for enhancing the reliability and security of QRNG systems in future large-scale quantum security applications.

We introduce a two-fluid mobility model incorporating fundamental aspects of electron-hole (e-h) scattering such as momentum conservation for simulating laser-driven semiconductor switches (LDSSs). Compared to previous works that use Matthiessen’s rule, the two-fluid mobility model predicts distinct ac responses of e-h plasmas in semiconductors. Based on the two-fluid mobility model, we develop a theory with very few adjustable parameters for simulating the switching performance of LDSSs based on high-purity indirect-gap semiconductors such as silicon ($\mathrm{Si}$). As a prototypical application, we successfully reproduce experimentally measured reflectance at around 320 GHz in a laser-driven $\mathrm{Si}$ switch. By injecting e-h plasmas with densities up to ${10}^{20}{\mathrm{cm}}^{-3}$, we reveal the importance of carrier-screening effects in e-h scattering and Auger recombination for carrier densities above the critical carrier density for exciton-plasma Mott transition. Our results also suggest a way to characterize the intrinsic momentum-relaxation mechanism, e-h scattering, and the intrinsic e-h recombination mechanism in indirect-gap semiconductors, Auger recombination. We reassess the ambipolar Auger coefficient of high-purity $\mathrm{Si}$ with high injection levels of e-h plasmas up to ${10}^{20}{\mathrm{cm}}^{-3}$ and find a minimal value of $1.8\times {10}^{-41}{\mathrm{cm}}^6/\mathrm{ns}$. This value is more than one order of magnitude smaller than the ambipolar Auger coefficient widely used for simulating LDSSs, $3.8\times {10}^{-40}{\mathrm{cm}}^6/\mathrm{ns}$, which was deduced from the minority-carrier lifetime in highly doped silicon more than four decades ago.

The majority of electrical fire incidents in power systems are caused by arc faults, highlighting the necessity for prompt detection of such faults. Traditional arc fault detection methods based on the system current and voltage often interfere with the existing power systems during data collection, whereas measurement of arc radio frequency (rf) signals avoids this type of interference. In contrast to traditional metal antennas, Rydberg-atom-based electric field sensors have nonmetallic, self-calibrating, and isotropic characteristics. Theoretically, they offer substantial advantages in terms of their working bandwidths, sensitivity, and other aspects. We investigate the response of a Rydberg-atom-based sensor to rf signals from a commercial arc lighter. We measure the spectrum of the megahertz-level arc rf signals directly, and the results obtained align well with those obtained using a traditional metal antenna. Finally, we also explore the effects of different detection distances on the measurement results. This work provides an alternative approach to the detection of arc faults and can serve as a reference for expanding the range of applications of Rydberg-atom-based sensors.

Efficient manipulation of perpendicular magnetic tunnel junctions is essential for the development of high-density and low-power spintronic devices. However, the general requirement for an external magnetic field restricts the application of emerging spin-orbit torque technologies that utilize the in-plane polarized spin current. In this work, we demonstrate an all-electrical method for manipulating perpendicular magnetic memory via tilted spin polarization. This unconventional spin current is generated by a ${\mathrm{Ru}\mathrm{O}}_2(101)$ writing channel, especially when the charge current flows along the [010] axis. The tunneling magnetoresistance ratio reaches 60%, and the critical switching current density is below 1 × ${10}^7$ A/${\mathrm{cm}}^2$. Field-free magnetization switching is further confirmed through anomalous Hall effect and magneto-optical Kerr effect measurements on Hall bar devices. These findings should advance the application of unconventional spin currents and shed light on high-density and low-power magnetic memories.

We numerically study the network dynamics and suppression of spray combustion instability in a backward-facing step combustor from the perspective of a complex network. We observe the appearance, disappearance, and reappearance of a scale-free topology in a turbulence network. A scale-free topology appears when a large-scale organized vortex is formed, whereas it disappears upon the collapse of the organized vortex. A spatially irregular retention of primary hubs occurs in the turbulence network, which is closely associated with the collapse motion of a large-scale organized vortex. Spray combustion instability is significantly suppressed by placing an obstacle at the location where connector communities are mostly formed in the turbulence network.

Subpicosecond time synchronization can enable future optical timekeeping networks [F. Riehle, Nat. Photonics 11 (1), 25 (2017)], including coherent phased array radar imaging at gigahertz levels [Collaboration, Event Horizon Telescope, Astrophys. J. Lett. 875 (2019)], intercontinental clock comparisons for the redefinition of the second [Dimarcq et al., Metrologia 61 (1), 012001 (2024)], chronometric leveling [Takamoto et al., Nat. Photonics 14 (7), 411 (2020)], and synchronization of remote assets, including future satellite-based optical time standards. With optical clocks now operating on mobile platforms [Roslund et al., Nature 628 (8009), 736 (2024)], free-space synchronization networks with compatible performance and the ability to operate under platform motion are essential to expand the reach of precision timing. Recently, femtosecond-level optical time-transfer techniques have been developed that can operate over hundreds of kilometers despite atmospheric turbulence, signal fade, and dropouts [Giorgetta et al., Nat. Photonics 7, 434 (2013); Caldwell et al., Nature 618 (7966), 721 (2023); Shen et al., Nature 610 (7933), 661 (2022)]. However, practical challenges remain for these pulsed techniques on moving platforms, including dispersion management for variable link lengths and Doppler-induced pulse ambiguities. Continuous-wave techniques overcome these limitations but are prone to femtosecond-level cycle slips during signal fades and dropouts [Gozzard et al., Phys. Rev. Lett. 128, 020801 (2022)]. Here we report an optical time-transfer scheme with femtosecond-level performance that overcomes the practical limitations of dispersion and dropout-induced cycle slips and can also support moving platforms. Importantly, the setup reduces hardware requirements by employing an integrated photonics transceiver and telecom-band lasers that are compatible with full photonic integration. Using this scheme, subfemtosecond synchronization was demonstrated over a free-space link of about 100 m for several hours. Similar performance was demonstrated over a 1-km fiber non-polarization-maintaining link without dispersion compensation. Finally, the link was used to syntonize two iodine optical clocks and then compare them over 4 days. Importantly, the system maintains femtosecond-level performance for received optical power as low as 1 picowatt, compatible with ground-to-satellite link-loss budgets.

In this paper, we present a device, designed to be used in a space payload, capable of correcting any arbitrary polarization state. The device consists of a set of three liquid-crystal variable-retarder (LCVR) cells, in which by selecting the appropriate values of the voltage to be applied in each of them, it can correct the undesirable effects. In particular, the degradation effects of the polarization states, such as ellipticity and rotation with respect to the reference system, are compensated. Among many other potential applications, this device has been conceived to be used in quantum-key-distribution (QKD) intersatellite link or satellite-ground polarization-coding protocols as a polarization-correction device (PCD). To demonstrate that these effects are corrected, we have carried out an experiment in which we have tested how the BB84 protocol states are compensated after being strongly perturbed, achieving polarization extinction ratio (PER) values around 39 dB. Our solution offers the advantage of being small and mechanism free, being the ideal solution to be implemented in small satellites. These satellites are the core to be used in future constellations required for a global quantum communications network.