Arguably, the lamp-pumped vapor-cell atomic clock is the workhorse of precise timekeeping in space. However, the present-day Rb atomic frequency standard (RAFS) has two clear deficiencies: (1) its short-term stability is limited by weak optical pumping, and (2) its long-term stability can be limited by light-shift-induced frequency noise. In both cases, the limitations trace to the use of an rf-discharge lamp for atomic signal generation and monitoring. Though replacement of the lamp by a pulsed diode laser can eliminate these problems, laser use in the RAFS presents space-systems engineers with a dilemma: trading better clock performance against potentially decreased clock reliability. Here, we discuss progress on development of the lamp and laser integrated pulsed optically pumped atomic clock, which is a “bridge technology” that can solve the space-system engineer’s dilemma.

Numerical gate design typically makes use of high-dimensional parametrizations enabling sophisticated, highly expressive control pulses. Developing efficient experimental calibration methods for such gates is a long-standing challenge in quantum control, as on-device calibration requires the optimization of noisy experimental data over high-dimensional parameter spaces. To improve the efficiency of calibrations, we present a systematic method based on singular value decomposition stacking for reducing the dimensionality of the parameter space traversed in gate calibration, starting from an arbitrary high-dimensional pulse representation. We use this approach to design and calibrate an $X_{\pi /2}$ gate robust against amplitude and detuning errors, as well as an $X_{\pi /2}$ gate robust against coherent errors due to a spectator qubit.

We demonstrate single-electron trapping and detection in a dual-plane printed-circuit-board Penning trap. We have performed deterministic single-electron loading, achieved a axial damping rate of ${\gamma }_z/(2\pi )=18(1)\mathrm{Hz}$, measured the axial temperature, and measured the magnetron-radius growth rate at magnetic fields below 0.15 T. These results establish a practical platform for dual-plane Penning traps.

We report an rf parametric amplifier that exploits the gate-tunable quantum capacitance of a semiconductor two-dimensional electron gas at cryogenic temperatures. The prototype amplifier exhibits a gain approximately 30 dB up to an input power of $-66\mathrm{dBm}$ and a noise temperature $T_N\sim 2\mathrm{K}$ at approximately 370 MHz while dissipating approximately a microwatt of power. Operable at tesla-scale magnetic fields and temperatures ranging from millikelvin to a few kelvin, the quantum capacitance parametric amplifier may find utility in the readout circuit of spin qubits. Here, a quantum dot system is used to verify amplifier performance, improving the signal-to-noise ratio of the readout by approximately 9 dB.

Most quantum error-correction (QEC) protocols for superconducting qubits assume spatially and temporally uncorrelated decoherence events; however, recent evidence suggests that cosmic radiation induces spatially correlated errors. We present a platform that sandwiches a superconducting transmon qubit between two microwave kinetic inductance detector (MKID) arrays, enabling real-time detection of radiation-induced phonon bursts. By synchronizing MKID event detection with single-shot measurements of qubit energy relaxation ($T_1$) and phase coherence ($T_2$), we observe statistically significant reductions in both $T_1$ and $T_2$—up to 30.5%—immediately following dual MKID events attributed to penetrating muons. Our findings directly link radiating events to correlated qubit decoherence. Furthermore, our experimental platform provides a foundation for systematic studies of radiation effects, the development of shielding and mitigation techniques, and the refinement of error-correction algorithms tailored to correlated noise sources.

Purpose: Parameter estimation from biexponential models is a notoriously ill-posed problem, often yielding inaccurate estimates that are strongly influenced by noise. We present a simple, nonconventional approach to regularizing nonlinear least-squares (NLLS) parameter estimation and apply it to myelin water fraction (MWF) mapping with magnetic resonance imaging. Motivated by the ridge regression theorem for linear least-squares problems, we introduce a nonlinear extension, which we denote $\lambda$-NL-RR, in which regularization is applied directly to the NLLS estimation of discrete parameters. Methods: We applied $\lambda$-NL-RR to simulated and experimental biexponential magnetic resonance imaging (MRI) data and compared parameter estimates with those obtained using conventional NLLS fitting. The regularization parameter $\lambda$ was selected using generalized cross validation. Results: Under conditions of modest signal-to-noise ratio and closely spaced exponential time constants, regimes in which conventional biexponential analysis performs particularly poorly, $\lambda$-NL-RR was found to reduce mean-square error by up to 80% relative to standard NLLS for simulated data, consistent with the known behavior of regularization in more conventional contexts. Improvements in root-mean-square error in the range of 11%–25% in parameter estimates were seen for in vivo brain data, although there is no true gold standard for underlying values in these experiments. Notably, these results are due to postprocessing only and require no modification of data acquisition strategy or duration. Conclusion: Regularization of the NLLS estimation problem for the biexponential model substantially improves parameter accuracy in MWF mapping. Although related ideas exist in scattered form in the applied mathematics literature, this work recognizes, formalizes, and demonstrates ridge-type regularization for discrete NLLS parameter estimation in MRI. These results establish a foundation for broader application of nonlinear regularization strategies in quantitative MRI and other fields.

We study quantum enhancement of sensitivity using squeezed light in a multiparameter quantum sensor, the hybrid dc and rf optically pumped magnetometer (hOPM) [M. Lipka et al., Multiparameter quantum sensing and magnetic communication with a hybrid dc and rf optically pumped magnetometer, Phys. Rev. Appl. 21, 034054 (2024)]. Using a single spin ensemble, the hOPM acquires both the dc field strength (scalar magnetometry) and resonantly detects one quadrature of the ac magnetic field at a chosen frequency (rf magnetometry). In contrast to the Bell-Bloom scalar magnetometer [C. Troullinou et al., Squeezed-light enhancement and backaction evasion in a high sensitivity optically pumped magnetometer, Phys. Rev. Lett. 127, 193601 (2021)], the backaction evasion in the hOPM is incomplete, leading to a nontrivial interplay of the three quantum noise sources in this system: photon shot noise, spin projection noise, and measurement backaction noise. We observe these interactions using squeezed light as a tool to control the distribution of optical quantum noise between $S_2$ and $S_3$ polarization Stokes components, and the resulting effect on readout quantum noise and measurement backaction. These results demonstrate quantum-enhanced sensitivity in a continuously operating multiparameter sensor and reveal fundamental trade-offs between sensitivity, backaction, and bandwidth.

Parametric resonance provides a versatile mechanism to manipulate wave signals in time-modulated metamaterials, but typically operates within narrow wavelength or frequency ranges. Here, we show that one-dimensional arrays of coupled resonators can generically exhibit parametric amplification across an entire band under standing-wave parametric modulation. We develop a general physical framework to describe the parametric resonance of isolated bands, derive modulation conditions needed to achieve full-band amplification for a given band, and demonstrate their validity in a model of active mechanical metamaterials based on tunable membrane resonator arrays. We numerically demonstrate applications of the proposed strategy in distortion-free amplification and temporal splitting of wave packets, with results in quantitative agreement with our theoretical calculations. Our strategies for broadband amplification can be generalized to other active wave media with nearly flat or cosinelike dispersion relations, with potential applications in loss mitigation, noise squeezing, and programmable wave manipulation.

Random spin-exchange collisions in dense thermal alkali vapors can induce coherent mode-mixing dynamics, enabling a minority species to inherit the majority species’ gyromagnetic ratio and long coherence time. However, conventional polarization Bloch equations rely on a phenomenological slowing-down factor $q(P)$ that does not fully capture transient dynamical coupling effects in the spin-exchange relaxation-free regime for multispecies systems. Here we introduce a hyperfine-resolved Bloch equation framework that explicitly retains hyperfine-resolved alkali-alkali spin-exchange channels and alkali-nuclear Fermi-contact couplings. In a thermal ${}^{39}\mathrm{K}\text{−}{}^{85}\mathrm{Rb}\text{−}{}^3\mathrm{He}$ vapor, we demonstrate that ${}^{39}\mathrm{K}$ coherently inherits the ${}^{85}\mathrm{Rb}$ precession frequency over a wide range of longitudinal field $B_z$, with the nuclear spins inducing rapid relaxation and nonlinear gyromagnetic-ratio shifts near a magnetically tunable critical point. Time- and frequency-domain measurements with species-selective readout show that the hyperfine-resolved model reproduces the measured field-dependent mode frequencies and critical hybridization of the trispin system. By resolving the mode frequencies, damping rates, and optical-readout response near the dynamical critical point, the present framework provides a practical response-function calibration tool for strongly coupled alkali-noble-gas sensors.

Qubit noise spectroscopy (QNS) is a valuable tool for both the characterization of a qubit’s environment and as a precursor to more effective qubit control to improve qubit fidelities. Existing approaches to QNS are what the classical spectrum estimation literature would call “nonparametric” approaches, in that a series of probe sequences are used to estimate noise power at a set of points or bands. In contrast, model-based approaches to spectrum estimation assume additional structure in the form of the spectrum and leverage this for improved statistical accuracy or other capabilities, such as superresolution. Here, we derive model-based QNS approaches using inspiration from classical signal processing, primarily through the recently developed Schrödinger wave autoregressive moving-average formalism for modeling correlated noise. We show, through both simulation and experimental data, how these model-based QNS approaches maintain the statistical and computational benefits of their classical counterparts, resulting in powerful estimation approaches. Beyond the direct application of these approaches to QNS and quantum sensing, we anticipate that the flexibility of the underlying models will find utility in adaptive feedback control for quantum systems, in analogy with their role in classical adaptive signal processing and control.

Non-Hermitian systems exhibit unique properties, with exceptional points (EPs) serving as a prominent example of singular spectral degeneracy that has attracted broad interest. This interest now encompasses magnon-polariton platforms. Here, we disclose a fundamentally distinct picture in a strongly driven magnon-polariton Floquet system: a pseudo-exceptional point (PEP). Analytical derivation and macrospin simulation reveal that both the mode’s frequency and its decay rate remain finite on both sides of the PEP—a definitive departure from the characteristic eigenvalue splitting of a conventional EP. Crucially, we demonstrate that the key spectral features, particularly in the weak-driving limit, are governed by the damping coefficient, confirming the non-Hermitian origin of the phenomenon. This picture originates from the Floquet dynamics of a single driven mode and provides a natural mechanism for key nonlinear phenomena, such as frequency combs. Our work clarifies the underlying physics of non-Hermitian critical behavior in driven-dissipative spin systems and establishes the PEP as a framework, offering a robust and designable route to generating nonlinear phenomena in Floquet-engineered systems.

Seismic nonlinear coupling noise is one of the limiting factors for high-accuracy torsion pendulums. Based on the common-mode rejection of dual torsion pendulums, a seismic coupling noise suppression approach is proposed and verified by modulation experiments. By mounting two nearly identical pendulums on a single frame within a vacuum chamber, seismic disturbances on the torque noise act as common-mode signals that can be effectively canceled. Modulation experiments are performed to evaluate the common-mode rejection (CMR) performance. Modulation results demonstrate that, by applying the driven vibration, the raw noise of the pendulum increases to $1.4\times {10}^{-14}\text{Nm}/{\mathrm{Hz}}^{1/2}$ at around 2 mHz, while the CMR-processed noise stays at $1.1\times {10}^{-15}\text{Nm}/{\mathrm{Hz}}^{1/2}$, which confirms a CMR noise suppression of over one order of magnitude. This validates a practical approach for improving the torsional pendulum accuracy in small-force scientific measurements.

Controlling the radiative dynamics of quantum emitters in structured environments is fundamental to quantum optics and light-matter interaction engineering. Photonic crystal slabs offer a versatile platform for engineering emission anisotropy, yet simultaneously achieving high directionality, radiative efficiency, and collective coherence remains a central challenge. In this work, we present a comprehensive framework that quantifies directional emission through an iso-frequency-contour straightness parameter combined with spatial Fourier analysis of the radiation field. For single emitters in square-lattice slabs, we show that dielectric and geometric tuning can simultaneously realize a high Purcell factor (PF) and robust unidirectional propagation. Extending the analysis to multiemitter arrays, however, reveals an intrinsic trade-off of open nanophotonic systems: the extreme local-field confinement that maximizes the PF inherently suppresses the long-range coherent coupling required for cooperative superradiance. By operating in a moderate Purcell regime with emitters centered in air holes, we demonstrate that highly directional propagation and scalable collective emission can coexist. We further examine lattice symmetry and find that, within the parameter regimes specifically evaluated, $C_{4v}$ square lattices provide more robust single-channel unidirectional emission than the triangular lattices considered. These results establish design principles for integrated quantum light sources with controlled spatiotemporal coherence.

Enclosures made of highly permeable and highly conductive materials are used as passive shields to reduce the static magnetic field and magnetic disturbances for fundamental physics experiments and biomedical diagnostics. The two most significant specifications of a magnetic shield are the shielding factor (SF) and the residual static magnetic field. We present an alternative strategy to improve the prediction of shielding factors, taking into account the influence of gaps and overlaps in the real complex structure, which reduce the shielding properties of the magnetically shielded rooms (MSRs) in comparison to virgin shielding material. The air gap $\delta$ between the sheets turns out to be the most critical parameter with respect to the shielding factor. By using a reasonable value $\delta$, it is possible to theoretically estimate a nonlinear equivalent permeability of the shielding wall structure, which leads to a realistic prediction of the frequency-dependent shielding factor up to about 20 Hz. The residual static field distribution inside the shield is hard to predict because it depends on the magnetic history of the shielding material. A reproducible residual field stable in time can only be obtained by a demagnetization process. The presented simulation of the complete demagnetization process, considering the complex structure of an MSR with material gaps, overlaps, feedthrough openings, and different arrangements of the degaussing coils, is able to predict the residual field distribution inside MSRs for the environmental field around the chamber. The prediction of the limit obtainable by degaussing can explain the surprisingly low magnitude of the residual field observed and reproduced inside a three-layer MSR.

We investigate spin-wave transport in hybrid two-dimensional magnonic crystals comprising a low-damping yttrium iron garnet (YIG) film coupled to a periodic array of CoFeB nanodisks. Using propagating spin-wave spectroscopy, super-Nyquist magneto-optical Kerr effect microscopy, and micromagnetic simulations, we show that these systems host pronounced, tunable bandgaps that do not originate from conventional Bragg scattering, i.e., from standing-wave quantization restricted to a single axis by translational invariance along the propagation direction. Instead, the gaps arise from hybridization between the fundamental magnonic-crystal mode and in-plane transverse standing modes of the nanodisk array. Their spectral position and width are controlled by the array geometry and by the magnetic state of the nanodisks, including their vortex configuration, which sets both the static and dynamic dipolar coupling to the film. At larger lattice periods, additional gaps emerge from hybridization with modes quantized both transverse and parallel to the propagation direction, reflecting two-dimensional dispersion folding. Our results establish mode hybridization as a versatile mechanism for engineering spin-wave band structures beyond the constraints of conventional Bragg scattering and provide a pathway toward reconfigurable magnonic devices based on dipolar-coupled hybrid architectures.

While the Josephson junction is central to qubit operation, overall quantum coherence requires thorough understanding of the entire metallic environment—leads, capacitive pads, and readout resonators. The established contributors to decoherence are surface oxides and hydroxides, as well as absorbed species, such as hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with nonoxidizing metals is a successful strategy to mitigate these issues. To achieve guided optimization, it is significant to understand the microscopic behavior of capped films. Although most attention was paid to the surface and interfaces, bulk properties were not systematically studied. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium gold and compare them to bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature and the lowest upper critical field, London penetration depth, and critical current. These observations agree well with the microscopic theory of anisotropic normal and superconducting states of niobium with varying amounts of scattering. We conclude that surface encapsulation significantly affects bulk properties, and gold-coated films are the cleanest. Since these changes reside in the bulk of the niobium rather than in the overlayer, they may in turn influence the processes that limit quantum coherence. Considering such a strong variation of properties, we believe that the question of whether the surface or bulk is the dominant contributor to quantum decoherence is wide open. However, they likely both contribute via different mechanisms, which makes it very hard to pinpoint where the bottleneck is.