This paper develops a method for estimating the locations of laser-induced stray charges on surface-electrode ion traps. How the electric potential at the trapped-ion location changes in the presence of laser-induced stray charges is theoretically modeled, and the magnitude of the shift in the secular frequency of a trapped ion is derived from the model. This model can be used inversely to find the locations of stray charges from the secular frequency shift under certain conditions. Typically, this inverse relationship is not unique. However, in surface-electrode ion traps, the secular frequency shift caused by the stray charges can be measured at multiple ion positions, and therefore, the required number of secular-frequency-shift measurements can be performed to determine the stray-charge locations uniquely. This model is experimentally evaluated by intentionally producing electric charges at a single location by irradiating a laser onto that spot and by measuring the resulting secular frequency shift. The location of produced charges is estimated from the measured result, and the estimated location is consistent with where the laser is irradiating. The developed method can be applied to characterize the laser-induced stray charges, which in turn can be used to design experiments to avoid or reduce the occurrence of stray charges.

According to the Heisenberg uncertainty principle between time and energy fluctuation, a concept of the quantum speed limit (QSL) has been established to determine the minimum evolutionary time between quantum states. Considerable theoretical and experimental efforts are invested in obtaining the QSL time bounds in various scenarios. However, it remains a long-standing goal to derive a meaningful QSL bound for a general quantum problem. Here, we propose a geometrical approach to derive a QSL bound for closed and open quantum systems. By solving a quantum brachistochrone problem in the framework of the Riemannian metric, we show that the QSL between a given initial state and a final state is determined not only by the entire dynamics of the system but also by the individual dynamics of a critical parameter. We exemplify the utility of the bound in three representative scenarios, demonstrating a pronounced advantage in finding a tight and meaningful QSL bound of a general quantum evolution problem.

We propose a hetero-homodyne receiver for quantum illumination (QI) target detection. Unlike prior QI receivers, it uses a cascaded positive operator-valued measurement (POVM) that does not require a quantum interaction between QI’s returned radiation and its stored idler. When used without sequential detection its performance matches the 3-dB quantum advantage over optimum classical illumination (CI) that Guha and Erkmen’s [Phys. Rev. A 80, 052310 (2009)] phase-conjugate and parametric amplifier receivers enjoy. When used in a sequential detection QI protocol, the hetero-homodyne receiver offers a 9-dB quantum advantage over a conventional CI radar, and a 3-dB advantage over a CI radar with sequential detection. Our work is a significant step forward toward a practical quantum radar for the microwave region, and, more generally, emphasizes the potential offered by cascaded POVMs for quantum radar.

A self-driven hybrid atomic spin oscillator is demonstrated in theory and experiment with a vapor $\mathrm{Rb}$-$\mathrm{Xe}$ dual-spin system. The raw signal of $\mathrm{Rb}$ spin oscillation is amplified, phase-shifted, and sent back to drive the $\mathrm{Xe}$ spins coherently. By fine-tuning the driving-field strength and phase, a self-sustaining spin-oscillation signal with zero frequency shift is obtained. The effective coherence time is infinitely prolonged beyond the intrinsic coherence time of $\mathrm{Xe}$ spins, forming a hybrid atomic spin oscillator. Spectral analysis indicates that a frequency resolution of 13.1 nHz is achieved, increasing the detection sensitivity for a magnetic field. Allan-deviation analysis shows that the spin oscillator can operate in continuous-wave mode like a spin maser. The prototype spin oscillator can be easily incorporated into other hybrid spin systems and can increase the detection sensitivity of alkali-metal–noble-gas comagnetometers.

A label-free, low-photodamage, low-cost, and in vivo high-resolution microscopic imaging technique has been challenging to develop. Ghost imaging (GI) is a computational imaging protocol with the spatially nonlocal property, which enables low-light imaging. Considering the advantages of GI, we propose a method, called high-resolution microscopic ghost imaging (HRMGI), to achieve high-resolution imaging by simply demagnifying the signal beam. The analytical formulas for resolution and depth of field (DOF) of HRMGI are provided. Experimental results show that the HRMGI system can have 10.5 times higher resolution than the conventional GI, which can exceed the pixel size limit of multipixel detectors, and 58 times greater DOF than the traditional diffraction-limited optical (TDLO) system. To present the performance of HRMGI, blood vessels within an ovarian section are also imaged. The proposed method is expected to facilitate the development of bioimaging techniques and promote the practical application of GI.

Nonreciprocal quantum devices with low loss are indispensable in quantum information processing but challenging to be realized. Here we report an experiment to realize the nonreciprocal phase shift larger than $\pi$ with low loss in a cavity-free and magnetic-free cold atomic ensemble based on electromagnetically induced transparency. Furthermore, a four-port all-optical circulator is demonstrated with an average isolation ratio of 22.25 dB and transmission of 0.94 for each channel. At its optimal working point, the circulator’s total isolation ratio reaches 63.47 dB, transmission reaches 0.86, and fidelity at about 0.99. Our work can advance further investigation of nonreciprocity with cold atoms in quantum information processing applications.

A high-efficiency spin-photon interface is an essential piece of quantum hardware necessary for various quantum technologies. Self-assembled ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dots have excellent optical properties: if embedded in an optical microcavity, they can show near-deterministic spin-photon entanglement and spin readout. To address the individual spin states, an external magnetic field is required, and a superconducting magnet is usually used. Here we show a compact cryogenically compatible $\mathrm{Sm}$-$\mathrm{Co}$ magnet design that delivers a 475-mT in-plane (Voigt-geometry) magnetic field at $5\mathrm{K}$, which is suitable for lifting the energy degeneracy of the electron spin states and trion transitions of a single ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dot. This quantum dot is embedded in a birefringent high-finesse optical microcavity that enables efficient collection of single photons emitted by the quantum dot. We demonstrate spin-state manipulation by addressing the trion transitions with a single laser field and two laser fields. The experimental data agree well with our model, which covers single- and two-laser cross-polarized resonance fluorescence, Purcell enhancement in a birefringent cavity, and variation of the laser powers.

Antiferromagnets that facilitate terahertz (THz) spin resonances have the potential to revolutionize high-speed electronics at the nanoscale. The electrical control of THz spin resonances is the key to such THz devices; however, experimental demonstration has remained elusive. In this work, we demonstrate electrically tunable THz spin resonance in an antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ heterostructure by employing both low-wavenumber Raman and continuous-wave THz spectroscopy techniques. A redshift of over 100 GHz in the $\mathrm{Ni}\mathrm{O}$ spin resonance frequency of around 1 THz is observed by applying charge currents along the adjacent $\mathrm{Pt}$ layer. A control experiment with $\mathrm{Ni}\mathrm{O}/\mathrm{Cu}$ and temperature-dependent measurement confirm that the dominant tuning mechanism is Joule heating. Finally, a prototype device is designed to achieve an electrical control of THz transmission at dual channels of 0.96 and 1 THz, leading to a Q factor of 56. This work opens up the possibility for the implementation of tunable THz devices utilizing antiferromagnetic spin resonance.

An acoustic dimer composed of two electronically controlled electroacoustic resonators is presented with the view of exploring one-dimensional topological phenomena. Active control allows real-time manipulation of the metamaterial’s properties, including its mechanical mass, resistance, compliance, and internal coupling. The latter enables active tuning of the topological phase transition in the effective acoustic Su-Schrieffer-Heeger system. An analytical model of the unit cell as well as the electronic control scheme is provided. Band structures derived from the analytical model, the finite-element simulation, and the experimental data consistently demonstrate the realization of a tunable one-dimensional topological insulator.

We investigate the optimal deposition conditions for permalloy (${\mathrm{Ni}}_{81}\mathrm{Fe}$${}_{19}$) thin films fabricated via magnetron sputtering to achieve soft magnetic films for magnetoresistive sensing applications. The films are grown with different deposition techniques such as sputtering and molecular-beam epitaxy and parameters, including varying inert gas pressure and deposition power, and with different magnetic fields applied during the growth. Our approach enables sputtering of permalloy films with low coercivity while keeping high anisotropic magnetoresistance values. We develop a robust method to characterize the intrinsic magnetic anisotropy of the films that is not dominated by local defects and we demonstrate the possibility of magnetic anisotropy suppression via implementing a rotating magnetic field during sputtering.

The Laser Interferometer Space Antenna (LISA) is an upcoming European Space Agency mission that will detect gravitational waves in space by interferometrically measuring the separation between free-falling test masses at picometer precision. To reach the desired performance, LISA will employ the noise reduction technique time-delay interferometry (TDI), in which multiple raw interferometric readouts are time shifted and combined into the final scientific observables. Evaluating the performance in terms of these TDI variables requires careful tracking of how different noise sources propagate through TDI, as noise correlations might affect the performance in unexpected ways. One example of such potentially correlated noise is the relative-intensity noise (RIN) of the six lasers aboard the three LISA satellites, which will couple into the interferometric phase measurements. In this article, we calculate the expected RIN levels based on the current mission architecture and the envisaged mitigation strategies. We find that strict requirements on the technical design reduce the effect from approximately $8.7\mathrm{pm}/\sqrt{\mathrm{Hz}}$ per interspacecraft interferometer to that of a much lower sub-$1$-$\mathrm{pm}/\sqrt{\mathrm{Hz}}$ noise, with typical characteristics of an uncorrelated readout noise after TDI. Our investigations underline the importance of sufficient balanced detection of the interferometric measurements.

Interaction of counterpropagating (CP) solitons in a high-$Q$ microcavity can lead to phase locking of the two microcombs, endowing a Vernier-like comb pair with high mutual coherence. Here, we study numerically and analytically the soliton dynamics in this phase-locking state. Our simulations unveil that the relative group velocity of CP solitons will change periodically as opposed to being locked to a fixed value. CP solitons are kicked into the frequency-locking state instead of via continuous interaction when assuming a point backscatterer. The group-velocity modulation and periodic soliton motion induce sidebands around the comb modes. The simulated locking dynamics can be analytically modeled by a generalized Adler equation, which accounts for injection locking among multiple comb-line pairs. Our work elucidates the relevance of considering the frequency-locking dynamics when using CP solitons for dual-comb or time-of-flight-based applications.

Greenberger-Horne-Zeilinger and $W$ states feature genuine tripartite entanglement that cannot be converted into each other by local operations and classical communication. Here, we present a dissipative protocol for deterministic interconversion between Greenberger-Horne-Zeilinger and $W$ states of three neutral ${}^{87}\mathrm{Rb}$ atoms arranged in an equilateral triangle of a two-dimensional array. With three atomic levels and diagonal van der Waals interactions of Rydberg atoms, the interconversion between tripartite entangled states can be efficiently accomplished in the Floquet-Lindblad framework through the periodic optical pump and dissipation engineering. We evaluate the feasibility of the existing methodology using the experimental parameters accessible to current neutral-atom platforms. We find that our scheme is robust against typical noises, such as laser phase noise and geometric imperfections of the atom array. In addition, our scheme can integrate the Gaussian soft quantum control technique, which further reduces the overall conversion time and increases the resilience to timing errors and interatomic distance fluctuations. The high-fidelity and robust tripartite entanglement interconversion protocol provides a route to save physical resources and enhance the computational efficiency of quantum networks formed by neutral-atom arrays.