Solving optimization problems using variational algorithms stands out as a crucial application for noisy intermediate-scale devices. Instead of constructing gate-based quantum computers, our focus centers on designing variational quantum algorithms within the analog paradigm. This involves optimizing parameters that directly control pulses, driving quantum states toward target states without the necessity to compile a quantum circuit. In this work, we introduce pulse-based variational quantum optimization (PBVQO) as a hardware-level framework. We illustrate the framework by optimizing external fluxes on superconducting quantum interference devices, effectively driving the wave function of this specific quantum architecture to the ground state of an encoded problem Hamiltonian. Given that the performance of variational algorithms relies heavily on appropriate initial parameters, we introduce a global optimizer as a metalearning technique to tackle a simple problem. The synergy between PBVQO and metalearning provides an advantage over conventional gate-based variational algorithms.

Optical and photoemission measurements were performed on alkali antimonide ${\mathrm{Na}}_2\mathrm{KSb}$ and ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_x\mathrm{Sb}$ photocathodes in order to determine their energy-band diagrams, elucidate the photoemission pathways, and explore the options for interface engineering in order to reach high quantum efficiencies of the photocathodes. This study is motivated by the recent discovery of optical orientation in ${\mathrm{Na}}_2\mathrm{KSb}$ and emission of spin-polarized electrons from ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_x\mathrm{Sb}$ photocathodes [V.S. Rusetsky et al., Phys. Rev. Lett. 129, 166802 (2022)]. We have shown that the band gap $E_g$ of ${\mathrm{Na}}_2\mathrm{KSb}$ at $T=295$ K lies within the range of 1.40–1.44 eV. The ${\mathrm{Na}}_2\mathrm{KSb}$ surface activation by the deposition of $\mathrm{Cs}$ and $\mathrm{Sb}$ results in effective electron affinity decrease by approximately 0.37 eV, and in an increase of the quantum efficiency up to 0.2 electrons per incident photon. The analysis of longitudinal energy distribution curves (EDCs) proves that the surface of activated ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_x\mathrm{Sb}$ photocathodes have negative effective electron affinity of approximately $-0.1$ and $-0.25$ eV at $T=295$ and 80 K, respectively. EDC measurements under increasing photon energy $\hslash \omega$ demonstrate the transition of photoemission pathway from the surface states’ photoionization at $\hslash \omega <E_g$ to the emission from the conduction-band bottom at $\hslash \omega \approx E_g$ and from the states with high kinetic energy in the conduction band at $\hslash \omega >E_g$. EDCs measured at 80 K reveal a highly directional photoelectron emission from the ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_x\mathrm{Sb}$ photocathode, as compared to the p-$\mathrm{Ga}\mathrm{As}$($\mathrm{Cs}$,$\mathrm{O}$) photocathode. This fact, along with the observed significant, by an order of magnitude, increase in the photoluminescence intensity under the ${\mathrm{Na}}_2\mathrm{KSb}$ surface activation by $\mathrm{Cs}$ and $\mathrm{Sb}$, indicates relatively weak diffuse scattering in the “quasiepitaxial” ${\mathrm{Cs}}_x\mathrm{Sb}$ activation layer of a ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_x\mathrm{Sb}$ photocathode, compared to strong scattering in the amorphous ($\mathrm{Cs}$,$\mathrm{O}$) activation layer of a p-$\mathrm{Ga}\mathrm{As}$($\mathrm{Cs}$,$\mathrm{O}$) photocathode.

Given a quantum system $S$ entangled with another system $I$, the entanglement-testing problem arises, prompting the identification of the system $S$ within a set of $m\ge 2$ identical systems. This scenario serves as a model for the measurement task encountered in quantum ranging and entanglement-assisted communication [Phys. Rev. Lett. 126, 240501, (2021)]. In this context, the optimal measurement approach typically involves joint measurements on all $m+1$ systems. However, we demonstrate that this is not the case when the subsystems containing system $S$ are subjected to entanglement-breaking noise. Our approach utilizes the recently developed measurement technique of correlation-to-displacement conversion. We present a structured design for the entanglement-testing measurement, implementable with local operations and classical communications (LOCC) on the $m+1$ systems, while joint between multiple identical copies. Furthermore, we prove that this measurement approach, combining Gaussian operations and on and off photon detection, achieves optimality in terms of error probability asymptotically under noisy conditions. When applied to quantum illumination, our measurement design enables optimal ranging in scenarios with low signal brightness and high levels of noise. Similarly, when applied to entanglement-assisted pulse-position-modulated classical communication, the measurement design leads to a significant relative advantage in communication rates, particularly in scenarios with low signal brightness.

Achieving high-fidelity direct two-qubit gates over meter-scale quantum interconnects is challenging, in part due to the multimode nature of such systems. One alternative scheme is to combine local operations with remote quantum state transfer or remote entanglement. Here, we theoretically study quantum state transfer and entanglement generation for two distant qubits, equipped with tunable interactions, over a common multimode interconnect. We model the performance of the superadiabatic transitionless driving (SATD) protocol for adiabatic passage and demonstrate various favorable improvements over the standard method. In particular, by suppressing leakage to a select (resonant) interconnect mode, SATD breaks the speed-limit relation imposed by the qubit-interconnect interaction $g$, where instead the operation time is limited by leakage to the adjacent modes, i.e., the free spectral range ${\mathrm{\Delta }}_c$ of the interconnect, allowing for fast operations even with weak $g$. Furthermore, we identify a multimode error mechanism for Bell-state generation using such adiabatic protocols, in which the even/odd modal dependence of qubit-interconnect interaction breaks down the dark-state symmetry, leading to detrimental adiabatic overlap with the odd modes growing as $(g/{\mathrm{\Delta }}_c{)}^2$. Therefore, adopting a weak coupling, imposed by a multimode interconnect, SATD provides a significant improvement in terms of operation speed and consequently sensitivity to incoherent error.

The emergence of two-dimensional (2D) hyperbolic materials, characterized by opposite-sign optical conductivities along two orthogonal axes within a specific band (known as the hyperbolic region), opens an avenue for optical device engineering. Broadening the hyperbolic region is essential for cutting-edge photonic applications. In this study, based on a correlation between the hyperbolic region and anisotropic electronic structures, we propose a strategic framework for identifying 2D natural hyperbolic materials (NHMs) with broadband hyperbolicity. Using this framework, we engineered a 2D lattice incorporating p and d orbitals, and discovered a series of 2D NHMs, MYZ (M = $\mathrm{Co},\mathrm{Pd},\mathrm{Ru},\mathrm{Rh}$; Y = $\mathrm{S},\mathrm{Se},\mathrm{Te}$; and Z = $\mathrm{Cl},\mathrm{Br},\mathrm{I}$). These materials exhibit broadband hyperbolicity that extends from the near-infrared to the visible-light spectrum. We have confirmed the directional propagation of surface plasmon polaritons on these 2D materials based on Maxwell’s equations. Our findings pave the way for future exploration and practical deployment of 2D NHMs in advanced technological applications.

The monolayer graphene-based terahertz (THz) perfect absorber has made significant progress in switchable devices with diverse functionalities. However, existing methods for achieving switchability rely on either multilayered metasurface with complex configurations or complicated patterned metasurfaces. A switchable absorber that switches between n-band and m-band modes (n, m correspond to the number of resonance peaks) using tunable materials is also interesting but has not yet been mentioned. Here, a dynamically switchable multiband perfect absorber with a digital coding graphene metasurface is proposed and demonstrated. By integrating multiple metasurface units with different Fermi levels into a newly generated period, we realize a multiband perfect absorber with a rather simple and straightforward configuration composed of a patterned metasurface. Through introducing a digital coding metasurface into the perfect absorber, we use the digital signals via FPGA (field programmable gate array) to convert n-band into m-band absorption modes. Moreover, the results reveal that the THz perfect absorber, possessing great impedance matching with the free space, has excellent angle tolerance and robustness. Such a perfect absorber offers a flexible tool for selecting THz channels and may pave the way for ${\mathrm{sixth}}^{-}$generation communication.

Motivated by our recent findings in [Phys. Rev. Lett. 128, 013902, 2022], which introduced a new class of electromagnetic bulk materials whose response is similar to conventional semiconductor transistors, here we propose a one-dimensional (1D) version of such materials based on transmission lines coupled with FET isolators. We demonstrate that the response of this 1D system is nonreciprocal and non-Hermitian, analogous to the idealized transistor-metamaterial, and is also characterized by a broken time-reversal symmetry. We analyze the wave propagation in the system and find that the interaction between the eigenmodes can either lead to gain or loss, depending on the propagation distance. Furthermore, it is also shown that the system may be operated at an exceptional point, wherein the response becomes singular, and the power gain is maximized. Finally, we demonstrate that the exceptional point coincides with the point of operation of typical microwave amplifiers, such as the distributed amplifier.

The research of magnetic tunnel junctions (MTJs) has scientific and technical significance for promoting the development of information storage technology, promoting spintronics and realizing higher-density integrated circuits. At present, mainly by improving the material and designing alternative structures, the performance and reliability of the MTJs can be enhanced. Recently, it has been reported that the monolayer $\mathrm{Cr}\mathrm{As}$ is expected to be a ferromagnetic half-metal with high Curie temperature. Here, using DFT+$U$, we investigate the magnetic and spin-dependent transport properties in a MTJ made of half-metallic $\mathrm{Cr}\mathrm{As}$ nanosheet. The magnetic anisotropy energy (MAE) along the $z$ axis is $637\text{μ}\mathrm{eV}$ per $\mathrm{Cr}$, and the ferromagnetism and out-of-plane easy magnetization are not affected by biaxial strain. In addition, the spin-dependent transport features of two MTJs with arsenene inserted in the $\mathrm{Cr}\mathrm{As}$ layer were theoretically developed and investigated. The findings reveal that tunneling magnetoresistance ratio peaks at approximately equal to ${10}^{10}$ and remains more than approximately equal to ${10}^6$ even when subjected to bias voltages as high as 1 V. Moreover, 100% spin-filtering effects can be observed in the devices. Our research offers viable MTJs possibilities for the next spintronic devices.

Reducing errors in quantum gates is critical to the development of quantum computers. To do so, any distortions in the control signals should be identified; however, conventional tools are not always applicable when part of the system is under high vacuum, cryogenic, or microscopic. Here, we demonstrate a method to detect and compensate for amplitude-dependent phase changes, using the qubit itself as a probe. The technique is implemented using a microwave-driven trapped-ion qubit, where correcting phase distortions leads to a threefold improvement in the error of single-qubit gates implemented with pulses of different amplitudes, to attain state-of-the-art performance benchmarked at $1.6(4)\times {10}^{-6}$ error per Clifford gate.

Efficient manipulation of spin waves (SWs) is considered as one of the promising means for encoding information with low power consumption in next-generation spintronic devices. The SW Doppler shift is a key phenomenon obtained by manipulating SW propagation. Here, we predict an efficient way to control the SW Doppler shift by voltage-control magnetic-anisotropy-boundary (MAB) movement in the FM-FE heterojunction. From the micromagnetic simulation, we verified that the SW Doppler shift aligns well with our theoretical predictions in $\mathrm{Fe}/{\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ heterostructure. The SW Doppler shift also shows an ultra-wide-band-shift (over 5 GHz) property with voltage. Such an efficient SW Doppler shift may provide one possible way to measure the Hawking radiation of an analogue black hole in the SW systems.

Optically pumped magnetometers (OPMs) based on the free-induction-decay (FID) configuration have recently attracted interest owing to their advantages, such as being calibration-free, easy to operate, and showing reduced light shifts induced by the pump light. Currently, alkali-metal vapor cells have been employed in most of the FID magnetometers. However, the relatively narrow linewidth of the magnetic resonance signal and nonlinear Zeeman (NLZ) effects generate bandwidth limitation and heading errors of these magnetometers in the Earth’s field, respectively. In this work, we propose a ${}^4\mathrm{He}$-based FID magnetometer, which not only has the advantages of being NLZ-free and showing a high bandwidth, but also maintains an enhanced sensitivity assisted by a multipass cell. This magnetometer demonstrates a magnetic field noise floor of $0.34\mathrm{pT}/{\mathrm{Hz}}^{1/2}$ with a Nyquist-limited bandwidth of $5$ kHz, which opens another route for atomic magnetometry using FID signals, and exhibits potential for applications in high-frequency magnetic field detection, such as biomagnetic measurements and magnetic communications in the geomagnetic field.

To derive the best oscillator phase noise when implementing a high-$Q$ resonator, the resonant spectral line shape must have high contrast and symmetry. Ideally, this line shape is second order and Lorentzian; however, in a high-mode-density spectral region, low-$Q$ background spurious modes interact and distort the resonance. For a sapphire-loaded cavity resonator operating with whispering-gallery modes confined within the sapphire crystal, we show that this high contrast and symmetry can be achieved by meticulously changing the dimensions of the surrounding metallic cavity shield to tune the background low-$Q$ structures into antiresonance. This works because the high-$Q$ resonances are primarily defined by the sapphire, while the background modes are defined by the cavity shield. Alternatively, it has been shown that a similar result can be achieved by exciting the high-$Q$ resonator with a balanced microwave dipole probe in a Mach-Zehnder interferometric configuration. The probe has been constructed from two separate coaxial electric field probes symmetrically inserted into a cylindrical-cavity resonator, from opposite sides with a small gap between them, so they can behave like an active wire-dipole antenna. The power into the two separate probes may be matched with an external variable attenuator in one of the arms of the interferometer. Conversely, the phase between the two electric field probes may be changed with an external variable phase shifter, which changes the nature of the field components to which the probe couples. The probe couples to the high-$Q$ resonant modes as well as low-$Q$ background modes, which can be made resonant or antiresonant with respect to the high-$Q$ modes by changing this external phase. When the background modes are in antiresonance, the line shape of the high-$Q$ mode can be made symmetric and with higher contrast. This technique has been applied to both whispering-gallery sapphire modes, as well as hollow-cavity resonators, without changing the dimensions of the cavity.

We present a design and implementation of a frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin-film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby niobium current feedline induces Meissner shielding currents in the resonator loop leading to a change in the kinetic part of the total inductance of the resonator. We demonstrate continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency up-conversion is achieved when a kilohertz ac modulation signal is superimposed onto the dc bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong rf pump signal in the feedline. The simple architecture is amenable to large-array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, up-converters, and parametric amplifiers operating above 4 K when alternative high-critical-temperature material with high kinetic inductance is used.

Much attention has focused on the transmon architecture for large-scale superconducting quantum devices; however, the fluxonium qubit has emerged as a possible successor. With a shunting inductor in parallel to a Josephson junction, the fluxonium offers larger anharmonicity and stronger protection against dielectric loss, leading to higher coherence times as compared to conventional transmon qubits. The interplay between the inductive and Josephson energy potentials of the fluxonium qubit leads to a rich dispersive-shift landscape when tuning the external flux. Here, we propose to exploit the features in the dispersive shift to improve qubit readout. Specifically, we report on theoretical simulations showing improved readout times and error rates by performing the readout at a flux-bias point with large dispersive shift. We expand the scheme to include different error channels and show that with an integration time of 155 ns, flux-pulse-assisted readout offers about a $5$-times improvement in the signal-to-noise ratio. Moreover, we show that the performance improvement persists in the presence of finite measurement efficiency combined with quasistatic flux noise and also when considering the increased Purcell rate at the flux-pulse-assisted readout point. We suggest a set of reasonable energy parameters for the fluxonium architecture that will allow for the implementation of our proposed flux-pulse-assisted readout scheme.