arXiv subjects

L. F. Wei

Publications and source records attributed to L. F. Wei.

At least 19 recordsLinked to original sources

arXiv2026

An input-output approach for giant atom scatterings beyond the dipole approximation

A giant atom is an artificial matter configuration whose spatial scale is comparable to the wavelength of the interacting electromagnetic wave, such that the usual electric-dipole approximation is no longer valid. As a consequence, certain quasi-direct scattering channels for the electromagnetic wave can arise. Given that the well-known input-output approach can only work for the usual point scattering configuration, wherein the electric-dipole approximation is well satisfied, here we develop a modified input-output approach, wherein an additional low-Q cavity channel is introduced, to treat the electromagnetic scattering problem of a giant atom. We demonstrate that, beyond the multiple coupling-point model used widely in recent publications, the present approach can well explain the Fano-type scattering spectra observed generically and extract certain physical parameters, including the energy dissipation parameter of a two-level giant atom and its coupling strength with the scattered electromagnetic wave. Consequently, we argue that various high-performance optical quantum devices, typically the giant-atom-based optical quantum switches, can be generated by engineering the Fano-type scatterings of giant atoms.

quant-ph↗

arXiv2026

Significantly enhanced detectability of dark photons with a steady-state excited microwave cavity

The resonant cavity system has been widely used to search for the electromagnetic response of dark photons, although its achievable detection sensitivity remains at a relatively low level. In this letter, we propose a feasible approach to significantly improve its achievable detection sensitivity by enhancing the detectability of the dark photon-photon dynamical effect, assisted with the steady-state excitation of the target mode in the cavity. Unlike in almost all the previous detection schemes, wherein where the cavity modes are kept in vacuum (and thus only the second-order energy signals can be detected), here the pre-excited steady-state field in the cavity can be used to achieve the coherent amplification of the dark photon response signal, thereby obtaining detectable first-order (rather than the conventional second-order) energy response signals of dark photons. Although the phase of the dark photon field and thus its electromagnetic response signal is stochastic, the amplitude of such a first-order energy response power signal can still be extracted by using mature IQ demodulation technology. As a consequence, we argue that, even considering the influence of the shot noise of the pre-excited steady-state field, the achievable detection sensitivity of this in-situ enhancement detectability, based on the steady-state excitation signal of the target mode, is still at least one order of magnitude higher than those of the current resonant cavity experiments with the same Q-quality factors. Based on existing microwave cavity and weak signal demodulation detection technologies, the feasibility of such a significantly enhanced detectability scheme is also discussed.

physics.optics↗

arXiv2025

A broadband single microwave-photon detector insensitive to the thermal noise

Thermal noise is one of the physical obstacles that constrain the achievable detection sensitivities of various detectors. Indeed, as we showed in a recent paper (PRB 111, 024501 (2025)), the usual Josephson threshold detector (JTD) operated in an equilibrium state can be utilized to implement a weak microwave signal, just approaching (but not arriving at) its energy quantum limit, even though its physical parameters have been optimized. In this letter, we further demonstrate numerically that the phase dynamics of a current-biased Josephson junction (CBJJ) can be insensitive to the always-on thermal noise if the sweep rate of the biased current is significantly high. As a consequence, the JTD can be operated alternatively in a non-equilibrium state. Based on the statistical binary detection criterion, we demonstrate how such a non-equilibrium JTD (NEJTD) can be utilized to implement the weak microwave signal, arriving at its energy quantum limit level. The dynamic range and photon-number resolvability of the proposed NEJTD are also discussed when it serves as a broadband single microwave-photon detector.

quant-ph↗

arXiv2025

Measuring weak microwave signals via current-biased Josephson Junctions II: Arriving at single-photon detection sensitivity

It is well known that the current-biased Josephson junction (CBJJ) can serve as a Josephson threshold detector (JTD) for the sensitive detection of weak microwave signals. Based on the recent work (PRB {\bf 111}, 024501 (2025)) on the detection sensitive limit of the usual equilibrium JTD, here we numerically demonstrate that a non-equilibrium JTD can be alternatively utilized to implement the higher sensitive detection of a weak microwave signal, arriving at its energy quantum limit. In the presence of thermal noise, we numerically simulate the phase dynamics for the CBJJ in the JTD with the different sweep rates of the biased currents, and find that the SCDs of the JTD with and without the microwave signal input show different behaviors. It is demonstrated that, depending on how high the sweep rate of the biased current being applied, the JTD can be operated in either the equilibrium- or the non-equilibrium state. Specifically, under the rapidly non-adiabatic driving, the SCDs of the JTD are obviously insensitive to the thermal noises, which means that the non-equilibrium JTD can possess a higher achievable detection sensitivity, compared with its equilibrium state counterpart. Consequently, the non-equilibrium JTD can be utilized to implement the desired single microwave-photon detection. Also, some of the achievable performance indexes, such as the dynamic range, detection bandwidth, and the photon-number resolvability, etc., of the non-equilibrium JTD have been estimated, when it serves as a wideband microwave single-photon detector.

quant-ph↗

arXiv2025

Switchable coherent state stored quantum batteries with large ergotropies

Quantum battery (QB) is a conceptually new energy storage and conversion device, which consists usually of a quantum charger and an energy store (called usually as the QB for simplicity). The demonstrated advantage of QB, over its classical counterpart, is that its charging efficiency can be significantly enhanced by using quantum entanglement resources. In this letter, we investigate alternatively how to realize the switchable charging and the lossless power detection of the charged QB. With the proposed QB configuration we show that, by adjusting the eigenfrequency of the qubit-based charger, the cavity-based QB can be switched on between the charging and power-off states and its stored energy can be non-destructively monitored by probing the transmitted spectrum of the external electromagnetic waves scattered by the qubit-based charger. As the qubit-based charger has never been excited, the proposed cavity-based QB could directly store the coherent state energy (rather than the single-photon one) and thus possess significantly large ergotropy. The physical realization of the proposed switchable QB configuration is demonstrated specifically with the experimental circuit quantum electrodynamical devices.

quant-ph↗

arXiv2025

Quantum detection of millimeter wave electric fields with driving surface-state electrons

We introduce a spin-based receiver to sensitively detect the electric fields of millimeter (mm) waves by using quantum interferometric approach. The proposed quantum sensor consists of many surface-state electrons trapped individually on liquid helium by an electrode-network at the bottom of the liquid helium film. A dc-current in this chip is biased to generate a strong spin-orbit coupling of each of the trapped electrons. The mm wave signals are conducted to non-dissipatedly drive the orbital motions of the trapped electrons and result in the Stark shifts of the spin-orbit dressed states of the electrons. As a consequence, the electric fields of the conducted mm waves could be detected sensitively by using the Hahn echo interferometry with the long-lived spin states of the electrons trapped on liquid helium.

quant-ph↗

arXiv2025

Experimental demonstrations of Josephson threshold detectors for broadband microwave photons detection

Current-biased Josephson junctions (CBJJs) have been demonstrated as sensitive Josephson threshold detectors (JTDs) in the infrared range. In this letter, we show this kind of detector could also be used to detect broadband microwave photons. Based on the numerical simulations of the noise-driving phase dynamics of an underdamped Josephson junction, driven by the low-frequency triangular wave current, we argue that the microwave photons flowing across the JJ can be detected by probing the voltage switched signals of the JJ. Experimentally, we designed and fabricated the relevant Al/AlOx/Al Josephson device and measured its response to microwave photons at 50~mK temperature. Experimental results indicate that the weak microwave signals could be threatened as the additional noises modify the phase dynamics of the CBJJ, which could thus be detected by the generated JTD. The detection sensitivity is characterized by using the Kumar-Caroll index to differentiate the junction switched duration distributions, with and without microwave signal input. Although the demonstrated detection sensitivity is just $-92$~dBm (corresponding to approximately 30~photon/ns) for the microwave photons at $\sim 5$GHz (which is manifestly deviated from the plasma frequency of the fabricated JJ), we argued, based on the relevant numerical simulations, that the generated JTD could be used to achieve the sensitive detection of the microwave photons at the plasma frequency of the JJ.

physics.app-ph↗

arXiv2024

Sensitively searching for microwave dark photons with atomic ensembles

Dark photon is one of the promising candidates of light dark matter and could be detected by using its interaction with standard model particles via kinetic mixings. Here, we propose a feasible approach to detect the dark photons by nondestructively probing these mixing-induced quantum state transitions of atomic ensembles. Compared with the scheme by probing the mixing-induced quantum excitation of single-atom detector, the achievable detection sensitivity can be enhanced theoretically by a factor of $\sqrt{N}$ for the ensemble containing $N$ atoms. Specifically, we show that the dark photons, in both centimeter- and millimeter-wave bands, could be detected by using the artificial atomic ensemble detector, generated by surface-state electrons on liquid Helium. It is estimated that, with the detectable transition probability of $10^{-4}$, the experimental surface-state electrons (with $N = 10^8$ trapped electrons) might provide a feasible approach to search for the dark photons in $18.61-26.88$ $μ$eV and $496.28-827.13$ $μ$eV ranges, within about two months. The confidence level can exceed 95\% for the achievable sensitivities being $10^{-14} \sim 10^{-13}$ and $10^{-12} \sim 10^{-11}$, respectively. In principle, the proposal could also be generalized to the other atomic ensemble detectors for the detection of dark photons in different frequency bands.

quant-ph↗

arXiv2024

Approaching energy quantum limit detection of microwave photons with Josephson Junctions

Single-photon detection is an energy quantum limit detection (EQLD) of a significantly weak electromagnetic wave. Given the sensitivity of the conventional electromagnetic induction microwave receiver working at room-temperature is very limited, due to the significantly strong thermal noise, here we analyze the possibility of approaching the EQLD of the weak microwave signal by using a current-biased Josephson Junction (CBJJ) detector. By numerically simulating the dynamics for the phase particle of the CBJJ, we propose an approach to describe the discriminability between the probabilistically escaped events of the phase particle with and without the microwave current driving, by measuring the minimum $d_{\rm KC}$-index. We predicate that, the experimentally demonstrated CBJJ detectors possess the ability to resolve about a dozen photons. The feasibility of the desired EQLD of microwave signal by using the CBJJ detector is also discussed.

quant-ph↗

arXiv2024

Wide-range quantum enhanced rotation sensing with 1+2 dimensional dynamical decoupling techniques

We propose a motional dynamical decoupling technique by utilizing a sequence of $π$-phase shifts, instead of the conventional $π$-pulses for spin flipping, to implement the quantum enhanced rotation sensing with a 1+2 dimensional hybrid atomic Sagnic interferometor. By fully disentangling the spin from the two-dimensional vibrational modes of the particle under rotation, the spin coherence time and thus the phase accumulation can be significantly increased. Consequently, both the achievable sensitivity and dynamic range of the rotation sensing can be significantly enhanced and extended simultaneously, compared to the previous schemes where the spin and motions of the particle were not completely decoupled. The experimental feasibility for the unambiguous estimation of the rotation parameters is also discussed. Hopefully, this technique holds promise for overcoming certain challenges existing in the usual matter-wave Sagnac interferometers with trapped particles, particularly for the practical inertial navigation that demands both high sensitivity and large dynamic range.

quant-ph↗

arXiv2023

Sub-SQL electronic field sensing by simultaneously using quantum entanglements and squeezings

Quantum entanglement and quantum squeezing are two most typical approaches to beat the standard quantum limit (SQL) of the sensitive phase estimations in quantum metrology. Each of them has already been utilized individually to improve the sensitivity of electric field sensing with the trapped ion platform, but the upper bound of the demonstrated sensitivity gain is very limited, i.e., the experimental 3dB and theoretical 6dB, over the SQL. Here, by simultaneously using the internal (spin)-external (oscillator) state entanglements and the oscillator squeezings to effectively amplify the accumulation phase, we show that these sensitivity gains can be effectively surpassed. Hopefully, the proposal provides a novel approach to the stronger beaten of the SQL for the sensitive sensings of the desired electric field and also the other metrologies.

quant-ph↗

arXiv2023

Experimental evidences of a current-biased Josephson junction device can be worked as a macroscopic "Boson" or "Fermion" and the combination

According to the statistical distribution laws, all the elementary particles in the real 3+1-dimensional world must and only be chosen as either bosons or fermions, without exception and not both. Here, we experimentally verified that a quantized current-biased Josephson junction (CBJJ), as an artificial macroscopic "particle", can be served as either boson or fermion, depending on its biased dc-current. By using the high vacuum two-angle electron beam evaporations, we fabricated the CBJJ devices and calibrated their physical parameters by applying low-frequency signal drivings. The microwave transmission characteristics of the fabricated CBJJ devices are analyzed by using the input-output theory and measured at 50mK temperature environment under low power limit. The experimental results verify the theoretical predictions, i.e., when the bias current is significantly lower than the critical one of the junction, the device works in a well linear regime and thus works as a harmonic oscillator, i.e., a "boson"; while if the biased current is sufficiently large (especially approaches to its critical current), the device works manifestly in the nonlinear regime and thus can be served as a two-level artificial atom, i.e., a "fermion". Therefore, by adjusting the biased dc-current, the CBJJ device can be effectively switched from the boson-type macroscopic particle to the fermion-type one, and thus may open the new approach of the superconducting quantum device application.

quant-ph↗

arXiv2023

Robust phase metrology with hybrid quantum interferometers against particle losses

Entanglement is an important quantum resource to achieve high sensitive quantum metrology. However, the rapid decoherence of quantum entangled states, due to the unavoidable environment noise, result in practically the unwanted sharp drop of the measurement sensitivity. To overcome such a difficulty, here we propose a spin-oscillator hybrid quantum interferometer to achieve the desirable precise estimation of the parameter encoded in the vibrations of the oscillator. Differing from the conventional two-mode quantum interferometers input by the two-mode NOON state or entangled coherent states (ECS), whose achievable sensitivities are strongly limited by the decoherence of the entangled vibrational states, we demonstrate that the present interferometer, input by a spin-dependent two-mode entangled state, possesses a manifest advantage, i.e., the measurement sensitivity of the estimated parameter is not influenced by the decoherence from the spin-oscillator entanglement. This is because that, by applying a spin-oscillator disentangled operation, the information of the estimated parameter encoded originally in the vibrational degrees can be effectively transferred into the spin degree and then can be sensitively estimated by the precise spin-state population measurements. As consequence, the proposed hybrid quantum interferometer possesses a manifest robustness against the particle losses of the vibrational modes. Interestingly, the achieved phase measurement sensitivity can still surpass the SQL obviously, even if relatively large number of particle loss occurs in one of the two modes. The potential application of the proposed spin-oscillator hybrid quantum interferometer is also discussed.

quant-ph↗

arXiv2022

Electric Current Induced by Microwave Stark Effect of Electrons on Liquid Helium

We propose a frequency-mixed effect of Terahertz (THz) and Gigahertz (GHz) electromagnetic waves in the cryogenic system of electrons floating on liquid helium surface. The THz wave is near-resonant with the transition frequency between the lowest two levels of surface state electrons. The GHz wave does not excite the transitions but generates a GHz-varying Stark effect with the symmetry-breaking eigenstates of electrons on liquid helium. We show an effective coupling between the inputting THz and GHz waves, which appears at the critical point that the detuning between electrons and THz wave is equal to the frequency of GHz wave. By this coupling, the THz and GHz waves cooperatively excite electrons and generate the low-frequency ac currents along the perpendicular direction of liquid helium surface to be experimentally detected by the image-charge approach [Phys. Rev. Lett. 123, 086801 (2019)]. This offers an alternative approach for THz detections.

quant-ph↗

arXiv2022

Sensitive detection of local magnetic field changes with atomic interferometry by using superconducting Meissner effects

Sensitive detection of magnetic field is one of the open problem in metrology. Here, we propose an Mach-Zehnder atomic interferometry to sensitively detect the very weak local magnetic field, which is expelled by the superconductor (as the "testing magnet") due to the Meissner effect. The induced magnetic field gradient near the superconductor provides a centripetal acceleration of the atomic motion in the interferometry and thus can be detected by using the atomic interferences. Given gravity acceleration of the atoms have been measured at the accuracy of $10^{-12}$ g, the measured sensitivity of the expelled local field could reach $10^{-14}$ T.

quant-ph↗

arXiv2021

Study of quasi-particle dynamics using the optical pulse response of asuperconducting resonator

We study the optical pulse response of a superconducting half-wavelength coplanar waveguide (CPW) resonator. We apply a short optical pulse to the center strip of the CPW resonator, where the current distribution shows antinodes or nodes for different resonance modes, and measure the frequency response. We develop a time-dependent variable inductance circuit model with which we can simulate the optical pulse response of the resonator. By fitting this model to experimental data, we extract the temporal kinetic inductance variations, which directly reflect the quasi-particle recombination with time and diffusion in space. We also retrieve the spatial size of the quasi-particle distribution and the quasi-particle diffusion constant. Our study is very useful for the design of photon-counting kinetic inductance detectors, and the method developed in this work provides a useful way to study the quasi-particle dynamics in the superconductor.

cond-mat.supr-con↗

arXiv2019

Three-dimensional compacted optical waveguide couplers designed by invariant engineerings

Due to the limitations either on the sizes of devices and signal routing channels, the current planar integrated optical waveguide circuits await for the further developments into the three-dimensional (3D) integrations, although their designs and fabrications are still challenges. In this paper we demonstrate an analytical method, basing on the invariant engineering, to overcome the complication in the usual method by numerically solving the relevant 3D coupled-mode equations for designing various 3D optical waveguide devices such as the typical couplers. Our method is based on the quantum-optical analogy, i.e., the Maxwell equation for the electrcomagnetic wave prorogating along the waveguide structure in the spatial domain is formally similar to the Schrödinger equation for the evolving quantum state in the time domain. We find that the spatial-domain invariants can be effectively constructed to solve the 3D coupled-mode equations, analogously to solve the dynamical evolutions of quantum systems in the time-domain. As a consequence, as long as appropriately set the coupling parameters between the 3D interconnected waveguides, the 3D three-waveguide couplers could be designed for various desirably power divisions. As the invariant method is a natural shortcut to the adiabaticity, the compacted devices designed by the invariant-based engineerings are robust against the coupling coefficient variations and the coupler lengths.

physics.optics↗

arXiv2018

Measurement of optical constants of TiN and TiN/Ti/TiN multilayer films for microwave kinetic inductance photon-number-resolving detectors

We deposit thin titanium-nitride (TiN) and TiN/Ti/TiN multilayer films on sapphire substrates and measure the reflectance and transmittance in the wavelength range from 400 nm to 2000 nm using a spectrophotometer. The optical constants (complex refractive indices), including the refractive index n and the extinction coefficient k, have been derived. With the extracted refractive indices, we propose an optical stack structure using low-loss amorphous Si (a-Si) anti-reflective coating and a backside aluminum (Al) reflecting mirror, which can in theory achieve 100% photon absorption at 1550 nm. The proposed optical design shows great promise in enhancing the optical efficiency of TiN-based microwave kinetic inductance photon-number-resolving detectors.

physics.optics↗