arXiv subjects

L. Q. Chen

Publications and source records attributed to L. Q. Chen.

At least 37 records · Page 2Linked to original sources

arXiv2025

Near-perfect broadband quantum memory enabled by intelligent spinwave compaction

Quantum memory, a pivotal hub in quantum information processing, is expected to achieve high-performance storage and coherent manipulation of quantum states, with memory efficiency exceeding 90% and quantum fidelity surpassing the non-cloning limit. However, the current performance falls short of these requirements due to the inherent trade-off between memory efficiency enhancement and noise amplification, which not only imposes significant demands on quantum purification but also fundamentally impedes continuous-variable quantum information processing. In this paper, we break through these constraints, enabling high-performance quantum memory and unlocking new possibilities for quantum technologies. We unveil a Hankel-transform spatiotemporal mapping for light-spinwave conversion in quantum memory, and propose an intelligent light-manipulated strategy for adaptive spinwave compaction, which can maximize the conversion efficiency and simultaneously suppress the excess noise. This strategy is experimentally demonstrated for a Raman quantum memory in warm 87Rb atomic vapor with an efficiency up to 94.6% and a low noise level of only 0.026 photons/pulse. The unconditional fidelity reaches 98.91% with an average of 1.0 photons/pulse for a 17-ns input signal. Our results successfully demonstrate a practical benchmark for broadband quantum memory, which may facilitate advancements in high-speed quantum networks, quantum state manipulation, and scalable quantum computation.

quant-ph↗

arXiv2025

Spin relaxation and transport behaviors in altermagnetic systems

The D'yakonov-Perel' (DP) spin-relaxation mechanism has traditionally been associated with either relativistic spin-orbit coupling, which breaks space-inversion symmetry, or inhomogeneous magnetization, which breaks both time-reversal and translational symmetries. Here, we investigate spin relaxation mechanism in altermagnetic systems which possess novel magnetic states characterized by sublattices connected through crystal-rotation symmetries and opposite spins with zero overall net magnetization and absence of spin-orbit coupling. We find that altermagnetic states exhibit DP-type spin relaxations in both strong- and weak-scattering regimes, with the spin relaxation rate decreasing to zero as the temperature approaches the critical temperature of the altermagnetic phase transition. However, the scattering time involved in this spin relaxation mechanism is not the momentum relaxation time, in contrast to the conventional DP spin relaxation. Using a kinetic approach incorporating rigorous microscopic scattering, we demonstrate that the spin Hall current is highly anisotropic and proportional to the degree of altermagnetic order.

cond-mat.mes-hall↗

arXiv2023

High-Sensitive Microwave Electrometry with Enhanced Instantaneous Bandwidth

Rydberg microwave (MW) sensors are superior to conventional antenna-based techniques because of their wide operating frequency range and outstanding potential sensitivity. Here, we demonstrate a Rydberg microwave receiver with a high sensitivity of $62\,\mathrm{nV} \mathrm{cm}^{-1} \mathrm{Hz}^{-1/2}$ and broad instantaneous bandwidth of up to $10.2\,\mathrm{MHz}$. Such excellent performance was achieved by the amplification of one generated sideband wave induced by the strong coupling field in the six-wave mixing process of the Rydberg superheterodyne receiver, which was well predicted by our theory. Our system, which possesses a uniquely enhanced instantaneous bandwidth and high-sensitivity features that can be improved further, will promote the application of Rydberg microwave electrometry in radar and communication.

quant-ph↗

arXiv2023

The effect of Quantum Statistics on the sensitivity in an SU(1,1) interferometer

We theoretically study the effect of quantum statistics of the light field on the quantum enhancement of parameter estimation based on cat state input the SU(1,1) interferometer. The phase sensitivity is dependent on the relative phase $θ$ between two coherent states of Schrödinger cat states. The optimal sensitivity is achieved when the relative phase is $π$% , i.e., odd coherent states input. For a coherent state input into one port, the phase sensitivity of the odd coherent state into the second input port is inferior to that of the squeezed vacuum state input. However, in the presence of losses the Schrödinger cat states are more resistant to loss than squeezed vacuum states. As the amplitude of Schrödinger cat states increases, the quantum enhancement of phase sensitivity decreases, which shows that the quantum statistics of Schrödinger cat states tends towards Poisson statistics from sub-Poisson statistics or super-Poisson statistics.

quant-ph↗

arXiv2023

Ultimate precision limit of SU(2) and SU(1,1) interferometers in noisy metrology

The quantum Fisher information (QFI) in SU(2) and SU(1,1) interferometers was considered, and the QFI-only calculation was overestimated. In general, the phase estimation as a two-parameter estimation problem, and the quantum Fisher information matrix (QFIM) is necessary. In this paper, we theoretically generalize the model developed by Escher et al [Nature Physics 7, 406 (2011)] to the QFIM case with noise and study the ultimate precision limits of SU(2) and SU(1,1) interferometers with photon losses because photon losses as a very usual noise may happen to the phase measurement process. Using coherent state and squeezed vacuum state as a specific example, we numerically analyze the variation of the overestimated QFI with the loss coefficient, and find its disappearance and recovery phenomenon.

quant-ph↗

arXiv2022

Ferroelectric Polycrystals: Structural and microstructural levers for property engineering via domain-wall dynamics

Ferroelectrics have a spontaneous electrical polarization that is arranged into domains and can be reversed by an externally applied field. This high versatility makes them useful in enabling components such as capacitors, sensors, and actuators. The key to tuning their dielectric, piezoelectric, and electromechanical performance is to control the domain structure and the dynamics of the domain walls. In fixed compositions, this is often realized by chemical doping. In addition, structural and microstructural parameters, such as grain size, degree of crystallographic texture or porosity play a key role. A major breakthrough in the field came with the fundamental understanding of the link between the local electric and mechanical driving forces and domain wall motion. Here, the impact of structure and microstructure on these driving forces is reviewed and an engineering toolbox is introduced. An overview of advances in the understanding of domain wall motion on the micro- and nanoscale is provided and discussed in terms of the macroscopic functional performance of polycrystalline ferroelectrics/ferroelastics. In addition, a link to theoretical and computational models is established. The review concludes with a discussion about beyond state-of-the-art characterization techniques, new approaches, and future directions toward non-conventionally ordered ferroelectrics for next-generation nanoelectronics and energy-storage applications.

cond-mat.mtrl-sci↗

arXiv2021

Quantum non-demolition measurement based on an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer

Quantum non-demolition (QND) measurement is an important tool in the field of quantum information processing and quantum optics. The atom-light hybrid interferometer is of great interest due to its combination of atomic spin wave and optical wave, which can be utilized for photon number QND measurement via the AC-Stark effect. In this paper, we present an SU(1,1)-SU(2)-concatenated atom-light hybrid interferometer, and theoretically study the QND measurement of photon number. Compared to the traditional SU(2) interferometer, the signal-to-noise ratio (SNR) in a balanced case is improved by a gain factor of the nonlinear Raman process (NRP) in this proposed interferometer. Furthermore, the condition of high-quality of QND measurement is analyzed. In the presence of losses, the measurement quality is reduced. We can adjust the gain parameter of the NRP in readout stage to reduce the impact due to losses. Moreover, this scheme is a multiarm interferometer, which has the potential of multiparameter estimation with many important applications in the detection of vector fields, quantum imaging and so on.

quant-ph↗

arXiv2021

Microscopic piezoelectric behavior of clamped and membrane (001) PMN-30PT thin films

Bulk single-crystal relaxor-ferroelectrics, like Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), are widely known for their large piezoelectricity. This is attributed to polarization rotation which is facilitated by the presence of various crystal symmetries for compositions near a morphotropic phase boundary (MPB). Relaxor-ferroelectric thin films, which are necessary for low-voltage applications, suffer a reduction in their piezoelectric response due to clamping by the passive substrate. To understand the microscopic behavior of this adverse phenomenon, we employ AC electric field driven in-operando synchrotron x-ray diffraction (XRD) on patterned device structures to investigate the piezoelectric domain behavior under an electric field for both a clamped (001) PMN-PT thin film on Si and a (001) PMN-PT membrane released from its substrate. In the clamped film, the substrate inhibits the field induced rhombohedral (R) to tetragonal (T) phase transition resulting in a reversible R to Monoclinic (M) transition with a reduced longitudinal piezoelectric coefficient d33 < 100 pm/V. Releasing the film from the substrate results in recovery of the R to T transition and results in a d33 > 1000 pm/V. Using diffraction with spatial mapping, we find that lateral constraints imposed by the boundary between active and inactive material also inhibits the R to T transition. Phase-field calculations on both clamped and released PMN-PT thin films simulate our experimental findings. Resolving the suppression of thin film piezoelectric response is critical to their application in piezo-driven technologies.

cond-mat.mtrl-sci↗

arXiv2021

Low-Voltage Magnetoelectric Coupling in Membrane Heterostructures

Strain-mediated magnetoelectric (ME) coupling in ferroelectric (FE) / ferromagnetic (FM) heterostructures offers a unique opportunity for both fundamental scientific research and low power multifunctional devices. Relaxor-ferroelectrics, like (1-x)Pb(Mg1/3Nb2/3)O3-(x)PbTiO3 (PMN-xPT), are ideal FE layer candidates due to their giant piezoelectricity. But thin films of PMN-PT suffer from substrate clamping which substantially reduces piezoelectric in-plane strains. Here we present the first demonstration of low voltage ME coupling in an all-thin-film heterostructure which utilizes the anisotropic strains induced by the (011) orientation of PMN-PT. We completely remove PMN-PT films from their substrate and couple with FM Ni overlayers to create membrane PMN-PT/Ni heterostructures showing 90 degree Ni magnetization rotation with 3V PMN-PT bias, much less than the bulk PMNPT ~100V requirement. Scanning transmission electron microscopy and phase-field simulations clarify the membrane response. These results provide a crucial step towards understanding the microstructural behavior of PMN-PT thin films for use in piezo-driven magnetoelectric heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

In-plane quasi-single-domain BaTiO$_3$ via interfacial symmetry engineering

The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO$_3$ thin films. Theoretical calculations predict the key role of the BaTiO$_3$ / PrScO$_3$ (110)$_O$ substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Effects of losses on the sensitivity of an actively correlated Mach-Zehnder interferometer

We theoretically studied the quantum Cramér-Rao bound of an actively correlated Mach-Zehnder interferometer (ACMZI), where the quantum Fisher information obtained by the phase-averaging method can give the proper phase-sensing limit without any external phase reference. We numerically calculate the phase sensitivities with the method of homodyne detection and intensity detection in the presence of losses. Under lossless and very low loss conditions, the ACMZI is operated in a balanced case to beat the standard quantum limit (SQL). As the loss increases, the reduction in sensitivity increases. However within a certain range, we can adjust the gain parameters of the beam recombination process to reduce the reduction in sensitivity and realize the sensitivity can continue to beat the SQL in an unbalanced situation. Our scheme provides an optimization method of phase estimation in the presence of losses.

quant-ph↗

arXiv2021

Coherence Protection of Electron Spin in Earth-field Range by All-optical Dynamic Decoupling

In recent years, unshielded atomic systems have been attracting researchers' attention, in which decoherence is one of the major problems, especially for high precision measurements. The nonlinear Zeeman effect and magnetic field gradient are the main decoherence sources of atomic electron spin in Earth-field range. Here, we propose a method to cancel out the two dominant broadening effects simultaneously by an all-optical dynamic decoupling approach based on Raman scattering in the 87Rb Zeeman sublevels. By adjusting the parameters of the Raman lasers, we realize spin control along an arbitrary direction. We analyze the state evolution of atomic spin under the Raman light control sequence in detail. The results show that both the nonlinear Zeeman effect and magnetic field gradient can be significantly suppressed.

physics.atom-ph↗

arXiv2020

Nonlinear phase estimation enhanced by an actively correlated Mach-Zehnder interferometer

A nonlinear phase shift is introduced to a Mach-Zehnder interferometer (MZI), and we present a scheme for enhancing the phase sensitivity. In our scheme, one input port of a standard MZI is injected with a coherent state and the other input port is injected with one mode of a two-mode squeezed-vacuum state. The final interference output of the MZI is detected with the method of active correlation output readout. Based on the optimal splitting ratio of beam splitters, the phase sensitivity can beat the standard quantum limit and approach the quantum Cramér-Rao bound. The effects of photon loss on phase sensitivity are discussed. Our scheme can also provide some estimates for units of $χ^{(3)}$, due to the relation between the nonlinear phase shift and the susceptibility $χ^{(3)}$ of the Kerr medium.

quant-ph↗

arXiv2020

Atom-light hybrid quantum gyroscope

A new type of atom-light hybrid quantum gyroscope (ALHQG) is proposed due to its high rotation sensitivity. It consists of an optical Sagnac loop to couple rotation rate and an atomic ensemble as quantum beam splitter/recombiner (QBS/C) based on atomic Raman amplification process to realize the splitting and recombination of the optical wave and the atomic spin wave. The rotation sensitivity can be enhanced by the quantum correlation between Sagnac loop and QBS/C. The optimal working condition is investigated to achieve the best sensitivity. The numerical results show that the rotation sensitivity can beat the standard quantum limit (SQL) in ideal condition. Even in the presence of the attenuation under practical condition, the best sensitivity of the ALHQG can still beat the SQL and is better than that of a fiber optic gyroscope (FOG). Such an ALHQG could be practically applied for modern inertial navigation system.

quant-ph↗

arXiv2020

Angular displacements estimation enhanced by squeezing and parametric amplification

We theoretically study the angular displacements estimation based on a modified Mach-Zehnder interferometer (MZI), in which two optical parametric amplifiers (PAs) are introduced into two arms of the standard MZI, respectively. The employment of PAs can both squeeze the shot noise and amplify the photon number inside the interferometer. When the unknown angular displacements are introduced to both arms, we derive the multiparameter quantum Cramér-Rao bound (QCRB) using the quantum Fisher information matrix approach, and the bound of angular displacements difference between the two arms is compared with the sensitivity of angular displacement using the intensity detection. On the other hand, in the case where the unknown angular displacement is in only one arm, we give the sensitivity of angular displacement using the method of homodyne detection. It can surpass the standard quantum limit (SQL) and approach the single parameter QCRB. Finally, the effect of photon losses on sensitivity is discussed.

quant-ph↗

arXiv2019

A novel two-mode squeezed light based on double-pump phase-matching

A novel two-mode non-degenerate squeezed light is generated based on a four-wave mixing (4WM) process driven by two pump fields crossing at a small angle. By exchanging the roles of the pump beams and the probe and conjugate beams, we have demonstrated the frequency-degenerate two-mode squeezed light with separated spatial patterns. Different from a 4WM process driven by one pump field, the refractive index of the corresponding probe field $n_{p}$ can be converted to a value that is greater than $1$ or less than $1$ by an angle adjustment. In the new region with $n_{p}<1$, the bandwidth of the gain is relatively large due to the slow change in the refractive index with the two-photon detuning. As the bandwidth is important for the practical application of a quantum memory, the wide-bandwidth intensity-squeezed light fields provide new prospects for quantum memories.

quant-ph↗

arXiv2019

Magnetoelectric Coupling by Giant Piezoelectric Tensor Design

Strain-coupled magnetoelectric (ME) phenomena in piezoelectric / ferromagnetic thin-film bilayers are a promising paradigm for sensors and information storage devices, where strain is utilized to manipulate the magnetization of the ferromagnetic film. In-plane magnetization rotation with an electric field across the film thickness has been challenging due to the virtual elimination of in-plane piezoelectric strain by substrate clamping, and to the requirement of anisotropic in-plane strain in two-terminal devices. We have overcome both of these limitations by fabricating lithographically patterned devices with a piezoelectric membrane on a soft substrate platform, in which in-plane strain is freely generated, and a patterned edge constraint that transforms the nominally isotropic piezoelectric strain into the required uniaxial strain. We fabricated 500 nm thick, (001) oriented [Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$]$_{0.7}$-[PbTiO$_3$]$_{0.3}$ (PMN-PT) unclamped piezoelectric membranes with ferromagnetic Ni overlayers. Guided by analytical and numerical continuum elastic calculations, we designed and fabricated two-terminal devices exhibiting Ni magnetization rotation in response to an electric field across the PMN-PT. Similar membrane heterostructures could be used to apply designed strain patterns to many other materials systems to control properties such as superconductivity, band topology, conductivity, and optical response.

physics.app-ph↗

arXiv2018

Phase estimation for an SU(1,1) interferometer in the presence of phase diffusion and photon losses

We theoretically study the quantum Fisher information (QFI) of the SU(1,1) interferometer with phase shifts in two arms taking account of realistic noise effects. A generalized phase transform including the phase diffusion effect is presented by the purification process. Based on this transform, the analytical QFI and the bound to the quantum precision are derived when considering the effects of phase diffusion and photon losses simultaneously. To beat the standard quantum limit with the reduced precision of phase estimation due to noisy, the upper bounds of decoherence coefficients as a function of total mean photon number are given.

quant-ph↗