arXiv subjects

Quancheng Liu

Publications and source records attributed to Quancheng Liu.

10 recordsLinked to original sources

arXiv2025

Relation between winding numbers and energy dispersions

Two-band Hamiltonians provide a typical description of topological band structures, in which the eigenfunctions can be characterized by a %Bloch vector field whose winding number that defines an integer topological invariant. This winding number is quantized and protected against continuous deformations of the Hamiltonian. Here we show that the Bloch vector and its winding number can be directly related to the gradient of the energy dispersion. Since the energy gradient is proportional to the group velocity, our result establishes an experimentally accessible correspondence between the Bloch vector field and angle-resolved photoemission spectroscopy measurements. We discuss a mapping between the gradient of the energy dispersion and the Bloch vector. This implies a direct and measurable relation between two-band Hamiltonians and their underlying topological structures.

cond-mat.mes-hall↗

arXiv2024

Entanglement of bosonic systems under monitored evolution

The evolution of non-interacting bosons in the presence of repeated projective measurements is studied. Following the established approach, this monitored evolution is characterized by the first detected return and the first detected transition probabilities. We show that these quantities are directly related to the entanglement entropy and to the entanglement spectrum of a bipartite system. Calculations with specific values for the number of bosons, the number of measurements and the time step between measurements reveal a sensitive and often strongly fluctuating entanglement entropy. In particular, we demonstrate that in the vicinity of special values for the time steps the evolution of the entanglement entropy is either stationary or performs dynamical switching between two or more stationary values. In the entanglement spectrum, on the other hand, this complex behavior can be associated with level crossings, indicating that the dominant quantum states and their entanglement respond strongly to a change of the system parameters. We discuss briefly the role of time averaging to remove the fluctuations of the entanglement entropy.

quant-ph↗

arXiv2024

Fractionally Quantized Recurrence Detection Times in Monitored Quantum Many-Body Systems

Recurrence time quantifies the duration required for a physical system to return to its initial state, playing a pivotal role in understanding the predictability of complex systems. In quantum systems with subspace measurements, recurrence times are governed by Anandan-Aharonov phases, yielding fractionally quantized recurrence times. However, the fractional quantization phenomenon in interacting quantum systems remains unexplored. Here, we address this gap by establishing universal lower and upper bounds for recurrence times in interacting many-body spin systems. Notably, we investigate scenarios where these bounds are approached, shedding light on the speed of quantum processes under monitoring. In specific cases, our findings reveal that the complex many-body system can be effectively mapped onto a dynamical system with a single quasi-particle, leading to integer-quantized recurrence times. Our work demonstrates a valuable link between recurrence times and the number of dark states in the system, thus providing a deeper understanding of the intricate interplay between Hilbert-space fragmentation, ergodicity breaking, measurements, and interaction effects. Finally, our findings have been implemented on an IBM quantum computer, revealing resonances and fractional quantization in agreement with theoretical predictions. This demonstrates the resilience of non-equilibrium topological fractional quantization to noise and highlights its potential use for benchmarking quantum devices and probing dark states.

cond-mat.stat-mech↗

arXiv2023

Inverse orbital Hall effect and orbitronic terahertz emission observed in the materials with weak spin-orbit coupling

The Orbital Hall effect, which originates from materials with weak spin-orbit coupling, has attracted considerable interest for spin-orbitronic applications. Here, we demonstrate the inverse effect of the orbital Hall effect and observe orbitronic terahertz emission in the Ti and Mn materials. Through spin-orbit transition in the ferromagnetic layer, the generated orbital current can be converted to charge current in the Ti and Mn layers via the inverse orbital Hall effect. Furthermore, the inserted W layer provides an additional conversion of the orbital-charge current in the Ti and Mn layers, significantly enhancing the orbitronic terahertz emission. Moreover, the orbitronic terahertz emission can be manipulated by cooperating with the inverse orbital Hall effect and the inverse spin Hall effect in the different sample configurations. Our results not only discover the physical mechanism of condensed matter physics but also pave the way for designing promising spin-orbitronic devices and terahertz emitters.

cond-mat.mes-hall↗

arXiv2023

Entanglement transition through Hilbert-space localization

We study Hilbert-space localization of the many-body dynamics due to ergodicity breaking and analyze this effect in terms of the entanglement entropy and the entanglement spectrum. We find a transition from a regime driven by quantum tunneling to a regime that is dominated by boson-boson interaction, where the latter exhibits ergodicity breaking. Properties of this transition are captured by observation time averaging, which effectively suppresses the large dynamical entanglement fluctuations near the critical point. We employ this approach to the experimentally available Bosonic Josephson Junction. In this example the transition from a tunneling regime to Hilbert-space localization reveals clear signatures in the entanglement entropy and entanglement spectrum. Interestingly, the transition point is reduced by quantum effects in comparison to the well-known result of the mean-field approximation in the form of self-trapping. This indicates that quantum fluctuations reduce the classical self-trapping. Different scaling with the respect to the number of bosons $N$ are found in the tunneling and the localization regime: While the entanglement entropy grows {\it logarithmically} with $N$ in the tunneling regime, it increases {\it linearly} in the localized regime. Our results indicate that entanglement provides a concept for a sensitive diagnosis for the transition from a quantum tunneling regime to Hilbert-space localization.

quant-ph↗

arXiv2022

Designing exceptional-point-based graphs yielding topologically guaranteed quantum search

Quantum walks underlie an important class of quantum computing algorithms, and represent promising approaches in various simulations and practical applications. Here we design stroboscopically monitored quantum walks and their subsequent graphs that can naturally boost target searches. We show how to construct walks with the property that all the eigenvalues of the non-Hermitian survival operator, describing the mixed effects of unitary dynamics and the back-action of measurement, coalesce to zero, corresponding to an exceptional point whose degree is the size of the system. Generally, the resulting search is guaranteed to succeed in a bounded time for any initial condition, which is faster than classical random walks or quantum walks on typical graphs. We then show how this efficient quantum search is related to a quantized topological winding number and further discuss the connection of the problem to an effective massless Dirac particle.

quant-ph↗

arXiv2022

No-go theorems for deterministic purification and probabilistic enhancement of coherence

The manipulation of quantum coherence is one of the principal issues in the resource theory of coherence, with two critical topics being the purification and enhancement of coherence. Here, we present two no-go theorems for the deterministic purification of coherence and the probabilistic enhancement of coherence, respectively. Specifically, we prove that a quantum state cannot be deterministically purified if it can be expressed as a convex combination of an incoherent state and a coherent state. Besides, we give an easy-to-verified sufficient and necessary condition to determine whether a state can be probabilistically enhanced via a stochastic strictly incoherent operation (sSIO). Our findings provide two feasibility criteria for the deterministic purification and the probabilistic enhancement of coherence, respectively. These results have repercussions on the understanding of quantum coherence in real quantum systems.

quant-ph↗

arXiv2021

Role of focusing distance in picosecond laser-induced Cu plasma spectra

To study the effects of focusing distance on the characteristics of copper plasma, a picosecond laser was utilized to ablate a pure copper plate to generate a plasma spectrum. Following numerous experiments on the subject, three significant factors have been determined: lens focal length, pulse energy and the lens-to-sample distance. These factors were employed to analyze the spectral intensity, plasma temperature and electron density in the local thermodynamic equilibrium (LTE) and optically thin condition. Due to the shielding effects of mixed plasma, the strongest spectral intensity can be obtained in the pre-focused case rather than on the focus, no matter how much beam irradiance was employed. The more intensive the beam irradiance is, the more the optimal position is distant from the focal point. Similarly, the evolution of plasma temperature and electron density was shown a peak in the pre-focused case, which is consistent with the trend of spectral intensity. For the case of extremely high irradiance (on the focus), the shielding effects become more apparent and the resultant above three factors decreased sharply. When a longer-focal-length lens was employed, the spectral intensity exhibited an obvious bimodal trend. In the pre-focused case, a longer-focal-length lens is helpful to eliminate the effects of the roughness of the target surface compared with a shorter one. Finally, the assumed LTE was validated by McWhirter relation, plasma relaxation time and diffusion length, and the optically thin condition also validated by spectral intensity ratio. We hope this work could be an important reference for the future design of highly optimized experiments for Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS).

physics.plasm-ph↗

arXiv2021

Driving quantum systems with repeated conditional measurements

We investigate the effect of conditional null measurements on a quantum system and find a rich variety of behaviors. Specifically, quantum dynamics with a time independent $H$ in a finite dimensional Hilbert space are considered with repeated strong null measurements of a specified state. We discuss four generic behaviors that emerge in these monitored systems. The first arises in systems without symmetry, along with their associated degeneracies in the energy spectrum, and hence in the absence of dark states as well. In this case, a unique final state can be found which is determined by the largest eigenvalue of the survival operator, the non-unitary operator encoding both the unitary evolution between measurements and the measurement itself. For a three-level system, this is similar to the well known shelving effect. Secondly, for systems with built-in symmetry and correspondingly a degenerate energy spectrum, the null measurements dynamically select the degenerate energy levels, while the non-degenerate levels are effectively wiped out. Thirdly, in the absence of dark states, and for specific choices of parameters, two or more eigenvalues of the survival operator match in magnitude, and this leads to an oscillatory behavior controlled by the measurement rate and not solely by the energy levels. Finally, when the control parameters are tuned, such that the eigenvalues of the survival operator all coalesce to zero, one has exceptional points that corresponds to situations that violate the null measurement condition, making the conditional measurement process impossible.

quant-ph↗

arXiv2018

Highly-efficient spintronic terahertz emitter enabled by metal-dielectric photonic crystal

Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility in compared with the commercial THz emitters, and thus attracts great interests recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption - more than 50% energy is wasted and the conversion efficiency is thus limited. Here, we theoretically propose and experimentally demonstrate a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency. The device, which consists of a metal-dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generations show one-to-one correspondence with the theoretical calculations. We achieve the strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics.

cond-mat.mes-hall↗