arXiv subjects

Yiming Pan

Publications and source records attributed to Yiming Pan.

At least 55 records · Page 3Linked to original sources

arXiv2023

Vibrational dichroism of chiral valley phonons

Valley degrees of freedom in transition-metal dichalcogenides influence thoroughly electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly-polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab-initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS$_2$ that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient timescales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persisting beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

cond-mat.mes-hall↗

arXiv2023

Low-energy Free-electron Rabi oscillation and its applications

We propose free-electron Rabi oscillation by creating an isolated two-level system in a synthetic energy space induced by laser. The π/2-pulse and π-pulse in synthetic Rabi dynamics can function as 'beam splitters' and 'mirrors' for free-electron interferometry, allowing us to detect local electromagnetic fields and plasmonic excitations. When the coupling field is quantized, we can observe quantum and vacuum Rabi oscillations of the two-level electron, which can be used to investigate the quantum statistics of optical excitations and electron-photon entanglement. Recent advances in laser control of electron microscopes and spectroscopes makes the experimental detection of synthetic Rabi oscillations possible. However, observing the quantum Rabi oscillation of electrons remains challenging. Our work has the potential to advance various fundamentals and applications of resonant light-matter interactions between low-energy electrons and quatum light.

quant-ph↗

arXiv2022

Quantum wavefunction reconstruction by free-electron spectral shearing interferometry

We propose a novel spectral method for reconstructing quantum wavefunction of an electron pulse, free-electron spectral shearing interferometry (FESSI). We employ a Wien filter to generate two time-delayed replicas of the electron wavepacket and then shift one replica in energy using a light-electron modulator driven by a mid-infrared laser. As a direct demonstration, we numerically reconstruct an ultrashort electron pulse with a kinetic energy of 10 keV. FESSI is experimentally feasible and enables us to fully determine distinct orders of spectral phases and their physical implications, providing a universal approach to characterize ultrashort electron pulses.

quant-ph↗

arXiv2022

Superluminal k-gap solitons in nonlinear photonic time-crystals

We propose superluminal solitons residing in the momentum gap (k-gap) of nonlinear photonic time-crystals. These gap solitons are structured as plane-waves in space while being periodically self-reconstructing wavepackets in time. The solitons emerge from modes with infinite group velocity causing superluminal evolution, which is opposite to the stationary nature of the analogous Bragg gap soliton residing at the edge of an energy gap (or a spatial gap) with zero group velocity. We explore the faster-than-light pulsed propagation of these k-gap solitons in view of Einstein's causality by introducing a truncated input seed as a precursor of signal velocity forerunner, and find that the superluminal propagation of k-gap solitons does not break causality.

nlin.PS↗

arXiv2022

Synthetic dimensions using ultrafast free electrons

We propose a synthetic dimension for ultrafast free electrons based on the discrete energy sidebands in photon-induced near-field electron microscopy (PINEM). The PINEM synthetic dimension can offer a powerful way to coherently shape or modulate free-electron wavefunctions in systems such as ultrafast transmission electron microscopes, dielectric laser accelerators, and quantum free-electron lasers. As examples of this paradigm, we demonstrate Bloch oscillations, diffraction management, and linear optics effects such as Talbot self-imaging in the PINEM lattice. These effects show the potential of PINEM synthetic dimensions as a novel quantum simulation platform.

quant-ph↗

arXiv2022

Quantum interference visibility spectroscopy in two-color photoemission from tungsten needle tips

When two-color femtosecond laser pulses interact with matter, electrons can be emitted through various multiphoton excitation pathways. Quantum interference between these pathways gives rise to a strong oscillation of the photoemitted electron current, experimentally characterized by its visibility. In this work, we demonstrate two-color visibility spectroscopy of multi-photon photoemission from a solid-state nanoemitter. We investigate the quantum pathway interference visibility over an almost octave-spanning wavelength range of the fundamental femtosecond laser pulses and their second-harmonic. The photoemission shows a high visibility of 90% +/- 5%, with a remarkably constant distribution. Furthermore, by varying the relative intensity ratio of the two colors, we find that we can vary the visibility between 0 and close to 100%. A simple but highly insightful theoretical model allows us to explain all observations, with excellent quantitative agreements. We expect this work to be universal to all kinds of photo-driven quantum interference, including quantum control in physics, chemistry and quantum engineering.

physics.optics↗

arXiv2022

Demonstration of weak measurements, projective measurements, and quantum-to-classical transitions in ultrafast free electron-photon interactions

How does the quantum-to-classical transition of measurement occur? This question is vital for both foundations and applications of quantum mechanics. Here, we develop a new measurement-based framework for characterizing the classical and quantum free electron-photon interactions and then experimentally test it. We first analyze the transition from projective to weak measurement in generic light-matter interactions and show that any classical electron-laser-beam interaction can be represented as an outcome of a weak measurement. In particular, the appearance of classical point-particle acceleration is an example of an amplified weak value resulting from weak measurement. A universal factor quantifies the measurement regimes and their transition from quantum to classical, where Gamma corresponds to the ratio between the electron wavepacket size and the optical wavelength. This measurement-based formulation is experimentally verified in both limits of photon-induced near-field electron microscopy and the classical acceleration regime using a dielectric laser accelerator. Our results shed new light on the transition from quantum to classical electrodynamics, enabling to employ the essence of wave-particle duality of both light and electrons in quantum measurement for exploring and applying many quantum and classical light-matter interactions.

quant-ph↗

arXiv2022

Nonparaxiality-triggered Landau-Zener transition in topological photonic waveguides

Photonic lattices have been widely used for simulating quantum physics, owing to the similar evolutions of paraxial waves and quantum particles. However, nonparaxial wave propagations in photonic lattices break the paradigm of the quantum-optical analogy. Here, we reveal that nonparaxiality exerts stretched and compressed forces on the energy spectrum in the celebrated Aubry-Andre-Harper model. By exploring the mini-gaps induced by the finite size of the different effects of nonparaxiality, we experimentally present that the expansion of one band gap supports the adiabatic transfer of boundary states while Landau-Zener transition occurs at the narrowing of the other gap, whereas identical transport behaviors are expected for the two gaps under paraxial approximation. Our results not only serve as a foundation of future studies of dynamic state transfer but also inspire applications leveraging nonparaxial transitions as a new degree of freedom.

cond-mat.mes-hall↗

arXiv2022

Asymmetric topological pumping in nonparaxial photonics

Topological photonics was initially inspired by the quantum-optical analogy between the Schrödinger equation for an electron wavefunction and the paraxial equation for a light beam. Here, we reveal an unexpected phenomenon in topological pumping observed in arrays of nonparaxial optical waveguides where the quantum-optical analogy becomes invalid. We predict theoretically and demonstrate experimentally an asymmetric topological pumping when the injected field transfers from one side of the waveguide array to the other side whereas the reverse process is unexpectedly forbidden. Our finding could open an avenue for exploring topological photonics that enables nontrivial topological phenomena and designs in photonics driven by nonparaxiality.

physics.optics↗

arXiv2021

Chirality of Valley Excitons in Monolayer Transition-Metal Dichalcogenides

By enabling control of valley degrees of freedom in transition-metal dichalcogenides, valley-selective circular dichroism has become a key concept in valleytronics. In this manuscript, we show that valley excitons -- bound electron-hole pairs formed at either the K or K' valleys upon absorption of circularly-polarized light -- are chiral quasiparticles characterized by a finite orbital angular momentum (OAM). We further formulate an ab-initio many-body theory of valley-selective circular dichroism and valley excitons based on the Bethe-Salpeter equation. Beside governing the interaction with circularly polarized light, the OAM confers excitons a finite magnetization which manifests itself through an excitonic Zeeman splitting upon interaction with external magnetic fields. The good agreement between our ab-initio calculations and recent experimental measurements of the exciton Zeeman shifts corroborate this picture.

cond-mat.mtrl-sci↗

arXiv2021

Photonic Floquet time crystals

The public and scientists constantly have different perspectives. While on a time crystal, they stand in line and ask: What is a time crystal? Show me a material that is spontaneously crystalline in time? This study synthesizes a photonic material of Floquet time crystals and experimentally observes its indicative period-2T beating. We explicitly reconstruct a discrete time-crystalline ground state and reveal using an appropriately-designed photonic Floquet simulator the rigid period-doubling as a signature of the spontaneous breakage of the discrete time-translational symmetry. Unlike the result of the exquisite many-body interaction, the photonic time crystal is derived from a single-particle topological phase that can be extensively accessed by many pertinent nonequilibrium and periodically-driven platforms. Our observation will drive theoretical and technological interests toward condensed matter physics and topological photonics, and demystify time crystals for the non-scientific public.

quant-ph↗

arXiv2020

Signatures of the $π$-mode anomaly in (1+1) dimensional periodically-driven topological/normal insulator heterostructures

Akin to zero-mode anomalies, such as the chiral anomaly of edge states in quantum Hall effect, in this work, a pi-mode anomaly is proposed in a 1+1 dimensional periodically-driven topological/normal insulator (TI/NI) heterostructure. Usually, when coupling in a background gauge field, the zero modes on domain walls would provide an anomalous current term that is eventually canceled by additional boundary contributions from the topological bulk, via the Callan-Harvey mechanism. This anomaly cancellation associated with the generalization of bulk-boundary correspondence is called anomaly inflow. Through our photonic modeling and setup of the Floquet TI/NI heterostructure, for the first time, we experimentally observed the $π$-mode domain wall in certain driven frequencies, which is always attached to the reminiscent Floquet gauge that plays the vital role of an emergent background field. Indeed, due to the possible emergence of Floquet gauge anomaly from the driven topological bulk, the resultant $π$-mode anomaly can be matched on the driven interface between Floquet domains. Prospectively, we believe our prediction and observation could pave a new avenue on exploring anomalies in both periodically-driven classical and quantum systems.

cond-mat.mes-hall↗

arXiv2020

Beyond Adiabatic Elimination in Topological Floquet Engineering

In quantum mechanics, adiabatic elimination is a standard tool that produces a low-lying reduced Hamiltonian for a relevant subspace of states, incorporating effects of its coupling to states with much higher energy. Suppose this powerful elimination approach is applied to quasi-energy states in periodically-driven systems, a critical question then arises that the violation of the adiabatic condition caused by driven forces challenges such a presence of spectral reduction in the non-equilibrium driven system. Here, both theoretically and experimentally, we newly reported two kinds of driven-induced eliminations universal in topologically-protected Floquet systems. We named them "quasi-adiabatic elimination" and "high-frequency-limited elimination", in terms of different driven frequencies that deny the underlying requirement for the adiabatic condition. Both two non-adiabatic eliminations are observed in our recently developed microwave Floquet simulator, a programmable test platform composed of periodically-bending ultrathin metallic coupled corrugated waveguides. Through the near-field imaging on our simulator, the mechanisms between the adiabatic and driven-induced eliminations are revealed, indicating the ubiquitous spectral decomposition for tailoring and manipulating Floquet states with quasi-energies. Finally, we hope our findings may open up profound and applicable possibilities for further developing Floquet engineering in periodically-driven systems, ranging from condensed matter physics to photonics.

quant-ph↗

arXiv2020

Weak-to-strong transition of quantum measurement in a trapped-ion system

Quantum measurement remains a puzzle through its stormy history from the birth of quantum mechanics to state-of-the-art quantum technologies. Two complementary measurement schemes have been widely investigated in a variety of quantum systems: von Neumann's projective 'strong' measurement and Aharonov's weak measurement. Here, we report the observation of a weak-to-strong measurement transition in a single trapped $40Ca^+$ ion system. The transition is realized by tuning the interaction strength between the ion's internal electronic state and its vibrational motion, which play the roles of the measured system and the measuring pointer, respectively. By pre- and post-selecting the internal state, a pointer state composed of two of the ion's motional wavepackets is obtained, and its central-position shift, which corresponds to the measurement outcome, demonstrates the transition from the weak-value asymptotes to the expected-value asymptotes. Quantitatively, the weak-to-strong measurement transition is characterized by a universal transition factor $e^{-Γ^2}$, where $Γ$ is a dimensionless parameter related to the system-apparatus coupling. This transition, which continuously connects weak measurements and strong measurements, may open new experimental possibilities to test quantum foundations and prompt us to re-examine and improve the measurement schemes of related quantum technologies.

quant-ph↗

arXiv2020

Universal presence of time-crystalline phases and period-doubling oscillations in one-dimensional Floquet topological insulators

In this work, we reported a ubiquitous presence of topological Floquet time crystal (TFTC) in one-dimensional periodically-driven systems. The rigidity and realization of spontaneous discrete time-translation symmetry (DTS) breaking in our model require necessarily coexistence of anomalous topological invariants (0 modes and $π$ modes), instead of the presence of disorders or many-body localization. We found that in a particular frequency range of the underlying drive, the anomalous Floquet phase coexistence between zero and pi modes can produce the period-doubling (2T, two cycles of the drive) that breaks the spontaneously, leading to the subharmonic response ($ω/2$, half the drive frequency). The rigid period-oscillation is topologically-protected against perturbations due to both non-trivially opening of 0 and $π$-gaps in the quasienergy spectrum, thus, as a result, can be viewed as a specific "Rabi oscillation" between two Floquet eigenstates with certain quasienergy splitting $π/T$. Our modeling of the time-crystalline 'ground state' can be easily realized in experimental platforms such as topological photonics and ultracold fields. Also, our work can bring significant interests to explore topological phase transition in Floquet systems and to bridge the gap between Floquet topological insulators and photonics, and period-doubled time crystals.

physics.optics↗

arXiv2019

Quantum Emergence of Linear Particle Accelerator and Anomalous Photon-induced Near-field Electron Microscopy in a Strong Coupling Regime

Photon-induced near-field electron microscopy (PINEM) is a currently developing spectral approach that characterizes quantum electron-light interactions in electron energy gain/loss spectrum, with symmetrically discretized gains or losses of light quanta ($\hbar ω$), coupled with a laser induced optical near-field. In this letter, we have demonstrated that Linear Particle Accelerator (LPA) and anomalous PINEM (APINEM) can analytically emerge from PINEM-kind interaction in a strong coupling regime, because of quantum interference of spectral photon sidebands overlap. Furthermore, we also found that the quantum interference in point-particle regime with pre-interaction drift can produce interesting optical spectral focusing and a periodically bunching of energy/momentum distribution that enable us to improve the spectral resolution of electron microscopy, imaging and spectroscopy. These observation of LPA and APINEM in strong laser physics can be of great interests for both theoretical and experimental communities, such as ultrafast electron microscopes, attosecond science and laser-driven accelerators.

quant-ph↗

arXiv2019

Beyond Fermi's Golden Rule in Free-Electron Quantum Electrodynamics: Acceleration/Radiation Correspondence

In this article, we present a unified reciprocal quantum electrodynamics (QED) formulation of quantum light-matter interaction. For electron-light interactions, we bridge the underlying theories of Photon-Induced Near-field Electron Microscopy (PINEM), Laser-induced Particle Accelerators and radiation sources, such as Free Electron Laser (FEL), transition radiation and Smith-Purcell effect. We demonstrate an electron-photon spectral reciprocity relation between the electron energy loss/gain and the radiation spectra. This "Acceleration/Radiation Correspondence" (ARC) conserves the electron-energy and photon-number exchanged and in the case of a Quantum Electron Wavepacket (QEW), displays explicit dependence on the history-dependent phase and shape of the QEW. It originates from an interaction-induced quantum interference term that is usually ignored in Fermi's Golden Rule analyses. We apply the general QED formulation to both stimulated interaction and spontaneous emission of classical and quantum light by the quantum-featured electrons. The 'spontaneous' emissions of coherent states ('classical' light) and squeezed states of light are shown to be enhanced with squeezed vacuum. This reciprocal ARC formulation has promise for extension to other fundamental research problems in quantum-light and quantum-matter interactions.

quant-ph↗

arXiv2019

Anomalous Photon-induced Near-field Electron Microscopy

We reveal the classical and quantum regimes of free electron interaction with radiation, common to the general variety of radiation sources (e.g. FEL, Smith-Purcell), Dielectric Laser Accelerator (DLA) and Photo-Induced Near-Field Electron Microscopy (PINEM). Modelling the electron with initial conditions of a coherent quantum electron wavepacket (QEW), its topology in phase-space uniquely defines a universal distinction of three interaction regimes (and their particle-wave duality transition): point-particle-like acceleration, quantum wavefunction (PINEM), and a newly reported regime of anomalous PINEM (APINEM). The quantum interference beat of APINEM is capable of improving the spectral resolution of post-selective electron microscopy, and the particle-wave duality transition reveals the history-dependent nature of quantum electron interaction with light.

quant-ph↗