Dimension-Dependent Stimulated Radiative Interaction of a Single Electron Quantum Wavepacket

In the foundation of quantum mechanics, the spatial dimensions of electron wavepacket are understood only in terms of an expectation value - the probability distribution of the particle location. One can still inquire how the quantum electron wavepacket size affects a physical process. Here we address the fundamental physics problem of particle-wave duality and the measurability of a free electron quantum wavepacket. Our analysis of stimulated radiative interaction of an electron wavepacket, accompanied by numerical computations, reveals two limits. In the quantum regime of long wavepacket size relative to radiation wavelength, one obtains only quantum-recoil multiphoton sidebands in the electron energy spectrum. In the opposite regime, the wavepacket interaction approaches the limit of classical point-particle acceleration. The wavepacket features can be revealed in experiments carried out in the intermediate regime of wavepacket size commensurate with the radiation wavelength

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

Spontaneous and Stimulated Emissions of Quantum Free-Electron Wavepackets - QED Analysis

Do the wavepacket-size of free-electron wavefunction and its history have physical effect in its interaction with light? Here we answer this problem by analyzing a QED model, considering both spontaneous and stimulated emission of quantized radiation field. For coherent radiation (Glauber state), we confirm that stimulated emission/absorption of photons has a dependence on wavepacket size that decays when it exceeds the interacting radiation wavelength, consistently and complementarily with Schrodinger equation analysis of wavepacket acceleration in classical electromagnetic field. Furthermore, the stimulated emission of modulated electron wavepacket with coherently-bunched profiles has characteristic harmonic emission spectrum that is also wavepacket size dependent but beyond the frequency cut-off. In either case, there is no wavepacket dependent emission of Fock state radiation, and particularly the vacuum state spontaneous emission is wavepacket-independent. The transition of radiation emission from the classical point-particle limit to the quantum electron wavefunction limit is demonstrated in electron wavepacket representation. It indicates a way for measuring the wavepacket size of single electron wavefunction, and suggests a new direction for exploring light-matter interaction fundamentally

Spontaneous and Stimulated Radiative emission of Modulated Free-Electron Quantum wavepackets - Semiclassical Analysis

Here we present a semiclassical analysis of spontaneous and stimulated radiative emission from unmodulated and optically-modulated electron quantum wavepackets. We show that the radiative emission/absorption and corresponding deceleration/acceleration of the wavepackets depend on the controllable 'history-dependent' wavepacket size. The characteristics of the radiative interaction when a wavepacket of size (duration) is short relative to the radiation wavelength, are close to the predictions of the classical point-particle modeling. On the other hand, in the long-sized wavepacket limit, the interaction is quantum-mechanical, and it diminishes exponentially at high frequency. We exemplify these effects through the scheme of Smith-Purcell radiation, and demonstrate that if the wavepacket is optically-modulated and periodically-bunched, it exhibits finite radiative emission at harmonics of the modulation frequency beyond the limit of high-frequency cutoff. Besides, the radiation analysis is further extended to the cases of superradiant emission from a beam of phase-correlated modulated electron wavepackets. This wavepacket-dependent emission process shows the radiative features of classical-to-quantum transition, indicates a way for measuring the quantum electron wavepacket size and suggests a new direction for exploring light-matter interaction

Pulse reverse-engineering for strong field-matter interaction

We propose a scheme to control the evolution of a two-level quantum system in the strong coupling regime based on the idea of reverse-engineering. A coherent control field is designed to drive both closed and open two-level quantum systems along user predefined evolution trajectory without utilizing the rotating-wave approximation (RWA). As concrete examples, we show that complete population inversion, an equally weighted coherent superposition, and even oscillationlike dynamics can be achieved. As there are no limitations on the coupling strength between the control field and matter, the scheme is attractive for applications such as accelerating desired system dynamics and fast quantum information processing

DataSheet1_Theory and design consideration of a THz superradiant waveguide FEL.pdf

We present theoretical analysis and design considerations of a THz superradiant FEL. We derive analytical expressions for the spectral parameter of THz radiation, emitted superradiantly in a rectangular waveguide using a Longitudinal Section Magnetic mode expansion. The results compare well with numerical simulations using UCLA GPTFEL code. GPT simulations of the accelerator e-beam transport show that the chirp provided by a hybrid photocathode RF gun, can produce tight bunching at the undulator site below σ = 100fs. This enables intense superradiant emission up to 3THz, limited by the beam bunching factor. Phase-space analysis of the beam transport indicates that keeping the beam bunching parameter small enough for higher THz frequency operation is limited by the energy spread of the beam in the gun.</p

Image2_Theory and design consideration of a THz superradiant waveguide FEL.jpeg

We present theoretical analysis and design considerations of a THz superradiant FEL. We derive analytical expressions for the spectral parameter of THz radiation, emitted superradiantly in a rectangular waveguide using a Longitudinal Section Magnetic mode expansion. The results compare well with numerical simulations using UCLA GPTFEL code. GPT simulations of the accelerator e-beam transport show that the chirp provided by a hybrid photocathode RF gun, can produce tight bunching at the undulator site below σ = 100fs. This enables intense superradiant emission up to 3THz, limited by the beam bunching factor. Phase-space analysis of the beam transport indicates that keeping the beam bunching parameter small enough for higher THz frequency operation is limited by the energy spread of the beam in the gun.</p

Image1_Theory and design consideration of a THz superradiant waveguide FEL.jpeg

We present theoretical analysis and design considerations of a THz superradiant FEL. We derive analytical expressions for the spectral parameter of THz radiation, emitted superradiantly in a rectangular waveguide using a Longitudinal Section Magnetic mode expansion. The results compare well with numerical simulations using UCLA GPTFEL code. GPT simulations of the accelerator e-beam transport show that the chirp provided by a hybrid photocathode RF gun, can produce tight bunching at the undulator site below σ = 100fs. This enables intense superradiant emission up to 3THz, limited by the beam bunching factor. Phase-space analysis of the beam transport indicates that keeping the beam bunching parameter small enough for higher THz frequency operation is limited by the energy spread of the beam in the gun.</p

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