Quantum Limitation to the Coherent Emission of Accelerated Charges

Abstract

Accelerated charges emit electromagnetic radiation. According to classical electrodynamics, if the charges move along sufficiently close trajectories they emit coherently; i.e., their emitted energy scales quadratically with their number rather than linearly. By investigating the emission by a two-electron wave packet in the presence of an electromagnetic plane wave within strong-field QED, we show that quantum effects deteriorate the coherence predicted by classical electrodynamics even if the typical quantum nonlinearity parameter of the system is much smaller than unity. We explain this result by observing that coherence effects are also controlled by a new quantum parameter which relates the recoil undergone by the electron to the width of its wave packet in momentum space.

Single and double nonlinear Compton scattering

“…In this paper we do not consider effects of finite-sized wave packets. See[22] for such effects in photon emission by a single electron, and[23] for two-electron wave packets and the difference in coherence compared to the classical prediction[24].…”

On quantum electrodynamic processes in plasmas interacting with strong lasers

“…Similarly, Classical Electrodynamics predicts that the emission should be coherent over a certain frequency range (for � ≲ 4 × 10 7 eV ), but overestimates the cutoff frequency until which coherent emission takes place by about an order of magnitude. This discrepancy is due to the kinematical indistinguishability condition introduced in this section; notice that it can be realized even for ≪ 1 (see Angioi and Di Piazza 2018).…”

“…In this section, we will show how quantum effects can introduce a frequency cutoff (without any classical analogue) above which coherent emission does not occur. We will see that the SF-QED predictions for the emission of radiation can depend non-trivially on the number of particles which participate in the process and this leads to qualitative differences with respect to the classical predictions (Angioi and Di Piazza 2018). In SF-QED, the lowest order process by which a charged particle in a strong field can emit radiation is called Nonlinear Single Compton Scattering (NSCS).…”

“…Although when computing NSCS with the initial state given by Eq. (4), the plane-wave conservation relations still apply, depending on the characteristics of the initial wave packet it can be fundamentally impossible to know which electron actually emitted the photon, and this prevents the reconstruction of the initial state from the final state; because of this, it is possible to observe coherent emission in the quantum spectra (Angioi and Di Piazza 2018). This "kinematical indistinguishability" condition for coherent emission in the quantum case is more restrictive than the one in the classical case, so that quantum effects can forbid coherent emission even at frequencies where classical mechanics predicts it to occur.…”

“…1 are the discrepancies between the NSCS rates calculated using a twoelectron initial state of the form given by Eq. 4, shown as a solid black line, with an analogous classical ensemble (for more details, see Angioi and Di Piazza 2018), shown as a dash-dotted red line.…”