Quantum Radiation Reaction: From Interference to Incoherence

Abstract

We investigate quantum radiation reaction in laser-electron interactions across different energy and intensity regimes. Using a fully quantum approach which also accounts exactly for the effect of the strong laser pulse on the electron motion, we identify in particular a regime in which radiation reaction is dominated by quantum interference. We find signatures of quantum radiation reaction in the electron spectra which have no classical analogue and which cannot be captured by the incoherent approximations typically used in the high-intensity regime. These signatures are measurable with presently available laser and accelerator technology.PACS numbers: 12.20.Ds, 41.75.Ht Intense light sources offer new prospects for observing quantum effects in laser-matter interactions. Phenomena such as particle beam spreading [1], cooling [2, 3] and trapping [4, 5] can all be phrased in terms of the quantum recoil experienced by particles interacting with laser pulses, recoil which dominates particle motion in certain regimes [6]. Because of this the topic of quantum recoil, also called quantum radiation reaction ("QRR"), now receives a great deal of attention [7][8][9][10][11][12].Investigations of QRR often focus on high-intensity regimes currently out of experimental reach. In such regimes QRR comes from multiphoton emission, and the shortness of the 'formation length' of quantum processes at high intensity implies that these emissions can be described as incoherent events [13, 14]. In this Letter we show that the nature of QRR varies significantly in different intensity and energy regimes, in particular regimes which are relevant to experiments soon to be performed. In particular we reveal a regime, accessible with the laser intensities and accelerator technology available today, in which QRR is dominated by coherent quantum effects with no classical analogue, effects which are distinct from those in the high-intensity regime and which cannot be described by the approximations or numerical methods used there. Further, we will find new kinematic delineations of the different regimes.Consider an electron interacting with a strong electromagnetic field. The classical Lorentz force equation predicts that the electron moves with some momentum π µ . A measurement of the electron momentum would however yield a different result P µ , because the Lorentz equation does not account for the fact that the electron radiates and, by conservation of momentum, recoils when it does so [15]. The impact of this radiation reaction ("RR") on the motion of the electron can be characterised simply by the difference between the actual momentum of the electron and that predicted by the Lorentz force: P µ −π µ is classical RR. The momentum P µ can be obtained as the classical or low-energy limit of a quantum mechanical observable, namely the expectation value of the electron momentum operatorP µ [16][17][18][19]. Hence P µ − π µ is a measure of QRR. The expectation value P µ can be calculated for arbitrary weak fields in perturbation theory...

Nonlinear Breit–Wheeler pair creation with bremsstrahlungγrays

“…This requires a 0 3 /χ ? 1, as will always be the case here [27]. (See [28][29][30][31] and references therein for a discussion of how the pair creation probability may be calculated exactly in the framework of strong-field QED.)…”

Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser

“…A full quantum description is thus required, and this is currently the subject of active theoretical research (see, for instance, Refs. [3][4][5][6][7][8][9][10]). Purely quantum effects can be triggered in these conditions, including the stochastic nature of photon emission [5,6], a hard cutoff in the maximum energy of the emitted photons [9], and pair production [10].…”

Reaching supercritical field strengths with intense lasers

“…These model a QED cascade of photon emission and pair creation by factorising it into a product of first-order processes [46,47], which occur along the particles' classical trajectories at positions determined by integration of QED probability rates that are calculated in the locally constant field approximation [8]. This 'semiclassical' approach is valid when c  a 1 e 0 3 because the formation lengths of the photons and electron-positron pairs are much smaller than the laser wavelength and interference effects are suppressed [48]. Details of the simulations are given in the appendix.…”