Pulsed Quantum Excitation

TL;DR

The paper addresses resonance fluorescence when the excitation source is pulsed quantum light from a cascaded two-level system. It develops a theoretical framework with a cascaded master equation and Gaussian pulses to compare classical versus quantum pulsed excitation, and analyzes single- and two-photon observables across three channels: source luminescence, target luminescence, and the photon-flux field. Key findings show that driving a target 2LS with quantum light yields stronger antibunching ($g^{(2)}(0)<1$) and higher indistinguishability (Hong-Ou-Mandel vis., up to ~95%), alongside spectral line narrowing and clear signatures of stimulated emission, with a robust agreement to recent experiments. The work provides a practical roadmap for exploiting pulsed quantum-light excitation in light-matter interfaces and suggests future directions such as chirped pulses and alternative quantum-light sources to further enhance performance.

Abstract

A two-level system (2LS) is the most fundamental building block of matter. Its response to classical light is well known, as it converts pulses of coherent light into antibunched emission. However, recent theoretical proposals have predicted that it is advantageous to illuminate two-level systems with Quantum Light; i.e., the light emitted from a quantum system. However, those proposals were done considering continuous excitation of the source of light. Here, we advance the field by changing the paradigm of excitation: we use the emission of a 2LS, itself driven by a laser pulse, to excite another 2LS. Thus, we present a thorough analysis of Resonance Fluorescence under pulsed quantum excitation and show, in particular, that the emission from a 2LS driven with quantum light is more antibunched and more indistinguishable than if it were driven with classical light. Our results reinforce the claim of the advantage of the excitation with quantum light, provide support to the recent experimental observations, and can be used as a road-map for the future of light-matter interaction research.

Figures (7)

• Figure 1: Sketch of the cascaded excitation. A classical pulse of light (blue line) is used to excite a two-level system, here depicted as a glowing halo around a pair energy levels. A fraction $\chi_1$ of the emission from the source 2LS becomes the source of quantum excitation of the target 2LS (line with blue-to-red gradient coloring). After the interaction between the target 2LS and the quantum excitation we can collect two fields: namely, a fraction $\chi_2$ corresponding to the photon flux of the entire system, and the remaining fraction $(1-\chi_2)$ associated to the individual emission from the target 2LS.
• Figure 2: Pulsed Rabi oscillations. (a) Intensity of the emission from a 2LS pulsed with laser with vanishing (dashed line) and finite but short length (solid line). (b) Intensity of the emission of the source 2LS as a function of both the area and the length of the pulse, showing that the Rabi oscillations are quickly suppressed as the length of the pulse is increased. (c) Visibility of the Rabi oscillations, as quantified by the normalized difference between the maximum near $A=\pi$ and the minimum near $A=2\pi$, for the source 2LS (dashed), the target 2LS (ranging between the upper two lines, from solid black to solid blue, covering the entire area shaded in blue) and the photon flux (ranging between the lower two lines, from dashed black to solid red, convering the entire area shaded in red). For the latter two, the gradient indicates the variation of the parameter $\chi_2$. (d) Extinction coefficients for the visibility lines showed in panel (c), for the source 2LS (dashed), the target 2LS (blue) and photon flux (red). (e, f) Same as in panel (b) but for the target 2LS and the photon flux, respectively; calculated for $\chi_2=1/2$. The panels in the bottom row share the same color encoding, and therefore can be compared directly.
• Figure 3: Emission spectra of dynamic Resonance Fluorescence. (a--c) Photoluminescence as a function of the area of the pulse, for the source 2LS, target 2LS and the photon flux, respectively. The variation in the intensity of the emission as the pulse area increases is an echo of the Rabi oscillations undergone by the 2LS. (d--f) Cuts of the spectra for $\pi$- (solid blue), $2\pi$- (solid red), $3\pi$- (dashed light blue) and $4\pi$-pulses (dot-dashed light red). In panels (e) and (f) the dotted gray line corresponds to the PL of the source 2LS for a $\pi$-pulse, showing the line narrowing of the target 2LS. The figures are obtained for pulses at resonance to the 2LSs, and setting $\mathcal{W}\gamma_\sigma = 1$ and $\chi_2=1/2$. The three upper panels can be compared directly, as they share the same color scaling.
• Figure 4: Time-dependent occupation of the pulsed emitters. Comparison between the averaged time-dependent population of the source 2LS (dashed gray), the photon flux (solid red) and the target 2LS (dot-dashed blue), for various pulse areas (noted on the top right corner of each panel). All the panels are obtained with a pulse of length $\mathcal{W}\gamma_\sigma = 1/2$ (which for reference is sketeched at the top of the figure) and letting $\chi_2 = 1/2$. The underlying quantum interference taking place in the field of the photon flux is particularly noticeable in panels (a) and (c), where a dip in the occupation can be distinguished at $t\gamma_\sigma \sim 2$. In all the panels, the origin of time is set by the position of the maximum of the classical pulse.
• Figure 5: Second-order correlation functions of the emitted photons. (a) Zero-delay $g^{(2)}(0)$ and (b) Photon indistinguishability, as captured by the visibility of the dip in a HOM interference. Here, we show the information for the source 2LS (dotted gray), the target 2LS (dot-dashed blue) and the photon flux (solid red). In both cases, the figures are obtained with a pulse of $\mathcal{W}\gamma_\sigma = 0.25$ and letting $\chi_2 = 1/2$.