Abstract We discuss two-photon correlations from the side peaks that are formed when a two-level system emitter is driven coherently, with a detuning between the driving source and the emitter (quasi-resonance fluorescence). We do so in the context of the theories of frequency-resolved photon correlations and homodyning, showing that their combination leads to a neat picture compatible with perturbative two-photon scattering that was popular in the early days of quantum electrodynamics. This should help to control, enhance and open new regimes of multiphoton emission. We also highlight a way to evidence the quantum coherent nature of the process from photoluminescence only, through the observation of a collapse of the symmetry of the lineshape accompanied by a surge of its intensity of emission. We discuss several of our results in the light of recent experimental works.
Abstract We discuss two-photon correlations from the side peaks that are formed when a two-level system emitter is driven coherently, with a detuning between the driving source and the emitter (quasi-resonance fluorescence). We do so in the context of the theories of frequency-resolved photon correlations and homodyning, showing that their combination leads to a neat picture compatible with perturbative two-photon scattering that was popular in the early days of quantum electrodynamics. This should help to control, enhance and open new regimes of multiphoton emission. We also highlight a way to evidence the quantum coherent nature of the process from photoluminescence only, through the observation of a collapse of the symmetry of the lineshape accompanied by a surge of its intensity of emission. We discuss several of our results in the light of recent experimental works.
Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser’s linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter’s single photon saturation or passively scattering light. Here, we propose a unified model that treats all fluorescence photons as spontaneous emission, one at a time, and can explain simultaneously both the spectral and correlation properties of the emission. We theoretically derive the excitation power dependencies, measurable at the single-photon incidence level, of the first-order coherence of the whole resonance fluorescence and super-bunching of the spectrally filtered, followed by experimental confirmation on a semiconductor quantum dot micro-pillar device. Furthermore, our model explains peculiar coincidence bunching observed in phase-dependent two-photon interference experiments. Our work provides an intuitive understanding of coherent light-matter interaction and may stimulate new applications. Standard descriptions of resonance fluorescence consider either absorption and re-emission in a single-photon picture or passive scattering. This study shows that resonance fluorescence can be described by a pure quantum state of the emitter and photon, revealing a coherent superposition that challenges the conventional view of spontaneous emission as an incoherent process.
Abstract Resonance fluorescence—the light emitted when exciting resonantly a two-level system—is a popular quantum source as it seems to inherit its spectral properties from the driving laser and its statistical properties from the two-level system, thus providing a subnatural-linewidth single-photon source (SPS). However, these two qualities do not actually coexist in resonance fluorescence, since an optical target detecting these antibunched photons will either be spectrally broad itself and not benefit from the spectrally narrow source, or match spectrally with the source but in this case the antibunching will be spoiled. We first explain this failure through a decomposition of the field-emission and how this gets affected by frequency resolution. We then show how to restore the sought joint subnatural linewidth and antibunched properties, by interfering the resonance fluorescence output with a coherent beam. We finally discuss how the signal that is eventually generated in this way features a new type of quantum correlations, with a plateau of antibunching which suppresses much more strongly close photon pairs. This introduces a new concept of perfect SPS.
Resonance fluorescence, the process in which a single two-level quantum emitter interacts with a near-resonant coherent light, is a cornerstone of quantum optics as a paradigmatic light-matter interaction. Quantum phenomena in resonance fluorescence have been accounted for in terms of one- and two-photon processes; however, the physical processes arising from the interaction of a single two-level atom with higher photon numbers remain unexplored experimentally. Here we experimentally reveal the multiphoton scattering nature of resonance fluorescence, which intrinsically gives rise to multiphoton entanglement. By accessing quantum fluctuations in the field emitted from a cavity-quantum electrodynamics system, we resolve third- and fourth-order photon scattering events from a single two-level atom. In the three-photon component, the scattered photons are shown to be genuinely energy-time entangled, as verified by a violation of Svetlichny's inequality. As a demonstration of an application, a quantum secret sharing protocol (QSS) is implemented using this entangled photon source. Our study refines the descriptive framework of resonance fluorescence and broadens the scope of its investigation. It also establishes a fundamentally simple route for generating multiphoton entangled states for quantum information processing and quantum metrology.
Multiphoton emitters are a sought-after resource in quantum photonics. Nonlinear interactions between a multilevel atomic system and a coherent drive can lead to resonant two-photon emission, but harvesting light from this process has remained a challenge due to the small oscillator strengths involved. Here we present a study of two-photon resonance fluorescence at microwave frequencies, using a superconducting, ladder-type artificial atom, a transmon, strongly coupled to a waveguide. We drive the two-photon transition between the ground and second-excited states at increasingly high powers and observe a resonance fluorescence peak whose intensity becomes comparable to single-photon emission until it splits into a Mollow-like triplet. We measure photon correlations of frequency-filtered spectral lines and find that while emission at the fundamental frequency stays antibunched, the resonance fluorescence peak at the two-photon transition is superbunched. Our results provide a route towards the realization of multiphoton sources in the microwave domain.
We study a system of two interacting, nonidentical quantum emitters driven by a coherent field. We focus on the particular condition of two-photon resonance and obtain analytical expressions for the stationary density matrix of the system and observables of the fluorescent emission. Importantly, our expressions are valid for the general situation of nonidentical emitters with different transition energies. Our results allow us to determine the regime of parameters in which coherent two-photon excitation, enabled by the coherent coupling between emitters, is dominant over competing, first-order processes. Using the formalism of quantum parameter estimation, we show that the features imprinted by the two-photon dynamics into the spectrum of resonance fluorescence are particularly sensitive to changes in the distance between emitters, making two-photon excitation the optimal driving regime for the estimation of interemitter distance. This can be exploited for applications such as super-resolution imaging of pointlike sources.
Abstract The coherent interaction between a two-level system and electromagnetic fields serves as a foundation for fundamental quantum physics and modern photonic quantum technology. A profound example is resonance fluorescence, where the non-classical photon emission appears in the form of a Mollow-triplet when a two-level system is continuously driven by a resonant laser. Pushing resonance fluorescence from a static to dynamic regime by using short optical pulses generates on-demand emissions of highly coherent single photons. Further increasing the driving strength in the dynamical regime enables the pursuit of exotic non-classical light emission in photon number superposition, photon number entanglement, and photon bundle states. However, the long-sought-after spectrum beyond the Mollow-triplet, a characteristic of dynamic resonance fluorescence under strong driving strength, has not been observed yet. Here we report the direct observation and systematic investigations of dynamic resonance fluorescence spectra beyond the Mollow-triplet in a solid-state cavity quantum electrodynamic system. The dynamic resonance fluorescence spectra with up to five pairs of side peaks, excitation detuning induced spectral asymmetry, and cavity filtering effect are observed and quantitatively modeled by a full quantum model with phonon scattering included. Time-resolved measurements further reveal that the multiple side peaks originate from interference of the emission associated with different temporal positions of the excitation pulses. Our work facilitates the generation of a variety of exotic quantum states of light with dynamic driving of two-level systems.