Resonance fluorescence of two-level quantum systems has emerged as a powerful tool in quantum information processing. Extension of this approach to higher-level systems provides new opportunities for quantum optics applications. Here we introduce an all-optical tuning functionality into a well-established resonance fluorescence coherent driving scheme. We accomplish this by resonant excitation of a three-level ladder system with two laser fields utilizing Autler-Townes and ac Stark effects. We propose theoretically and demonstrate experimentally the feasibility of this approach toward all-optical spectral tuning of quantum-dot-based single-photon sources and investigate photon indistinguishability and purity levels. Our tuning technique allows for fast optical control of the quantum emitter spectrum which paves the way toward temporal and spectral shaping of the single photons, formation of topological Floquet states, or generation of high-dimensional frequency-encoded quantum states of light.
Resonance fluorescence of two-level quantum systems has emerged as a powerful tool in quantum information processing. Extension of this approach to higher-level systems provides new opportunities for quantum optics applications. Here we introduce an all-optical tuning functionality into a well-established resonance fluorescence coherent driving scheme. We accomplish this by resonant excitation of a three-level ladder system with two laser fields utilizing Autler-Townes and ac Stark effects. We propose theoretically and demonstrate experimentally the feasibility of this approach toward all-optical spectral tuning of quantum-dot-based single-photon sources and investigate photon indistinguishability and purity levels. Our tuning technique allows for fast optical control of the quantum emitter spectrum which paves the way toward temporal and spectral shaping of the single photons, formation of topological Floquet states, or generation of high-dimensional frequency-encoded quantum states of light.
We describe how a coherent optical drive that is near resonant with the upper rungs of a three-level ladder system, in conjunction with a short-pulse excitation, can be used to provide a frequency-tunable source of on-demand single photons. Using an intuitive master equation model, we identify two distinct regimes of device operation: (i) for a resonant drive, the source operates using the Autler-Townes effect, and (ii) for an off-resonant drive, the source exploits the ac Stark effect. The former regime allows for a large frequency-tuning range but coherence suffers from timing jitter effects, while the latter allows for high indistinguishability and efficiency, but with a restricted tuning bandwidth due to high required drive strengths and detunings. We show how both these negative effects can be mitigated by using an optical cavity to increase the collection rate of the desired photons. We apply our general theory to semiconductor quantum dots, which have proven to be excellent single-photon sources, and find that scattering of acoustic phonons leads to excitation-induced dephasing and increased population of the higher-energy level which limits the bandwidth of frequency tuning achievable while retaining high indistinguishability. Despite this, for realistic cavity and quantum dot parameters, indistinguishabilities of over 90 are achievable for energy shifts of up to hundreds of $\ensuremath{\mu}\mathrm{eV}$, and near-unity indistinguishabilities for energy shifts up to tens of $\ensuremath{\mu}\mathrm{eV}$. Additionally, we clarify the often-overlooked differences between an idealized Hong-Ou-Mandel two-photon interference experiment and its usual implementation with an unbalanced Mach-Zehnder interferometer, pointing out the subtle differences in the single-photon visibility associated with these different setups.
Single semiconductor quantum dots, which are considered as two-level systems in the artificial atom model, are promising structures for the realization of integrated devices such as single photon sources for quantum information applications. However, contrary to genuine atoms, they are condensed matter systems which suffer from the coupling to their solid environment, leading to a degradation of the coherence of the emitted photons. One possibility to avoid this coupling, thereby improving the coherence of the photons until the one imposed by the radiative limit T2 = 2T1, is to perform strictly resonant excitation of a quantum dot at low temperature. Using an experimental setup that spatially decouples the excitation and detection paths, we showed that a fine control of the electrostatic environment related to the nanostructures residual doping remains a major challenge to answer the widely encountered problem of the inhibition of the resonance fluorescence of a quantum dot. Meanwhile, thanks to quantum optics experiments and high resolution optical spectroscopy, we demonstrated that the resonant Rayleigh scattering regime where quantum dots emit single photons with the laser coherence time, paves the way to the development of "ultra-coherent" sources of single photons with high degrees of indistinguishability. In this regime where the laser tailors the coherence of the photons and where the radiative limit becomes a secondary requirement, two photons emitted successively by the same quantum dot are indistinguishable on time scales never obtained before for a solid nano-emitter (up to ten nanoseconds, comparing to the carriers lifetime and decoherence time of about a hundred of picoseconds).
Quantum optics provides a fundamental framework for understanding the interaction between light and matter at the quantum level. Recently, it has been shown that under incoherent pumping, the resonance fluorescence spectrum dramatically changes. Engineering the resonance fluorescence spectrum paves the way towards solid-state-based single-photon sources. In this paper, we start by reviewing and reproducing some of the results concerning the resonance fluorescence spectrum, single-photon sources, dressed-state lasers, and luminescence spectrum of a quantum dot in a microcavity. Photon correlations in quantum optical systems and spectral properties of radiation emitted by atomic and semiconductor systems interacting with external fields are investigated. The well known Mollow triplet structure of the emission spectrum is discussed, together with the role of dressed states in explaining the origin of the three spectral peaks. Furthermore, the luminescence spectra of quantum emitters coupled to microcavities are reviewed. The numerical results presented here contribute to the theoretical understanding of resonance fluorescence, photon correlations, and engineered emission in quantum optical systems. These studies highlight the rich physical properties arising from light matter interaction at the quantum level and demonstrate their relevance for emerging quantum technologies.
Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter re-excitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity-enhancement of biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter re-excitation and naturally introduces spectral separation of excitation laser and emitted single photon. Supported by first experimental results, we demonstrate theoretically that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently g(2)(0), and HOM visibility V, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot based deterministic single-photon sources.
Next-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments.
Optimal quantum control (OQC) enabled by femtosecond laser pulse shaping techniques provides a highly flexible approach to quantum states manipulation in solid-state qubit systems. Here we apply OQC to optimize the form of the triggering laser pulse for a single photon source. The use of a frequency-swept laser pulse possessing a notch resonant with the transition energy of the quantum emitter enables the spectral isolation of the emitted photon stream in conjunction with resonant driving for good indistinguishability. We show that the robustness of this scheme would enable spectral multiplexing of quantum light sources using a single triggering laser pulse.