Collective excitation of spatio-spectrally distinct quantum dots enabled by chirped pulses
Florian Kappe, Yusuf Karli, Thomas K. Bracht, Saimon Filipe Covre da Silva, Tim Seidelmann, V. M. Axt, Armando Rastelli, Gregor Weihs, Doris E. Reiter, Vikas Remesh
Abstract Nanoscale bright sources that produce high-purity single photons and high-fidelity entangled photon pairs are the building blocks to realize high security quantum communication devices. To achieve high communication rates, it is desirable to have an ensemble of quantum emitters that can be collectively excited, despite their spectral variability. In case of semiconductor quantum dots, Rabi rotations are the most popular method for resonant excitation. However, these cannot assure a universal, highly efficient excited state preparation, due to the sensitivity to excitation parameters. In contrast, adiabatic rapid passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we show that the robustness of ARP holds true for the simultaneous excitation of the biexciton states in multiple, spatially separated and spectrally different quantum dots. For positive chirps, we also find a regime where the influence of phonons relax the sensitivity to spectral detunings and lower the needed excitation power. Being able to generate high-purity photons from spatially multiplexed quantum dot sources using the biexciton to ground state cascade is a big step towards the implementation of high photon rate, entanglement-based quantum key distribution protocols.
Abstract Nanoscale bright sources that produce high-purity single photons and high-fidelity entangled photon pairs are the building blocks to realize high security quantum communication devices. To achieve high communication rates, it is desirable to have an ensemble of quantum emitters that can be collectively excited, despite their spectral variability. In case of semiconductor quantum dots, Rabi rotations are the most popular method for resonant excitation. However, these cannot assure a universal, highly efficient excited state preparation, due to the sensitivity to excitation parameters. In contrast, adiabatic rapid passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we show that the robustness of ARP holds true for the simultaneous excitation of the biexciton states in multiple, spatially separated and spectrally different quantum dots. For positive chirps, we also find a regime where the influence of phonons relax the sensitivity to spectral detunings and lower the needed excitation power. Being able to generate high-purity photons from spatially multiplexed quantum dot sources using the biexciton to ground state cascade is a big step towards the implementation of high photon rate, entanglement-based quantum key distribution protocols.
For a scalable photonic device producing entangled photons, it is desirable to have multiple quantum emitters in an ensemble that can be collectively excited, despite their spectral variability. For quantum dots, Rabi rotation, the most popular method for resonant excitation, cannot assure a universal, highly efficient excited state preparation, because of its sensitivity to the excitation parameters. In contrast, Adiabatic Rapid Passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we advocate the robustness of ARP for simultaneous excitation of the biexciton states of multiple quantum dots. For positive chirps, we find that there is also regime of phonon advantage that widens the tolerance range of spectral detunings. Using the same laser pulse we demonstrate the simultaneous excitation of energetically and spatially distinct quantum dots. Being able to generate spatially multiplexed entangled photon pairs is a big step towards the scalability of photonic devices.
A scalable source of single photons is a key constituent of an efficient quantum photonic architecture. To realize this, it is beneficial to have an ensemble of quantum emitters that can be collectively excited with high efficiency. Semiconductor quantum dots hold great potential in this context due to their excellent photophysical properties. Spectral variability of quantum dots is commonly regarded as a drawback introduced by the fabrication method. However, this is beneficial to realize a frequency-multiplexed single-photon platform. Chirped pulse excitation, relying on the so-called adiabatic rapid passage, is the most efficient scheme to excite a quantum dot ensemble due to its immunity to individual quantum dot parameters. Yet, the existing methods of generating chirped laser pulses to excite a quantum emitter are bulky, lossy, and mechanically unstable, which severely hampers the prospects of a quantum dot photon source. Here, we present a compact, robust, and high-efficiency alternative for chirped pulse excitation of solid-state quantum emitters. Our simple plug-and-play module consists of chirped fiber Bragg gratings, fabricated via femtosecond inscription, to provide high values of dispersion in the near-infrared spectral range, where the quantum dots emit. We characterize and benchmark the performance of our method via chirped excitation of a GaAs quantum dot, establishing high-fidelity single-photon generation. Our highly versatile chirping module coupled to a photon source is a significant milestone toward realizing practical quantum photonic devices.
To realize a scalable source of frequency-multiplexed single photons, it is important to have an ensemble of quantum emitters that can be collectively excited with high efficiency. Semiconductor quantum dots hold great potential here, due to their excellent photophysical properties. The most efficient scheme to excite a quantum dot ensemble employs chirped laser pulses, relying on the so-called adiabatic rapid passage, is due to its robustness against spectral and intensity fluctuations. Yet, the existing methods to generate chirped laser pulses coupled to a quantum emitter are bulky, lossy, less flexible, and mechanically unstable, which severely hampers the prospects of a practical quantum dot device. Here, we present a compact, robust, and high-efficiency alternative for chirped pulse excitation of solid-state quantum emitters. Our simple plug-and-play module consists of chirped fiber Bragg gratings (CFBGs) that provide high dispersion, tailored to ~ 800 nm with a ~ 5 nm bandwidth that accommodates the quantum dot spectral variability. To characterize and benchmark the performance of our method, we demonstrate the chirped excitation of a GaAs/AlGaAs quantum dot that shows excellent agreement with theoretical simulations and establish high-fidelity single-photon generation. Our method can be tailored for a wide spectral range and dispersion requirements and is a significant milestone toward realizing a direct fiber-coupled, plug-and-play quantum dot photon source.
A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network. Among solid state QE systems, semiconductor quantum dots (QD) are particularly attractive due to their high radiative quantum efficiencies, their strong optical coupling, enabling fast and arbitrary qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. Adiabatic rapid passage (ARP) is a state preparation scheme that enables the excitation and de-excitation of a QE with a high fidelity even in the presence of fluctuations in experimental parameters such as intensity and frequency of the control laser pulse. This excitation scheme may be used for quantum state initialization in quantum information platforms and for triggering quantum light sources. This thesis work concerns an exploration of the efficacy of ARP for the quantum control of excitons in semiconductor QDs. The experiments carried out in this work show that the impact of phonons may be suppressed for ARP, pointing to the possibility of high-fidelity inversion at elevated temperatures. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the QD. The experiments performed in this thesis work also quantify the detuning tolerance of ARP. ARP was also used to demonstrate parallel quantum state initialization in two different QDs with different transition energies and dipole moments.
In this thesis, several optical excitation schemes for single-photon generation in nanowire semiconductor quantum dots were investigated. Fluctuations in the excitation pulse can degrade photon quality and reduce the reliability of the source. These limitations motivated the development of a new excitation scheme called chirped stimulated adiabatic rapid passage. It is designed to be robust against fluctuations in both the energy levels of the quantum system and the laser parameters, including pulse power, spectral detuning, and time delay. Building upon the principles of adiabatic rapid passage, the proposed approach employs an additional chirped stimulation pulse, yielding an enhancement in photon generation by a factor of 1.47 compared to adiabatic rapid passage. These features make the technique promising for the stable generation of single photons, with improved performance under realistic experimental conditions.
Self-assembled semiconductor quantum dots are often referred to as artificial atoms. They are bright single photon sources implemented in an environment easier scalable then implementations with single atoms. These properties promote quantum dots as promising candidates for quantum information technology. A high fidelity state preparation reduces the error rate in quantum information protocols and is hence an essential requirement. \n \nIn this thesis the technique of rapid adiabatic passage is implemented with ultra-fast pulse parameters and is used to study single self-assembled quantum dots by detecting the resonance fluorescence signal following an optical excitation. Different experimental schemes provide insights into different aspects of a quantum dot and its environment. \n \nThe negative trion transition in a single quantum dot represents a good approach to creating an ideal two-level system. With this two-level system, the interaction of the electron trapped in the QD and phonons in the host material is studied with a single chirped pulse. Measuring the resonance fluorescence response of the quantum dot as a function of the pulse area for a set of chirp parameters reveals the electron-phonon interaction. Phonons in the semiconductor environment are a source of decoherence. The non-monotonic behaviour of the coupling to phonons is experimentally demonstrated. Furthermore, a decoupling regime where the electron oscillation is too fast for phonons to follow is reached. A high fidelity state preparation with a vanishing coupling to phonons is demonstrated. Experimental results are affirmed with an excellent agreement with simulations. \n \nIn another scheme, a three-level system consisting of the crystal ground state of the quantum dot, the neutral exciton and the biexciton is investigated. The goal in this experiment is a coherent, resonant and high fidelity preparation of a biexciton state robust against system parameter fluctuations. A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. An excitation with chirped pulses applying the technique of rapid adiabatic passage is the key for the biexciton preparation scheme. In contrast to other state of the art techniques, an interaction with phonons is here intentionally minimized reducing the dephasing and maintaining at the same time a robustness with respect to pulse area and detuning. A fidelity close to one is reached over a pulse area range of more than π. Also in this case, the interpretation of the experiment is confirmed by an excellent agreement with simulations which include a microscopic coupling to phonons. \n \n \nIn a third experiment, a sequence of two chirped pulses with different point spread functions is used to overcome the diffraction limit in spectroscopy. A universal technique - optical nanoscopy via quantum control - to perform diffraction-unlimited spectroscopy on two-level systems is presented. A model for simulating the system is developed and gives a prediction for the expected spatial resolution as a function of the pulse parameters. The concept is demonstrated and the prediction is fulfilled on self-assembled semiconductor quantum dots. A resolution down to 30 nm with an excitation wavelength of 950 nm is reached.