Chirped Pulses Meet Quantum Dots: Innovations, Challenges, and Future Perspectives
Florian Kappe, Yusuf Karli, Grant R. Wilbur, Ria G. Krämer, Sayan Ghosh, René Schwarz, Moritz Kaiser, Thomas K. Bracht, Doris E. Reiter, Stefan Nolte, Kimberley C. Hall, Gregor Weihs, Vikas Remesh
Abstract Shaped laser pulses have been remarkably effective in investigating various aspects of light–matter interactions spanning a broad range of research. Chirped laser pulses exhibiting a time‐varying frequency, or quadratic spectral phase, form a crucial category in the group of shaped laser pulses. This type of pulses have made a ubiquitous presence from spectroscopic applications to developments in high‐power laser technology, and from nanophotonics to quantum optical communication, ever since their introduction. In the case of quantum technologies recently, substantial efforts are being invested toward achieving a truly scalable architecture. Concurrently, it is important to develop methods to produce robust photon sources. In this context, semiconductor quantum dots hold great potential, due to their exceptional photophysical properties and on‐demand operating nature. Concerning the scalability aspect of semiconductor quantum dots, it is advantageous to develop a simple, yet robust method to generate photon states from it. Chirped pulse excitation has been widely demonstrated as a robust and efficient state preparation scheme in quantum dots, thereby boosting its applicability as a stable photon source in a real‐world scenario. Despite the rapid growth and advancements in laser technologies, the generation and control of chirped laser pulses can be demanding. Here, an overview of a selected few approaches is presented to tailor and characterize chirped pulses for the efficient excitation of a quantum dot source. By taking the chirped‐pulse‐induced adiabatic rapid passage process in quantum dot as an example, numerical design examples are presented along with experimental advantages and challenges in each method and conclude with an outlook on future perspectives.
Abstract Shaped laser pulses have been remarkably effective in investigating various aspects of light–matter interactions spanning a broad range of research. Chirped laser pulses exhibiting a time‐varying frequency, or quadratic spectral phase, form a crucial category in the group of shaped laser pulses. This type of pulses have made a ubiquitous presence from spectroscopic applications to developments in high‐power laser technology, and from nanophotonics to quantum optical communication, ever since their introduction. In the case of quantum technologies recently, substantial efforts are being invested toward achieving a truly scalable architecture. Concurrently, it is important to develop methods to produce robust photon sources. In this context, semiconductor quantum dots hold great potential, due to their exceptional photophysical properties and on‐demand operating nature. Concerning the scalability aspect of semiconductor quantum dots, it is advantageous to develop a simple, yet robust method to generate photon states from it. Chirped pulse excitation has been widely demonstrated as a robust and efficient state preparation scheme in quantum dots, thereby boosting its applicability as a stable photon source in a real‐world scenario. Despite the rapid growth and advancements in laser technologies, the generation and control of chirped laser pulses can be demanding. Here, an overview of a selected few approaches is presented to tailor and characterize chirped pulses for the efficient excitation of a quantum dot source. By taking the chirped‐pulse‐induced adiabatic rapid passage process in quantum dot as an example, numerical design examples are presented along with experimental advantages and challenges in each method and conclude with an outlook on future perspectives.
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.
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.
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.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.