Gallium arsenide is piezoelectric, so it is possible to generate coupled mechanical and electrical surface acoustic waves (SAWs) by applying a high-frequency voltage to a transducer on the surface of GaAs. By combining SAWs with existing low-dimensional nanostructures one can create a series of dynamic quantum dots corresponding to the minima of the travelling electric wave, and each dot carries a single electron at the SAW velocity (∼ 2800 m/s). These devices may be of use in developing future quantum information processors, and also offer an ideal environment for probing the quantum mechanical behaviour of single electrons. This thesis describes a numerical and theoretical study of the dynamics of an electron in a range of geometries. The numerical techniques for solving the time-dependent Schr ̈dinger equation with an arbitrary time-dependent potential will be described in Chapter 2, and then applied in Chapter 3 to calculate the transmission of an electron through an Aharonov-Bohm (AB) ring. It will be seen that an important property of the techniques used in this thesis is that they can be easily adapted to study realistic geometries, and we will see features in the AB oscillations which do not arise in simplified analytic descriptions. In Chapter 4, we will then study a device consisting of two parallel SAW channels separated by a controllable tunnelling barrier. We will use numerical simulations to investigate the effect of electric and magnetic fields upon the electron dynamics, and develop an analytic model to explain the simulation results. From the model, it will be apparent that it is possible to use this device to rotate the state of the electron to an arbitrary superposition of the first two eigenstates. We then introduce coherent and squeezed states in Chapter 5, which are ex- cited states of the quantum harmonic oscillator. Coherent and squeezed electronic states may be of use in quantum information processing, and could also arise due to unwanted perturbations in a SAW device. We will discuss how these states can be controllably generated in a SAW device, and also discuss how they could then be detected. In Chapter 6 we describe how to use the motion of a SAW to create a rapidly- changing potential in the frame of the electron, leading to a nonadiabatic excita- tion. The nonadiabatically-excited state oscillates from side to side within a 1D channel on a few-picosecond timescale, and this motion can be probed by placing a tunnelling barrier at one side of the channel. Numerical simulations will be performed to show how this motion can be controlled, and the simulation results will be seen to be in good agreement with recent experimental work performed by colleagues. Finally, we will show that this device can be used to measure the initial state of an electron which is an arbitrary superposition of the first two eigenstates.
Gallium arsenide is piezoelectric, so it is possible to generate coupled mechanical and electrical surface acoustic waves (SAWs) by applying a high-frequency voltage to a transducer on the surface of GaAs. By combining SAWs with existing low-dimensional nanostructures one can create a series of dynamic quantum dots corresponding to the minima of the travelling electric wave, and each dot carries a single electron at the SAW velocity (∼ 2800 m/s). These devices may be of use in developing future quantum information processors, and also offer an ideal environment for probing the quantum mechanical behaviour of single electrons. This thesis describes a numerical and theoretical study of the dynamics of an electron in a range of geometries. The numerical techniques for solving the time-dependent Schr ̈dinger equation with an arbitrary time-dependent potential will be described in Chapter 2, and then applied in Chapter 3 to calculate the transmission of an electron through an Aharonov-Bohm (AB) ring. It will be seen that an important property of the techniques used in this thesis is that they can be easily adapted to study realistic geometries, and we will see features in the AB oscillations which do not arise in simplified analytic descriptions. In Chapter 4, we will then study a device consisting of two parallel SAW channels separated by a controllable tunnelling barrier. We will use numerical simulations to investigate the effect of electric and magnetic fields upon the electron dynamics, and develop an analytic model to explain the simulation results. From the model, it will be apparent that it is possible to use this device to rotate the state of the electron to an arbitrary superposition of the first two eigenstates. We then introduce coherent and squeezed states in Chapter 5, which are ex- cited states of the quantum harmonic oscillator. Coherent and squeezed electronic states may be of use in quantum information processing, and could also arise due to unwanted perturbations in a SAW device. We will discuss how these states can be controllably generated in a SAW device, and also discuss how they could then be detected. In Chapter 6 we describe how to use the motion of a SAW to create a rapidly- changing potential in the frame of the electron, leading to a nonadiabatic excita- tion. The nonadiabatically-excited state oscillates from side to side within a 1D channel on a few-picosecond timescale, and this motion can be probed by placing a tunnelling barrier at one side of the channel. Numerical simulations will be performed to show how this motion can be controlled, and the simulation results will be seen to be in good agreement with recent experimental work performed by colleagues. Finally, we will show that this device can be used to measure the initial state of an electron which is an arbitrary superposition of the first two eigenstates.
Observation of coherent single-electron dynamics is severely limited by experimental bandwidth. We present a method to overcome this using moving quantum dots defined by surface acoustic waves. Each dot holds a single electron, and travels through a static potential landscape. When the dot passes abruptly between regions of different confinement, the electron is excited into a superposition of states, and oscillates unitarily from side to side. We detect these oscillations by using a weak, repeated measurement of the current across a tunnel barrier, and find close agreement with simulations.
Over the last few decades there has been strong interest in quantum information systems based on photonic qubits. Devices able to emit pure single-photon states on demand and detect incoming single photons in the system are fundamental building blocks for the realisation of a quantum photonic scheme. Advances in controlling single carriers have made it possible to transport single electrons using surface acoustic waves (SAWs). In a lateral n-i-p junction SAW-driven single-electron transport can be exploited to pump single electrons across the junction and generate single photons. In this work, devices have been developed in which both electrons and holes can be induced in an undoped GaAs/AlGaAs 15nm quantum well by gates to form a lateral n-i-p junction. The junction is confined into a quasi-1D channel laterally by etching and side gates. Light emission is observed when a 1 or 3 GHz SAW drives a current, by pumping electrons over the potential hill in the intrinsic region. SAW-driven electroluminescence has been characterised in the regime where less than one electron is transported per cycle on average. Time-resolved electroluminescence has been used to extract the electron recombination time and to quantify the contributions from electromagnetic crosstalk and the SAW. In a device without significant crosstalk, the degree of second-order coherence, g(2)(0) , shows the signature of antibunching. Hole SAW-pumping has been investigated as an alternative to electron pumping, showing a shorter recombination time. The dynamics has been further investigated with time-resolved and energy measurements at the junction, using a sub-micron positioner with lens assembly to scan along the pumping direction. A key requirement for practical quantum devices is the scalable integration of single-photon sources, detectors and linear optical elements on a common platform. Using the same type of heterostructure, photons are detected by collecting and measuring the photo-generated carriers. An external light source sends photons into the system, and when they are absorbed in the quantum well, they create electron-hole pairs. The photo-generated carriers are separated and transported by a SAW together with an electric field applied by gates on the surface. The carriers are then collected by ohmic contacts, in order to extract information about the number of photons absorbed. Using the same 15nm GaAs/AlGaAs quantum well as for the single- photon source, an acousto-electric photon detector has been realised and characterised. Single-photon sensitivity can be achieved by measuring the photo-generated charges at single-particle level. The possibility to include single-electron and single-hole charge readout using quantum dots and quantum point contacts has been investigated.
Imaging GHz surface acoustic wave modes in electrostricted LaAlO3/SrTiO3 heterostructures
The LaAlO3/SrTiO3 (LAO/STO) interface hosts a gate-tunable superconducting two-dimensional electron gas (2DEG) which can be programmed to create quantum devices such as ballistic electron waveguides and quantum dots. To fully exploit this platform for quantum transport, a key requirement is the ability to shuttle single electrons, electron pairs, and other exotic states between spatially separated devices with precision. Surface acoustic waves (SAWs), which travel along the surface of a solid, offer a powerful route to achieve this through their moving electrical potential that captures and transfers electrons. %acoustoelectric coupling. In particular, SAWs in the GHz regime enable fast, controlled transport of individual quantum particles. Although this approach is well-explored in GaAs-based 2DEG, SAW generation in STO remains largely unexplored due to the lack of intrinsic piezoelectricity at room temperature. Here, we investigate room-temperature SAWs in LAO/STO and observe SAW modes up to 2.2 GHz with very low propagation loss of the order 10−3 dB per wavelength. To directly visualize these modes, we employ Atomic Acoustic Force Microscopy (AAFM), achieving sub-micron resolution imaging of the SAW wave forms, providing insight into the electrostriction-induced SAW generation mechanism. Our measurements indicate a shear horizontal-type mode, which provides the ability to couple to in-plane degrees of freedom for future acoustoelectric and quantum device applications. This work studies the fundamentals of SAW excitation and propagation on STO, a widely used and commercially available substrate, enabling straightforward coupling of SAWs to a broad range of materials that can be grown or transferred onto STO.
Surface acoustic waves (SAWs) are mechanical vibrations that propagate on the surface of solids while dissipating little power, consequently enabling them to propagate freely over long distances. The speed and wavelength of SAWs are reduced a five order of magnitude compared to when light is used as a carrier at gigahertz frequencies. The unique properties of SAWs combined with the possibility to let them interact with artificial atoms, discovered and shown for the very first time in appended Paper I of this thesis, open up for exploration of new regimes of quantum physics. The appended Paper II is a book chapter providing an overview of many of the new areas of research, as well as going into depth of the method and significance of the results ofthe appended Paper I.The essential interaction between artificial atoms and SAWs was further investigated by using Autler-Townes splitting to achieve fast control of the interactions. The appended Paper IV, shows a transmitted field extinction of 80 %, and provides proof of concept for a SAW router in the quantum regime. In addition, due to the artificial atom's highly frequency dependent coupling to SAWs, electromagnetically induced transparency (EIT) could be observed in the appended Paper V. Furthermore, the EIT region was distinguished from the Autler-Townes splitting region by a threshold in the applied power. The results produce parallel findings to quantum optics, but are perhaps best described as part of a different field, quantum acoustics.Among the many possible areas of research emerging as an outcome of this work, a variety of potential quantum experiments would benefit greatly from a higher conversion efficiency between electric signals and SAWs. Due to this, focus was put on improving this conversion efficiency by studying superconducting unidirectional transducers (UDTs), making use of advances in classical SAW devices. The appended Paper III shows that 99.4~\\% of the acoustic power can be focused in a desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, thereby eliminating the largest source of loss of symmetric inter-digital transducers. There is, however, a trade-off between conversion efficiency and bandwidth. This finding allows tailoring of quantum experiments based on SAWs that may pave the way towards measuring quantum sound.
This thesis presents work on a series of devices for the generation of photonic quantum states based on self-assembled InAs quantum dots, which are among the most technologically mature candidates for practical quantum photonic applications due to their high internal quantum efficiency, narrow linewidth, tunability and straightforward integration with photonic and electric components. The primary results presented concern sources of multi-photon entangled states and single-photon sources with high repetition rate, both of which are crucial components for emerging photonic quantum technologies. First, we propose a scheme for the sequential generation of entangled photon chains by resonant scattering of a laser field on a single charged particle in a cavity-enhanced quantum dot. The charge has an associated spin that can determine the time bin of a photon, allowing for information encoding in this degree of freedom. We demonstrate coherent operations on this spin and realize a proof-of-principle experiment of the proposed scheme by showing that the time bin of a single-photon is dependent on the measured state of the trapped spin. The second main avenue of work investigates the effects of a surface acoustic wave, a mechanical displacement wave confined to the surface of a substrate, on the optical properties of quantum dots. In particular, we exploit the dynamic acoustically-induced tuning of the emission energy to modulate the Purcell effect in a pillar microcavity. Under resonant optical excitation we demonstrate the conversion of the continuous wave laser into a pulsed single-photon stream inheriting the acoustic frequency of 1 GHz as the repetition rate. High resolution spectroscopy reveals the presence of narrow sidebands in the emission spectrum, whose relative intensity can be controlled by the acoustic power and laser detuning. Furthermore, we develop a platform for analogous in-plane experiments by transferring GaAs membranes hosting quantum dots onto LiNbO3 substrates and patterning them into whispering gallery mode optical resonators. In addition to Purcell enhancement and acoustic tuning of the emission, the devices exhibit strong localized mechanical resonances. Finally, we perform initial experiments on the effects of a surface acoustic wave on the spin of a charge trapped in a quantum dot. We integrate acoustic transducers with charge-tunable diodes, where the charge state of the dot can be precisely controlled by an applied bias voltage, and demonstrate the frustration of optical spin pumping by the acoustic wave.
One of the most fundamental phenomena occurring in nature is the interaction between single atoms and electromagnetic fields. This is studied in quantum optics to probe the quantum nature of the system. In appended Paper I, we demonstrated that an artificial atom can couple to propagating sound, which is the acoustic analogue of quantum optics. This thesis covers experiments where surface acoustic waves (SAWs) are used in superconducting circuits at gigahertz frequencies with the aim for quantum applications.Since SAWs are mechanical vibrations that propagate on the surface of solids and dissipate little power, they can propagate freely over long distances before and after they interact with the artificial atom. Their five order of magnitude slower speed and smaller wavelength than light at gigahertz frequencies, open up for exploring new regimes of quantum physics. In addition, the coupling between the artificial atom and the SAWs is promising for reaching ultrastrong coupling limits, which is hard in other types of systems. Many of the future applications are discussed in appended Paper II, together with a more detailed description of the theory and fabrication of the devices.Although the possibilities are many, a variety of potential experiments would benefit from higher conversion efficiency between electric signals and SAWs. Therefore, improvements of this conversion are studied in appended Paper III, making use of the many advances in classical SAW devices. More specifically, the conversion of unidirectional transducers (UDTs) on lithium niobate is studied and compared to symmetric interdigital transducers (IDTs). The results show that 99.4 % of the acoustic power can be focused in the desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, eliminating the largest contribution to loss of the IDTs. However, there is a trade-off between conversion efficiency and bandwidth. This knowledge allows us to better tailor potential quantum experiments based on SAWs, possibly towards measuring quantum sound.