Indium gallium arsenide self-assembled quantum dots have attracted a lot of attention due to their ability to trap single electrons and holes whose spin can be manipulated optically. This makes them attractive as qubits and light sources in various quantum computing and communication schemes. However, the spin of electrons and holes rapidly decoheres due to hyper-fine interaction with the atomic nuclei of the dot. The theme of this thesis is to find ways of overcoming this decoherence, in particular to allow generation of photonic cluster states from quantum dots. This was first approached by designing theoretical schemes to measure and compensate for the source of the decoherence, which were experimentally tested. Two new systems were then theoretically designed where the effects of decoherence could be mitigated. It is shown theoretically that exciting a quantum dot with a laser of well-defined polarisation and monitoring the polarisation of emitted photons, it is possible to determine the vector polarisation of the nuclear spin ensemble. It is shown through simulation that this measurement can be performed on and possibly faster than the time-scale of nuclear fluctuations. The fundamental concept behind the measurement procedure is proved in an experiment using coupled quantum dots. Through the course of the experiment anomalous behaviour of the dots was discovered. A second theoretical proposal is made for a system allowing the fast application of an effective field to compensate for the decoherence mechanism. It is then shown by simulation that a coupled dot system with a prepared in-plane nuclear spin polarisation, can allow optical spin rotation and entanglement generation. A different system is then theoretically proposed where the electron spin in quantum dot can be replaced with another qubit, such as embedded manganese atoms. It is shown through simulation that this system also allows the generation of photonic cluster states.
Indium gallium arsenide self-assembled quantum dots have attracted a lot of attention due to their ability to trap single electrons and holes whose spin can be manipulated optically. This makes them attractive as qubits and light sources in various quantum computing and communication schemes. However, the spin of electrons and holes rapidly decoheres due to hyper-fine interaction with the atomic nuclei of the dot. The theme of this thesis is to find ways of overcoming this decoherence, in particular to allow generation of photonic cluster states from quantum dots. This was first approached by designing theoretical schemes to measure and compensate for the source of the decoherence, which were experimentally tested. Two new systems were then theoretically designed where the effects of decoherence could be mitigated. It is shown theoretically that exciting a quantum dot with a laser of well-defined polarisation and monitoring the polarisation of emitted photons, it is possible to determine the vector polarisation of the nuclear spin ensemble. It is shown through simulation that this measurement can be performed on and possibly faster than the time-scale of nuclear fluctuations. The fundamental concept behind the measurement procedure is proved in an experiment using coupled quantum dots. Through the course of the experiment anomalous behaviour of the dots was discovered. A second theoretical proposal is made for a system allowing the fast application of an effective field to compensate for the decoherence mechanism. It is then shown by simulation that a coupled dot system with a prepared in-plane nuclear spin polarisation, can allow optical spin rotation and entanglement generation. A different system is then theoretically proposed where the electron spin in quantum dot can be replaced with another qubit, such as embedded manganese atoms. It is shown through simulation that this system also allows the generation of photonic cluster states.
In this Thesis, we study open quantum spin systems from di↵erent perspectives.The first part is motivated by technological challenges of quantum computation.An important building block for quantum computation and quantum communication networks is an interface between material qubits for storage and data processing and travelling photonic qubits for communication.We propose the realisation of a quantum interface between a travelling-wave light field and the nuclear spins in a quantum dot strongly coupled to a cavity.Our scheme is robust against cavity decay as it uses the decay of the cavity to achieve the coupling between nuclear spins and the travelling-wave light fields.A prerequiste for such a quantum interface is a highly polarized ensemble of nuclear spins.High polarization of the nuclear spin ensemble is moreover highly desirable as it protects the potential electron spin qubit from decoherence.Here we present the theoretical description of an experiment in which highly asymmetric dynamic nuclear spin pumping is observed in a single self-assembled InGaAs quantum dot.The second part of this Thesis is devoted to fundamental studies of dissipative spin systems.We study general one-dimensional spin chains under dissipation and propose a scheme to realize a quantum spin system using ultracold atoms in an optical lattice in which both coherent interaction and dissipation can be engineered and controlled.This system enables the study of non-equilibrium and steady state physics of open and driven spin systems.We find, that the steady state expectation values of di↵erent spin models exhibit discontinuous behaviour at degeneracy points of the Hamiltonian in the limit of weak dissipation.This e↵ect can be used to dissipatively probe the spectrum of the Hamiltonian.We moreover study spin models under the aspect of state preparation and show that dissipation drives certain spin models into highly entangled state.Finally,
Self-assembled quantum dots can act as an interface between single spins and photons. They combine the flexibility and scalability of semiconductor systems with near-perfect two level systems, emitting highly coherent single photons. In potential quantum information processing architectures based upon optical networks, quantum dots permit light-matter coupling between matter qubits and photonic links. Single trapped charge carriers in quantum dots can be addressed optically, generating spin-photon entanglement between the matter qubits and \lq flying' photon qubits, a key first step in creation of a network. Such an optically linked network of spins in quantum dots has yet to be realized. The semiconductor environment presents a key challenge; it broadens the spectrum of the emitted light, which reduces the fidelity of schemes based upon photon exchange or interference. In addition, these quantum dots are subject to hyperfine coupling to a bath of > 105 nuclear spins. While this presents an opportunity to study the physics of a central spin in a spin bath, it degrades the spin coherence of a trapped electron or hole. The suggested gate operations to generate entanglement, and the spin measurements to verify a created state, rely upon photon detection. Thus a second limitation occurs due to low efficiency of detectors and photon collection probability. Recent developments in detector technology have enabled us to bring generation and verification of entangled states with quantum dots into the realm of possibility. This dissertation begins with a detailed study of the interaction of a single electron spin with the solid state environment of the quantum dot, analysing the intensity noise of scattered light under resonant excitation to uncover the timescales and relative amplitudes of identified noise sources. A technique for rapid characterization of samples is presented, focusing in particular upon comparison of spectral broadening of the optical transitions relevant to a spin-photon interface. Having identified the extent of environmental factors acting upon the quantum dots, we turn to a demonstration of photon-mediated entanglement between two distant electron spins using a probabilistic scheme. We are able to generate entangled states at 7 kHz, the fastest yet reported for photon-mediated protocols. The average state fidelity of the created Bell states is 63.5 ± 3.0 \%., violating the classical limit by over 4 standard deviations of the mean. In addition, we demonstrate full optical phase tuning of the Bell state ∣Ψ=21(∣↑∣↓+eiϕ∣↓∣↑). This is a proof of principle experiment, and sets a fidelity benchmark upon which gains can be made by mitigation of environmental dynamics.
Electron spins confined in quantum dots (QDs) are among the leading contenders for implementing quantum information processing. In this Thesis we address two of the most significant technological challenges towards developing a scalable quantum information processor based on spins in quantum dots: (i) decoherence of the electronic spin qubit due to the surrounding nuclear spin bath, and (ii) long-range spin-spin coupling between remote qubits. To this end, we develop novel strategies that turn the unavoidable coupling to the solid-state environment (in particular, nuclear spins and phonons) into a valuable asset rather than a liability. In the first part of this Thesis, we investigate electron transport through single and double QDs, with the aim of harnessing the (dissipative) coupling to the electronic degrees of freedom for the creation of coherence in both the transient and steady-state behaviour of the ambient nuclear spins. First, we theoretically show that intriguing features of coherent many-body physics can be observed in electron transport through a single QD. To this end, we first develop a master-equation-based formalism for electron transport in the Coulomb-blockade regime assisted by hyperfine (HF) interaction with the nuclear spin ensemble in the QD. This general tool is then used to study the leakage current through a single QD in a transport setting. When starting from an initially uncorrelated, highly polarized state, the nuclear system experiences a strong correlation buildup, due to the collective nature of the coupling to the central electron spin. We demonstrate that this results in a sudden intensity burst in the electronic tunneling current emitted from the QD system, which exceeds the maximal current of a corresponding classical system by several orders of magnitude. This gives rise to the new paradigm of electronic superradiance. Second, building upon the insight that the nuclear spin dynamics are governed by collective interactions giving rise to coherent effects such as superradiance, we propose a scheme for the deterministic generation of steady-state entanglement between the two nuclear spin ensembles in an electrically defined double quantum dot. Because of quantum interference in the collective coupling to the electronic degrees of freedom, the nuclear system is actively driven into a two-mode squeezedlike target state. The entanglement buildup is accompanied by a self-polarization of the nuclear spins towards large Overhauser field gradients. Moreover, the feedback between the electronic and nuclear dynamics is shown to lead to intriguing effects such as multistability and criticality in the steady-state solutions. In the second part of this Thesis, our focus turns towards the realization of long-range spin-spin coupling between remote qubits. We propose a universal, on-chip quantum transducer based on surface acoustic waves in piezo-active materials. Because of the intrinsic piezoelectric (and/or magnetostrictive) properties of the material, our approach provides a universal platform capable of coherently linking a broad array of qubits, including quantum dots, trapped ions, nitrogen-vacancy centers or superconducting qubits. The quantized modes of surface acoustic waves lie in the gigahertz range, can be strongly confined close to the surface in phononic cavities and guided in acoustic waveguides. We show that this type of surface acoustic excitations can be utilized efficiently as a quantum bus, serving as an on-chip, mechanical cavity-QED equivalent of microwave photons and enabling long-range coupling of a wide range of qubits. In summary, this thesis provides contributions towards developing a scalable quantum information processor based on spins in quantum dots in two different aspects. The first part is dedicated to a deeper understanding of the nuclear spin dynamics in quantum dots. In the second part we put forward a novel sound-based strategy to realize long-range spin-spin coupling between remote qubits. This completes a broad picture of spin-based quantum information processing which integrates different perspectives, ranging from the single-qubit level to a broader quantum network level.
Electron spins confined to self-assembled quantum dots are considered as nodes for a coherent optical network capable of supporting distributed quantum states. Through a series of experiments, the work contributing to this dissertation examines some of the key criteria for constructing such a network. First, the ability to optically extract a coherent spin state from the quantum dot without perturbing the nuclear environment is explored: nuclear feedback is an issue that has frustrated previous studies into electron spin coherence in these systems. With the novel techniques we develop, we identify and characterise the previously undetermined intrinsic mechanisms that govern the coherence of the central spin. We show how the coherence of the electron spin is intimately related to the growth of these strained nanostructures. Second, a model network is constructed in which two spins confined to separate quantum dots are projected into a highly entangled state. This is the first time electron spins in distant quantum dots have been entangled, and in doing so we demonstrate controllable entanglement generation at the highest rates recorded for optically accessible qubit definitions. We investigate the realisation of a hybrid quantum network by demonstrating the first interconnect between wholly different single quantum systems: a semiconductor quantum dot and a trapped ytterbium ion. In forming an optical link between these two complementary qubit definitions, we show that we can circumvent their intrinsic optical differences through coherent photon generation at the quantum dot. A network built from these diverse constituents could combine the ultrafast operations self-assembled quantum dots enable with the long coherence times states in trapped ions experience. Finally, in a step towards truly scalable entanglement generation between quantum dot spins, we design minimally invasive structures that will funnel large proportions of the optical dipole field from the optically dense material that surrounds the quantum dot. The techniques developed in this work and the knowledge gained from their operation should enable the demonstration the creation of high-order nonlocal states between quantum dot spins, single photons and trapped ions, as well as the development of new optically active systems that will benefit from enhanced spin coherence.
Semiconductor quantum dots can be used as sources of entangled single photons, which constitute a crucial resource for optical quantum computing. We present theoretical research on entanglement verification and nuclear spin physics, leading to results that are relevant to both experimental work and the theory of quantum optics and mesoscopic quantum systems. Optical quantum computing requires large entangled photonic states, yet characterizing even few-photon states is a challenge in current experiments due to low photon detection efficiencies. We present a lower bound on a measure of computational usefulness of a potentially large quantum state that requires only measured values of three-photon correlations. Hence this bound provides a simple and applicable benchmarking method for quantum dot experiments. We then turn to the critical issue of the interaction between electron and nuclear spins in quantum dots. This interaction gives rise to decoherence that stands in the way of generating entangled photons as well as nuclear phenomena that might help to overcome this challenge. We formulate a quantum mechanical model of the nuclear spin system in a quantum dot driven by continuous-wave laser light. Based on the analytical steady state solution of this model we predict a novel nuclear spin effect, giving rise to nuclear spin polarization that counteracts the effect of an external magnetic field. Beyond the decoherence problem nuclear spins give rise to randomly time-varying transition energies. A quantum mechanical model of this noise as well as the effect of photon scattering is developed, leading to the insight that optical driving can continuously probe the electron transition energy and thereby prevent it from changing.
Spins in a solid-state environment and photons are both promising physical systems regarding the implementation of qubits. Whereas electron and nuclear spins act very well as localized qubits that are able to store and process quantum information, photons are the primary candidates to transmit quantum information between distant locations. Entanglement, on the other hand, is a fundamental resource of quantum information processing enabling, e.g., secure communication over long distances. The generation and manipulation of entanglement are thus fundamental steps in order to leverage the benefits of quantum information processing. The aim of this dissertation is therefore to theoretically investigate entangling mechanisms for various physical systems. A functioning source of entangled photons is a basic hardware component required for quantum communication. To this end, we study a system composed of semiconductor quantum wells inside a microcavity, known as intersubband cavity system, in which the interaction of the electronic system with the cavity field can reach the ultrastrong coupling regime. The ground state contains a finite number of photons, and we find that these photons are entangled. The amount of entanglement is quantified analytically, and maximal entanglement is found to be possible. The technique of entanglement purification allows to restore maximal entanglement that has decreased, e.g. due to decoherence. Purification requires quantum memory, a role for which electron spins in electrically-defined quantum dots are well suited. However, existing purification protocols are rather unpractical in this case. Here, the concept of asymmetric bilateral two-qubit operations is introduced to purify spin entanglement by harnessing the typical interaction between electrons in neighboring quantum dots. As it turns out, this concept can be applied to a variety of qubit systems, e.g. superconducting qubits or spins in nitrogen-vacancy centers in diamond. The latter example offers several possibilities to implement a qubit, of which the intrinsic nitro- gen nuclear spin has proven its viability in many ways. We develop a scheme to deterministically couple two nuclear spin qubits, in which the interaction is mediated by an optical cavity. It is found that an entangling two-qubit gate, also required for universal quantum computation, can be implemented with operation times below 100 nanoseconds, i.e. several orders of magnitude faster compared to the decoherence time of the nuclear spin. The verification of entanglement, which is required e.g. in entanglement-based quantum com- munication to detect an eavesdropping attack, typically involves a measurement of qubit states. Using the input-output formalism, we derive a fully quantum-mechanical model of an optical readout scheme to measure the spin state of an electron in a self-assembled quantum dot.