Coherence and entanglement are the two key concepts that distinguish quantum mechanics from classical mechanics. Many novel phenomena occuring in the quantum world are due to these two “physical quantities”. They also play essential roles in quantum computation and quantum information. For example, coherence, which says that a quantum mechanical system could be in a superposition state, makes the quantum parallel computing scheme possible; and entanglement, which says that two quantum systems separated in space could be in an intervened state, is the key factor in various quantum teleportation algorithms. We have studied entanglement generation in various systems. We found that with atomic coherence, entanglement could be generated between two thermal fields with arbitrarily high temperatures. We also found that temperature difference instead of the purity of state is essential for the entanglement generation between an atom and a thermal field. We discovered that correlated spontaneous emission lasers (CELs) could be used to generate bright entanglement laser beams. As a special case of CEL systems, we studied entanglement generation in Non-degenerate Optical Parametric Amplifiers (NOPAs). We performed the input-output calculations for a NOPA system and showed that the two output optical beams are still entangled. This justifies our idea that CEL (or NOPA) systems can be used as an ideal entanglement source for various quantum information schemes. From an experimental point of view, we considered the effects of pumping fluctuations on entanglement generation in CEL and NOPA systems. We found that these fluctuations, especially the phase diffusion processes, in the pump laser would greatly reduce the entanglement generated in such systems.

Most applications in quantum information processing make either explicit or implicit use of entanglement. It is thus important to have a good understanding of entanglement and the role it plays in these protocols. However, especially when it comes to multipartite entanglement, there still remain a lot of mysteries. This thesis is devoted to getting a better understanding of multipartite entanglement, and its role in various quantum information protocols. First, we investigate transformations between multipartite entangled states that only use local operations and classical communication (LOCC). We mostly focus on three qubit states in the GHZ class, and derive upper and lower bounds for the successful transformation probability between two states. We then focus on absolutely maximally entangled (AME) states, which are highly entangled multipartite states that have the property that they are maximally entangled for any bipartition. With them as a resource, we develop new parallel teleportation protocols, which can then be used to implement quantum secret sharing (QSS) schemes. We further prove the existence of AME states for any number of parties, if the dimension of the involved quantum systems is chosen appropriately. An equivalence between threshold QSS schemes and AME states shared between an even number of parties is established, and further protocols are designed, such as constructing ramp QSS schemes and open-destination teleportation protocols with AME states as a resource. As a framework to work with AME states, graph states are explored. They allow for efficient bipartite entanglement verification, which makes them a promising candidate for the description of AME states. We show that for all currently known AME states, absolutely maximally entangled graph states can be found, and we were even able to use graph states to find a new AME state for seven three-dimensional systems (qutrits). In addition, the implementation of QSS schemes from AME states can be conveniently described within the graph state formalism. Finally, we use the insight gained from entanglement in QSS schemes to derive necessary and sufficient conditions for quantum erasure channel and quantum error correction codes that satisfy the quantum Singleton bound, as these codes are closely related to ramp QSS schemes. This provides us with a very intuitive approach to codes for the quantum erasure channel, purely based on the entanglement required to protect information against losses by use of the parallel teleportation protocol.

We consider several applications in black-box quantum computation in which untrusted physical quantum devices are connected together to produce an experiment. By examining the outcome statistics of such an experiment, and comparing them against the desired experiment, we may hope to certify that the physical experiment is implementing the desired experiment. This is useful in order to verify that a calculation has been performed correctly, that measurement outcomes are secure, or that the devices are producing the desired state. First, we introduce constructions for a family of simulations, which duplicate the outcome statistics of an experiment but are not exactly the same as the desired experiment. This places limitations on how strict we may be with the requirements we place on the physical devices. We identify many simulations, and consider their implications for quantum foundations as well as security related applications. The most general application of black-box quantum computing is self-testing circuits, in which a generic physical circuit may be tested against a given circuit. Earlier results were restricted to circuits described on a real Hilbert space. We give new proofs for earlier results and begin work extending them to circuits on a complex Hilbert space with a test that verifies complex measurements. For security applications of black-box quantum computing, we consider device independent quantum key distribution (DIQKD). We may consider DIQKD as an extension of QKD (quantum key distribution) in which the model of the physical measurement devices is replaced with an adversarial model. This introduces many technical problems, such as unbounded dimension, but promises increased security since the many complexities hidden by traditional models are implicitly considered. We extend earlier work by proving security with fewer assumptions. Finally, we consider the case of black-box state characterization. Here the emphasis is placed on providing robust results with operationally meaningful measures. The goal is to certify that a black box device is producing high quality maximally entangled pairs of qubits using only untrusted measurements and a single statistic, the CHSH value, defined using correlations of outcomes from the two parts of the system. We present several measures of quality and prove bounds for them.

This thesis describes experimental generation, manipulation and measurement of quantum information using photon pairs emitted in bulk crystals. Multi-photon sources engineered during the course of this thesis have proven to be ideal for original contributions in the field of optical quantum information. In the first part of this dissertation, we study nonlocality, bound entanglement and measurement-based quantum computing using entangled resources produced by our source. First, we produced and characterised three-photon GHZ polarisation states. We then experimentally violate the long-standing Svetlichny's inequality with a value of 4.51, which is greater than the classical bound by 3.6 standard deviations. Our results agree with the predictions of quantum mechanics, rule out nonlocal hidden-variable theories and certify the genuine tripartite entanglement achievable by our source. Second, with four-photon polarisation states, we demonstrate bound entanglement in Smolin states and realize all of their conceptually important characteristics. Our results highlight the difficulties to achieve the critical condition of undistillability without completely losing entanglement. We conclude the first part by simulating, for the first time, valence-bond solid states and use them as a resource for measurement-based quantum computing. Affleck-Kennedy-Lieb-Tasaki states are produced with 87% fidelity and single-qubit quantum logic gates reach an average fidelity of 92% over all input states and rotations. In the second part of this dissertation, we explore controlled waveform manipulation at the single-photon level. Specifically, we shrink the spectral bandwidth of a single photon from 1740 GHz to 43 GHz and demonstrate tunability over a range 70 times that bandwidth. The results are a considerable addition to the field of quantum frequency conversion and have genuine potential for technological applications.

No description available.

Most applications in quantum information processing make either explicit or implicit use of entanglement. It is thus important to have a good understanding of entanglement and the role it plays in these protocols. However, especially when it comes to multipartite entanglement, there still remain a lot of mysteries. This thesis is devoted to getting a better understanding of multipartite entanglement, and its role in various quantum information protocols. First, we investigate transformations between multipartite entangled states that only use local operations and classical communication (LOCC). We mostly focus on three qubit states in the GHZ class, and derive upper and lower bounds for the successful transformation probability between two states. We then focus on absolutely maximally entangled (AME) states, which are highly entangled multipartite states that have the property that they are maximally entangled for any bipartition. With them as a resource, we develop new parallel teleportation protocols, which can then be used to implement quantum secret sharing (QSS) schemes. We further prove the existence of AME states for any number of parties, if the dimension of the involved quantum systems is chosen appropriately. An equivalence between threshold QSS schemes and AME states shared between an even number of parties is established, and further protocols are designed, such as constructing ramp QSS schemes and open-destination teleportation protocols with AME states as a resource. As a framework to work with AME states, graph states are explored. They allow for efficient bipartite entanglement verification, which makes them a promising candidate for the description of AME states. We show that for all currently known AME states, absolutely maximally entangled graph states can be found, and we were even able to use graph states to find a new AME state for seven three-dimensional systems (qutrits). In addition, the implementation of QSS schemes from AME states can be conveniently described within the graph state formalism. Finally, we use the insight gained from entanglement in QSS schemes to derive necessary and sufficient conditions for quantum erasure channel and quantum error correction codes that satisfy the quantum Singleton bound, as these codes are closely related to ramp QSS schemes. This provides us with a very intuitive approach to codes for the quantum erasure channel, purely based on the entanglement required to protect information against losses by use of the parallel teleportation protocol.

In this thesis, I study quantum transport of electron (e.g., current and noise) in quantum dots exploring microscopic processes responsible for spin-relaxation in double quantum dots in Pauli spin blockade regime. This is a regime where current is blocked due to the spin configuration of electrons in the dot. The Pauli spin blockade provides a means for preparation, manipulation and readout in spin qubits. Hence, understanding the underlying mechanism which lifts this blockade is extremely important. First, I have developed a theory of spin-flip cotunneling (higher order tunneling) processes in double quantum dots in the Pauli spin blockade regime. Utilizing this theory, I have calculated the full analytical dependence of the stationary current on applied magnetic fields, gate voltages, and an inter-dot tunnel coupling in Pauli spin blockade. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e.g., hyperfine coupling to nuclear spins or spin-orbit coupling) are weak, including silicon and carbon nanotube or graphene quantum dots. This theory explains recent experiments on carbon nanotubes and silicon double quantum dot. In addition, I propose a new scheme based on the current noise to probe spin relaxation mechanisms in double quantum dot in the Pauli spin blockade regime, where spin-selection rule applies. As a result, I provide a simple closed-form expression which can be used to fit experimental data to extract multiple spin-relaxation rates, even at very low energy splitting. This method allows for the characterization of different aspects of decay process in these systems.

In this thesis, I study quantum transport of electron (e.g., current and noise) in quantum dots exploring microscopic processes responsible for spin-relaxation in double quantum dots in Pauli spin blockade regime. This is a regime where current is blocked due to the spin configuration of electrons in the dot. The Pauli spin blockade provides a means for preparation, manipulation and readout in spin qubits. Hence, understanding the underlying mechanism which lifts this blockade is extremely important. First, I have developed a theory of spin-flip cotunneling (higher order tunneling) processes in double quantum dots in the Pauli spin blockade regime. Utilizing this theory, I have calculated the full analytical dependence of the stationary current on applied magnetic fields, gate voltages, and an inter-dot tunnel coupling in Pauli spin blockade. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e.g., hyperfine coupling to nuclear spins or spin-orbit coupling) are weak, including silicon and carbon nanotube or graphene quantum dots. This theory explains recent experiments on carbon nanotubes and silicon double quantum dot. In addition, I propose a new scheme based on the current noise to probe spin relaxation mechanisms in double quantum dot in the Pauli spin blockade regime, where spin-selection rule applies. As a result, I provide a simple closed-form expression which can be used to fit experimental data to extract multiple spin-relaxation rates, even at very low energy splitting. This method allows for the characterization of different aspects of decay process in these systems.

No description available.

Quantum information processing has the potential of implementing faster algorithms for numerous problems, communicating with more secure channels, and performing higher precision sensing compared to classical methods. Recent experimental technology advancement has brought us a promising future of harnessing such quantum advantage. Yet, quantum engineering entails wise control and strategy under the current noisy intermediate-scale quantum era. Developing robust and efficient approaches to manipulating quantum systems based on constrained and limited resources is imperative. This dissertation focuses on two major topics theoretically. In the first part, this work present how to conceive robust quantum control on matter-based qubits with a geometric approach. We have proposed the method of designing noise robust control pulses suitable for practical devices by combining spatial curves, filter functions, and machine learning. In the second part, this work stresses the topic of photonic multipartite entangled graph states. An improved protocol of generating arbitrary graph states is introduced. We show that one can efficiently find the deterministic photon emission circuit with minimal overhead on the number of quantum emitters. / Doctor of Philosophy / As classical information technology has revolutionized our modern world, theoretically, quantum information technology outperforms its classical counterpart and has the potential to achieve further progress. Utilizing the non-classical features unique to quantum physics, one can build quantum computers capable of accelerating data searching, breaking most current cryptographic systems, simulating molecular-level dynamics, and enhancing artificial intelligence. Furthermore, one can use the entangled quantum resource to establish secure communication or increase the capacity of the classical communication channel. Although numerous applications may reshape our daily life, industry, and scientific research, the mastery of quantum information technology is still challenging since quantum systems are more susceptible to noise than classical systems. Unlike classical signal processing, reading out an unknown quantum state will irreversibly change the state, while copying an unknown quantum state is strictly infeasible. Therefore, detecting and correcting errors from quantum data can be tricky. Depending on different platforms, establishing a complicated quantum network can also be constrained by the near-term noisy device. Mainly, what this work attempts to innovate are the previous results on quantum dynamical control pulse design and the protocol of entangling photons. For the former, the goal of this work is to develop control pulses that can decouple coherent noise in the quantum computer when manipulating the quantum information. This work combines the mathematical framework of spatial curve quantum control with filter functions and machine learning to yield new outcomes. The flexibility of this framework enables us to give neat mathematical analysis and obtain satisfying control pulses design for physical implementations through numerical experiments. For the latter, this work studies a promising scheme of deterministically producing an entangling photonic quantum network, where quantum emitters are treated as the media to build up quantum non-locality. Achieving this emission process in the real world is desirable for distributing entangled quantum resources and realizing measurement-based quantum computation. In this case, we analyze the emitter overhead needed to generate an entangling photonic resource state, specifically when sending photons back for interactions is inaccessible. At last, we propose an efficient algorithm for producing the generation protocol along with several practical examples whose overheads on quantum emitters number are strictly optimized.