Currently, a major challenge for solid-state spin qubit systems is achieving one-qubit operations on a timescale shorter than the spin coherence time, T2*, a goal currently two orders of magnitude away. By taking advantage of the quasi-one-dimensional structure of a nanowire and the strong spin-orbit interaction of InAs, it is estimated that π-rotations can be implemented using electric dipole spin resonance on the order of 10 ns. To this end, a procedure for the fabrication of homogeneous InAs nanowire quantum dot devices is presented herein for future investigations of solid state spin qubits as a test bed for quantum computing. Both single and double quantum dot systems are formed using local gating of InAs nanowires. Single quantum dot systems were characterized through electron transport measurements in a dilution refrigerator; in one case, the charging energy was measured to be 5.0 meV and the orbital energy was measured to be 1.5-3.5 meV. The total capacitance of the single quantum dot system was determined to be approximately 30 aF. An estimate of the quantum dot geometry resulting from confinement suggests that the quantum dot is approximately 115 nm long. The coupling energy of the double quantum dot system was measured to be approximately 4.5 meV. The electron temperature achieved with our circuitry in the dilution refrigerator is estimated to be approximately 125 mK.

Over the last few decades, quantised excitations of the electromagnetic field have proven to be an ideal system with which to investigate and harness quantum optical phenomena. The techniques developed have enabled fundamental tests of quantum mechanics as well as practical applications in quantum metrology and quantum information processing. Advancing to larger-scale entangled quantum systems will open up new regimes of quantum many-body physics, allowing us to probe the limits of quantum mechanics and enabling truly quantum-enhanced technologies. However, moving towards this goal will require further experimental and theoretical innovations. The work described in this thesis focuses on several different aspects of optical quantum information, but are ultimately all linked by this long-term aim. The first part of this thesis describes a novel method for strain-based active control of quantum optical circuits and a new method for the characterisation of high efficiency detectors. Building on this, I discuss in detail two different fields of quantum optics that stand to benefit from these techniques. I initially consider quantum-enhanced metrology, including work aimed towards demonstrating a truly better-than-classical phase measurement, and a theoretical exploration of multiple-phase estimation. Finally, I focus on linear-optical quantum information processing, exploring in detail the use of time-frequency encodings for quantum computing.

Currently, there is interest in the development of molecular-scale devices for use in quantum information processing (QIP). With this application in mind, physical studies on antiferromagnetically coupled molecular wheels [Cr7MF3(Etglu)(O2CtBu)15(phpy)], where M is a divalent metal cation (M = Mn2+, Zn2+, Ni2+) have been pursued. The heterometallic wheels contain an octagon of metal centres, which are bridged by fluoride ions, pivalate groups and a chiral N-ethyl-D-glutamine molecule which is penta-deprotonated and bound to the metal sites through all available O-donors. They are deep purple in colour and they have been named purple-Cr7M. There is antiferromagnetic coupling between adjacent metal centres, J » -8 cm-1, resulting in a non-zero net spin ground state. The spin-Hamiltonian parameters of this family have been determined.At the heterometal site of purple-Cr7M wheels there is a terminal ligand which can be substituted for a variety of N-donor organic ligands. A series of bidentate N-donor linkers has been used to link Cr7Ni wheels (each wheel Seff = 1/2) to create prototype two-qubit systems. Multi-frequency EPR spectroscopy and SQUID magnetometry has been used to extract the spin-Hamiltonian parameters of this family. It has been shown that the single wheels can be linked together electronically as well as chemically. It has been found that for the unsaturated linkers, there is a weaker interaction between Cr7Ni wheels when longer linkers are used. The strength of interaction is smaller for the saturated linkers than for the unsaturated linkers.The formation of 'green'-Cr7M wheels is different, being templated around a cation. Two new types of wheels have been studied: [tBuCONHC6H12NH2C6H12NHCOtBu][Cr7M2+F8(O2CtBu)16] and [Cs?Cr7MF8(O2CtBu)16]·0.5MeCN (where, M = Mn2+, Zn2+, Ni2+), where the former is templated around a long dialkylammonium group and the latter around a caesium cation. The effect of the templating cation on spectroscopic properties has been determined.Physical studies on a family of antiferromagnetically-coupled homometallic clusters have been pursued. They consist of cyclic arrays of homometallic Cr3+ ions in either a octametallic wheel or hexametallic horseshoes. The horseshoes have the general formula: [CrxFx+5L2x-2]n3- (where L = carboxylate). Cr3+ centres are bridged by pivalate groups and fluorides, while Cr3+ centres at the ends of the chain have terminal fluorides completing their coordination sphere. These terminal fluoride groups are labile enough to be substituted, e.g. [EtNH2][Cr6F7(O2CtBu)10(acac)2] is the product of a substitution reaction with acetylacetone.

Electron spin resonance (ESR) experiments are used to assess the possibilities for processing quantum information in the electronic and nuclear spins of endohedral fullerenes. It is shown that ¹⁵N@C₆₀ can be used for universal two-qubit quantum computing. The first step in this scheme is to initialize the nuclear and electron spins that each store one qubit. This was achieved with a magnetic field of 8.6 T at 3 K, by applying resonant RF and microwave radiation. This dynamic nuclear polarization technique made it possible to show that the nuclear T₁ time of ¹⁵N@C₆₀ is on the order of twelve hours at 4.2 K. The electronic T₂ is the limiting decoherence time for the system. At 3.7 K, this can be extended to 215 μs by using amorphous sulphur as the solvent. Pulse sequences are described that could perform all single-qubit gates to the two qubits independently, as well as CNOT gates. After these manipulations, the value of the qubits should be measured. Two techniques are demonstrated for this, by measuring the nuclear spin. Sc@C₈₂ could also be useful for quantum computation. By comparing ESR measurements with density functional theory calculations, it is shown how the orientation of a Sc@C₈₂ molecule in an applied magnetic field affects the molecule's Zeeman and hyperfine coupling. Hence the g- and A-tensors are written in the coordinate frame of the molecule. Pulsed ESR measurements show that the decoherence time at 20 K is 13 μs, which is 20 times longer than had been previously reported. Carbon nanotubes have been filled with endohedral fullerenes, forming 1D arrays that could lead to a scalable quantum computer. N@C₀₆ and Sc@C₈₂ have been used for this filling in various concentrations. ESR measurements of these samples are consistent with simulations of the dipolar coupling.

Quantum information processing (QIP), which inherently utilizes quantum mechanical phenomena to perform information processing, may outperform its classical counterpart at certain tasks. As one of the physical implementations of QIP, the electron-spin based architecture has recently attracted great interests. Endohedral fullerenes with unpaired electrons, such as N@C₆₀, are promising candidates to embody the qubits because of their long spin decoherence time. This thesis addresses several fundamental aspects of the strategy of engineering the N@C₆₀ molecules for applications in QIP. Chemical functionalization of N@C₆₀ is investigated and several different derivatives of N@C₆₀ are synthesized. These N@C₆₀ derivatives exhibit different stability when they are exposed to ambient light in a degassed solution. The cyclopropane derivative of N@C60 shows comparable stability to pristine N@C₆₀, whereas the pyrrolidine derivatives demonstrate much lower stability. To elucidate the effect of the functional groups on the stability, an escape mechanism of the encapsulated nitrogen atom is proposed based on DFT calculations. The escape of nitrogen is facilitated by a 6-membered ring formed in the decomposition of the pyrrolidine derivatives of N@C₆₀. In contrast, the 4-membered ring formed in the cyclopropane derivative of N@C₆₀ prohibits such an escape through the addends. Two N@C₆₀-porphyrin dyads are synthesized. The dyad with free base porphyrin exhibits typical zero-field splitting (ZFS) features due to functionalization in the solid-state electron spin resonance (ESR) spectrum. However, the nitrogen ESR signal in the second dyad of N@C₆₀ and copper porphyrin is completely suppressed at a wide range of sample concentrations. The dipolar coupling between the copper spin and the nitrogen spins is calculated to be 27.0 MHz. To prove the presence of the encapsulated nitrogen atom in the second dyad, demetallation of the copper porphyrin moiety is carried out. The recovery of approximately 82% of the signal intensity confirms that the dipolar coupling suppresses the ESR signal of N@C₆₀. To prepare ordered structure of N@C₆₀, the nematic matrix MBBA is employed to align the pyrrolidine derivatives of N@C₆₀. Orientations of these derivatives are investigated through simulation of their ESR spectra. The derivatives with a –CH3 or phenyl group derived straightforward from the N-substituent of the pyrrolidine ring are preferentially oriented based on their powder-like ESR spectra in the MBBA matrix. An angle of about is also found between the directors of fullerene derivatives and MBBA. In contrast, the derivatives with a –CH₂ group inserted between the phenyl group and the pyrrolidine ring are nearly randomly distributed in MBBA. These results illustrate the applicability of liquid crystal as a matrix to align N@C₆₀ derivatives for QIP applications.

Developments in the interdisciplinary field of quantum information open up previously impossible abilities in the realms of information processing and communication. Quantum entanglement has emerged as one property of quantum systems that acts as a resource for quantum information processing and, in particular, enables teleportation and secure cryptography. Therefore, the creation of entangled resources is of key importance for the application of these technologies. Despite a great deal of research the efficient creation of entanglement over long distances is limited by inevitable noise. This problem can be overcome by creating entanglement between nodes in a network and then performing operations to distribute the entanglement over a long distance. This thesis contributes to the field of entanglement distribution within such quantum networks. Entanglement distribution has been extensively studied for one-dimensional networks resulting in "quantum repeater" protocols. However, little work has been done on higher dimensional networks. In these networks a fundamentally different scaling, called "long distance entanglement distribution", can appear between the resources and the distance separating the systems to be entangled. I reveal protocols that enable long distance entanglement distribution for quantum networks composed of mixed state and give a few limitations to the capabilities of entanglement distribution. To aid in the implementation of all entanglement distribution protocols I finish by introducing a new system, composed of an optical nanofibre coupled to a carbon nanotube, that may enable new forms of photo-detectors and quantum memories.

Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer. In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.

Nanostructures are promising building blocks for quantum technologies due to their reproducible nature and ability to self-assemble into complex structures. However, the need to control these nanostructures represents a key challenge. Hence, this thesis investigates the manipulation and creation of quantum states in certain nanostructures. The results of this thesis can be applied to quantum information processing and to extremely sensitive magnetic-field measurements. In the first research chapter, we propose and examine methods for entangling two (remote) nuclear spins through their mutual coupling to a transient optically excited electron spin. From our calculations we identify the specific molecular properties that permit high entangling power gates for different protocols. In the next research chapter, we investigate another method to create entanglement; this time between two remote electronic spins. This method uses a very sensitive magnetic-field sensor based on a crystal defect that allows the detection of single magnetic moments. The act of sensing the local field constitutes a two-qubit projective measurement. This entangling operation is remarkably robust to imperfections occurring in an experiment. The third research chapter presents an augmented sensor consisting of a nitrogen-vacancy centre for readout and an `amplifier' spin system that directly senses tiny local magnetic fields. Our calculations show that this hybrid structure has the potential to detect magnetic moments with a sensitivity and spatial resolution far beyond that of a sensor based on only a nitrogen-vacancy centre, and indeed this may be the physical limit for sensors of this class. Finally, the last research chapter investigates measurements of magnetic-field strength using an ensemble of spin-active molecules. Here, we describe a quantum strategy that can beat the common standard strategy. We identify the conditions for which this is possible and find that this crucially depends on the decoherence present in the system.

Donors in silicon, which combine an electron and nuclear spin, are some of the most promising candidates for quantum information. The electron spin has been proposed as a register with fast manipulation and the nuclear spin as a memory with long coherence times. However, this division reduces the complexity of the donor system, in particular behaviors emerging from their interaction. In natural silicon, there is also the presence of ²⁹Si nuclear spins in the donor environment; though they are generally seen as a source of decoherence, they are quantum systems that can be investigated too. The main subject of this thesis is the study of the interactions between these various spins, using different methods to probe and control them. I first concentrate on the coupling between the donor and the ²⁹Si spins. This coupling can be perturbed by the application of dynamical decoupling on the donor electron spin, whose evolution can be made sensitive to the number of ²⁹Si spins interacting together. I then propose an error correction scheme using the donor and ²⁹Si spins, showing key requirements such as coherence times and methods for manipulation and initialization. Secondly, I focus on the donor itself, in a regime where the hyperfine and Zeeman couplings compete with each other. Here, the spin transitions can have different sensitivities to the magnetic environment, and can even be suppressed to first order, resulting in coherence times up to seconds with electron spin-like manipulation times. Controlling this sensitivity was also used to probe the effect of the donor on the ²⁹Si spin bath evolution. Finally, I use electric fields to modulate the hyperfine coupling within the donor. I first characterize the spins’ sensitivity to the electric field, and then demonstrate electrical switching of the nuclear spin response to an external magnetic excitation.

Photons have proven to be an effective test-bed for the fundamental concepts and elements of quantum-enhanced technologies. As systems become increasingly complex, however, practical considerations make the traditional approach of bulk optics and free-space propagation progressively more difficult. The major obstacles are the physical space necessary to realise and operate such a complex system, its stability, and maintaining low losses. In order to address these issues, quantum optical technologies can take a cue from their classical counterparts and look towards an integrated architecture to provide miniaturisation, greatly enhanced stability, less alignment, and low loss interfaces between different system components. In this thesis the feasibility of chip-based waveguides as a platform for metrology and information processing will be explored. In Part I, the necessary criteria for a metrology system to out-perform its classical counterpart will be investigated. It will be found that loss is a major barrier to this aim and, critically, that it is unlikely to have been achieved to date by any experiment which consumes resources of a fixed photon number. The issue of loss will be addressed by developing a scalable heralded source of a class of entangled photonic states which are both robust to losses and practically feasible to prepare. A novel tomographic technique will be developed to characterize these states and it will be explicitly demonstrated how it is possible to beat some bounds on classical performance without being able to out-perform a comparable classical system. Finally, a proof of principle demonstration of a waveguide-based interferometer with an integrated phase-shifter will be undertaken. It will be shown that the device preserves quantum interference, making it suitable for use in quantum-enhanced metrology applications. In Part II, integrated optics in the context of information processing will be discussed. First, a novel characterization technique will be developed which enables the behaviour of complex circuits to be predicted. The technique is independent of loss in the device being characterized. A method of simulating these circuits will be outlined that takes advantage of the computational speed-up available from parallelisation and sparse matrix operations. A key increase in complexity for integrated photonic systems will be demonstrated by showing quantum interference of three photons from two separate sources in eight spatial modes. The resulting interference has a visibility which beats all possible classical interference visibilities for similar circuits. Finally, a fully integrated waveguide-coupled photon-number-resolving detector will be developed and demonstrated. This proof of concept demonstration will show good resolution of different photon number events. The device will be modelled and routes to high efficiency operation will be explored.