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No / We consider two distant mesoscopic SQUID rings, approximated with two-level systems, interacting with two-mode microwaves. The Hamiltonian of the system is used to calculate its time evolution. The cases with microwaves which at t = 0 are in separable states (classically correlated) or entangled states (quantum mechanically correlated) are studied. It is shown that the Josephson currents in the two SQUID rings are also correlated.

Information processors that harness quantum mechanics may be able to outperform their classical counterparts at certain tasks. Quantum information processing (QIP) can utilize the quantum mechanical phenomenon of entanglement to implement quantum algorithms. Endohedral fullerenes, where atoms, ions or clusters are trapped in a carbon cage, are a class of nanomaterials that show great promise as the basis for a solid-state QIP architecture. Some endohedral fullerenes are spin–active, and offer the potential to encode information in their spin-states. This thesis addresses the challenges of how to engineer the components of a scalable QIP architecture based on endohedral fullerenes. It focuses on the synthesis and characterization of molecules which may, in the future, permit the demonstration of entanglement; the optical read-out of quantum states; and the creation of quasi-one-dimensional molecular arrays. Due to its long spin decoherence time, N@C₆₀ is the selected as the basic molecular unit for ‘coupled’ fullerene pairs, molecular systems for which it may be possible to demonstrate entanglement. To this end, isolated fullerene pairs, in the form of spin-bearing fullerene dimers, are created. This begins with the processing of N@C₆₀ at the macroscale and leads towards the synthesis of ¹⁵N@C₆₀-¹⁵N@C₆₀ dimers at the microscale. High throughput processing is introduced as the most efficient technique to obtain high purity N@C₆₀ on a reasonable timescale. A scheme to produce symmetric and asymmetric fullerene dimers is also demonstrated. EPR spectroscopy of the dimers in the solid-state confirms derivatization, whilst permitting the modelling of spin–spin interactions for 'coupled' fullerene pairs. This suggests that the optimum inter–spin separation for which to observe spin–spin coupling in powders is circa 3 nm. Motivated by the properties of the trivalent erbium ion for the optical detection of quantum states, optically–active erbium–doped fullerenes are also investigated. These erbium metallofullerenes are synthesized and isolated as individual isomers. They are characterized by low temperature photoluminescence spectroscopy, emitting in the infra- red at a wavelength of 1.5 μm. The luminescence is markedly different where a C₂ cluster is trapped alongside the erbium ions in the fullerene cage. Er₂C₂@C₈₂ (isomer I) exhibits emission linewidths that are comparable to those observed for Er³⁺ in crystals. Finally, the discovery of a novel praseodymium-doped fullerene is reported. The balance of evidence favours the structure being assigned as Pr₂@C₇₂. This novel endohedral fullerene forms quasi-one-dimensional arrays in carbon nanotubes, which is a useful proof-of-principle of how a scaled fullerene-based architecture may be achieved.

Entanglement is one of the most technologically important quantum phenomena and its con-trolled creation brings us a step closer to the realisation of a quantum computer. Hybrid electron and nuclear spin systems which combine long nuclear decoherence times with the high polarisation and rapid processing times of electron spins are considered reliable candidates for the representation of the fundamental building block of a quantum computer, the qubit. In the literature electron spins quite often play the role of a mediator which can access, manipulate and couple states with long coherence times, beneficial for storing quantum information. Despite the fact that an electron spin can be a useful resource for nuclear spin systems, its permanent presence can be a source of decoherence. The use of transient photoexcited electron spins provide an additional advantage and once the operations which involve the electron spin are completed, the electron spin can decay and not interfere further with the evolution of the system. In this thesis we report magnetic resonance experiments and density functional theory calculations for the demonstration of nuclear - nuclear entanglement using photoexcited triplet states. We study homonuclear and heteronuclear fullerene derivatives and we identify in each case the relevant parameters that can lead to high fidelity entangling operations. The hyperfine interaction in a homonuclear system is the key parameter which determines the degree of entanglement between the nucelar spins according to a recent theoretical proposal. We measure and calculate the hyperfine interaction in homonuclear systems with ¹³C nuclear spins in order to prove the feasibility of this scheme. Further experiments on a fullerene system with two nuclear spins a ³¹P and a ¹H show that entangling operations of high fidelity which involve the demonstration of CNOT gates, are possible within the lifetime of the triplet state.

The ability to address and manipulate individual information carriers in a deterministic, coherent, and scalable manner is a central theme in quantum information processing. Neutral atoms trapped by laser light are amongst the most promising candidates for storing and processing information in a quantum computer or simulator, so a scalable and flexible scheme for their control and manipulation is paramount. This thesis demonstrates a fast and versatile method to address and dynamically control the position (the motional degrees of freedom) of neutral atoms trapped in optical tweezers. The tweezers are generated by using the direct image of a Spatial Light Modulator (SLM) which can control and shape a large number of optical dipole-force traps. Trapped atoms adapt to any change in the potential landscape, such that one can re-arrange and randomly access individual sites within atom-trap arrays. A diffraction limited imaging system is used to map the intensity distribution of the SLM onto a cloud of cold atoms captured and cooled using a Magneto Optical Surface Trap (MOST).

Quantum information processing devices promise to solve some problems more efficiently than their classical counterparts. The source of the speedup is the structure of quantum theory itself. In that sense, the physical units that are the building blocks of such devices are its power. The quest then is to find or manufacture a system that behaves according to quantum theory, and yet is controllable in such a way that the desired algorithms can be implemented. Candidate systems are benchmarked against general criteria to evaluate their success. In this thesis, I advance a particular system and present the progress made towards each of these criteria. The system is a three-qubit 13C solid-state nuclear magnetic resonance (NMR) based quantum processor. I report results concerning system characterization and control, pseudopure state preparation, and quantum error correction. I also report on using the system to test a central question in the foundation of quantum mechanics.

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.

<p>Among the various physical systems considered for scalable quantum information processing (QIP), individually trapped ions or neutral atoms have emerged as promising candidates. Recent experiments using these systems have demonstrated the basic building blocks required for a useful quantum computer. In many of these experiments, precisely tuned lasers control and manipulate the quantum bit (qubit) represented in the electronic energy levels of the ion or atom. Scaling these systems to the necessary number of qubits needed for meaningful calculations, requires the development of scalable optical technology capable of delivering laser resources across an array of ions or atoms. That scalable technology is currently not available.</p><p>In this dissertation, I will report on the development, design, characterization, and implementation of an optical beam steering system utilizing microelectromechanical systems (MEMS) technology. Highly optimized micromirrors enable fast reconfiguration of multiple laser beam paths which can accommodate a range of wavelengths. Employing micromirrors with a broadband metallic coating, our system has the flexibility to simultaneously control multiple beams covering a wide range of wavelengths. </p><p>The reconfiguration of two independent beams at different wavelengths (780 and 635 nm) across a common 5x5 array of target sites is reported along with micromirror switching times as fast as 4 us. The optical design of the system minimizes residual intensity at neighboring sites to less than 40 dB below the peak intensity. Integration of a similar system into a neutral atom QIP experiment is reported where 5 individually trapped atoms are selectively manipulated through single qubit rotations with a single laser source. This demonstration represents the first application of MEMS technology in scalable QIP laser addressing.</p> / Dissertation

Quantum information processing devices promise to solve some problems more efficiently than their classical counterparts. The source of the speedup is the structure of quantum theory itself. In that sense, the physical units that are the building blocks of such devices are its power. The quest then is to find or manufacture a system that behaves according to quantum theory, and yet is controllable in such a way that the desired algorithms can be implemented. Candidate systems are benchmarked against general criteria to evaluate their success. In this thesis, I advance a particular system and present the progress made towards each of these criteria. The system is a three-qubit 13C solid-state nuclear magnetic resonance (NMR) based quantum processor. I report results concerning system characterization and control, pseudopure state preparation, and quantum error correction. I also report on using the system to test a central question in the foundation of quantum mechanics.