The quantum information science (QIS) is relatively young, but quite broad and emerging field of modern physics. This is an interdisciplinary field where quantum mechanics, computer science, mathematics and computing technologies meet together. Using the laws of quantum mechanics to deal with information opens many new opportunities. However, these opportunities can only be used if one can construct a quantum logic device to implement QIS methods. In the last decade many researchers proposed the variety of physical systems, that can be used as building blocks of the so called quantum computer. But the question of building an effective connection between different blocks remains open. Recently, the idea to build a ’quantum wire’ using chains of permanently coupled spins was studied by a number of authors. The purpose of this thesis is to investigate and develop advanced schemes for using quantum chains as wires. The first part of the thesis shortly describes the concepts of quantum computing and quantum state transfer. We then briefly introduce different schemes to couple two superconducting qubits and analyze more closely one of them. Then we analyze the process of the state transfer for the chain of flux qubits. Next, we propose a new method of improved quantum state transfer which was created as a part of this thesis. Finally, we study the effects and the role of interference in quantum state transfer via spin chains.
The quantum information science (QIS) is relatively young, but quite broad and emerging field of modern physics. This is an interdisciplinary field where quantum mechanics, computer science, mathematics and computing technologies meet together. Using the laws of quantum mechanics to deal with information opens many new opportunities. However, these opportunities can only be used if one can construct a quantum logic device to implement QIS methods. In the last decade many researchers proposed the variety of physical systems, that can be used as building blocks of the so called quantum computer. But the question of building an effective connection between different blocks remains open. Recently, the idea to build a ’quantum wire’ using chains of permanently coupled spins was studied by a number of authors. The purpose of this thesis is to investigate and develop advanced schemes for using quantum chains as wires. The first part of the thesis shortly describes the concepts of quantum computing and quantum state transfer. We then briefly introduce different schemes to couple two superconducting qubits and analyze more closely one of them. Then we analyze the process of the state transfer for the chain of flux qubits. Next, we propose a new method of improved quantum state transfer which was created as a part of this thesis. Finally, we study the effects and the role of interference in quantum state transfer via spin chains.
The concept of a quantum computer was invented in the beginning of the 1980s as a quantum generalization of the reversible classical computer. After discoveries in the middle of the 1990s of quantum algorithms, which wouldsolve some problems considered intractable for classical computers, the field of quantum computing has developed rapidly. A wide range of quantum systems are investigated for their possible ability to implement quantum algorithms in practice. The most promising solid state implementations are superconducting electrical circuits based on the Josephson effect, and the Coulomb blockade effect. Superconducting circuits are macroscopic quantum systems which can be fabricated using standard lithography technologies. Superconducting qubits - basic building blocks of a quantum computer - have been developed at several research laboratories, among others the MC2 here at Chalmers. This thesis presents a theoretical investigation of controllable coupling of superconducting qubits based on the single Cooper-pair box. Two coupling schemes are investigated in detail: current-controlled coupling via large Josephson junctions, and coupling via a superconducting stripline cavity. Both coupling designs are scalable and suitable for the charge regime of the single Cooper-pair box, as well as for the charge-phase regime where the qubit is better protected from environmental noise. Quantum gate protocols which are relevant for these physical couplings are discussed. The investigations show that, for both considered designs, the simplest two-qubit gate is the conditional phase-shift gate. This gate is universal for quantum computing, and it can create maximallyentangled qubit pairs in only one run. The investigation combines methods from electrical engineering, quantum mechanics and quantum computer science.
One of the most essential processes in classical computation is that related to the information manipulation; each component or register of a computer needs to communicate to others by exchanging information encoded in bits and transforming it through logical operations. Hence the theoretical study of methods for information transfer and processing in classical information theory is of fundamental importance for telecommunications and computer science, along with study of errors and robustness of such proposals. When adding the quantum ingredient, there arises a whole new set of paradigms and devices, based on manipulations of \\emph{qubits}, the quantum analogues of conventional data bits. Such systems can show enormous advantage against their classical analogues, but at the same time present a whole new set of technical and conceptual challenges to overcome. The full and detailed understanding of quantum processes and studies of theoretical models and devices therefore provide the first logical steps to the future technological exploitation of these new machines. In this line, this thesis focuses on spin chains as such theoretical models, formed by series of coupled qubits that can be applied to a wide range of physical systems, and its several potential applications as quantum devices. In this work spin chains are presented as reliable devices for quantum communication with high transfer fidelities, entanglement generation and distribution over distant parties and protected storage of quantum information. Methods to design these tools to have some robustness against errors and noise are provided, giving optimism for future quantum technologies.
A quantum computer is an information processing machine, much like an ordinary classical computer, but its function is based on quantum mechanical principles. To be able to construct such a machine would be a fantastic accomplishment---to have total control over a quantum system is a dream for both physicists and science-fiction enthusiasts. The basic information unit in a quantum computer is the quantum bit, or qubit for short. A quantum computer consists of many coupled qubits. To get a single qubit to work properly, would be a major step towards building this machine. Here we study two different qubit ideas. The central element in both setups is the superconducting tunnel junction---the Josephson junction. By connecting the Josephson junctions to standard electronics in a clever way, a qubit can be realised. With these constructions it is in principle very easy to manipulate and read out the quantum probabilities, by varying voltages and currents in time. However, this ease of manipulation has a cost: strong interactions with uncontrolled degrees of freedom of the environment transfer information from the qubit. For superconducting qubits this decoherence is typically very fast. There are ways to deal with the decoherence. One way is to tune the circuit parameters so that the decoherence becomes minimal. Another way is to engineer the qubits so fast so that the effect of decoherence becomes small. In this thesis, we will apply both these strategies. Specifically, the measurement speed of the second qubit we study, turns out to be very sensitive to the topology of the phase space of the detector variables.
This thesis examines some of the more fundamental requirements of a successful quantum computation, namely the ability to transmit quantum information with maximum efficiency, and the creation of entanglement. I focus specifically on neutron entanglement, showing that the spins of two or more distinct neutrons can be measurably entangled by forward-scattering from an isotropic medium. The interpretation of `time' in scattering experiments is also discussed. I present a simple treatment based on the Heisenberg S-matrix, from which it emerges that in certain situations the quantum-mechanical time parameter appearing in the effective time-evolution operator for the spin system has an intuitive physical interpretation. The final part of the thesis deals with quantum information transfer in arrays of permanently coupled dipolar systems. It is shown that spin chains with dipolar couplings offer high fidelity long-distance state transmission, but transfer times in unmodulated chains are unfeasibly long. Possible optimization methods are discussed, concluding with a review of recent achievements in this field.
In the last few years quantum information has played an increasingly central role in the research activities of many scientists within wide ranging areas of physics, mathematics and computer science as it known to be more efficient than its classical counterpart. The impact and advantages of quantum information protocols emerge in numerous situations. In the case of cryptography, quantum dynamics guarantees secure protocols, and in quantum computation, factorization of large numbers, intractable with classical algorithms, can be solved much faster with a quantum computer. It is now widely believed that quantum information will play a leading role in future technologies. Together with the ongoing development of more efficient schemes to solve both new and old tasks in information science, a great deal of interest has been devoted to selecting suitable physical systems where one could realize these ideas. In particular, the quest for large scale integrability has stimulated an increasing interest in the field of solid state physics. Nanoelectronics seems the natural arena to realize physical implementation of quantum hardware. Qubits made out of solid-state devices, such as spins/charges in quantum dots or superconducting nanocircuits, may offer a greater advantage in this respect because fabrication techniques allow for scalability to a large number of coupled qubits. Recent experimental breakthroughs in semiconducting and superconducting nanostructures constitute the first important steps towards the realization of a solid-state quantum computer. In addition the interest in solid state quantum computation has stimulated a large body of research aimed at understanding the properties of entanglement in solid state systems. This Focus issue of New Journal of Physics gathers together contributions from leading experts in the area of solid state quantum information with the aim of providing a panorama of the most exciting scientific questions currently being investigated in the field. Focus on Solid State Quantum Information Contents Quasiparticle entanglement: redfinition of the vacuum and reduced density matrix approach P Samuelsson, E Sukhorukov and M Buttiker Pseudospin quantum computation in semiconductor nanostructures V W Scarola, K Park and S Das Sarma Superconducting qubit network with controllable nearest-neighbour coupling M Wallquist, J Lantz, V S Shumeiko and G Wendin Weak coupling Josephson junction as a current probe: effect of dissipation on escape dynamics J M Kivioja, T E Nieminen, J Claudon, O Buisson, F Hekking and J P Pekola The single Cooper-pair box as a charge qubit K Bladh, T Duty, D Gunnarsson and P Delsing Quantum state transfer in arrays of flux qubits A O Lyakhov and C Bruder Spin filling of a quantum dot derived from excited-state spectroscopy L H Willems van Beveren, R Hanson, I T Vink, F H L Koppens, L P Kouwenhoven and L M K Vandersypen Clauser–Horne inequality for the full counting statistics F Taddei, R Fazio and E Prada Recent advances in exciton based quantum information processing in quantum dot nanostructures H J Krenner, S Stufler, M Sabathil, E C Clark, P Ester, M Bichler, G Abstreiter and J J Finley Spatially highly resolved study of AFM scanning tip-quantum dot local interaction S Kicin, A Pioda, T Ihn, M Sigrist, A Fuhrer, K Ensslin, M Reinwald and W Wegscheider Transfer of entanglement from electrons to photons by optical selection rules M Titov, B Trauzette, B Michealis and C W J Beenakker Non-Abelian Chern-Simons models with discrete gauge groups on a lattice B Doucot and L B Ioffe Landau–Zener transitions in qubits controlled by electromagnetic fields Martijn Wubs, Keiji Saito, Sigmund Kohler, Yosuke Kayanuma and Peter Hänggi Phase-slip flux qubits J E Mooij and C J P M Harmans Mediated tunable coupling of flux qubits Alec Maassen van den Brink, A J Berkley and M Yalowsky Divergent beams of nonlocally entangled electrons emitted from hybrid normal-superconducting structures Elsa Prada and Fernando Sols Decoherence from ensembles of two-level fluctuators Josef Schriefl, Yuriy Makhlin, Alexander Shnirman and Gerd Schön Semiconductor quantum dots for electron spin qubits W G van der Wiel, M Stopa, T Kodera, T Hatano and S Tarucha Rosario Fazio, NEST-INFM and Scuola Normale Superiore, Pisa, Italy
The faithful exchange of quantum information will soon become one of the challenges of the emerging quantum information technology. One of the possible solutions is to transfer a superposition through a chain of properly coupled spins. Such a system is called a quantum wire. We discuss the transfer in a quantum wire \cite{christ,niko1,niko2}, when the process of thermalization of the state takes place together with the free evolution. We investigate which encoding scheme is more faithful in certain thermal conditions.