It is demonstrated that the universality of any quantum computing element can be understood and verified via a precise mathematical criterion which tests for the controllability of an associated quantum control system. This relation between quantum computing and quantum control is deeper in that tools of coherent control of quantum dynamics may be used to arrive at specific designs for quantum computing devices. \textcopyright{} 1996 The American Physical Society.
It is demonstrated that the universality of any quantum computing element can be understood and verified via a precise mathematical criterion which tests for the controllability of an associated quantum control system. This relation between quantum computing and quantum control is deeper in that tools of coherent control of quantum dynamics may be used to arrive at specific designs for quantum computing devices. \textcopyright{} 1996 The American Physical Society.
This paper investigates the intersection of quantum chaos and quantum computing control. It explores the potential influence of chaotic behavior within quantum systems on computational processes and proposes control strategies to mitigate these effects. The core argument centers on the recognition of quantum chaos as a crucial, yet often overlooked, element in the precise control required for reliable quantum computation. Through the application of chaos theory, this work analyzes the manifestation of chaotic dynamics in quantum systems, detailing the implications for qubit stability and gate fidelity. Specific control techniques, including feedback mechanisms and tailored Hamiltonian designs, are examined and discussed, ultimately aiming to harness, rather than suppress, chaotic behavior to potentially enhance certain quantum algorithms. The research contributes to a deeper understanding of the complex relationship between quantum systems and chaos, providing a foundation for the development of more robust and adaptable quantum control protocols. The study utilizes mathematical formalism to rigorously define and analyze the concepts explored, focusing on key equations governing quantum dynamics and chaotic systems.
A quantum computer is similar to an ordinary computer, except that instead of bits it uses two-level quantum systems (qubits), and the usual logical operations on bits are replaced with noiseless quantum dynamics in the form of unitary gates or Hamiltonian interactions. Many sets of resources that are universal for quantum computation (that is, capable of implementing any quantum computation) have been found, but general conditions for universality are still only partially understood. Improving our understanding of these conditions contributes to two broad goals. On the one hand, understanding what class of physical systems is suitable for quantum computation allows us choose the systems that are most easily controlled or built in the laboratory. On the other hand, identifying the unifying characteristics of such a class provides insight into the fundamental physics responsible for the much greater computational power of quantum computers. The original results presented in this thesis are a contribution to our understanding of the physical requirements for universal quantum computation. These results lead to the identification of a new, large class of universal dynamics. More specifically, the main problem that is addressed is the following: consider a physical system that is made up of a finite number of qubits that are subject to a natural, fixed interaction described by a two-body Hamiltonian. Suppose we have the ability to control the system only by applying fast one-qubit operations. Under what conditions is this system capable of universal quantum computation? The first original result of the thesis is to prove that universal quantum computation is possible under these conditions given the single extra constraint that the Hamiltonian must entangle, or connect, all of the qubits. In other words, since a two-body Hamiltonian is expressible as a sum of coupling terms each of which acts nontrivially on a pair of qubits, consider the graph whose vertices correspond to qubits and whose edges connect qubits that are coupled by a term in the Hamiltonian. Then this Hamiltonian, together with fast one-qubit unitaries, is universal for quantum computation precisely when the corresponding graph is connected. This result suggests many potential generalizations, three of which are explored in this thesis. The first generalization is to show that the same result holds for a two-body Hamiltonian that acts on D-dimensional quantum systems (qudits), given fast one-qudit unitaries. A second extension is to the case of a many-body Hamiltonian, that is, a Hamiltonian that has coupling terms that act on more than two qubits. Again, we find that under mild restrictions, such Hamiltonians are universal provided they are connected and that arbitrary one-qubit unitaries are available. We also discover and explore some surprising structure in many-body Hamiltonians that is associated with the parity of the couplings. A third related problem that is addressed is the question of what two-qubit unitaries are universal for quantum computation. This problem was originally solved by J.-L. and R. Brylinski [BB02], who gave a technical proof of the universality of any two-qudit gate that is not the swap gate or a product of one-qudit unitaries, given one-qudit unitaries as a free resource. We develop an elementary and constructive proof for the case of qubits, and prove that our construction is very close to optimal in the number of uses of the two-qubit gate. Given a qualitative understanding of what dynamical resources are universal for quantum computation, it is natural to consider developing a quantitative measure of the usefulness of quantum dynamics for quantum information processing. This is done in the final chapter of the thesis, in which the beginnings of a quantitative theory of quantum dynamics are developed.
This paper explores the potential of quantum chaos control for advanced quantum computation and information processing. We propose a system leveraging the inherent properties of quantum chaos, specifically the extreme sensitivity to initial conditions, to achieve precise control and manipulation of quantum systems. The core mechanism involves inducing and managing quantum chaos within a system of qubits by carefully tuning their parameters. This approach aims to exploit the chaotic dynamics for quantum computation, offering a novel pathway beyond traditional quantum control methods. The theoretical framework presented outlines the design considerations and potential applications of this technique, focusing on harnessing the unique characteristics of chaotic quantum systems. This work establishes a foundation for future research into utilizing quantum chaos as a powerful tool in the development of more robust and versatile quantum computing architectures.
Weak values of quantum observables are a powerful tool for investigating quantum phenomena. Some methods for measuring weak values in the laboratory require weak interactions and postselection, while others are deterministic, but require statistics over a number of experiments that grows linearly with the dimension of the measured system in the worst case over all possible observables. Here we propose a deterministic dimension-independent scheme for estimating weak values of arbitrary observables. The scheme is based on controlled operations and associates states and observables in the mathematical expression of the weak value to preparations devices and measurement devices in the experimental setup, respectively. Thanks to this feature, it provides insights into the relation between states of two identical quantum systems at a single moment of time and states of a single quantum system at two moments of time, also known as two-time states. Specifically, our scheme provides an alternative expression for two-time states and establishes a link between two-time states accessible through the controlled- scheme and bipartite quantum states with positive partial transpose. Published by the American Physical Society 2024
Based on a number of experimentally verified physical observations, it is argued that the standard principles of quantum mechanics should be applied to the Universe as a whole. Thus, a paradigm is proposed in which the entire Universe is represented by a pure state wavefunction contained in a factorisable Hilbert space of enormous dimension, and where this statevector is developed by successive applications of operators that correspond to unitary rotations and Hermitian tests. Moreover, because by definition the Universe contains everything, it is argued that these operators must be chosen self-referentially; the overall dynamics of the system is envisaged to be analogous to a gigantic, self-governing, quantum computation. The issue of how the Universe could choose these operators without requiring or referring to a fictitious external observer is addressed. The processes by which conventional physics might be recovered from this fundamental, mathematical and global description of reality are particularly investigated. Specifically, it is demonstrated that by considering the changing properties, separabilities and factorisations of both the state and the operators as the Universe proceeds though a sequence of discrete computations, familiar notions such as classical distinguishability, particle physics, space, time, special relativity and endo-physical experiments can all begin to emerge from the proposed picture. A pregeometric vision of cosmology is therefore discussed, with all of physics ultimately arising from the relationships occurring between the elements of the underlying mathematical structure.