At the moment, quantum computer development is in the NISQ (Noisy Intermediate Scale Quantum) stage. This means that quantum computers are relatively small and exhibit large amounts of noise. To...Show moreAt the moment, quantum computer development is in the NISQ (Noisy Intermediate Scale Quantum) stage. This means that quantum computers are relatively small and exhibit large amounts of noise. To run any mean- ingful computation, small noise-resistant circuits are necessary. This work proposes a new algorithm, QASNEAT, for finding small noise-resistant circuits. The performance is evaluated by ground-state energy estimation of three small molecules with shot noise and physical depolarizing noise. QASNEAT is able to find small accurate circuits both in noisy and noise- less casesShow less
In quantum information theory, the presence of Bell non-local correlations is a key indicator of non-classical behavior in multipartite quantum systems. However, non-locality is not exclusive to...Show moreIn quantum information theory, the presence of Bell non-local correlations is a key indicator of non-classical behavior in multipartite quantum systems. However, non-locality is not exclusive to quantum mechanics; more general theories with stronger non-local correlations than those achievable within the quantum formalism can be constructed. While distinguishing classical (local) correlations from non-local correlations can, in principle, be accomplished by a finite number of linear constraints called Bell inequalities, distinguishing between quantum and post-quantum correlations requires solving a hierarchy of SDP relaxations. To simplify the certification of quantum correlations, a whole line of research has focused on searching for an operational principle that can explain the limited strength of quantum correlations. Among the proposed principles, information causality (IC) stands out as the most promising, though deriving general correlation bounds from it is also very complex. We review the various attempts to formalise IC and their effectiveness in constraining bipartite non-locality, as well as the challenges encountered in studying this principle. In particular, we perform numerical experiments to showcase the insufficiency of all the currently proposed IC bounds to capture the full potential of the principle for correlations near the quantum boundary. Furthermore, we demonstrate the instability of two out of three bounds under non-locality distillation.Show less
This presentation investigates the potential of applying reinforcement learning to quantum control settings through the theoretical framework of Markov decision processes (MDPs). Barry et al....Show moreThis presentation investigates the potential of applying reinforcement learning to quantum control settings through the theoretical framework of Markov decision processes (MDPs). Barry et al. formulated Quantum Observable MDPs (QOMDPs) as a model for quantum environments which Tamon claims to generalize with their introduced Quantum Partially Observable MDPs (QPOMDPs). We construct a formalism of behavioural equivalence of decision process models in order to evaluate expressibility of models through distinguishibility of models. We show that all quantum experiments can be described as POMDPs and specific environments can be modelled by varying types and formulations of decision processes with their respective advantages and disadvantages. By conducting experiments on a quantum cartpole environment, this research investigates the effects of varying environmental specifications on learning behavior and performance in quantum control problems generalized by QOMDPs in order to determine which setting is more appropriate for accurately modeling the dynamics of the system. The insights gained in this thesis can aid with appropriate model specification which is important for learning in quantum control settings. This research also contributes to the understanding of the practical implications of environmental specifications in quantum control problems, with findings having implications for the development of more effective and efficient learning algorithms tailored to quantum control settings.Show less
Characterizing quantum states is a central, yet involved, task in quantum information processing. In experiments, the unknown quantum state of interest must be prepared and measured multiple times...Show moreCharacterizing quantum states is a central, yet involved, task in quantum information processing. In experiments, the unknown quantum state of interest must be prepared and measured multiple times to learn its properties. Unfortunately, a full tomographic description is prohibitive by the exponential scaling of the quantum state description with the system size. In practice, only a few quantities are of interest for which protocols involving informationally incomplete measurements are preferable. After studying existing data acquisition protocols, we discuss classical shadow estimation, a particular experimentally feasible method for estimating many system properties. We extend the applicability to quantum many-body systems with higher dimensional subspaces and derive similar performance guarantees to the qubit case. Ultimately we implement the generalized protocol in a modular and economic numerical framework and demonstrate the accuracy along with the favourable scaling of classical shadow estimation in unbiased numerical experiments. In particular, we suggest and discuss the near-term application to 4-photon OAM entangled systems.Show less
The aim of this project is to study and characterize the vectors of entropies that are compatible with a multipartite quantum state for the von Neumann entropy. We review the background theories...Show moreThe aim of this project is to study and characterize the vectors of entropies that are compatible with a multipartite quantum state for the von Neumann entropy. We review the background theories that are required for studying the entropy cones. We numerically nd and characterize the extreme rays of the cone for 4-partite systems. We also investigate the ground states of the Lipkin-Meshkov-Glick model as a physically relevant model arising from asymmetric local Hamiltonian. Moreover, we use the Fourier-Motzkin elimination method to project the entropy cone to smaller subspaces, where its characterization might be more tractable.Show less
The preparation of a qubit register in the ground state of a given Hamiltonian is a challenging problem in the field of quantum algorithms. Its solution is relevant to enable the study of chemistry...Show moreThe preparation of a qubit register in the ground state of a given Hamiltonian is a challenging problem in the field of quantum algorithms. Its solution is relevant to enable the study of chemistry, condensed matter and nuclear physics models on a quantum computer Inspired by the way natural systems can be driven to a low energy and low entropy state by coupling them to a cold bath, we show how a single ancilla qubit periodically measured and reset can be used to drive a system towards the ground state of an Hamiltonian simulated on a digital quantum computer. We identify caveats that might compromise this method, like violation of the energy conservation principle and non-ergodicity caused by symmetries, and study strategies to circumvent them. We define and optimize two implementations of an elementary de-excitation procedures based on the simulation of Hamiltonian evolution and a single non-unitrary operation on the ancilla. By iteration of this procedure, we construct two protocols that can prepare approximate ground states of composite qubit systems. Using results of numerical simulations, we show that one protocol can prepare an arbitrary-fidelity approximation of the ground state a system of non-interacting qubits with random energies in polynomial time. The second protocol is designed to have a small computational cost, with the aim of being experimentally realizable in small near-term quantum computers. Both protocols are demonstrated to succeed in preparing approximate ground states of interacting spin-1/2 chain with transverse-field Ising Hamiltonians. We propose that the methods studied in this thesis can be extended and applied to develop a novel class of non-unitary quantum algorithms based on ancilla-mediated non-unitary operationsShow less
