Quantum information is a field with many potential applications in future technology. One of the major challenges in the practical implementation of quantum information protocols is the difficulty in synthesizing and maintaining controllable quantum systems. One of the reasons for this is because quantum systems are susceptible to decoherence that reduces the quality of the quantum state of a system. There have been several advances toward mitigating this decoherence such as the design of codes to combat errors due to incoherent quantum evolution, known as quantum error correction, and the design of novel quantum states that are less susceptible to decoherence. In this dissertation, we provide a rigorous analysis of the mechanisms that introduce decoherence into select quantum states and propose methods to compensate for it. We find that squeezing a macroscopic quantum optical state known as a Schrödinger cat state can reduce the amount of decoherence introduced by optical parametric amplification. We also find that noiseless amplification can be used to compensate for decoherence due to polarization dependent loss in single-photonic polarization qubits. In the process of understanding decoherence and its compensation we also introduce new and exciting mathematical and physical concepts such as delta functions that admit complex arguments, known as the generalized delta function, and a noiseless attenuator made using an optical parametric amplifier.
Quantum information is a field with many potential applications in future technology. One of the major challenges in the practical implementation of quantum information protocols is the difficulty in synthesizing and maintaining controllable quantum systems. One of the reasons for this is because quantum systems are susceptible to decoherence that reduces the quality of the quantum state of a system. There have been several advances toward mitigating this decoherence such as the design of codes to combat errors due to incoherent quantum evolution, known as quantum error correction, and the design of novel quantum states that are less susceptible to decoherence. In this dissertation, we provide a rigorous analysis of the mechanisms that introduce decoherence into select quantum states and propose methods to compensate for it. We find that squeezing a macroscopic quantum optical state known as a Schrödinger cat state can reduce the amount of decoherence introduced by optical parametric amplification. We also find that noiseless amplification can be used to compensate for decoherence due to polarization dependent loss in single-photonic polarization qubits. In the process of understanding decoherence and its compensation we also introduce new and exciting mathematical and physical concepts such as delta functions that admit complex arguments, known as the generalized delta function, and a noiseless attenuator made using an optical parametric amplifier.
Loss and decoherence are major problems in the transmission of nonclassical states of light over large distances. It was recently shown that the effects of decoherence can be reduced by applying a probabilistic noiseless attenuator before transmitting a quantum state through a lossy channel, followed by probabilistic noiseless amplification [M. Micuda, I. Straka, M. Mikova, M. Dusek, N. J. Cerf, J. Fiurasek, and M. Jezek, Phys. Rev. Lett. 109, 180503 (2012)]. Here we show that similar results can be obtained for certain kinds of macroscopic quantum states by squeezing the signal before transmission, followed by deterministic amplification and antisqueezing to restore the original amplitude of the state. This approach can greatly reduce the effects of decoherence in the transmission of non-Gaussian states, such as Schr\"odinger cat states, without any reduction in the data transmission rate.
Quantum decoherence, a fundamental challenge in quantum information processing, represents the loss of quantum coherence due to interaction with the environment. This paper explores a novel mathematical framework for understanding and quantifying this phenomenon, proposing a 'Quantum Decoherence Metric' – a rigorously defined function that captures the rate of decoherence and offers a precise tool for analysis. This work addresses a longstanding difficulty in modeling and analyzing decoherence, aiming for a more mathematically solid and controllable approach. The core mechanism involves the creation of a metric specifically designed to quantify the degradation of quantum states. This development builds upon existing theoretical concepts, providing a quantitative lens through which to examine and potentially mitigate decoherence effects. The resulting framework offers a significant step towards a deeper understanding of quantum systems and their stability.
The development of a quantum computer presents one of the greatest challenges in science and engineering to date. The promise of more efficient computing based on entangled quantum states and the superposition principle has led to a worldwide explosion of interest in the fields of quantum information and computation. Among the number of hurdles which must first be cleared before we witness a physical realization are problems associated with environment-induced decoherence and noise more generally. However, the discovery of quantum error correction and the establishment of the accuracy threshold theorem provide us with the hope of someday harnessing the potential power a functioning fault-tolerant quantum information processor has to offer. This dissertation contributes to this effort by investigating a particular class of quantum error correcting codes, namely noiseless subsystem encodings. The passive approach to error correction taken by these encodings provides an efficient means of protection from symmetrically coupled system-environment interactions. Here I will present methods for determining the subsystem-preserving evolutions for noiseless subsystem encodings supported by arbitrary-dimensional physical quantum systems. Implications for universal, collective decoherence-free quantum computation using the derived operations are discussed. Moreover, I will present a proposal for an optical device which is capable of preparing a variety of these noiseless subsystem encodings through a postselection strategy.
Real photonic devices are subject to photon losses that can decohere quantum information encoded in the system. In the absence of full fault tolerance, quantum error mitigation techniques have been introduced to help manage errors in noisy quantum devices. In this paper, we introduce an error mitigation protocol inspired by probabilistic error cancellation (a popular error mitigation technique in discrete variable systems) for continuous variable systems. We show that our quantum error cancellation protocol can undo photon losses in expectation value estimation tasks. To do this, we analytically derive the (nonphysical) inverse photon loss channel and decompose it into a sum over physically realizable channels with potentially negative coefficients. The bias of our ideal expectation value estimator can be made arbitrarily small at the cost of increasing the sampling overhead. The protocol requires a noiseless amplification followed by a series of photon subtractions. While these operations can be implemented probabilistically, for certain classes of initial state one can avoid the burden of carrying out the amplification and photon subtractions by leveraging Monte Carlo methods to give an unbiased estimate of the ideal expectation value. We validate our proposed mitigation protocol by simulating the scheme on squeezed vacuum states, cat states, and entangled coherent states. Published by the American Physical Society 2024
Quantum information processing opens up completely new possibilities in communication and computation, that would be impossible in the realms of classical physics. While the benefits of quantum information processing are well understood in theory, it remains a formidable challenge to experimentally implement even simple quantum information processing tasks. Numerous physical architectures have been proposed for preparing and coherently manipulating quantum information. Amongst these, quantum optics has been identified as a particularly promising candidate. In particular this arises from the long decoherence times of single photon states, which can be used to encode logical qubits in a variety of ways. Nonetheless, there remain formidable challenges to implementing large scale quantum information processing using optical qubits. Most notably these include: the preparation of pure indistinguishable photons; efficient number-resolving photo-detection; mode-matching; photon loss; and the non-determinism of entangling gates. Any future large scale implementation of optical quantum information processing protocols will require significant advances on most, if not all these fronts. In this thesis we aim to present a comprehensive overview of the major requirements for optical quantum information processing, understand the effects they have, and how they can be modeled. From this we aim to understand what the realistic technological requirements are to achieve scalable optical quantum information processing, and, where possible, suggest means by which to help achieve these goals.
Quantum technologies have shown immeasurable potential to effectively solve several information processing tasks such as prime number factorization, unstructured database search or complex macromolecule simulation. As a result of such capability to solve certain problems that are not classically tractable, quantum machines have the potential revolutionize the modern world via applications such as drug design, process optimization, unbreakable communications or machine learning. However, quantum information is prone to suffer from errors caused by the so-called decoherence, which describes the loss in coherence of quantum states associated to their interactions with the surrounding environment. This decoherence phenomenon is present in every quantum information task, be it transmission, processing or even storage of quantum information. Consequently, the protection of quantum information via quantum error correction codes (QECC) is of paramount importance to construct fully operational quantum computers. Understanding environmental decoherence processes and the way they are modeled is fundamental in order to construct effective error correction methods capable of protecting quantum information. In this thesis, the nature of decoherence is studied and mathematically modelled; and QECCs are designed and optimized so that they exhibit better error correction capabilities.