Quantum Entanglement and Coherence in Molecular Magnets
Shiddiq, Muhandis (author)
Hill, S. (Stephen Olof) (professor directing dissertation)
Fajer, Peter G. (university representative)
Chiorescu, Irinel (committee member)
Bonesteel, N. E. (committee member)
Myers, Edmund G. (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Physics (degree granting department)
Quantum computers are predicted to outperform classical computers in certain tasks, such as factoring large numbers and searching databases. The construction of a computer whose operation is based on the principles of quantum mechanics appears extremely challenging. Solid state approaches offer the potential to answer this challenge by tailor-making novel nanomaterials for quantum information processing (QIP). Molecular magnets, which are materials whose energy levels and magnetic quantum states are well defined at the molecular level, have been identified as a class of material with properties that make them attractive for quantum computing purpose. In this dissertation, I explore the possibilities and challenges for molecular magnets to be used in quantum computing architecture. The properties of molecular magnets that are critical for applications in quantum computing, i.e., quantum entanglement and coherence, are comprehensively investigated to probe the feasibility of molecular magnets to be used as quantum bits (qubits). Interactions of qubits with photons are at the core of QIP. Photons can be used to detect and manipulate qubits, after which information can then be transferred over long distances. As a potential candidate for qubits, the interactions between Fe8 single-molecule magnets (SMMs) and cavity photons were studied. An earlier report described that a cavity mode splitting was observed in a spectrum of a cavity filled with a single-crystal of Fe8 SMMs. This splitting was interpreted as a vacuum Rabi splitting (VRS), which is a signature of an entanglement between a large number of SMMs and a cavity photon. However, find that large absorption and dispersion of the magnetic susceptibility are the reasons for this splitting. This finding highlights the fact that an observation of a peak splitting in a cavity transmission spectrum neither represents an unambiguous indication of quantum coherence in a large number of spins, nor a signature of entanglement between the spins and photons. Furthermore, the potential application of SMMs as qubits requires a quantum mechanical coupling between two or more SMMs to each other or to other components of a device, while maintaining the intrinsic single-molecule properties of each SMMs. I report a study of [MnIII 6 O2(O2CMe)6(dpd)3](I3)2, which is a dimer of Mn3 SMMs ([Mn3]2). This study reveals a quantum entanglement of the two Mn3 SMM units in the dimer, both in solid state and, for the first time, in solution phase. The quantum entanglement is brought about by a ferromagnetic coupling between Mn3 units which manifests in the splitting of electron paramagnetic resonance (EPR) spectra. The demonstration of quantum entanglement between two SMMs in a solution has paved the way to deposit covalently-linked SMMs on surfaces for device studies. Another requirement for qubits is a long coherence time, which can be achieved by knowing and suppressing decoherence processes. I report a new mechanism to suppress the dechorence in holmium polyoxometalate (HoPOM) molecular magnets by using clock transitions (CTs). The Frequency of a CT is insensitive (at least to first order) to variations in the magnetic field. This insensitivity to the fluctuations of the surrounding magnetic fields causes the spin coherence times at CTs to be longer than those at any other spin transition. In the HoPOM system, the enhancement of coherence times by using CTs for a concentrated sample (i.e., 10% concentration) is equivalent to the enhancement from diluting the sample a hundred times. These studies show that there is much potential for encoding qubits within molecular magnets.
Electron Spin Resonance, Molecular Magnets, Quantum Coherence, Quantum Computing, Quantum Entanglement
July 8, 2015.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Stephen Hill, Professor Directing Dissertation; Peter G. Fajer, University Representative; Irinel Chiorescu, Committee Member; Nicholas Bonesteel, Committee Member; Edmund Myers, Committee Member.
Florida State University
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