Physics
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Item Simulating many-body quantum spin models with trapped ions(2021) Kyprianidis, Antonis; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Richard Feynman in 1981 suggested using a quantum machine to simulate quantum mechanics.Peter Shor in 1994 showed that a quantum computer could factor numbers much more efficiently than a conventional one. Since then, the explosion of the quantum information field is attesting to how motivation and funding work miracles. Research labs in the field are multiplying, commercial companies manufacturing prototypes are proliferating, undergraduate Physics curricula incorporate more than one courses in aspects of quantum information, quantum advantage over classical computers has been claimed, and the United States and European Union will be spending more than \$$10^9$ each in quantum information over the next few years. Naturally, this expansion has led to diversification of the devices being developed. The quantum information systems that cannot simulate an arbitrary evolution, but are specialized in a specific set of Hamiltonians, are called quantum \emph{simulators}. They enjoy the luxury of being able to surpass computational abilities of classical computers \emph{right now}, at the expense of only doing so for a narrow type of problem. Among those systems, ions trapped in vacuum by electric fields and manipulated with light have proved to be a leading platform in emulating quantum magnetism models. In this thesis I present trapped-ion experiments realizing a prethermal discrete time crystal. This exotic phase occurs in non-equilibrium matter subject to an external periodic drive. Normally, the ensuing Floquet heating maximizes the system entropy, leaving us with a trivial, infinite-temperature state. However, we are able to parametrically slow down this heating by tuning the drive frequency. During the time window of slow thermalization, we define an order parameter and observe two different regimes, based on whether it spontaneously breaks the discrete time translation symmetry of the drive or it preserves it. Furthermore, I demonstrate a simple model of electric field noise classically heating an ion in an anharmonic confining potential. As ion traps shrink, this kind of noise may become more significant. And finally, I discuss a handful of error sources. As quantum simulation experiments progress to more qubits and complicated sequences, accounting for system imperfections is becoming an integral part of the process.Item STUDIES IN NONEQUILIBRIUM QUANTUM THERMODYNAMICS(2019) Smith, Andrew Maven; Jarzynski, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The first part of this thesis focuses on verifying the quantum nonequilibrium work relation in the presence of decoherence. The nonequilibrium work relation is a generalization of the second law of thermodynamics that links nonequilibrium work measurements to equilibrium free energies via an equality. Despite being well established for classical systems, a quantum work relation is conceptually difficult to construct for systems that interact with their environment. We argue that for a quantum system which undergoes decoherence but not dissipation, these conceptual difficulties do not arise and the work relation can be proven similarly to the case of an isolated system. This result is accompanied by an experimental demonstration using trapped ions. The second part of this thesis examines the relationship between quantum work and coherence by constructing analogous quantities in classical physics. It has recently been shown that quantum coherence can function as a resource for work extraction. Furthermore, it has been suggested that this property could be a truly quantum aspect of thermodynamics with no classical analog. We examine this assertion within the framework of classical Hamiltonian mechanics and canonical quantization. For classical states we define a so called non-uniformity measure and show that it is a resource for work extraction similar to quantum coherence. Additionally, we show that work extracted from non-uniformity and coherence agree in the classical limit. This calls into question the idea that coherence qualitatively separates classical and quantum thermodynamics. The final part of this thesis explores the connection between decoherence and adiabatic (quasistatic) driving. This topic is inspired by an experiment where it was seen that strong dephasing suppressed energy level transitions. Using a perturbative method we investigate this mechanism in the regime of small to moderate decoherence rate and ask if decoherence can help suppress energy transitions when compared with an adiabatic process without decoherence. We find that strategies that include decoherence are inferior to those where decoherence is absent.Item The Response of Molecular Gases and Modulated Plasmas to Short Intense Laser Pulses(2011) Pearson, Andrew; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we study the response of two systems to short, intense laser pulses. The first system is a gas of diatomic molecules whose ensemble-averaged alignment features rotational revivals. We analyze the effect of a background plasma on the revival peaks. Both the revivals and the plasma are the result of a laser pulse passing through the gas. The second system is a density-modulated plasma channel. We study the generation of electromagnetic radiation by a laser pulse passing through this structure. The molecules in the gas are modeled as rigid rotors that interact first with the cycle-averaged electric field of the laser pulse, and second with the fluctuating electric field of the background plasma. The laser pulse generates a broad superposition of angular momentum eigenstates, resulting in the transient alignment of the molecules. Because of the time evolution properties of the angular momentum states, the alignment re-occurs periodically in field-free conditions. The alignment is calculated using a density matrix, and the background plasma is modeled using dressed particles. The result is decoherence between the phases of the basis states of the wavefunction, which causes decay of subsequent alignment peaks. We find that field-induced decoherence is competitive with collisional decoherence for small ionization fractions. The corrugated plasma channel is modeled using linear plasma theory, and the laser pulse is non-evolving. Corrugated channels support EM modes that have a Floquet dispersion relation, and thus consist of many spatial harmonics with subluminal phase velocities. This allows phase matching between the pulse and the EM modes. Since the pulse bandwidth includes THz frequencies, significant THz generation is possible. Here we consider realistic density profiles to obtain predictions of the THz power output and mode structure. We then estimate pulse depletion effects. The fraction of laser energy converted to THz is independent of laser pulse energy in the linear regime, and we find it to be around one percent. Extrapolating to a pulse energy of 0.5 J gives a THz power output of 6 mJ, with a pulse depletion length of less than 20 cm.Item Hydrogenic Spin Quantum Computing in Silicon and Damping and Diffusion in a Chain-Boson Model(2006-08-08) Skinner, Andrew J.; Hu, Bei-Lok; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We propose an architecture for quantum computing with spin-pair encoded qubits in silicon. Electron-nuclear spin-pairs are controlled by a DC magnetic field and electrode-switched on and off hyperfine interaction. This digital processing is insensitive to tuning errors and easy to model. Electron shuttling between donors enables multi-qubit logic. These hydrogenic spin qubits are transferable to nuclear spin-pairs, which have long coherence times, and electron spin-pairs, which are ideally suited for measurement and initialization. The architecture is scaleable to highly parallel operation. We also study the open-system dynamics of a few two-level systems coupled together and embedded in a crystal lattice. In one case, superconducting quantum interference devices, or SQUIDs, exchange their angular momenta with the lattice. Some decaying oscillations can emerge in a lower energy subspace with a longer coherence time. In another case, the exchange coupling between spins-1/2 is strained by lattice distortions. At a critical point energy level crossing, four well-spaced spins dissipate collectively. This is partially true also for the two- or three-SQUID-chain. These collective couplings can improve coherence times.