Physics
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Item Optical and quantum interferences in strong field ionization and optimal control(2017) Foote, David B.; Hill, Wendell T.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For decades, ultrafast laser pulses have been used to probe and control strong-field molecular dynamics, including in optimal control experiments. While these experiments successfully recover the optimal control pulses (OCPs), they have a limitation -- it is generally unknown how the OCP guides the target system to its final state. This thesis is concerned with "unpacking" OCPs to explain how they achieve their control goals. The OCPs that inspired this work consisted of pulse trains; a twin-peaked pulse (TPP) is the simplest example. Consequently, TPPs with variable interpeak delay and relative phase were employed in this work to study ionization, the first step in many control experiments. Two types of interference influence ionization from a TPP: optical interference (OI) between the electric fields of the two peaks, and quantum interference (QuI) between the electron wavepackets produced by the two peaks. Two sets of experiments were performed to determine what roles OI and QuI play in controlling ionization from a TPP and how they in turn influence subsequent molecular dynamics. The first set of experiments measured the total ionization yield induced by the TPPs. It was found that OI was principally responsible for changing the ion yield; QuI-induced oscillations were not observed. Small imperfections in the shape of the TPP (i.e., pedestals and subordinate peaks) were found to have a surprisingly large influence in the OI, highlighting the need for researchers in molecular control experiments to characterize the temporal profile of their pulses accurately. A time-dependent perturbation theory simulation showed that the signatures of QuI in the ionic continuum vanish when measuring {\it total} electron yield, but appear in {\it energy-resolved} electron yields. The second set of experiments measured photoelectron energy distributions from a TPP with a velocity map imager to search for QuI. The experiments were performed at high intensities (~10^14 W/cm^2) where the ponderomotive energy tends to wash out the fine energy structures of QuI. The thesis ends by proposing a modified, low-intensity experiment that will allow for the first unambiguous observation of QuI in non-resonant, multiphoton ionization.Item DEMONSTRATION OF A QUANTUM GATE WITH ULTRAFAST LASER PULSES(2017) Wong Campos, Jaime David; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits. Quantum computers based on trapped atomic ions are one of the most mature and promising platforms for quantum information processing, exhibiting excellent coherence properties, near-perfect qubit detection efficiency, and high-fidelity entangling gates. Entangling operations between multiple ions in a chain typically rely on qubit state-dependent forces that modulate their Coulomb-coupled normal modes of motion. However, scaling these operations to large qubit numbers in a single chain must account for the increasing complexity of the normal mode spectrum, and can result in a gate time slowdown or added complexity of the control forces. In this thesis, I present an alternative route to the scalability problem using optical interactions faster than any state evolution. The experiments shown here represent a proof of principle for quantum manipulation of atoms in the strong coupling regime. This work relies on spin dependent forces (SDK) with short laser pulses and use it as our fundamental building block for thermometry and non-trivial motional state preparation. Together with a robust stabilization of the ion trap and high light collection efficiency, we demonstrate two-ion entanglement with ten ultrafast pulses. Due to the nature of the interaction, the demonstrated entangling operation can be made arbitrarily fast only limited by laser engineering.Item STUDY OF THE FEMTOSECOND DYNAMICS AND SPECTROSCOPY OF LASER IONIZED PLASMAS.(2015) Elle, Jennifer; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-short laser pulses are used to ionize gas in different configurations and study the plasma and ionization dynamics. The variation in non-linear index of refraction as a function of time is used to diagnose laser-plasma interactions. First, a proposed novel method to stimulate lasing in the atmosphere is studied. A few mJ pulse is used to ionize nitrogen gas in a long column without dissociating the molecular nitrogen. A 140ps laser is used to heat the resulting electron population in an attempt to generate a population inversion between the C3u and B3g states of molecular nitrogen. No evidence of lasing from this transition is observed. Next, a few mJ pulse is used to ionize xenon gas, creating Xe+ plasma. Ionization in Xe+ is observed far below the threshold predicted by multiphoton ionization theory due to resonant multiphoton ionization of collisionally excited states. To my knowledge, this is the first observation of resonant ionization involving multiple resonances. Finally, construction of an experiment to detect predicted birefringence in a relativistic laser-plasma interaction is described, with preliminary testing of diagnostics included.Item Ultrafast Control of Spin and Motion in Trapped Ions(2013) Mizrahi, Jonathan Albert; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Trapped atomic ions are a promising medium for quantum computing, due to their long coherence times and potential for scalability. Current methods of entangling ions rely on addressing individual modes of motion within the trap and applying qubit state dependent forces with external fields. This approach can limit the speed of entangling gates and make them vulnerable to decoherence due to coupling to unwanted modes or ion heating. This thesis is directed towards demonstrating novel entanglement schemes which are not limited by the trap frequency, and can be made almost arbitrarily fast. Towards this goal, I report here on the first experiments using ultrafast laser pulses to control the internal and external states of a single trapped ion. I begin with experiments in ultrafast spin control, showing how a single laser pulse can be used to completely control both spin degrees of freedom of the ion qubit in tens of picoseconds. I also show how a train of weak pulses can be used to drive Raman transitions based on a frequency comb. I then discuss experiments using pulses to rapidly entangle the spin with the motion, and how careful spectral redistribution allows a single pulse to execute a spin-dependent momentum kick. Finally, I explain how these spin-dependent momentum kicks can be used in the future to create an ultrafast entangling gate. I go over how such a gate would work, and present experimentally realizable timing sequences which would create a maximally entangled state of two ions in a time faster than the period of motion in the trap.Item Structured Plasma Waveguides and Deep EUV Generation Enabled by Intense Laser-Cluster Interactions(2012) Layer, Brian; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Using the unique properties of the interaction between intense, short-pulse lasers and nanometer scale van-der-Waals bonded aggregates (or `clusters'), modulated waveguides in hydrogen, argon and nitrogen plasmas were produced and extreme ultraviolet (EUV) light was generated in deeply ionized nitrogen plasmas. A jet of clusters behaves as an array of mass-limited, solid-density targets with the average density of a gas. Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with periodic structure within a laser-ionized cluster jet. The propagation of ultra-intense femtosecond laser pulses with intensities up to 2x1017 W/cm2 has been experimentally demonstrated in waveguides generated using both methods, limited by available laser energy. The first uses a `ring grating' to impose radial intensity modulations on the channel-generating laser pulse, which leads to axial intensity modulations at the laser focus within the cluster jet target. This creates a waveguide with axial modulations in diameter with a period between 35 μm and 2 mm, determined by the choice of ring grating. The second method creates modulated waveguides by focusing a uniform laser pulse within a jet of clusters with flow that has been modulated by periodically spaced wire obstructions. These wires make sharp, stable voids as short as 50 μm with a period as small as 200 μm within waveguides of hydrogen, nitrogen, and argon plasma. The gaps persist as the plasma expands for the full lifetime of the waveguide. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides. Simulations show that these `slow wave' guiding structures could allow direct laser acceleration of electrons, achieving gradients of 80 MV/cm and 10 MV/cm for laser pulse powers of 1.9 TW and 30 GW, respectively. Results are also presented from experiments in which a nitrogen cluster jet from a cryogenically cooled gas valve was irradiated with relativistically intense (up to 2x1018 W/cm2) femtosecond laser pulses. The original purpose of these experiments was to create a transient recombination-pumped nitrogen soft x-ray laser on the 2p3/2→1s1/2 (λ = 24.779 Å) and 2p1/2→1s1/2 (λ = 24.785 Å) transitions in H-like nitrogen (N6+). Although no amplification was observed, trends in EUV emission from H-like, He-like and Li-like nitrogen ions in the 15 - 150 Å spectral range were measured as a function of laser intensity and cluster size. These results were compared with calculations run in a 1-D fluid laser-cluster interaction code to study the time-dependent ionization, recombination, and evolution of nitrogen cluster plasmas.Item Sum Frequency Generation in Laser Safety and Quantum Telecommunications Applications(2011) Houston, Jemellie; Clark, Charles W; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes the implications of sum-frequency generation in both laser safety and quantum telecommunications applications. Green laser pointer technology uses frequency doubling of invisible 1064 nm infrared radiation to visible 532 nm green radiation. An inexpensive green laser pointer was found to emit infrared leakage primarily due to the lack of an infrared-blocking filter. An experimental setup using common household materials was presented to detect unwanted infrared radiation from such devices. Also reported, is the design and characterization of a high-speed versatile 780 nm pump source up to 1.25 GHz through second harmonic generation from a wavelength of 1560 nm. The 780 nm source is currently being used for the production of correlated photon pairs, one of which is at 656 nm, the hydrogen Balmer alpha line. The final goal will be to generate a high-speed entanglement source after some adjustments in the correlated pair source assembly. This will improve an operational quantum key distribution system.