Physics
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Item QUANTUM CONTROL AND MEASUREMENT ON FLUXONIUMS(2022) Xiong, Haonan; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting circuit is a promising platform for quantum computing and quantum simulation. A number of efforts have been made to explore the physics in transmon systems and optimize the qubit performance. Compared to transmon, fluxonium is a relatively new type of qubit and attracts more attention recently due to its high coherence time and large anharmonicity. In this thesis, we summarize recent progress toward high fidelity two-qubit gate and readout for fluxonium qubits. We report improved fluxonium coherence either in cavity or cavityless environment. In the former case, we demonstrate single-shot joint readout for two fluxonium qubits and explore various two-qubit gate schemes such as controlled-Z(CZ) gate, controlled-phase(CP) gate, bSWAP gate and cross-resonance(CR) gate. The CZ gate realized by near-resonantly driving the high transitions exhibits 99.2% fidelity from randomized benchmarking. A continuous CP gate set can be implemented by off-resonantly driving the high transitions and shows an average 99.2% fidelity from the cross-entropy benchmarking technique. Other gates involving only computational states are also explored to further improve the gate fidelity, which can take advantage of the high coherence of the fluxonium lower levels. In the cavityless environment, we demonstrate fluorescence shelving readout with 1.7 MHz radiative decay rate for the readout transition while maintaining 52 us coherence time for the qubit transition. Our research explores the basic elements for fluxonium-based quantum processors. The results suggest that fluxonium can be an excellent candidate for not only universal quantum computation but also quantum network and quantum optics studies.Item Developing New Experimental Techniques to Understand Neuronal Networks(2021) Aghayee, Samira Sadat; Losert, Wolfgang; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying the propagation of action potentials across neuronal networks and how information is stored and accessed in their dynamic firing patterns has always been the essence of neuroscience. Emerging evidence shows that information in the brain is encoded in the simultaneous or avalanche-like firing of multiple, spatially separated groups of neurons. Thus, understanding the collective behavior of neurons is essential for understanding how the brain processes information and encodes memory. Since its discovery, the advent of optogenetics has brought upon a revolution in neuroscience, where individual neuronal circuits are able to be selectively probed and their connections decoded. This ability has been used by many groups to great effect, with some groups even using optogenetic stimulation to create phantom sensations, which are typically encoded in the functional activity of distinct neuronal populations. However, in-vivo optogenetic excitation relies inherently on the quality and accuracy of the stimulation method, with many problems arising due to biological effects such as animal motion, the scattering nature of brain tissue, and cell health. Typically, groups either use digital micromirror arrays or spatial light modulators, with the former lacking transmission efficiency and the latter having a high technical skill barrier due to its propensity to induce artifacts into intended patterns of light. This dissertation attempts to reduce the barrier towards the use of spatial light modulators in optogenetics by improving targeting accuracy, reducing the effects of unmodulated light and related artifacts, and developing new methods of stimulation which reduce the power density directed at neurons. To accomplish the first step, improving targeting accuracy, I created and demonstrated a real-time capable particle-based motion tracking algorithm to correct for animal motion. To reduce the effects of optical artifacts, I developed and patented a method of using Fresnel lenses convolved with intended light patterns to project higher orders of diffraction and un-diffracted light axially away from the object plane. To improve cell health during stimulation, I researched the use of optical vortices to stimulate neurons, allowing for ion channel activation with reduced power per unit area. Finally, I set the stage for new science by creating neuroimaging platforms integrating these techniques and capable of imaging activity across multiple scales. Other avenues for improvement are outlined as well in this dissertation, as well as new scientific questions that can be asked, leveraging these developments contained within.Item RESOLUTION IMPROVEMENT OF PHOTOLITHOGRAPHIC TECHNIQUES BASED ON VISIBLE LIGHT(2017) Tomova, Zuleykhan; Fourkas, John T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The semiconductor industry is planning to use Extreme Ultraviolet lithography as its next-generation patterning technique. However, this technique has run into many roadblocks due to its cost and complexity. An alternative approach employs light in the near-UV. A 2-color photolithographic technique based on combination of two colors on the near-UV or visible light has shown promising results in creating structures with sizes at a fraction of the excitation light wavelength. One color of light excites photoinitiator molecules to a chemically active state that leads initiation of polymerization. A second color of light deactivates photoinitiator molecules before they form radicals, inhibiting polymerization. In this thesis we show how extending 2-color lithography to include a third color (3CL) can achieve super-resolution for applications requiring fabrication of closely packed structures. The advantage of the 3CL process is in its separation of polymerization initiation and deactivation steps by involving different chemical states that allow for more efficient deactivation and for increased resolution. Some of the crucial elements needed to achieve an optimized scheme for 3CL are the determination of the intramolecular transitions that participate in the process, the lifetimes of the photoinitiators, and the exposure parameters. Several photoinitiators were studied to determine the optimal exposure conditions. Polymerization action spectra and deactivation action spectra were used to determine the combinations of excitation and deactivation parameters resulting in the most efficient deactivation. The 2-beam initiation threshold (2-BIT) method was introduced for in situ measurement of the order of eective nonlinearity of photoresists. The order of the effective nonlinearity was determined for a series of photoinitiators under various excitation wavelengths and fabrication velocities. Additionally, a photoinitiator with a proportional velocity (PROVE) dependence, in which feature size increases with the velocity, was found to undergo efficient self-deactivation at increased temperatures. This dependence was demonstrated by gradually heating the sample and analyzing the fabricated feature sizes. Spot heating with a laser beam was also used to locally prevent polymerization. The correlation between polymerization rate and temperature opens opportunities for high speed fabrication that uses temperature gradients to create finer structures.Item Optical and magnetic measurements of a levitated, gyroscopically stabilized graphene nanoplatelet(2017) Coppock, Joyce Elizabeth; Wellstood, Frederick; Kane, Bruce; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I discuss the design and operation of a system for levitating a charged, $\mu$m-scale, multilayer graphene nanoplatelet in a quadrupole electric field trap in high vacuum. Levitation decouples the platelet from its environment and enables sensitive mechanical and magnetic measurements. First, I describe a method of generating and trapping the nanoplatelets. The platelets are generated via liquid exfoliation of graphite pellets and charged via electrospray ionization. Individual platelets are trapped at a pressure of several hundred mTorr and transferred to a trap in a second chamber, which is pumped to UHV pressures for further study. All measurements of the trapped platelet's motion are performed via optical scattering. Second, I present a method of gyroscopically stabilizing the levitated platelet. The rotation frequency of the platelet is locked to an applied radio frequency (rf) electric field $\bm{E}_{\mathrm{rf}}$. Over time, frequency-locking stabilizes the platelet so that its axis of rotation is normal to the platelet and perpendicular to $\bm{E}_{\mathrm{rf}}$. Finally, I present optical data on the interaction of a multilayer graphene platelet with an applied magnetic field. The stabilized nanoplatelet is extremely sensitive to external torques, and its low-frequency dynamics are determined by an applied magnetic field. Two mechanisms of interaction are observed: a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation. A model is constructed to describe this data, and experimental values are compared to theory.Item Nitrogen-Vacancy Coupling in Nanodiamond Hybrid Nanostructures(2017) Steinsultz, Nathaniel; Ouyang, Min; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrogen-vacancy centers (NVs) are an atomic defect in diamond which possess remarkable fluorescence and spin properties which can be used for multiple metrological applications, particularly when the NV is hosted in nanodiamond, which can be easily integrated with a variety of nanoscale systems. A new class of nanodiamond hybrid nanostructures was developed using bottom-up synthesis methods. In this work, coupling between NV centers and plasmonic, excitonic and magnetic nanoparticles in these nanodiamond hybrid nanostructures is investigated using fluorescence lifetime measurements, spin relaxometry measurements and modeled using finite element method (FEM) and Monte Carlo simulations. This work not only characterizes the properties of these nanodiamond-hybrid nanostructures but also facilitates design guidelines for future hybrid structures with enhanced metrological and imaging capabilities.Item In Situ Characterization of Optical Absorption by Carbonaceous Aerosols: Calibration and Measurement(2016) You, Rian; Zachariah, Michael R.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light absorption by aerosols has a great impact on climate change. A Photoacoustic spectrometer (PA) coupled with aerosol-based classification techniques represents an in situ method that can quantify the light absorption by aerosols in a real time, yet significant differences have been reported using this method versus filter based methods or the so-called difference method based upon light extinction and light scattering measurements. This dissertation focuses on developing calibration techniques for instruments used in measuring the light absorption cross section, including both particle diameter measurements by the differential mobility analyzer (DMA) and light absorption measurements by PA. Appropriate reference materials were explored for the calibration/validation of both measurements. The light absorption of carbonaceous aerosols was also investigated to provide fundamental understanding to the absorption mechanism. The first topic of interest in this dissertation is the development of calibration nanoparticles. In this study, bionanoparticles were confirmed to be a promising reference material for particle diameter as well as ion-mobility. Experimentally, bionanoparticles demonstrated outstanding homogeneity in mobility compared to currently used calibration particles. A numerical method was developed to calculate the true distribution and to explain the broadening of measured distribution. The high stability of bionanoparticles was also confirmed. For PA measurement, three aerosol with spherical or near spherical shapes were investigated as possible candidates for a reference standard: C60, copper and silver. Comparisons were made between experimental photoacoustic absorption data with Mie theory calculations. This resulted in the identification of C60 particles with a mobility diameter of 150 nm to 400 nm as an absorbing standard at wavelengths of 405 nm and 660 nm. Copper particles with a mobility diameter of 80 nm to 300 nm are also shown to be a promising reference candidate at wavelength of 405 nm. The second topic of this dissertation focuses on the investigation of light absorption by carbonaceous particles using PA. Optical absorption spectra of size and mass selected laboratory generated aerosols consisting of black carbon (BC), BC with non-absorbing coating (ammonium sulfate and sodium chloride) and BC with a weakly absorbing coating (brown carbon derived from humic acid) were measured across the visible to near-IR (500 nm to 840 nm). The manner in which BC mixed with each coating material was investigated. The absorption enhancement of BC was determined to be wavelength dependent. Optical absorption spectra were also taken for size and mass selected smoldering smoke produced from six types of commonly seen wood in a laboratory scale apparatus.Item Transmon qubits coupled to superconducting lumped element resonators(2015) Suri, Baladitya; Wellstood, Frederick C.; Palmer, Benjamin S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I discuss the design, fabrication and measurement at millikelvin-temperatures of Al/AlO$_x$/Al Josephson junction-based transmon qubits coupled to superconducting thin-film lumped element microwave resonators made of aluminum on sapphire. The resonators had a center frequency of around $6\,$GHz, and a total quality factor ranging from 15,000 to 70,000 for the various devices. The area of the transmon junctions was about $150\, \mathrm{nm} \times 150\, \mathrm{nm}$ and with Josephson energy $E_J$ such that $10\,\text{GHz} \leq E_J / h \leq 30\,$GHz. The charging energy of the transmons arising mostly from the large interdigital shunt capacitance, was $E_c / h \approx 300\,$MHz. I present microwave spectroscopy of the devices in the strongly dispersive regime of circuit quantum electrodynamics. In this limit the ac Stark shift due to a single photon in the resonator is greater than the linewidth of the qubit transition. When the resonator is driven coherently using a coupler tone, the transmon spectrum reveals individual ``photon number'' peaks, each corresponding to a single additional photon in the resonator. Using a weighted average of the peak heights in the qubit spectrum, I calculated the average number of photons $\bar{n}$ in the resonator. I also observed a nonlinear variation of $\bar{n}$ with the applied power of the coupler tone $P_{rf}$. I studied this nonlinearity using numerical simulations and found good qualitative agreement with data. In the absence of a coherent drive on the resonator, a thermal population of $5.474\,$GHz photons in the resonator, at an effective temperature of $120\,$mK resulted in a weak $n=1$ thermal photon peak in the qubit spectrum. In the presence of independent coupler and probe tones, the $n=1$ thermal photon peak revealed an Autler-Townes splitting. The observed effect was explained accurately using the four lowest levels of the dispersively dressed Jaynes-Cummings transmon-resonator system, and numerical simulations of the steady-state master equation for the coupled system. I also present time-domain measurements on transmons coupled to lumped-element resonators. From $T_1$ and Rabi oscillation measurements, I found that my early transmon devices (called design LEv5) had lifetimes ($T_1 \sim 1\,\mu$s) limited by strong coupling to the $50\,\Omega$ transmission line. This coupling was characterized by the the rate of change of the Rabi oscillation frequency with the change in the drive voltage ($\mathrm{d}f_{Rabi}\, / \mathrm{d}V$) -- also termed the Rabi coupling to the drive. I studied the design of the transmon-resonator system using circuit analysis and microwave simulations with the aim being to reduce the Rabi coupling to the drive. By increasing the resonance frequency of the resonator $\omega_r/2\pi$ from 5.4$\,$GHz to 7.2$\,$GHz, lowering the coupling of the resonator to the transmission line and thereby increasing the external quality factor $Q_e$ from 20,000 to 70,000, and reducing the transmon-resonator coupling $g/2\pi$ from 70$\,$MHz to 40$\,$MHz, I reduced the Rabi coupling to the drive by an order of magnitude ($\sim$ factor of 20). The $T_1 \sim 4\,\mu$s of devices in the new design (LEv6) was longer than that of the early devices, but still much shorter than the lifetimes predicted from Rabi coupling, suggesting the presence of alternative sources of noise causing qubit relaxation. Microwave simulations and circuit analysis in the presence of a dielectric loss tangent $\tan \delta \simeq 5\times10^{-6}$ agree reasonably well with the measured $T_1$ values, suggesting that surface dielectric loss may be causing relaxation of transmons in the new designs.Item Graphene chemical-vapor-deposited on platinum: synthesis, characterization and magneto-transport properties(2013) Ping, Jinglei; Fuhrer, Michael Sears; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene with large grain size and high electronic mobility was synthesized by ambient-pressure chemical vapor deposition on platinum and transferred to a variety of substrates for characterization by electrical transport, Raman spectroscopy, and transmission electron microscopy. The grain boundaries and pyramid-like multilayer structures of graphene samples prepared in this way were imaged with dark-field transmission electron microscopy, and a method was developed to use differences in first- and second-order diffraction intensities to characterize the layer-number and stacking-order of graphene up to at least seven layers. Combining this dark-field method with secondary electron microscopy, electron backscatter diffraction, Raman microscopy, and electronic transport measurements, it was also discovered that nano-crystalline carbon impurities distributed inhomogeneously under mono-layer graphene. These impurities were distributed inhomogenously, exhibiting micron-sized islands of denser impurity concentration whose shapes depended on the orientation of the grains of the Pt substrate. In such impurity-decorated samples both linear and quadratic magnetoresistance was observed. The linear magnetoresistance was found for carrier densities well beyond filling the ground Landau level, therefore Abrikosov's quantum magnetoreistance is ruled out. Sample 1, 2, and 3 with suppressing inhomogeneity were synthesized by controlling growing conditions in the chemical vapor deposition process. The magnetoresistance positively correlates with the density of inhomogeneity. The magnetoresistance in samples with widely varying impurity concentrations can be described by a unique function of the ratio of carrier-density inhomogeneity to gate-induced carrier density, and can therefore be attributed to impurity-induced inhomogeneity.Item Organic Molecular Thin Films on Device-Relevant Substrates(2013) Groce, Michelle Anne; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Organic thin films are central to many cutting-edge electronic devices. Improving our understanding of the characteristics of thin films is important not only to the development of condensed matter physics but also to our ability to engineer specialized devices that we demand be ever smaller, less expensive, and more efficient. This thesis applies the experimental techniques of scanning tunneling microscopy and spectroscopy to the task of characterizing submonolayer thin films of two types: the organic semiconductor C60 on silicon oxide, and self-assembling porous networks of trimesic acid on graphite. Capture zone analysis of the initial nucleation regime for C60 on ultrathin silicon oxide is reported. The critical nucleus size, reflecting the largest unstable cluster of particles on a surface, is found to have a parabolic dependence on temperature rather than a monotonically increasing one. Between stages of stable monomers (i=0$) at < 300 K and > 480 K, a peak corresponding to i=1 is found at 386±3 K. This unique temperature dependence is attributed to defect-like variation in the silicon oxide surface. The first successful room-temperature UHV STM of trimesic acid on graphite is also presented here. These exploratory studies indicate the potential for a variety of porous hexagonal networks of trimesic acid to exist on a graphitic surface at room temperature. Significant electronic effects on graphite from trimesic acid lattices are shown via scanning tunneling spectroscopy, including an electronic state at -0.14 V that appears in networks whose pores are filled with excess TMA guest molecules. Ultimately, if the growth of TMA films could be extended to graphene, then the periodicity of electronegative oxygen atoms in molecules physisorbed on the graphene surface is predicted to provide a slight energy shift between the degenerate sublattices, opening a band gap. Promising directions for future research in these areas are also discussed.Item TAILORING PROPERTIES AND FUNCTIONALITIES OF NANOSTRUCTURES THROUGH COMPOSITIONS, COMPONENTS AND MORPHOLOGIES(2013) WENG, LIN; Ouyang, Min; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The field of nanoscience and nanotechnology has made significant progresses over the last thirty years. Sophisticated nanostructures with tunable properties for novel physics and applications have been successfully fabricated, characterized and underwent practical test. In this thesis, I will focus on our recent efforts to develop new strategies to manipulate the properties of nanostructures. Particularly, three questions have been answered from our perspective, based on the nanomaterials synthesized: (1) How does the composition affect a novel nanostructure? We started from single-molecule precursors to reach nanostructures whose bulk counterparts only exist under extreme conditions. Fe3S and Fe3S2 are used as examples to demonstrate this synthetic strategy. Their potential magnetic properties have been measured, which may lead to interesting findings in astronomy and materials science. (2) How to achieve modularity control at nanoscale by a general bottom-up approach? Starting with reviewing the current status of this field, our recent experimental progresses towards delicate modularity control are presented by abundant novel heteronanostructures. An interesting catalytic mechanism of these nanostructures has also been verified, which involves the interaction between phonons, photons, plasmons, and excitons. (3) What can the morphology difference tell us about the inside of nanostructures? By comparing a series of data from three types of CdSe/CdS core-shell structures, a conclusion has been reached on the CdS growth mechanism on CdSe under different conditions, which also may lead to a solution to the asymmetry problem in the synthesis of CdSe/CdS nanorods. Finally this thesis is concluded by a summary and future outlook.