Quantum information is a growing field showing exciting possibilities for computational speed-up and communications. For the successful implementation of quantum computers, high-precision control is required to reach fault-tolerant thresholds. Control of quantum systems pertains to the manipulation of states and their evolution. In order to minimize the effects of the environment on the control operations of the qubits, control pulses should be made time-optimal. In addition, control pulses should be made robust to noise in the system, dispersion in energies and coupling elements, and uncertain parameters. In this thesis, we examine a robust time-optimal gradient ascent technique which is used to develop controls of the motional degrees of freedom for an ensemble of neutral atoms in a one-dimensional optical lattice in the high dispersion regime with shallow trapping potentials. As such, the system is analyzed in the delocalized basis. The system is treated as an ensemble of atoms with a range of possible quasimomenta across the first Brillouin zone. This gives the ensemble of Hamiltonians, indexed by the quasimomenta, a distinct spectra in their motional states and highly inhomogeneous control Hamiltonians. Thus, the optical lattice is seen as a model system for robust control. We find optimized control pulses designed using an ensemble modification of gradient-ascent pulse engineering robust to any range of quasimomentum. We show that it is possible to produce rotation controls with fidelities above 90\% for half of the first Brillouin zone with gate times in the order of several free oscillations. This is possible for a spectrum that shows upwards of 75\% dispersion in the energies of the band structure. We also show that NOT controls for qubit rotations on the entire Brillouin zone fidelities above 99\% were possible for 0.6\% dispersion in energies. The gate times were also in the order of several free oscillations. It is shown that these solutions are palindromic in time due to phase differences in some of the energy couplings when comparing one half of the Brillouin zone to another. We explore the limits of discretized sampling of a continuous ensemble for control.

Laser beams with precisely controlled intensity profiles are essential for many areas. We developed a beam shaping system based on the digital micromirror device (DMD) for ultra-cold atom experiments and other potential applications. The binary DMD pattern was first designed by the error diffusion algorithm based on an accurate measurement of the quasi-Gaussian incident beam from a real-world laser. The DMD pattern was projected to the image plane by a bandwidth-limited 4f telescope that converted this pattern to the grayscale image. The system bandwidth determined the theoretical limit of image precision by the digitization error. In addition, it controlled the spatial shape of the point spread function (PSF) that reflected the tradeoff between image precision and spatial resolution. PSF was used as a non-orthogonal basis set for iterative pattern refinement to seek the best possible system performance. This feedback process, along with stable performance of DMD, the blue-noise spectrum of the error diffusion algorithm, and low-pass filtering, guaranteed high-precision beam shaping performance. This system was used to produce various beam profiles for different spatial frequency spectra. First, we demonstrated high-precision slowly-varying intensity beam profiles with an unprecedented high intensity accuracy. For flattop and linearly-tilted flattop beams, we achieved 0.20-0.34% root-mean-square (RMS) error over the entire measurement region. Second, two-dimensional sinusoidal-flattop beams were used to evaluate image precision versus system bandwidth. System evaluation confirmed that this system was capable of producing any spatial pattern with <3% RMS error for the most system bandwidth. This experiment extended the beam shaping to any system bandwidth and provided a reference to estimate the output image quality based on its spatial spectrum. Later experiment using a Lena-flattop beam profile demonstrated the arbitrary beam profile generation. We implemented this system for applications on the homogenous optical lattice and dynamic optical trap generation. The DMD pattern was optimized by the iterative refinement process at the image feedback arm, and projected through a two-stage imaging system to form the desired beam profile at the working plane. Experiments demonstrated a high-precision beam shaping as well as a fast and dynamic control of the generated beam profile. / text

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In this thesis, I report on experiments with cold 85Rb atoms in a far-detuned one-dimensional optical lattice. These experiments are focused on creating efficient coupling between the quantized vibrational states of atoms in the optical lattice, on controlling and maintaining coherence between the vibrational states, and on developing a spectroscopy method to characterize the decay of coherence. First, I present an experimental study of the application of simple and compound pulses consisting of time-dependent spatial translations to coupling vibrational states of ultracold 85Rb atoms in the optical lattice. Experimental results show that a square pulse consisting of lattice displacements and a delay is more efficient than single-step and Gaussian pulses. The square pulse can be seen as an example of coherent control. Numerical calculations are in strong agreement with the experimental results. In addition, it is shown numerically that the vibrational state coupling due to such lattice manipulations is more efficient in shallow lattices than in deep lattices, in which the coupling probability approaches the harmonic oscillator limit. Next, the effectiveness of these pulses in reviving oscillations of atoms in vibrational superposition states using a pulse-echo technique is examined. Experimental results show that the square and Gaussian pulses result in higher echo amplitudes than the single-step pulse. These echo amplitudes are an order of magnitude larger than the echo amplitudes observed previously for the motional states of atoms in optical lattices. With the aim of the optimized square echo pulse, echo amplitude is measured at much longer times, where a surprising coherence freeze (plateau) is observed. To investigate mechanisms responsible for the observed echo decay and the coherence freeze, we developed a new two-dimensional pump-probe spectroscopy technique to monitor the evolution of frequency-frequency correlations in the system, a necessary input for understanding the decay of coherence. Through this 2D technique, we have characterized the temporal decay of frequency memory and through our simulations we find that coherence freeze is related to the shape of this memory loss function. This technique is general in that it can be applied in a variety of quantum information candidate systems to probe the nature of their decoherence.

In this thesis, I report on experiments with cold 85Rb atoms in a far-detuned one-dimensional optical lattice. These experiments are focused on creating efficient coupling between the quantized vibrational states of atoms in the optical lattice, on controlling and maintaining coherence between the vibrational states, and on developing a spectroscopy method to characterize the decay of coherence. First, I present an experimental study of the application of simple and compound pulses consisting of time-dependent spatial translations to coupling vibrational states of ultracold 85Rb atoms in the optical lattice. Experimental results show that a square pulse consisting of lattice displacements and a delay is more efficient than single-step and Gaussian pulses. The square pulse can be seen as an example of coherent control. Numerical calculations are in strong agreement with the experimental results. In addition, it is shown numerically that the vibrational state coupling due to such lattice manipulations is more efficient in shallow lattices than in deep lattices, in which the coupling probability approaches the harmonic oscillator limit. Next, the effectiveness of these pulses in reviving oscillations of atoms in vibrational superposition states using a pulse-echo technique is examined. Experimental results show that the square and Gaussian pulses result in higher echo amplitudes than the single-step pulse. These echo amplitudes are an order of magnitude larger than the echo amplitudes observed previously for the motional states of atoms in optical lattices. With the aim of the optimized square echo pulse, echo amplitude is measured at much longer times, where a surprising coherence freeze (plateau) is observed. To investigate mechanisms responsible for the observed echo decay and the coherence freeze, we developed a new two-dimensional pump-probe spectroscopy technique to monitor the evolution of frequency-frequency correlations in the system, a necessary input for understanding the decay of coherence. Through this 2D technique, we have characterized the temporal decay of frequency memory and through our simulations we find that coherence freeze is related to the shape of this memory loss function. This technique is general in that it can be applied in a variety of quantum information candidate systems to probe the nature of their decoherence.

Quantum information is a growing field showing exciting possibilities for computational speed-up and communications. For the successful implementation of quantum computers, high-precision control is required to reach fault-tolerant thresholds. Control of quantum systems pertains to the manipulation of states and their evolution. In order to minimize the effects of the environment on the control operations of the qubits, control pulses should be made time-optimal. In addition, control pulses should be made robust to noise in the system, dispersion in energies and coupling elements, and uncertain parameters. In this thesis, we examine a robust time-optimal gradient ascent technique which is used to develop controls of the motional degrees of freedom for an ensemble of neutral atoms in a one-dimensional optical lattice in the high dispersion regime with shallow trapping potentials. As such, the system is analyzed in the delocalized basis. The system is treated as an ensemble of atoms with a range of possible quasimomenta across the first Brillouin zone. This gives the ensemble of Hamiltonians, indexed by the quasimomenta, a distinct spectra in their motional states and highly inhomogeneous control Hamiltonians. Thus, the optical lattice is seen as a model system for robust control. We find optimized control pulses designed using an ensemble modification of gradient-ascent pulse engineering robust to any range of quasimomentum. We show that it is possible to produce rotation controls with fidelities above 90\% for half of the first Brillouin zone with gate times in the order of several free oscillations. This is possible for a spectrum that shows upwards of 75\% dispersion in the energies of the band structure. We also show that NOT controls for qubit rotations on the entire Brillouin zone fidelities above 99\% were possible for 0.6\% dispersion in energies. The gate times were also in the order of several free oscillations. It is shown that these solutions are palindromic in time due to phase differences in some of the energy couplings when comparing one half of the Brillouin zone to another. We explore the limits of discretized sampling of a continuous ensemble for control.

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Cold bosonic atoms trapped in optical lattices formed by standing wave interference patterns of multiple laser beams constitute excellent emulators of models of strongly correlated quantum systems of bosons. In this thesis, we develop and deploy strong-coupling expansion (i.e., an expansion in terms of the ratio of the inter-site hopping amplitude of the bosons to the strength of their interactions) techniques for studying the properties of three different instances of such systems. In the first instance, we have used strong coupling expansion techniques to calculate the density pro le for bosonic atoms trapped in an optical lattice with an overall harmonic trap at finite temperatures and large on site interaction in the presence of super fluid regions. Our results match well with quantum Monte Carlo simulations at finite temperature. We present calculations for the entropy per particle as a function of temperature which can be used to calibrate the temperature in experiments. Our calculations for the scaled density in the vacuum-to-super fluid transition agree well with the experimental data for appropriate temperatures. We also discuss issues connected with the demonstration of universal quantum critical scaling in the experiments. Experimental realizations of “atomtronic" Josephson junctions have recently been created in annular traps in relative rotation with respect to potential barriers that generate the weak links. If these devices are additionally subjected to optical lattice potentials, then they can incorporate strong-coupling Mott physics within the design, which can modify the behaviour and can allow for interesting new configurations of system generated barriers and of super fluid ow patterns. we have examined theoretically the behavior of a Bose super fluid in an optical lattice in the presence of an annular trap and a barrier across the annular region which acts as a Josephson junction. As the fluid is rotated relative to the barrier, it generates circulating super-currents until, at larger speeds of rotation, it develops phase slips which are typically accompanied by vortices. We use a finite temperature strong-coupling expansion about the mean- held solution of the Bose Hubbard model to calculate various properties of the device. In addition, we discuss some of the rich behavior that can result when there are Mott regions within the system. Rubidium-Cesium dipolar molecule formation through Feshbach resonance is an area of great current interest, for, the dipolar molecules, once formed, interact via v long range dipolar forces, leading to possibilities of novel phases. Experimentalists currently make such systems mostly using trial and error, and the resulting efficiencies for molecule formation tend to be low. With a goal to assist cold-atom experimentalists to achieve higher e ciencies of molecule formation, we have estimated the trap parameters for Rb and Cs atoms in a 3D optical lattice required to create single occupancy per site Mott phase for both the species in the same regions of the trap. We thus identify the ne tuning of the external magnetic held near Rb-Cs Feshbach resonance required to achieve highest probability for creating single Rb-Cs Feshbach molecules in the system. We have used the Falicov-Kimball model to describe the relevant system and strong-coupling expansions about the mean- held solution to calculate the density pro les for both species and efficiency for molecule formation, determined by overlapping regions of single occupancy for both Rb and Cs, up to second order in the expansion. We also calculate the entropy per particle which serves as an estimation of the temperature in the experimental system