UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item SPIN-PHOTON INTERFACE USING CHARGE TUNABLE QUANTUM DOTS(2021) Luo, Zhouchen; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The unconditional security of quantum networks and unparalleled acceleration of quantum algorithms enabled by “quantum computers” has motivated significant research across scientific communities. Among these different architectures to achieve such quantum information processing paradigms, a promising and straightforward proposal is the use of photons as flying qubits to transfer quantum information as well as local quantum memories to store and process quantum information. Toward this goal, it is very important to develop a type of quantum memory that can efficiently interface with photons while possessing good qubit properties, including a long coherence time and good scalability. To date, researchers have developed promising solid state quantum memory platforms, such as defects in diamond and other group IV compounds, rare earth ions hosted in various materials and self-assembled quantum dots. While each platform has its strengths and challenges, this thesis will focus on charge tunable InAs quantum dots grown inside a GaAs matrix that is doped into a PN junction. Though not long after the first demonstration of optically active self-assembled quantum dots, researchers have already developed the idea to sandwich them inside a PN junction to tune their charge status. The spin manipulation in the strong coupling regime has been mostly using these dots without PN junction doping, which has resulted in limited dot-cavity cooperativity and spin lifetime due to electron tunneling. In this thesis, I will first show the design, fabrication and characterization of several common photonic cavities, with their performance compared. Second I will show strong coupling between a negatively charged quantum dot and photonic crystal cavity, where the resonant cavity reflectivity is strongly dependent on the spin state. Third I will show that the electron spin lifetime (T1) can be significantly shortened by an off-resonant laser that reaches the device surface. While the exact reason for this undesired effect is not clear yet, we did observe the thickness of the electron tunnel barrier of the quantum dot wafer can result in distinct spin properties. I will present electron spin T1 characterization across several different quantum dot samples with different electron tunneling barrier thickness. Lastly, I will present coherent control of electron spin using picosecond laser pulse and sidebands of modulated continuous wave laser with limited spin rotation fidelity due to the off-resonant laser induced deterioration of the spin properties.Item Random Routing and Concentration in Quantum Switching Networks(2008) Ratan, Rahul; Oruc, Ahmet Y; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flexible distribution of data in the form of quantum bits or qubits among spatially separated entities is an essential component of envisioned scalable quantum computing architectures. Accordingly, we consider the problem of dynamically permuting groups of quantum bits, i.e., qubit packets, using networks of reconfigurable quantum switches. We demonstrate and then explore the equivalence between the quantum process of creation of packet superpositions and the process of randomly routing packets in the corresponding classical network. In particular, we consider an n × n Baseline network for which we explicitly relate the pairwise input-output routing probabilities in the classical random routing scenario to the probability amplitudes of the individual packet patterns superposed in the quantum output state. We then analyze the effect of using quantum random routing on a classically non-blocking configuration like the Benes network. We prove that for an n × n quantum Benes network, any input packet assignment with no output contention is probabilistically self-routable. In particular, we prove that with random routing on the first (log n-1) stages and bit controlled self-routing on the last log n stages of a quantum Benes network, the output packet pattern corresponding to routing with no blocking is always present in the output quantum state with a non-zero probability. We give a lower bound on the probability of observing such patterns on measurement at the output and identify a class of 2n-1 permutation patterns for which this bound is equal to 1, i.e., for all the permutation patterns in this class the following is true: in every pattern in the quantum output assignment all the valid input packets are present at their correct output addresses. In the second part of this thesis we give the complete design of quantum sparse crossbar concentrators. Sparse crossbar concentrators are rectangular grids of simple 2 × 2 switches or crosspoints, with the switches arranged such that any k inputs can be connected to some k outputs. We give the design of the quantum crosspoints for such concentrators and devise a self-routing method to concentrate quantum packets. Our main result is a rigorous proof that certain crossbar structures, namely, the fat-slim and banded quantum crossbars allow, without blocking, the realization of all concentration patterns with self-routing. In the last part we consider the scenario in which quantum packets are queued at the inputs to an n × n quantum non-blocking switch. We assume that each packet is a superposition of m classical packets. Under the assumption of uniform traffic, i.e., any output is equally likely to be accessed by a packet at an input we find the minimum value of m such that the output quantum state contains at least one packet pattern in which no two packets contend for the same output. Our calculations show that for m=9 the probability of a non-contending output pattern occurring in the quantum output is greater than 0.99 for all n up to 64.