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Ever since its discovery, quantum mechanics has remained an intensely active field, and its real-world applications continue to unfold rapidly. In 1982, Richard Feynman proposed a new type of computer operating directly under quantum mechanics laws: – the quantum computer \cite{Feynman1982}. Compared with the classical computer, whose information is encoded in “bits”, the quantum computer, whose information is encoded in “quantum bits”, or “qubits”, will be able to perform calculations exponentially faster for such problems as factoring large integers into primes and simulating complicated quantum systems. Due to their extremely powerful calculation speeds and abilities, quantum computers have been the long-pursued dreams for both experimentalists and theorists in many research groups, government agencies, industrial companies, etc., and the fast-paced developments in their architecture and speed continue to make them more and more attractive. There are two principal models of quantum computing: the circuit model and the measurement-based model. The circuit model is similar to a traditional computer where there are inputs, gates and outputs. The measurement-based model is different, as it is crucially based on the cluster state, a type of highly entangled quantum state. In this new model, quantum computing begins with an initial cluster state and then carries out calculations by physical measurements of the cluster state itself along with feedforward. Thus, the cluster state serves as the material and resource for the entire set of calculations, and it is an extremely important part of measurement-based quantum computing. This thesis will discuss an experimental and theoretical work that holds the world record for the largest entangled cluster state ever created whose 60 qumodes (optical versions of qubits) are all available simultaneously. Moreover, the entangled state we created is not random, and it is a cluster state which meets the specific requirements for implementing quantum computing. In the race to build a practical quantum computer, the ability to create such a large cluster state is paramount. Also, our creative optical method to generate massive entanglement advances many other methods due to its high efficiency, super-compactness and large scalability. The entanglement proceeds from interfering multiple EPR entangled pairs, which are generated from the down-converting process of a nonlinear crystal in an optical parametric oscillator, into a very long dual-rail wire cluster state. Moreover, many copies of the same state can easily be obtained by merely adjusting the frequencies of the pump lasers. These cluster states serve as building blocks of the universal quantum computer, and also are, in their own right, important resources for studying and exploring quantum mechanics in large systems.
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This thesis demonstrates the presence of two-mode squeezing within a broadband squeezed light field generated by an OPO pumped just below threshold. Past experiments have confirmed the existence of this phenomenon, but we have demonstrated the technique over an approximately 1 MHz frequency range, with a highly tunable measurement technique. Through the use of a two-frequency local oscillator generated by a double sideband modulation of an EOM, we are able to measure many two-mode squeezed fields within the spectrum of one OPO output mode. Such a technique has great potential for observing larger, more densely packed optical cluster states, which could be used to construct a quantum computer.
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spin torque nano oscillator
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Quantum interference of twin optical beams, a typical example of correlated boson fields, is of great interest from the viewpoint of both fundamental and applied physics. Intensity correlated optical beams and their quantum interference bring the possibility of performing sub-shot-noise interferometry experiments which yield high-sensitivity in practical applications. A type-II doubly resonant optical parametric oscillator (OPO) has been built for research in quantum optics, to provide a source of ultrastable quantum correlated (twin) optical beams with continuously tunable frequency difference from zero up to several THz. Three servo loops have been built to control the frequency difference of the twin optical beams and sub-hertz stability has been achieved for frequency difference up to 100MHz. In particular, frequency degenerate twin beams are indistinguishable macroscopic quantum systems. A series of experimental results has been produced with these OPO-generated ultrastable twin beams: the observation of the generalized Hong-Ou-Mandel interference of untrastable twin optical beams, sub-shot-noise homodyne interferometry, and sub-shot-noise heterodyne polarimetry. Note: Abstract extracted from PDF text
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Terahertz-frequency Quantum Cascade Lasers (THz QCLs) are compact electrically-driven sources of narrowband, coherent radiation in the 1–5 THz band. Although peak output powers in excess of 1 W have been demonstrated, most potential applications of THz QCLs as a local oscillator (LO) in THz astronomy and atmospheric spectroscopy require frequency stability better than 1 ppm and narrow linewidth, with low phase noise sidebands. However, temperature and current bias fluctuations in the gain media of the QCL can cause the refractive index to change with time, which affects the lasing frequency. Phase locking improves the stability by using a negative-feedback system that combines a current-controlled QCL with a phase comparator and a stable reference so that the QCL maintains a constant phase angle relative to a reference signal. Since it is challenging to find compact and stable THz-frequency source to use as a reference, a THz mixer is needed to down-convert the signal to a lower frequency where frequency and phase comparisons are possible. A number of groups have accomplished QCL phase locking using a Hot Electron Bolometer (HEB) or Semiconductor Superlattice nonlinear device (SSL) as mixers. However, these mixers require an additional cryo-cooler, which increases the size and the complexity of the phase locking system. Furthermore, room temperature SSL devices exhibit conversion loss of 80 dB or more, which makes phase locking difficult. Therefore, a room-temperature, solid-state mixer with lower conversion loss is desirable to produce more compact phase-locked THz sources. This thesis describes the phase locking of free running 2.518 THz and a 2.6906 THz QCLs, which have achieved a spectral resolution of approximately 1010. To phase lock THz QCLs, a room temperature Schottky diode based WM-86 (WR0.34) 1.8-3.2 THz harmonic mixer is developed. The mixer consists of quartz-based Local Oscillator (LO) and Intermediate-Frequency (IF) circuits and a GaAs based beam-lead THz circuit with an integrated diode. Measurements of the mixer are performed using a 2 THz solid state source and 2.6906 THz QCL, and a conversion loss of 27 dB for the 3rd harmonic mixing is achieved. This is the first time the development of a WM-86 (WR0.34) harmonic mixer with a beam-lead THz circuit for frequencies above 2.5 THz is demonstrated and the result represents the best Schottky-based harmonic mixer in this frequency range. Similarly, this thesis also prescribes the phase locking of QCLs at 2.518 THz and at 2.6906 THz using a room temperature Schottky diode for the first time after a 2.32 THz QCL phase locking was reported by a group at University of Massachusetts. A phase locked QCL can be used to build a heterodyne interferometer in the far-infrared range and high-resolution heterodyne tunable spectroscopy for different applications such as radio astronomy, molecular spectroscope, and plasma diagnostics.
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This dissertation documents a second Bose-Einstein Condensate production apparatus at the University of Virginia. The apparatus produces condensates of 87Rb atoms to be used as the wave source for atom interferometry experiments. Additionally, a new magnetic trap was developed which provides a harmonic potential with cylindrical symmetry and also supports the atoms against gravity. This trap is based on a time-orbiting potential. To characterize the trapping potential, a condensate was loaded into the trap and perturbed by suddenly changing the confinement field during loading. This had the effect of inducing harmonic oscillations which were measured at varying trap parameters. We expect that this trap will be useful for the implementation of a compact atom interferometer-based gyroscope. Finally, an asymmetric Bragg splitting pulse was developed which allowed for the implementation of two simple interferometers. These interferometers developed a phase which depended on the recoil frequency of 87Rb and served as a proof of principle for this new apparatus.
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Cold Rydberg atoms are a useful medium to study physical properties for several reasons. The trapped and cooled atoms move on the order of 0.24 µm/µs - essentially stationary on the timescale of our experiments. In addition Rydberg Rubidium atoms have one loosely bound electron at a distance from the ion core ∝ n 2 . When a dressing field is applied to that electron, it oscillates between states creating a single atom dipole which is a useful object of study. This thesis presents experimental studies of dressed state Rydberg atoms. It was found that the dressed state is highly sensitive to the polarization of the dressing field. We characterized dressed state cold Rydberg atoms - how they behave under changes in dressing frequency, dressing power, polarization of dressing and probe fields, and density. Note: Abstract extracted from PDF text
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The hippocampus is involved in the generation and propagation of epileptic seizures. Although it has been stated that seizures are hypersynchronized activity of neurons, some studies showed desynchronization during seizure onsets. Most of the previous studies were conducted in brain slices. However, the structure of synchronization and propagation during hippocampal seizures in vivo is still undeveloped. The purpose of this dissertation was to understand the dynamic properties of neuronal firing and quantify the synchrony of high frequency firing in both anesthetized and awake rats during epileptogenesis. A high-speed dynamic recording apparatus was constructed with a microelectrode array to record electrically evoked seizures. We found that as the epileptogenesis progresses, firing of neurons in CA1 region become more synchronized and the propagation is along the lamellar axis in CA1, but not the septotemporal axis. The synchrony of neurons was measured by using the following methods: cross correlation, theta phase synchronization and event synchronization. Cross correlation revealed that the firing pattern was highly correlated during evoked seizures along the lamellar axis but not the septotemporal. Both theta phase synchronization and event synchronization demonstrated that neuronal synchrony increased along the lamellar axis of CA1 pyramidal neurons as kindling progressed, while synchrony along the septotemporal axis remained at a relatively low level. Additionally, the theta phase distribution demonstrated that the firings of CA1 pyramidal cells became preferential for the negative peak of the theta oscillations as the seizure progresses. This was only true in the lamellar direction. Lastly, event synchronization shows that neuronal firings along the lamellar axis were more synchronized than those along the septotemporal axis. The effects of two commonly prescribed antiepileptic drugs, phenytoin and levetiracetam, on seizure activity and neuronal synchronization in the electrically kindled model were examined. There was a marked decrease in synchronization and propagation after treatment with both phenytoin and levetiracetam. The preferred firing phase, which is around the negative peak of theta oscillation, was lost after both drug treatments. In this dissertation, we discovered the temporal relationship between the structure of synchrony in CA1 and seizure severity. It can be used to help design a new deep brain stimulation algorithm to interrupt synchrony during epileptogenesis.
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I use empirical/statistical models and physically based general circulation models to assess the capacity for the Arctic Oscillation (AO) and the El Nino Southern Oscillation (ENSO) to influence terrestrial ecosystems, and the potential for those ecosystems to feedback to the climate system. AO warming leads to modest reductions in Eurasian carbon stocks; ~17 Pg carbon are lost to the atmosphere, primarily from increased soil decomposition. Precipitation reductions in southern Africa associated with increased frequency of El Nino events lead to a reduction in tree cover and expansion of grasslands in the north and a reduction in grass cover in drier areas. Here half the carbon cycle changes are driven by the loss of tree cover, leading to a net loss of ~5 Pg of carbon to the atmosphere. Over southern Africa, positive soil moisture anomalies lead to reduced precipitation through enhanced subsidence and reduced moisture convergence. Higher snow cover alone in Eurasia leads to minor albedo increases and moderate localized cooling (3 o -5 o C), mostly at very high latitudes (>70 o N) and during the spring season. When vegetation is allowed to interact, increased snow cover leads to southward retreat of boreal vegetation, widespread cooling, and persistent snow cover over much of the boreal region during the boreal summer, with cold anomalies of up to 15 o C. In southern Africa, the feedback experiments suggest a negative feedback between soil moisture and precipitation over the same area, implying this region may be resistant to externally forced changes in precipitation. In Eurasia, a persistent high phase of the AO leads to winter warming, but the feedback response is complicated. Warming during this season has been associated with increased snowfall, which could increase snow cover and 2 albedo, countering the AO warming. Conversely, increased temperatures could lead to increased snow melting and decreased albedo, amplifying the AO warming. Note: Abstract extracted from PDF text
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In recent years, there has been a paradigm shift ushered in by “More-than-Moore” technologies which has focused on functional diversification of modern circuits rather than geometric scaling. One of the promising technologies in this field has been spintronics devices which exploits the spin of an electron instead of its charge. Furthermore, the integration of magnetic spintronics devices with MOSFET circuits--demonstrated by commercial devices such as STT-MRAM--has opened the possibility of Systems-on-Chip (SoC) integrated circuits with both types of components. While there are many physics-based simulators which can study the detailed dynamics of magnetic materials and many CAD tools for large scale circuit design, there is a dearth of simulation tools for circuit designers working with spintronics devices. This dissertation proposes a Verilog-A behavioral hardware model of multiferroic and spin transfer torque (STT) devices which can be incorporated within traditional large scale CAD tools such as Synopsys HSPICE. Using this simulation platform, this work explores how spintronics devices can implement several More-than-Moore applications and proposes circuit and architecture-level designs to realize those applications. This dissertation considers two promising developments in magnetic spintronics devices--logic using multiferroic materials and applications using spin-torque nano-oscillators (STNO). Multiferroic materials describe a class of materials which exhibit both ferroelectric and ferromagnetic behaviors. The combination of these two attributes allows for the control of the magnetic state of the material using an electric field rather than a magnetic field—a process known as electrically assisted magnetic switching (EAMS). This work develops a Verilog-A model which captures the EAMS process in multiferroic materials through a compact thermodynamic model. This model demonstrates that multiferroic nanopillars can not only be used to represent binary logic bits, but they also provide a third state that can be used for reconfiguration similar to traditional field programmable gate arrays (FPGAs). This dissertation describes the operations of a reconfigurable array of magnetic automata (RAMA) based on multiferroic nanopillars which can perform ultra-low power computation. Yet another method to control the magnetization of materials using electrical currents is through the spin transfer torque (STT) effect. The STT effect manifests in magnetic tunnel junctions (MTJ) when a DC current is applied through the junctions. The modularity of the proposed Verilog-A model can be modified to include the STT effect to simulate the behavior of spin torque nano-oscillators. Furthermore, this model shows that connecting multiple STNOs leads to complex behaviors such as synchronization. In an array of parallel-connected STNOs, this synchronization can be exploited for pattern recognition applications. Finally, this dissertation explores applications using STNOs as on-chip RF components such as bandpass and bandstop filters. The nanoscale dimensions, electrically tunable frequencies and integration with MOSFETs make STNOs an attractive option for future RF components of SoC integrated circuit.
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