92,872 research outputs found
Quantum Phase Transition in a Resonant Level Coupled to Interacting Leads
An interacting one-dimensional electron system, the Luttinger liquid, is
distinct from the "conventional" Fermi liquids formed by interacting electrons
in two and three dimensions. Some of its most spectacular properties are
revealed in the process of electron tunneling: as a function of the applied
bias or temperature the tunneling current demonstrates a non-trivial power-law
suppression. Here, we create a system which emulates tunneling in a Luttinger
liquid, by controlling the interaction of the tunneling electron with its
environment. We further replace a single tunneling barrier with a
double-barrier resonant level structure and investigate resonant tunneling
between Luttinger liquids. For the first time, we observe perfect transparency
of the resonant level embedded in the interacting environment, while the width
of the resonance tends to zero. We argue that this unique behavior results from
many-body physics of interacting electrons and signals the presence of a
quantum phase transition (QPT). In our samples many parameters, including the
interaction strength, can be precisely controlled; thus, we have created an
attractive model system for studying quantum critical phenomena in general. Our
work therefore has broadly reaching implications for understanding QPTs in more
complex systems, such as cold atoms and strongly correlated bulk materials.Comment: 11 pages total (main text + supplementary
Quantum Control of Open Systems and Dense Atomic Ensembles
Controlling the dynamics of open quantum systems; i.e. quantum systems that decohere because of interactions with the environment, is an active area of research with many applications in quantum optics and quantum computation. My thesis expands the scope of this inquiry by seeking to control open systems in proximity to an additional system. The latter could be a classical system such as metal nanoparticles, or a quantum system such as a cluster of similar atoms. By modelling the interactions between the systems, we are able to expand the accessible state space of the quantum system in question.For a single, three-level quantum system, I examine isolated systems that have only normal spontaneous emission. I then show that intensity-intensity correlation spectra, which depend directly on the density matrix of the system, can be used detect whether transitions share a common energy level. This detection is possible due to the presence of quantum interference effects between two transitions if they are connected. This effect allows one to asses energy level structure diagrams in complex atoms/molecules. By placing an open quantum system near a nanoparticle dimer, I show that the spontaneous emission rate of the system can be changed ``on demand by changing the polarization of an incident, driving field. In a three-level, system, this allows a qubit to both retain high qubit fidelity when it is operating, and to be rapidly initialized to a pure state once it is rendered unusable by decoherence. This type of behaviour is not possible in a single open quantum system; therefore adding a classical system nearby extends the overall control space of the quantum system. An open quantum system near identical neighbours in a dense ensemble is another example of how the accessible state space can be expanded. I show that a dense ensemble of atoms rapidly becomes disordered with states that are not directly excited by an incident field becoming significantly populated. This effect motivates the need for using multi-directional basis sets in theoretical analysis of dense quantum systems. My results demonstrate the shortcomings of short-pulse techniques used in many recent studies. Based on my numerical studies, I hypothesize that the dense ensemble can be modelled by an effective single quantum system that has a decoherence rate that changes over time. My effective single particle model provides a way in which computational time can be reduced, and also a model in which the underlying physical processes involved in the system\u27s evolution are much easier to understand. I then use this model to provide an elegant theoretical explanation for an unusual experimental result called ``transverse optical magnetism\u27\u27. My effective single particle model\u27s predictions match very well with experimental data
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Nanoscale oxygen defect gradients in UO2+x surfaces.
Oxygen defects govern the behavior of a range of materials spanning catalysis, quantum computing, and nuclear energy. Understanding and controlling these defects is particularly important for the safe use, storage, and disposal of actinide oxides in the nuclear fuel cycle, since their oxidation state influences fuel lifetimes, stability, and the contamination of groundwater. However, poorly understood nanoscale fluctuations in these systems can lead to significant deviations from bulk oxidation behavior. Here we describe the use of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy to resolve changes in the local oxygen defect environment in [Formula: see text] surfaces. We observe large image contrast and spectral changes that reflect the presence of sizable gradients in interstitial oxygen content at the nanoscale, which we quantify through first-principles calculations and image simulations. These findings reveal an unprecedented level of excess oxygen incorporated in a complex near-surface spatial distribution, offering additional insight into defect formation pathways and kinetics during [Formula: see text] surface oxidation
Controlling decoherence of a two-level-atom in a lossy cavity
By use of external periodic driving sources, we demonstrate the possibility
of controlling the coherent as well as the decoherent dynamics of a two-level
atom placed in a lossy cavity.
The control of the coherent dynamics is elucidated for the phenomenon of
coherent destruction of tunneling (CDT), i.e., the coherent dynamics of a
driven two-level atom in a quantum superposition state can be brought
practically to a complete standstill. We study this phenomenon for different
initial preparations of the two-level atom. We then proceed to investigate the
decoherence originating from the interaction of the two-level atom with a lossy
cavity mode. The loss mechanism is described in terms of a microscopic model
that couples the cavity mode to a bath of harmonic field modes. A suitably
tuned external cw-laser field applied to the two-level atom slows down
considerably the decoherence of the atom. We demonstrate the suppression of
decoherence for two opposite initial preparations of the atomic state: a
quantum superposition state as well as the ground state. These findings can be
used to the effect of a proficient battling of decoherence in qubit
manipulation processes.Comment: 12 pages including 3 figures, submitted for publicatio
Decoherence control in different environments
We investigate two techniques for controlling decoherence, focusing on the
crucial role played by the environmental spectrum. We show how environments
with different spectra lead to very different dynamical behaviours. Our study
clearly proves that such differences must be taken into account when designing
decoherence control schemes. The two techniques we consider are reservoir
engineering and quantum-Zeno control. We focus on a quantum harmonic oscillator
initially prepared in a nonclassical state and derive analytically its
non-Markovian dynamics in presence of different bosonic thermal environments.
On the one hand we show how, by modifying the spectrum of the environment, it
is possible to prolong or reduce the life of a Schr\"odinger cat state. On the
other hand we study the effect of nonselective energy measurements on the
degradation of quantumness of initial Fock states. In this latter case we see
that the crossover between Zeno (QZE) and anti-Zeno (AZE) effects, discussed by
Maniscalco et al. [Phys. Rev. Lett. 97, 130402 (2006)], is highly sensitive to
the details of the spectrum. In particular, for certain types of spectra, even
very small variations of the system frequency may cause a measurement-induced
acceleration of decoherence rather than its inhibition.Comment: 8 pages, 4 figures, new figures added, text modified for clarit
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