1,595 research outputs found
Demonstration of a quantum logic gate in a cryogenic surface-electrode ion trap
We demonstrate quantum control techniques for a single trapped ion in a
cryogenic, surface-electrode trap. A narrow optical transition of Sr+ along
with the ground and first excited motional states of the harmonic trapping
potential form a two-qubit system. The optical qubit transition is susceptible
to magnetic field fluctuations, which we stabilize with a simple and compact
method using superconducting rings. Decoherence of the motional qubit is
suppressed by the cryogenic environment. AC Stark shift correction is
accomplished by controlling the laser phase in the pulse sequencer, eliminating
the need for an additional laser. Quantum process tomography is implemented on
atomic and motional states using conditional pulse sequences. With these
techniques we demonstrate a Cirac-Zoller Controlled-NOT gate in a single ion
with a mean fidelity of 91(1)%.Comment: 11 pages, 5 figures, 4 table
Molecular Dynamics Study of Self-Diffusion in Zr
We employed a recently developed semi-empirical Zr potential to determine the
diffusivities in the hcp and bcc Zr via molecular dynamics simulation. The
point defect concentration was determined directly from MD simulation rather
than from theoretical methods using T=0 calculations. We found that the
diffusion proceeds via the interstitial mechanism in the hcp Zr and both the
vacancy and interstitial mechanisms give contribution in diffusivity in the bcc
Zr. The agreement with the experimental data is excellent for the hcp Zr and
for the bcc Zr it is rather good at high temperatures but there is a
considerable disagreement at low temperatures
Fe and N self-diffusion in non-magnetic Fe:N
Fe and N self-diffusion in non-magnetic FeN has been studied using neutron
reflectivity. The isotope labelled multilayers, FeN/57Fe:N and Fe:N/Fe:15N were
prepared using magnetron sputtering. It was remarkable to observe that N
diffusion was slower compared to Fe while the atomic size of Fe is larger
compared to N. An attempt has been made to understand the diffusion of Fe and N
in non-magnetic Fe:N
Coherent manipulations of trapped 88Sr+ using the 4D5/2 --> 5S1/2 transition
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.In title on t.p. 88 and + appear as superscript; 5/2 and 1/2 appear as subscript.Includes bibliographical references (p. 89-92).The ability to control quantum systems with high fidelity is of fundamental importance to a variety of scientific experiments. This thesis describes the frequency stabilization of a laser which is used to coherently manipulate both the internal electronic state and quantum harmonic motion of a trapped 88Sr+ ion via its 4D5/2 5S1/2 transition. The laser is locked with a linewidth on the order of 1Hz to the resonance of a high-finesse optical cavity. However, acoustic and thermal noise in the cavity broadens the laser's linewidth to several hundreds of Hertz. Despite the noise, we find that the ion's internal electronic state can be manipulated by the laser with a high degree of precision: 17 consecutive Rabi oscillations can be driven with a contrast exceeding 80 percent, and Ramsey inteferometry indicates that phase coherence decays with a time constant of 342±6/[mu]s. Because the laser's dominant spectral broadening occurs on a slow timescale, phase coherence can be maintained by spin echoes for over 5ms. In addition, laser pulses detuned to the first blue motional sideband are able to control the ion's quantum state of motion well enough to use the ground and first excited states as a second qubit. This allows a two-qubit controlled-NOT gate, the quantum analog of an XOR gate and a fundamental building block of quantum computation, to be demonstrated on a single 88Sr+ ion with a classical fidelity of 0.89±0.02.by Ruth Shewmon.S.B
Weak Localization Thickness Measurements of Si:P Delta-Layers
We report on our results for the characterization of Si:P delta-layers grown
by low temperature molecular beam epitaxy. Our data shows that the effective
thickness of a delta-layer can be obtained through a weak localization analysis
of electrical transport measurements performed in perpendicular and parallel
magnetic fields. An estimate of the diffusivity of phosphorous in silicon is
obtained by applying this method to several samples annealed at 850 Celsius for
intervals of zero to 15 minutes. With further refinements, this may prove to be
the most precise method of measuring delta-layer widths developed to date,
including that of Secondary Ion Mass Spectrometry analysis
New Models for UO2 Fuel Structure Evolution under Irradiation in Fast Reactors
On the base of analysis of experimental observations and critical assessment
of existing models for oxide fuel structure evolution under operation
conditions of fast reactors, new models for fuel restructuring and coring are
proposed. The restructuring model describes coherent motion in the temperature
gradient of various voids (gas bubbles, sintering pores and large lenticular
pores) and grain boundaries, to which the voids are attached. As a result, the
model explains elongation of thermally growing equiaxed grains and formation of
columnar grains, and predicts a rapid formation of extended columnar grain zone
during a relatively short initial period of fast reactor irradiation. The
coring model describes formation and growth of the central void in the fuel
pellet, activated by mass transport from the inner to the outer zone of the
pellet under stresses induced by inhomogeneous fuel densification in the
initial period of irradiation.Comment: 17 pages, 7 Figure
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Few-Body Collisions in a Quantum Gas Mixture of 40K and 87Rb Atoms
This thesis describes experiments on few-body interactions in a mixture of ultracold bosonic 87Rb and fermionic 40K atoms. Ultracold atoms are celebrated as a platform to explore fundamental quantum physics because their internal states, external potentials, and interactions with each other can be controlled by straightforward electrical and optical tools. In the case of Bose-Fermi mixtures, control comes at the cost of stability when strong interactions give rise to inelastic collisions that destroy the atomic samples. I present detailed measurements of the cross sections for these inelastic processes and discuss how they fit into an emerging pattern of resonances that is leading to better models of few-atom interactions. These phenomena have their roots in universal Efimov physics, which is a paradigm of quantum three-body interactions that was originally inspired by exotic nuclear states. I also discuss several methods to control the formation of KRb Feshbach molecules within the finite lifetime of the interacting K and Rb atomic mixture
Individual addressing of ions using magnetic field gradients in a surface-electrode ion trap
Dense array of ions in microfabricated traps represent one possible way to
scale up ion trap quantum computing. The ability to address individual ions is
an important component of such a scheme. We demonstrate individual addressing
of trapped ions in a microfabricated surface-electrode trap using a magnetic
field gradient generated on-chip. A frequency splitting of 310(2) kHz for two
ions separated by 5 um is achieved. Selective single qubit operations are
performed on one of two trapped ions with an average of 2.2+/-1.0% crosstalk.
Coherence time as measured by the spin-echo technique is unaffected by the
field gradient.Comment: 3 pages, 3 figures; submitted to AP
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