December 13, 2007, Ampel 311, UBC

Titles and Abstracts
Please note that we are still waiting for more titles and
abstracts.
Mahananda Dasgupta: Decoherence in nuclear fusion?
New measurements of fusion cross sections at energies well below (pure
tunnelling regime) and well above the fusion barrier are not
simultaneously explained [1] by the current, most successful model of
nuclear reactions. In the current models the colliding nuclei are assumed
to be in a coherent linear superposition of the nuclear states until they
reach a critical distance where fusion is simulated by introducing a
sudden onset of dissipation. The fundamental principle that dissipation
cannot occur without decoherence is ignored. It is proposed that a new
approach explicitly including gradual decoherence is needed to obtain a
consistent description of nuclear fusion. An understanding of decoherence
in nuclear fusion has implications in nucleosynthesis and may provide an
unique opportunity to investigate decoherence where the system is isolated
from an "external" environment.
[1] M. Dasgupta et al., Phys. Rev. Lett. 99 (2007) 192701
Luming Duan: Quantum simulation with ultracold atoms and supersolid
from general Hubbard model
With a few examples, I will show how to use ultracold atoms to simulate
various strongly correlated models and how to detect the resulting phases.
In particular, I will show that a general Hubbard model with particle
assisted tunneling will emerge as an effective Hamiltonian to describe
strongly interacting fermions (with Feshbach resonance) in an optical
lattice. Numerical simulation shows this Hamiltonian has a possible
supersolid phase under a quais1D configuration even if the onsite
interaction is repulsive.
David John Hinde: Coherence and Decoherence in Collisions of Complex
Nuclei
Atomic nuclei are complex mesoscopic systems with many (but finite)
internal degrees of freedom. Nuclear collisions may involve reversible
processes (e.g. excitation to the first few excited states, or
irreversible processes (e.g. fusion or breakup). The coherent
timeindependent coupledchannels model has successfully described many
aspects of nuclear collisions, including the direct measurement of the
eigenchannels of the system at the Coulomb barrier. Our recent
measurements probing closer to the region of irreversibility (inside the
barrier) show significant departures from this picture, which we interpret
for the first time as evidence of decoherence, and breakdown of the
"standard model" of nuclear reactions.
Andrew Hines: Decoherence in quantum walks and quantum
computers
To ever become reality, a quantum computer must be implemented
faulttolerantly  the ideal quantum computation must be reliably
performed using nonideal components. The threshold theorem for quantum
computation asserts that this is possible with quantum errorcorrection,
given reasonable assumptions about errors occurring in the underlying
hardware. Estimates for faulttolerant thresholds depend greatly on both
the error model employed and only recently have error models which make
some direct connection with physical models of decoherence been
considered. However, when applied to realistic quantum environments, these
results have little relation to standard decoherence calculations. We
approach this problem by looking at 'quantum walks'. Quantum walks refer
to the dynamics of a particle on some arbitrary mathematical graph.
One can make a lot of useful mappings between qubit Hamiltonians,
Hamiltonians for spin chains and other spin networks, quantum gate
systems, and quantum walk Hamiltonians. In the study of decoherence such
mappings can be very useful, since powerful techniques can be brought to
bear on the dynamics of quantum walks in the presence of an environment. I
will describe how one may systematically map between quantum walk
Hamiltonians and qubit circuits, and viceversa. Decoherence and
dissipation in the walker dynamics is described by couplings to a quantum
environment, composed of either oscillators or spins, and I will discuss
how generic couplings transform between representations. Using simple
examples, I will illustrate how different implementations of the quantum
walk imply different forms of decoherence, as well as the effects of a
quantum environment.
Ross McKenzie: Electronic excited states in optically active
biomolecules: quantum systems with a tuneable environment
interaction
Optically active molecules (chromophores) are crucial to the function of
wide range of biomolecules. Examples, include the green flourescent
protein, porphyrins associated with photosynthesis, and retinal associated
with vision. The electronic states of the chromophores can be viewed as
discrete quantum systems which are interacting with an environment
composed of the surrounding protein and water. The interaction of the
chromophore with its environment may be modelled quantum mechanically by
an independent boson model which describes a twolevel quantum system
interacting with a bath of harmonic oscillators. Femtosecond laser
spectroscopy experiments give a parametrisation of the spectral density
describing the systemenvironment interaction for a wide range of
chromophores and proteins. This spectral density completely determines the
quantum dynamics and decoherence of electronic excited states. We have
recently proposed and analysed several continuum dielectric models of the
environment[1]. Our results provide a framework to understand experimental
measurements and molecular dynamics simulations, including the relative
importance of the contributions of the protein, the water bound to the
surface of the protein, and the bulk water to decoherence. Our results
show that because biomolecules function in a "hot and wet" environment,
quantum coherence will generally not be significant for processes occuring
slower than a picosend, the timescale for the dielectric relaxation of
water. The "collapse" of the quantum state of the chromophore due to
continuous measurement of its state by the environment occurs on the
timescale of 10's femtoseconds.
[1] J. Gilmore and R.H. McKenzie, quantph/0609075.
Gerard Milburn: Measurement and decoherence in quantum
dots
Recent experimental work on mesocopic quantum dots enable the real time
monitoring of a single quantum dot and coupled quantum dots. In this talk
I will discuss these experiments focussing on measurement induced
decoherence and conditional quantum dynamics. I will also discuss a
molecular nanomechanical device in which transport through a dot is
coupled to the vibrational motion. The system exhibits a limit cycle and
we give a description of decoherence and quantum phase diffusion on the
limit cycle.
Zohar Nussinov: Gauge like symmetries, topological quantum orders,
and deconfinement on pyrochlore lattices
In recent years, the new paradigm of "topological quantum order" (TQO)
has emerged. These orders cannot be characterized by local quantities and
have led to a wealth of ideas and results motivated, in part, by the
prospect of fault tolerant quantum computing. We will show that known
examples of topological quantum order display a symmetry that lies midway
between local symmetries (that of gauge theories) and global symmetries.
Apart from prominent examples of TQO, these symmetries also appear in
orbital models, some frustrated magnets, cold atoms systems, and Josephson
junction schemes. We will further show that by duality transformations,
some of these systems can be mapped onto systems with global symmetries
(and orders). These mappings enable us to assess the effect of thermal
fluctuations. We will present an exact solution to a spin1/2 pyrochlore
antiferromagnet in which some of these notions are fleshed and illustrate
that this antiferromagnet displays an exact deconfined critical point.
Robert Raussendorf: On measurementbased quantum computation with
the
toric code states
We study measurementbased quantum computation (MQC) using as quantum
resource the planar code state on a twodimensional square lattice (planar
analogue of the toric code). It is shown that MQC with the planar code
state can be efficiently simulated on a classical computer. The is
surprising because the planar code state obeys the entanglement area law
[Zanardi04]. That is, the entanglement entropy of a block of spins is
proportional to its perimeter. Thus bipartite entanglement in the planar
code state is large. Diverging amounts of entanglement, measured in terms
of the socalled entanglement width, are required for universality as well
as the hardness of classical simulation of MQC [van den Nest et al.,
2006]. However, our result is that MQC with the planar code state as the
quantum resource can be simulated efficiently classically. This unexpected
property of the planar code state can be attributed to the exact
solvability of the Ising model on a planar graph. Thus, although large
entanglement in the resource state is necessary for universal MQC, it is
not sufficient.
Joint work with Sergey Bravyi, IBM
Barry Sanders: Efficient algorithm for universal simulation of
Hamiltonian evolution
We show how Hamiltonian evolution can be simulated in a blackbox
setting with a cost that is nearly linear in time and nearly constant in
size of the system given a sparse Hamiltonian.
Collaboration with G. Ahokas, D. Berry, R. Cleve, P. Hoyer, and
N. Wiebe.
Moshe Schechter: Quantum phase transitions in anisotropic dipolar
magnets
Anisotropic dipolar systems are emerging as a central system in the study
of classical and quantum phase transitions. As a result of the moderate
strength and the angular dependence of the dipolar interaction, one can
realize in these systems the Ising model in the presence of an effective
random field and an effective quantum term, in both spinglass and
ferromagnetic phases. Moreover, as a result of the strong anisotropy and
the strong hyperfine interactions, the effective fields are independently
tunable, allowing the first realization of the random field Ising model in
a ferromagnetic system, and the study of classical and quantum phase
transitions with controlled randomness.
Lu J Sham: Density functional theory for quantum phase
transitions
Density functional theory changes the functional dependence of the ground
state on the field parameters to that on the conjugate ground state
properties. Does the new dependence throw new light on the quantum phase
transitions? Based on work done with Lianao Wu, Marcelo Sarandy, and
Daniel Lidar.
Kirill Shtengel: NonAbelian Anyon Interferometry
Topologicallyordered states supporting excitations with nonAbelian
braiding statistics are expected to occur at several observed fractional
quantum Hall plateaux. I will discuss interferometric experiments designed
to detect such nonAbelian quasiparticle statistics  one of the hallmark
characteristics of the MooreRead and ReadRezayi states, which are likely
candidates for the observed fractional quantum Hall plateaux at nu=5/2 and
12/5 respectively. Aside from their potential utility for experimental
verification of nonAbelian anyonic statistics, such interferometric
experiments appear to provide the most promising route for qubit read out
in a topological quantum computation. With these potential applications in
mind, I will also address interferometric measurements of states having
superpositions of anyonic charges and discuss their measurement collapse
behavior.
Philip Stamp: Quantum Phase transitions & the Spin Bath:
Dynamics
If one takes a lowT quantum system through a quantum phase transition the
final state depends on both the characteristics of the system (including
the speed at which it is scanned past the critical point) and the nature
of the quantum environment to which it is coupled. Many systems exhibiting
quantum phase transitions couple to a ‘spin bath’ environment – I
will focus here on examples of insulating magnets, but also discuss the
more general case. This theory is relevant to existing experiments on
tunneling magnetic molecules, and possible future experiments on a wide
variety of systems. One can also make useful remarks about adiabatic
quantum computation using this theoretical framework.
Bill Unruh: Bell's Theorem and Locality (is quantum mechanics
nonlocal?)
Carl Williams: Pairing and structure in trapped atomic
systems
Fei Zhou: Tunable QuantumFluctuationControlled Coherent Spin
Dynamics
