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 quais-1D configuration even if the on-site 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 time-independent coupled-channels 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 fault-tolerantly - the ideal quantum computation must be reliably performed using non-ideal components. The threshold theorem for quantum computation asserts that this is possible with quantum error-correction, given reasonable assumptions about errors occurring in the underlying hardware. Estimates for fault-tolerant 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 vice-versa. 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 two-level quantum system interacting with a bath of harmonic oscillators. Femtosecond laser spectroscopy experiments give a parametrisation of the spectral density describing the system-environment 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, quant-ph/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 spin-1/2 pyrochlore antiferromagnet in which some of these notions are fleshed and illustrate that this antiferromagnet displays an exact deconfined critical point.

Robert Raussendorf: On measurement-based quantum computation with the toric code states
We study measurement-based quantum computation (MQC) using as quantum resource the planar code state on a two-dimensional 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 bi-partite entanglement in the planar code state is large. Diverging amounts of entanglement, measured in terms of the so-called 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 black-box 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 spin-glass 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: Non-Abelian Anyon Interferometry
Topologically-ordered states supporting excitations with non-Abelian braiding statistics are expected to occur at several observed fractional quantum Hall plateaux. I will discuss interferometric experiments designed to detect such non-Abelian quasiparticle statistics -- one of the hallmark characteristics of the Moore-Read and Read-Rezayi 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 non-Abelian 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 low-T 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 non-local?)

Carl Williams: Pairing and structure in trapped atomic systems

Fei Zhou: Tunable Quantum-Fluctuation-Controlled Coherent Spin Dynamics