Scientific Series

What if the second law of thermodynamics, in the hierarchy of physical laws, were at the same level as the fundamental laws of mechanics, such as the great conservation principles? What if entropy were an intrinsic property of matter at the same level as energy is universally understood to be? What if irreversibility were an intrinsic feature of the microscopic dynamical law of all physical objects, including an individual qubit or qudit? This talk will show how positive answers to these questions need not contradict any of the known results of quantum mechanics. We construct a logically consistent, mathematically sound and definite, physically intriguing, non-relativistic and non-statistical quantum theory, in which the second law of thermodynamics is embedded as a fundamental microscopical law. The theory hinges upon a nonlinear extension of unitary Hamiltonian dynamics which for uncorrelated and noninteracting systems reduces to the usual Schroedinger equation for the zero entropy states, but in general generates a group (not a semi group) of irreversible time evolutions, where the non-Hamiltonian entropy generating term in the evolution equation attracts the state towards the direction of maximal entropy increase. Various examples and features of this highly non-conventional dynamical theory are discussed. References available at http://www.quantumthermodynamics.org/

Renner\'s global quantum de Finetti theorem establishes that if the state of a quantum system is invariant under permutations of its systems, then almost all of its subsystems are almost in the same state and independent of each other. Motivated by this result, we show that the most straightforward classical analogue of Renner\'s theorem is false. Joint work with Matthias Christandl (Cambridge).

This will be an introductory talk about Topological Quantum Computation. TQC is attractive because it is intrinsicaly decoherence free. We introduce the basic notions, such as non abelian anyons, quantum symmetries and topological order. A topologically ordered phase is a gapped phase in which the basic degrees of freedom are of a topological nature (denoted as anyons), charactetized by their fusion and braiding properties. If time permits possible implementations based on Quantum Hall systems will be discussed as well.

I will discuss a new method of inflaton potential reconstruction that combines the flow formalism, which is a stochastic method of inflationary model generation, with an exact numerical calculation of the mode equations of quantum fluctuations. This technique allows one to explore regions of the inflationary parameter space yielding spectra that are not well parameterized as power-laws. We use this method to generate an ensemble of generalized spectral shapes that provide equally good fits to current CMB and LSS as data as do simpler power-law spectra. Within this ensemble are spectra that exhibit a strong running on large angular scales (where cosmic variance is large) that turns off on small scales. Such strongly running spectra are accompanied by large tensor components that lie outside the 1 and 2 sigma limits of current WMAP3 and SDSS data. This demonstrates that the generalization of the spectral shape adversely impacts our ability to constrain key inflationary observables. The inflationary models giving rise to such spectra are characterized by an initially fast rolling inflaton, in marked contrast to the dominant paradigm of slow roll inflation.

If some form of string theory indeed describes the ultra high energy physics of our universe, then there are two ingredients which are very likely to remain at low energies. The first, is a fifth force in the form of an additional abelian gauge group. The second, and more dramatic, is supersymmetry. Both may be observed at the upcoming Large Hadron Collider. In this talk I will explore a possible intimate connection between these two ingredients which leads to surprising predictions.

The one clean qubit model is a model of quantum computation in which all but one qubit starts in the maximally mixed state. One clean qubit computers are believed to be strictly weaker than standard quantum computers, but still capable of solving some classically intractable problems. I\'ll discuss my recent work in collaboration with Peter Shor which shows that evaluating a certain approximation to the Jones polynomial at a fifth root of unity for the trace closure of a braid is a complete problem for the one clean qubit complexity class.

The simplest algebraic curves of genus one are the nonsingular cubics in two-dimensional complex projective space. Interpreting CP^2 as the space of pure quantum states associated with a Hilbert space of dimension three, I will show how various properties of d=3 symmetric informationally complete positive operator valued measures can be understood in terms of the geometry of such curves. The resulting structure, although of considerable complexity, is very beautiful from a mathematical perspective.

The detection of primordial non-Gaussianity could provide a powerful means to rule out various inflationary scenarios. Although scale-invariant non-Gaussianity is currently best constrained by the Cosmic Microwave Background, single-field inflation models with changing sound speed can have strongly scale dependent non-Gaussianity. I will discuss the theoretical motivation for such models and present work on the likely ability of current and future large scale structure measurements to constrain them.

I will present a construction of supersymmetric Wilson loop operators in N=4 SYM for an arbitrary path on an S3 subspace of space-time. I will show how they are evaluated in AdS and in particular that the string world-sheet is a generalized calibration with respect to an almost-complex structure associate to the supersymmetries preserved by the loop. I will present some special examples and in the case when the loop is restricted to an S2, some evidence that the calculation reduces to a perturbative calculation in YM in 2-dimensions on S2. This in turn is exactly soluble in terms of a 0-dimensional matrix model.

We give a convenient representation for any map which is covariant with respect to an irreducible representation of SU(2), and use this representation to analyze the evolution of a quantum directional reference frame when it is exploited as a resource for performing quantum operations. We introduce the moments of a quantum reference frame, which serve as a complete description of its properties as a frame, and investigate how many times a quantum directional reference frame represented by a spin-j system can be used to perform a certain quantum operation with a given probability of success. We provide a considerable generalization of previous results on degradation of reference frame, from which follows a classification of the dynamics of spin-j system under the repeated action of any covariant map with respect to SU(2). Joint work with Lana Sheridan, Martin Laforest and Stephen Bartlett

Laser cooling and precision spectroscopy provide powerful tools for exploring quantum measurement and metrology using atoms as sensors. In this talk I will discuss our ongoing work to bring together abstract ideas of quantum parameter estimation and concrete physical details of atom-photon interactions in the specific context of magnetometry. I will also present some new ideas on how laser probing of cold atoms could provide a basis for developing entanglement-enhanced spin gyroscopes.