Talks

Recent advances in scaling photonics for universal quantum computation spotlight the need for a thorough understanding of practicalities such as distinguishability in multimode quantum interference.  Rather than the usual second quantized approach, we can bring quantum information concepts to bear in first quantization.  Distinguishability can then be modelled as entanglement between photonic degrees of freedom, where loss of interference is caused by decoherence due to correlations with an environment carried by the particles themselves.  This shows that multiparticle, multimode Fock states can have natural Schmidt decompositions, corresponding to what has been called unitary-unitary duality in the representation theory of many-body physics.  
We present results along two lines: 
(i) Generalised Hong-Ou-Mandel interference as unambiguous state discrimination (arXiv:1806.01236, with S. Stanisic); we find optimised interferometers that discriminate between indistinguishable and interesting distinguishable states by projecting onto non-symmetric irreps of the unitary group of interferometers.  We give analytic and numerical results for up to nine photons in nine modes.
(ii) Quantum simulation of noisy boson sampling (arXiv:1803.03657, with A. Moylett); we provide quantum circuits that simulate bosonic sampling with arbitrarily distinguishable particles, based on the quantum Schur-Weyl transform.  This makes clear how particle distinguishabililty leads to decoherence in the standard quantum circuit model.  We show how ideal samples can be postselected, how photon loss can also be modelled, and how standard results in the classical simulation of noisy quantum computations fail to apply to the boson sampling case.

Infrared sensitivity of the de Sitter decay rate due to particle creation requires that gravitational backreaction be taken into account on the horizon scale. At lowest order, backreaction can be studied by Linear Response of the geometry to quantum matter perturbations around the Bunch-Davies state. In Linear Response the scalar degree of freedom derived from the conformal anomaly gives rise to scalar gravitational waves that grow without bound on the de Sitter horizon scale,  which implies substantial non-linear quantum backreaction effects in cosmology. Fluctuations in the conformalon scalar are potentially responsible for the primordial density fluctuations observed in the CMB anisotropy without an inflaton, and can also lead to dynamical, spacetime dependent dark energy. Possible observational tests of the conformal origin of primordial density fluctuations is suggested by the prediction of equality of scalar and tensor weight spectral indices, and the bispectral shape function of non-Gaussian correlations in the CMB predicted by conformal invariance.

Over the last few years gravitational wave (GW) detections have marked
the beginning of a new era of astrophysical observations. When the
emitters include a compact object like a neutron star, the GW signal
is accompanied by emissions in different bands, e.g. X-rays,
gamma-rays, optical and neutrinos. The interpretation of such
multimessenger signals allows us to gain a deeper understanding of the
interiors of compact objects. One main challenge is to link our
knowledge of nuclear interactions to macroscopic properties of dense
objects in the Universe. In this talk I will discuss selected aspects
along the interface of nuclear physics and astrophysics