Hopf solitons arise in deformations of a deformed version of the O(3) sigma model and possess topological charge described by the Hopf index, which cannot be written locally. Gorsky Shifman and Yung proposed a more field-theoretical model that reduces to the sigma model at hand, and constructed within it an object with a non-zero Hopf index. We describe the construction of such an object, in particular in the case where it can be considered to be BPS i.e. acquiring extra stability through preservation of supercharges in the theory.

The evolution of quantum systems is characterised by the interference of coherent amplitudes associated with single or many-particle paths. By fixing the phase relation between these amplitudes, symmetries of the system fashion the resulting dynamics. I will briefly mention two examples where the exchange symmetry between identical particles plays an important role: totally destructive many-particle interference and dynamics of partially distinguishable bosons. I will then focus on a single-particle problem inspired by energy transport in biological photosynthetic complexes: the transfer of an excitation across a random network of two-level systems. Symmetry of the Hamiltonian under reflection is known to favour efficient transport despite the disordered nature of the system. However, this is incompatible with the large detuning of excitation energies observed in biological systems, where coupling to vibrational degrees of freedom is necessary to bridge the energy gap. We therefore consider periodically driven random networks and show that a generalised symmetry on Floquet-Hilbert space leads to an enhancement of the transfer probability.

When the temperature of a supercooled liquid is reduced below its melting point, its relaxation time skyrockets on a small temperature window. An explanation for this phenomenon is still under debate but one of the competing theories (RFOT) predicts the existence of a thermodynamic phase transition at a (Kauzmann) temperature below the experimental glass transition. The order parameter of this putative phase transition is the degree of similarity (or overlap) between a liquid configuration and a reference quenched equilibrium configuration. When a thermodynamic field is coupled to the overlap, a first-order transition line is expected to emerge from the Kauzmann point and to end by a critical point at a higher temperature and finite coupling. I will present computer simulations of a model glass-former in 3 dimensions that support the existence of this critical point and the associated first-order transition line.

$T^3$ in Grande Ourse*
Nishita will teach us how to use
CheckMATE https://arxiv.org/abs/1611.09856 :

*tea/coffee/biscuits break around 4pm

Optical metamaterials are man-made materials exhibiting unusual optical properties. They behave as homogeneous materials (at least for some frequency ranges) whereas their optical properties arise from multiple scattering events i.e. from collective effects. This is illustrated in Fig.1.

It has been suggested recently to introduce quantum emitters into metamaterials [1,2] in order to benefit from quantum effects to manipulate the propagation of light. As an example, quantum metamaterials could present a photonic band-gap oscillating in time with the Rabi frequency of the quantum emitters [2].

The starting point of the modeling of quantum metamaterials is the choice of a Hamiltonian that describes the quantum dynamics of the emitters and the (quantized) electromagnetic field. Although this seems as old a topic as quantum optics itself, we will show that this choice is not free of “surprises”. We will explain that a result as old as the Power-Zienau-Woolley transform[3], intensively used to model the interaction of the quantum electromagnetic field with matter, is actually not correct [4]. Indeed, we will show that it breaks the gauge-independence of the electromagnetic field and we will draw the consequences of the breaking of the gauge-independence at the quantum level.

References

1. A. M. Zagoskin, D. Felbacq, and E. Rousseau, EPJ Quantum Technol. 3, 2 (2016).

2. A. L. Rakhmanov et al., Phys. Rev. B 77, 144507 (2008)

3. E. A. Power and S. Zienau, Phil. Trans. Roy. Soc. Lond. A: Math., Phys. and Eng. Sc. 251, 427-454 (1959)

4. E. Rousseau and D. Felbacq Sci. Rep. 7:11115 (2017)

Nitrogen-vacancy defects (NV) in diamond are potent sensors for a variety of measures, ranging from magnetic and electric fields to temperature and strain. For sensing applications, a most remarkable property is their atomic size. As the spin is essentially confined to a single diamond lattice site, the NV can be regarded as a point sensor. Using multiple NV defects provides spatial information, as signals can be correlated with the respective defect position. Nanoscale constellations of NV defects offer the potential to reconstruct complex environments, as long as the relative positions of NVs are known and information can be extracted from individual NVs.

Strong magnetic field gradients and superresolution microscopy offer adequate solutions for these tasks. Magnetic gradients on the order of 100 μT/nm, such as the write field of commercial hard disk drive recorders, affect separated spins individually through their Zeeman interaction. Effectively, overlapping spin resonances can be separated and addressed. Moreover, the large bandwidth and high field strengths enable additional degrees of freedom to manipulate single spins [1,2]. Superresolution microscopy, on the other hand, allows to image and localize defects beyond the diffraction limit of optical wavelengths. Here, methods that allow optically detect magnetic resonance of NV spins, such as charge state depletion microscopy [3], are of particular interest to read out information from single NVs.

Combined, these methods enable selective manipulation, read-out, and localization of closely spaced spins and can ultimately offer a unique glimpse in the nanoscale world.

[1] I. Jakobi, et al. Nature Nanotechnology 12, 67-72 (2017)
[2] S. Bodenstedt, et al. Nano Letters 18, 5389-5395 (2018)
[3] K. Y. Han, et al. Nano Letters 10, 3199-3203 (2010)

It is known that purely repulsive self-propelled colloids can undergo bulk liquid-vapor phase separation. In experiments and large scale simulations, however, more complex steady states are also seen, comprising a dynamic population of dense clusters in a sea of vapor, or dilute bubbles in a liquid. Here we show that these microphase-separated states should emerge generically in active matter, without any need to invoke system-specific details. We give a coarse-grained description of them, and predict transitions between regimes of bulk phase separation and microphase separation. We achieve these results by extending the \phi^4 field theory of passive phase separation to allow for all local currents that break detailed balance at leading order in the gradient expansion. These local active currents, whose form we show to emerge from coarse-graining of microscopic models, include a mixture of irrotational and rotational contributions, and can be viewed as arising from an effective nonlocal chemical potential. Such contributions influence, and in some parameter ranges reverse, the classical Ostwald process that would normally drive bulk phase separation to completion.

Uncovering the nature of the electroweak (EW) phase transition in the early Universe would be key to shed light on the origin of the cosmic matter-antimatter asymmetry, and on the cosmological history of the Higgs vacuum. I will discuss searches for new physics beyond the Standard Model at the LHC as a probe of the EW phase transition, as well as the prospects for future colliders such as FCC-ee/hh and CLIC. I will also discuss their complementarity to gravitational wave searches with the future space-based observatory LISA, which will directly access the EW epoch of the early Universe.

We develop and extend a method to compute the local yield stresses at the atomic scale in model two-dimensional Lennard-Jones glasses produced via differing quench protocols [1-2]. This technique allows us to sample the plastic rearrangements in a non-perturbative manner for different loading directions on a well controlled length scale. Plastic activity upon shearing correlates strongly with the locations of low yield stresses in the quenched states. The distribution of local yield stresses is also shown to strongly depend on the quench protocol: the more relaxed the glass, the higher the local plastic thresholds. The local yield stress varies with the shear orientation tested and strongly correlates with the plastic rearrangement locations when the system is sheared correspondingly. Plastic rearrangements appear therefore as the consequence of shear transformation zones encoded in the glass structure that possess weak slip planes along different orientations. We then justify the length scale employed in this work and extract the yield threshold statistics as a function of the size of the probing zones. Finally, the evolution of local yield stress distributions upon shear deformation is studied [3]. We show how the plasticity-induced anisotropy in amorphous solids can be explained by the depletion of the soft zones in the direction of deformation with respect to the reverse direction. In the most relaxed system, systematically presenting a localization of the plastic deformation through the formation of a shear band, the persistence of plastic activity is explained by local softening. It is thus argued that this method makes it possible to derive physically grounded models of plasticity for amorphous materials by directly revealing the relevant details of the shear transformation zones that mediate this process.