Meet our team!

#### Christian MINIATURA

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### Education

- Alumnus ENS Cachan
- B.Sc. (Physics), M.Sc., ENS Cachan and University Paris 11 (1981-1986)
- PhD (Physics), University Paris 13 (1990)
- French Habilitation (Physics), University of Nice Sophia (2001)

### Professional Status

- Employed by CNRS since 1989
- Research Director since 2005
- Research Visiting Professor at CQT (FoS, NUS, since 2008) and PAP (SPMS, NTU, since 2016)
- Fellow of Institute of Advanced Studies (NTU, since 2009)

### Awards

CNRS Young Researcher Award (aka CNRS Bronze Medal) in 1994 for my experimental and theoretical work on atomic interferometry

### Research Areas

Ultracold matter, Quantum transport, Artificial gauge fields, Light-Matter interactions

### Social Media

### Recent Research Interests

From 2004 onwards, my theoretical research interests have mainly focussed on quantum transport and strongly-correlated systems from a quantum gases perspective.

Indeed, the remarkable degree of control and precision achieved nowadays by experiments using ultracold quantum gases (Bose condensates, degenerate Fermi gas) has allowed the systematic study of effects previously observed and studied in condensed matter.

Paradigmatic examples are temeprature-driven and quantum phase transitions (Mott-Hubbard, Kosterlitz-Thouless), graphene physics, artificial gauge fields and Anderson localization. Below is a brief summary of some of my favourite research topics.

**Weak and strong localization of matter waves in an optical speckle**

Interference phenomena surviving statistical averages in spatially random media are of primary importance in order to understand the transport properties of bulk matter. Already in weakly disordered media, spectacular deviations from the conventional Boltzmann picture of transport have been discovered and studied.

Important examples include weak localization and universal conductance fluctuations in mesoscopic electronic systems, or coherent backscattering (CBS) and intensity correlations in speckle patterns in the context of wave transport in disordered media. In strongly scattering systems, transport can even be completely suppressed, due to Anderson localization (AL).

For several years now, this phase-coherent inhibition of transport by disorder has been under active scrutiny in widely different systems using light, ultrasound, microwaves, and more recently ultracold atoms.

Our main works in the field address the momentum distribution of a coherent wave packet launched with finite velocity inside a random potential.

Phase-coherent multiple scattering effects are clearly evidenced by the CBS peak, originating from the constructive interference of counterpropagating scattering amplitudes.

At the onset of AL, the amplitudes cease to propagate, and the emergence of a novel interference effect, the coherent forward scattering (CFS) peak is triggered in the opposite, forward direction. As a net effect, the momentum distribution shows a remarkable twin-peak structure, a genuine signature of Anderson localization in the bulk.

It turns out that the CBS width and the CFS contrast provide critical quantities from which the critical exponents of the 3D AL can be extracted through finite-time scaling.

**Artificial gauge fields**

Ultracold quantum gases provide a rather unique testing bed where theorists’ dreams can be turned into carefully designed experimental situations.

As such quantum Hall effects did not escape the trend, with the catch, however, that atoms are neutral. One thus needs to mimic gauge fields that give rise to effective magnetic fields acting on the atoms. Versatile and promising schemes have thus been introduced, some even realized, based on light-atom interactions.

These light-induced artificial gauge fields, encompassing Abelian and non-Abelian situations, have opened the door to a whole class of model Hamiltonians that are addressing diverse physical situations ranging from artificial Dirac monopoles, spin-orbit coupling and topological phases, to non-Abelian particles, and mixed dimensional systems.

Our main works in the field address laser configurations able to generate an artificial U(3) monopole or a U(3) spin-orbit coupling for non-interacting atoms, and U(2) gauge fields generating nontrivial spin textures (Mermin-Ho vortex and vortex-antvortex pairs) for the interacting groundstate of a 2-component spinor condensate loaded in a 2D harmonic trap.

**Superradiance and Self-organization**

Cavity QED (CQED) investigates the interaction of atoms with confined electromagnetic field modes.

These hybrid systems open the way to new physical situations since the usual free space dipole force experienced by atoms is strongly enhanced and, at the same time, the back-action of atoms on the confined light field cannot be ignored any longer. As a consequence both atoms and light must be treated on the same footing and the dynamics for the atomic motion and the cavity field becomes strongly nonlinear.

In the dispersive regime, the light fields impart forces on the atoms which thus move. In turn, the light fields pick up phase shifts induced by the refractive index of this moving atomic dielectric medium. This alters the light forces, thus the atomic motion, thus the accumulated dispersive phase shifts, and this combined atom-field process loops self-consistently.

As a result, when the pump field strength exceeds some critical value, the atomic cloud scatters constructively the pump photons into the cavity modes (superradiance) and the atoms achieve self-organization into the effective optical lattice created by the pump and cavity fields.

Our main work in the field has been to examine the conditions for these atoms to self-organize into stable 2D triangular and honeycomb lattices (the graphene lattice). We have also studied how to drive atoms from one lattice structure to another by dynamically changing the phase of the cavity fields with respect to the pump laser.

**Graphene-type physics with cold atoms**

In 2004, researchers in Manchester isolated one-atom thick sheets of carbon atoms, with the atoms organized in a planar honeycomb structure.

Graphene is of utmost importance in condensed-matter physics since by stacking it one gets the graphite structure and by wrapping it one gets carbon nanotubes and fullerenes. Graphene is also of great theoretical interest because it provides a physical realization of two-dimensional field theories with quantum anomalies.

Indeed, the effective theory that describes the low-energy electronic excitations in graphene is that of two-dimensional massless Weyl-Dirac fermions. Triggered by the Manchester discovery (which led to Geim and Novoselov sharing the 2010 Nobel Prize for Physics), an intense activity has flourished in the field.

Our main work in the field was to highlight that some important features of the graphene physics could be reproduced by loading ultracold atoms in a two-dimensional optical lattice with honeycomb symmetry.

We have in particular shown that the Dirac points were well protected from experimental imperfections (imbalance of laser intensities, angle mismatches), hence demonstrating its experimental feasibility in the cold atom community. The first experiment were later conducted by Esslinger’s group at ETH (Zurich).

### Selected Publications

1) Coherent Backscattering of light by cold atoms, G. Labeyrie et al., PRL 83, 5266 (1999).

2) Multiple scattering of light by atoms with internal degeneracy, C. A. Müller and C. Miniatura, J. Phys. A: Math. Gen. 35, 10163 (2002).

3) Localization of Matter Waves in 2D-Disordered Optical Potentials, R. C. Kuhn et al., PRL 95, 250403 (2005).

4) Coherent forward scattering peak induced by Anderson localization, T. Karpiuk et al., PRL 109, 190601 (2012).

5) U(3) artificial gauge fields for cold atoms, Y.-X. Hu, Ch. Miniatura, D. Wilkowski, and B. Grémaud, PRA 90, 023601 (2014).