Quantum optics with many-body systems of atoms and photons: From quantum networks to entanglement measurement
le jeudi 22 juin 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé :

The physics of light-matter interactions plays a fundamental role for two important research areas in quantum technology: quantum information and quantum simulation.
On the one hand, quantum optics theory allows to design robust protocols for the processing of quantum information in quantum networks.
On the other hand, in the context of quantum simulation, light-assisted interactions between atoms provide the toolbox to prepare and probe many-body phases of complex Hamiltonians, which are related to long-standing problems in condensed matter (e.g. quantum magnetism or fractional quantum Hall states).

In this talk, I will discuss these two topics. I will present our recent results on quantum information processing in quantum networks, and on the engineering of new tools for quantum simulation.
At the technical level, I will show how we combine atomic physics, quantum optical techniques and numerical methods borrowed from condensed matter physics (such as Matrix-Product-State (MPS) techniques) to study these types of complex open many-body systems.

In the first part I will discuss some of our works related to photonic quantum networks.
After a general introduction, I will present a theoretical description of these systems based on the formalism of waveguide QED.
This will then allow me to present recent results on the development of robust Quantum State Transfer protocols [1,2,3], and to introduce our MPS techniques for the description of the dynamics of quantum networks beyond the standard treatment of quantum optics.

The second part of the seminar will be devoted to quantum simulators. I will first explain the challenge of measuring entanglement, which is essential to characterize various phases in condensed matter physics (such as Fractional Quantum Hall effect or Haldane phase).
I will then show how to measure the entanglement spectrum of ground states of generic Hamiltonians based on direct engineering of the entanglement Hamiltonian [4].
Our method, based on the Bisognano-Wichmann theorem [5], allows one to measure entanglement spectra via standard spectroscopy and can be implemented in all quantum simulation platforms.
I will provide numerical examples to support this result and give examples of AMO implementations of entanglement Hamiltonians.
If time allows, I will present a complementary method based on Random Matrix Theory, which would allow to measure the entanglement growth in a many-body localised (MBL) system [6].

[1] C Dlaska, BV and P. Zoller et al Quantum Sci. Technol. 2 015001 (2017).

[2] BV, PO Guimond, H. Pichler and P. Zoller et al Phys. Rev. Lett. 118, 133601 (2017).

[3] Berit Vogell, BV, T. Northup, B. Lanyon and C. Muschik arxiv:1704.06233.

[4] M. Dalmonte, BV and P. Zoller, in preparation.

[5] Bisognano and Wichmann, J. Math. Phys. 17, 303 (1976).

Many-Body Quantum Physics with Photons
le vendredi 23 juin 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : We review recent developments in the context of many-body quantum physics with microwave photons in superconducting quantum electrodynamics networks and Josephson junction arrays. First, we show how the Jaynes-Cummings lattice model yields an analogy with the Bose-Hubbard model and can allow to engineer a Mott-superfluid transition of photons. We discuss the challenges to achieve such a transition, requiring the coupling to AC perturbations and the necessity to include dissipation effects. We also discuss progress in methods and probes. Then, we discuss realizations of topological phases and robust photonics by analogy to progress in quantum materials and ultra-cold atoms, and address disorder and interaction effects. We also show the simulation of novel topological chain devices with superconducting and Josephson circuits. Experimental progress and realizations are discussed. Such systems also offer novel platforms to address and probe dissipative and driven quantum impurity physics, such as the Kondo effect.

Ground-state and asymptotic dynamical properties of 1D ultracold gases in the presence of a mobility edge
le lundi 26 juin 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Anna Minguzzi ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : In the first part of the talk we explore the ground-state properties of cold atomic gases focusing on the cases of
noninteracting fermions and hard-core (Tonks-Girardeau) bosons, trapped by the combination of two potentials
(bichromatic lattice) with incommensurate periods. In the tight-binding limit, the single-particle states in the
lowest occupied band show a localization transition, as the strength of the second potential is increased above a
certain threshold. In the continuum limit, when the tight-binding approximation does not hold, a mobility edge is
found, instead, whose position in energy depends upon the strength of the second potential. Here, we study how
the crossover from the discrete to the continuum behavior occurs, and prove that signatures of the localization
transition and mobility edge clearly appear in the generic many-body properties of the systems. Specifically, we
evaluate the momentum distribution, which is a routinely measured quantity in experiments with cold atoms,
and demonstrate that, even in the presence of strong boson-boson interactions (infinite in the Tonks-Girardeau
limit), the single-particle mobility edge can be observed in the ground-state properties. In the second part we
study the dynamical many-body response of for a one-dimensional fermionic gas in a mono- and bi-chromatic
optical potential following the sudden switching-on of a delta-like barrier at some at the center of the system.
Specifically we look at the Loschmidt echo as a figure of merit to characterize the response of the system and
its long time behavior. In order to evaluate the echo we employ two complementary approaches: (1) functional
determinants (Levitov) which gives the exact numerical solution for time- and therefore frequency-resolved
responses and (2) a perturbative approach (Linked Cluster Expansion) which provides an accurate evaluation
of the contribution of different physical processes involved in the dynamics. Again we focus on the two limits
of tight-binding and continuum showing that the phenomenon of the orthogonality catastrophe can be observed
in such systems which, unlike their condensed matter counterpart, are nowadays created and controlled with a
very high accuracy.

The arrow of time for continuous quantum measurements
le vendredi 30 juin 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The question of the time reversibility of quantum mechanics with
measurements is one that has been debated for some time. In this
talk, I will present new work exploring our ability to distinguish the
forward from the time-reverse measurement records of continuous
quantum measurements. The question involves both the conditions for
the time-reversibility of the quantum trajectory equations of motion,
as well as statistical distinguishability of the arrow of time. For a
continuous qubit measurement example, we demonstrate that
time-reversed evolution is physically possible, provided that the
measurement record is also negated. Despite this restoration of
dynamical reversibility, a statistical arrow of time emerges, and may
be quantified by the log-likelihood difference between forward and
backward propagation hypotheses. We then show that such reversibility
is a universal feature of non-projective measurements, with forward or
backward Janus measurement sequences that are time-reversed inverses
of each other.
J. Dressel, A. Chantasri, A. N. Jordan, A. N. Korotkov, arXiv:1610.03818

Maximally entangled states, pair-superfluidity and MORE in a many-body interacting system
le vendredi 30 juin 2017 à 13h30

Séminaire LPMMC

Personne à contacter : Dominique Spehner ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : In this talk I will present interesting results about the study of quantum correlations
between two species of ultra-cold bosons living on a ring lattice. In the first part, I
going to show that the presence of synthetic magnetic fields can lead to the formation
of entangled states between pair of qudits (high dimensional qubits). Notably,
maximally entangled eigenstates are possible to find for well-defined values of the
Aharonov-Bohm phase of the synthetic magnetic field, which are zero-energy
eigenstates of both the kinetic and interacting parts of the Bose-Hubbard Hamiltonian
[1]. This latter property makes them exeptional and robust for applications. In the
second part, I will focus on the eigenstates of the lowest-energy band in the regime
of large interaction where a pair-superfluid phase naturally emerge for the ground
state. In this scenario, the analysis of the interference pattern in the momentum
distribution indicates a strong connection between entanglement and the pair-
superfluid phase. This is further highlighted by the fact that for maximally entangled
eigenstates any single order tunneling process is naturally suppressed [2]. Thus the
observation of features of a pair-superfluid behavior can be used as a signature of
the presence of entanglement. This might be an important tool for the
characterization of the entanglement in the ground state. Finally, I will discuss the
perspective of using this setting with two type of particles as a benchmark to
investigate the connection between phase coherence and entanglement in many-
body quantum systems.

References

[1]
S. A. Reyes, L. Morales-Molina, M. Orszag, and D. Spehner, EPL 108, 20010
(2014).

[2] L. Morales-Molina, S. A. Reyes and E. Arevalo, EPL 115, 36004 (2016).