The center for Theoretical Physics (CPHT) at Ecole Polytechnique gathers research scientists working in diverse domains of fundamental and applied Physics. The overall coherence is assured by the corpus of common, transposable, mathematical and numerical methods.
CPHT is a joint research unit of CNRS and Ecole Polytechnique, and has a partnership with the Collège de France. His director is Jean-René Chazottes, Senior Researcher at CNRS.
CPHT is on the campus of Ecole Polytechnique, buildings 5 and 6. The reception offices are located in building 6 , offices 06.1046 and 06.1045.
 

Postal Address :
CPHT 
Ecole Polytechnique 
91128 Palaiseau cedex 
France

Secretary phone number : 01 69 33 42 01 (from abroad: +33 169 334 201)

Write an email to someone at CPHT :  : firstname.lastname@polytechnique.edu

 

 

Tuesday, March 12 at 14:30

at CPHT, Conference room Louis Michel

Eleanor Crane (MIT, USA)

Advantages of Digital Qubit-Boson Hardware for Quantum Simulation

Abstract: Finding a straightforward, scalable and universal framework for quantum simulation of strongly correlated fermions and bosons is important from material science to high-energy physics. Here, we develop hybrid qubit-oscillator operations for microwave cavities coupled to transmon qubits required for implementing dynamics of bosonic matter, fermionic matter, and Abelian gauge fields in (2+1)D. We then expand the method to ground state preparation and propose measurement of various long-range correlation functions required for the study of phase transitions. We implement numerical proof of principle experiments for a (1+1)D Z2 Bose Hubbard (BH) gauge theory and the U(1) Schwinger model. We include the main sources of hardware noise, which we mitigate through post-selection based on Gauss' law. This new approach motivates us to uncover the phase diagram of the Z2 BH model, relevant to the Higgs sector. We discover a new phase of matter which exhibits strong density fluctuations which we dub the `clump' phase. Finally, we perform a complexity analysis and find that for one Trotter step of these example models, qubit systems require higher gate counts than our proposal by three orders of magnitude. Our correspondingly higher circuit fidelities may help us to successfully capture the essential physics of these theories in the near-term.

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Le CMAP, le CMLS et le CPHT organisent des journées thématiques dont l'audience visée est un large public de mathématicien-ne-s et de physicien-ne-s. La prochaine journée aura pour thème l'écologie, thème qui sera abordé sous différents points de vue par :

Ada Altieri (Laboratoire Matière et Systèmes Complexes, Université Paris Cité)
Guy Bunin (Physics departement, Technion)
Olivier Hénard (LMO, université Paris-Saclay)
Gaël Raoul (CMAP, CNRS, école polytechnique)

Cette journée aura lieu le 15 novembre 2023 de 10h30 à 16h15.

Plus d'informations sont disponibles à l'adresse suivante :
https://indico.math.cnrs.fr/e/themaEcologie15nov2023

Pour des raisons logistiques, l'inscription est gratuite mais obligatoire. Elle s'effectue à l'adresse suivante :
https://indico.math.cnrs.fr/event/10692/registrations/975/

En espérant vous y voir nombreux et nombreuses,
Pour l'organisation, Anne-Sophie de Suzzoni Jean-René Chazottes Anne de Bouard Cécile Huneau

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Conférence grand public à l'occasion de la fête de la science

Samedi 14 octobre 2023 à 15h15 

Amphithéâtre Becquerel 

Tout est quantique !
par Laurent Sanchez-Palencia

La physique quantique a révolutionné notre compréhension du monde à l’échelle atomique et au-delà. Venez en découvrir les mystères, mais également comment elle a envahi les technologies du quotidien et ses promesses pour une nouvelle révolution technologique.

Plaquette de la fête de la science

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Tuesday, October 17 at 14:00

at CPHT, Conference room Louis Michel

Ilya Kull (University of Vienna)

Lower bounds on ground-state energies of local Hamiltonians through the renormalization group

Abstract: "Quantum many-body systems present us with intractable optimization problems, e.g. finding the ground state of a many-body system. This problem is most often addressed through the variational ansatz approach, where physical insight can guide us in constructing the correct ansatz for a given system. Yet, variational methods can only provide us with a one sided estimate---an upper bound on the ground state energy. To certify a variational solution a lower bound is also required. Lower bounds can be obtained through relaxation methods, in which the optimization problem is simplified by relaxing some of the constraints that define the set of admissible optimization variables. Relaxation methods have been applied to many-body systems since the 50s and have also proven to be an essential tool in tackling various problems in quantum information theory and conformal field theory (the numerical bootstrap). Such methods, however, suffer from the drawback of exponential scaling of their complexity with the accuracy of the solution. In contrast, variational algorithms such as the density matrix renormalization group (DMRG), and other tensor-network algorithms which are based on the renormalization group idea, exhibit polynomial scaling.
In this talk I will describe how one can incorporate the power of the renormalization group approach into the relaxation framework to efficiently compute lower bounds. I will present the results we obtained with this method for translation-invariant spin chains where we observe a polynomial scaling of the complexity with the accuracy.

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Pauline Besserve will defend her thesis on Friday, October 6th in Collège de France (11 place Marcelin Berthelot, 75005 Paris.
The defense will take place in room 4 at 2pm and will be followed by a pot in room 8.

Title: Hybrid quantum-classical algorithms for the quantum many-body problem

Advisors: Michel Ferrero (Centre de Physique Théorique, Collège de France) and Thomas Ayral (Eviden Quantum Laboratory).

The thesis explores the quantum-classical implementation of the celebrated Dynamical Mean-Field Theory to study strong electronic correlations in materials. These exhibit rich phases diagrams, spanning - among others - metallic, insulating, high-Tc superconducting behaviours according to external parameters, but remain notoriously difficult to tackle due to the exponential scaling of computational resources their description involves. Quantum computers are widely hailed as promising devices to ease the bottleneck of DMFT and the first, imperfect prototypes of such devices are now becoming available. The thesis explores the use of such imperfect devices concomitantly with that of classical computational power for DMFT. It addresses both of the impediments to a quantum-classical implementation of DMFT on current devices, namely low coherence time and low qubit count, by setting forth an improved variational state preparation scheme and using hardware noise as a resource to alleviate errors inherent to the low qubit count.

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The 13th CPHT Young Researchers Seminar will be held on Friday the 22nd of September at 15h00 in Salle de conférence Louis Michel: 

Erik Linnér : "A theoretical description of competing instabilities in strongly correlated fermionic systems" 

Matthieu Vilatte : "Probing the asymptotic structure of gravity"

Adrien Loty : "Does string theory saturate unitarity bounds?"

The seminar will be followed by a high tea.

English

 

Colloquium Friedmann 28-9-2023 17:00 Amphi Faure

Slava Mukhanov (Ludwig-Maximilians-Universität München)
How predictive are cosmological theories ?

Affiche Colloquium

Biséminaire Friedmann CPHT-LLR  29-9-2023 salle de conférences du LLR (aile 5 - centre)

11:00
Oleg Lebedev (University of Helsinki)
Gravity, dark matter and its detection

14:30
Slava Mukhanov (Ludwig-Maximilians-Universität München)
False vacuum decay: thick wall approximation

Affiche Biséminaire

 

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Tuesday, September 19 at 14:00

at CPHT, Conference room Louis Michel

Hareram Swain (IIT Madras)

A semi-holographic approach to strange metallic behaviour

Abstract: "Strange metals" exhibit an anomalous temperature dependence of the low temperature resistivity. This suggests the absence of electron quasiparticles which are the elementary excitations of the Fermi sea. This property is at odds with conventional Fermi liquids, justifying the “strangeness” of this phase. The measurement of the spectral function via ARPES has given us key insights into the nature of elementary constituents in strongly correlated electronic systems that do not admit quasiparticle description, and also demonstrate a rich variety of novel superconducting, metal, and insulating phases. The holographic approach, though allows one to dispense with the notion of quasiparticles, at the same time, this makes it difficult to understand what are the effective microscopic degrees of freedom of the system. In my talk, I will explain how we construct a semi-holographic description for such behaviours. Based on the key insights provided by Faulkner and Polchinski, we propose an effective theory in which the electron of a two-dimensional band hybridizes with a fermionic operator of a critical holographic sector, while also interacting with other bands that preserve quasiparticle characteristics. Besides the scaling dimension $\nu$ of the fermionic operator in the holographic sector, the effective theory has two dimensionless couplings α and γ determining the holographic and Fermi-liquid-type contributions to the self-energy respectively. In the case of DC conductivity that irrespective of the choice of the holographic critical sector, there exists a ratio of the effective couplings for which we obtain linear-in-T resistivity for a wide range of temperatures. This scaling persists to arbitrarily low temperatures when $\nu$ approaches unity in which limit we obtain a marginal Fermi liquid with a specific temperature dependence of the self-energy. Interestingly, we explain the origin of the linear-in-T resistivity and strange metallic behavior as a consequence of the emergence of a universal form of the spectral function which is independent of the model parameters when the ratio of the two couplings takes optimal values determined only by the critical exponent. This universal form fits well with photoemission data of copper oxide samples for under/optimal/over-doping with a fixed exponent over a wide range of temperatures. We further obtain a refined Planckian dissipation scenario in which the scattering time τ=f*h_bar/(k_B*T), with f being O(1) at strong coupling, but O(10) at weak coupling.

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Ephraim Bernhardt will publicly defend his thesis work on September 18, 2023 at 10:30 a.m. at the CPHT in the Louis Michel conference room.

Title: Topology and disorder effects in interacting and open quantum spin systems and analogues

Advisor: Karyn Le Hur

Abstract

Jury:

Alberto Rosso, Directeur de recherche au CNRS, LPTMS, Université Paris-Saclay (Rapporteur)
Thomas Schmidt, Professeur, Université du Luxembourg (Rapporteur)
Cristiano Ciuti, Professeur, MPQ, Université Paris-Cité (Examinateur)
Michel Ferrero, Chargé de recherche au CNRS, CPHT, École Polytechnique (Examinateur)
Loic Henriet, Chief Technology Officer, PASQAL (Examinateur)
Peter P. Orth, Professeur, Universität des Saarlandes (Examinateur)
Matteo Rizzi, Professeur, Universität zu Köln, (Examinateur)
Karyn Le Hur, Directrice de recherche au CNRS, CPHT, École Polytechnique (Directrice de thèse)
Cyril Elouard, Junior Professor, LPCT, Université de Lorraine (Invité)

The defense will be followed by a reception in the Jean Lascoux room at the CPHT.

After the reception, there will be a workshop on « Quantum many-body physics - new directions and modern applications ».

English

 

Chair: Karyn Le Hur, thesis supervisor, CNRS, CPHT, École Polytechnique

- 14h15 - 14h45 Matteo Rizzi (Universität zu Köln): Phase Diagram Detection via Gaussian Fitting of Number Probability Distribution

- 14h45 - 15h15 Thomas Schmidt (Université du Luxembourg) Topology and semiclassical transport in interacting electron system

-15h15 - 15h45 Peter P. Orth (Universität des Saarlandes) Nonlinear interrogation of quantum materials: why higher order response tells you more

- 15h45 - 16h15 Cyril Elouard (LPCT, Université de Lorraine) Extending the laws of thermodynamics for arbitrary autonomous quantum systems

16h15 - 16h30 Café et en-cas

- 16h30 - 17h00 Cristiano Ciuti (MPQ, Université Paris-Cité) New quantum many-body effects in high-impedance multi-mode circuit QED

- 17h00 - 17h30 Loic Henriet (Pasqal) tba

- 17h30 - 18h00 Alberto Rosso (CNRS, LPTMS, Université Paris-Saclay) tba

- 18h00 - 18h30 Michel Ferrero (CNRS, CPHT, École Polytechnique) tba

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