Seminars in Condensed Matter Theory
Organisers: Charles Creffield and Fernando Sols
This is a series of seminars dealing with topics related to condensed matter physics. We interpret condensed matter quite broadly, including mesoscopic physics, atomic and molecular physics as well as more traditional topics. Our standard seminar time is Wednesday at 12 o' clock, and they are held in the seminar room of the Departamento de Fisica de Materiales in the Universidad Complutense (sala 124, on the second floor of the east wing).
Academic year 2025 - 2026
17 April Yutaka Shikano, University of Tsukuba & Institute for Quantum Studies, Chapman University
Ontic- or epistemic-distinguishability of identical particles by using the Aharonov-Bohm effect
The setup of the Einstein-Podolsky-Rosen (EPR) paradox, which is the most profound problem in quantum mechanics, leads to an observer-dependent description of the quantum state from a quantum information perspective. While this problem is originally based on a single-particle system, it can be extended to systems with many identical particles. We propose an experimental approach to clarify the quantum state description for identical particles, utilizing the three particle Aharonov-Bohm effect [A. Noguchi, YS, K. Toyoda, and S. Urabe, Nat. Comm. 5, 3868 (2014)]. In this seminar, we review the EPR paradox from a quantum information viewpoint and point out the discrepancy between the two parties problem and two particles problem. Finally, we provide the different quantum-state descriptions with and without knowing the situation to reveal ontic or epistemic properties of the identical particle.
28 November Andrei Khrennikov, Linnaeus University (Växjö, Sweden)
Violation of Bell inequality from incompatibility of local observables
This talk is aimed to dissociate nonlocality from quantum theory. We demonstrate that the tests on violation of the Bell type inequalities can be interpreted as special statistical tests of local incompatibility of observables. In fact, these are tests on violation of the Bohr complementarity principle. Thus, the attempts to couple experimental violations of the Bell type inequalities with “quantum nonlocality” is misleading. These violations are explained in the quantum theory as exhibitions of incompatibility of observables for a single quantum system, e.g., the spin projections for a single electron or the polarization projections for a single photon. Of course, one can go beyond quantum theory with the hidden variables' models (as was suggested by Bell) and then discuss their possible nonlocal features. However, conventional quantum theory is local.
5 November Alberto Rodríguez, Universidad de Salamanca
Quantum chaos in a system of interacting bosons: Stationary and dynamical features
We discuss quantum chaos in a paradigmatic system of interacting bosons modelled by the Bose-Hubbard Hamiltonian. The emergence of an ergodic phase as a function of energy and interaction strength can be assessed by the structural properties of the system's eigenstates, and witnessed in the dynamical behaviour of observables. After reviewing the main features of the chaotic phase in terms of stationary figures of merit, we focus on its dynamical fingerprints. In particular, we analyse the time evolution of initial density configurations typically used in quenched dynamics experiments with cold atoms, and unveil the conditions to systematically observe ergodic dynamics in the temporal behaviour of experimentally accessible observables within realistic time scales .
17 September Karen Kheruntsyan, University of Queensland, Brisbane (Australia)
Thermodynamic Maxwell relations for pair correlations
Maxwell relations enable the determination of challenging-to-measure thermodynamic properties from more accessible ones. This is an extremely powerful tool, as it allows for the full characterisation of the equilibrium states of complex materials—for example, through the determination of the thermodynamic equation of state. In this talk, I will introduce generalised Maxwell relations that connect arbitrary thermodynamic quantities (e.g. entropy, heat capacity, isothermal compressibility, magnetisation) to local pair correlation functions. To illustrate the utility of these relations, we apply them to iconic quantum many-body models—including the Lieb–Liniger, Fermi–Hubbard, and transverse-field Ising models—deducing thermodynamic properties from known particle–particle or spin–spin correlations. These results demonstrate the potential of correlation measurements as a route to determining thermodynamic properties. This universal approach is experimentally accessible thanks to advances such as quantum gas microscopy in ultracold atom quantum simulators, where correlation function measurements have become routine. Thus, the Maxwell relations for pair correlations open new pathways to uncover and understand emergent phases of matter in condensed-matter systems where direct thermodynamic measurements are challenging, such as atomically thin materials.
15 September David W. Snoke, University of Pittsburgh
Superfluids of light
It is possible to engineer the properties of photons in an optical medium to have an effective mass and repulsive interactions, so that they act like a gas of atoms. These "renormalized photons" are called polaritons. In the past decade, several experiments have demonstrated many of the canonical effects of Bose-Einstein condensation and superfluidity of polaritons. In this talk I will review some of this past work and present recent results with polaritons that have very long lifetime, including our recent results on persistent circulation of a polariton condensate.
Academic year 2024 - 2025
11 June Francisco Matute-Cañadas and Alfredo Levy Yeyati, Universidad Autónoma de Madrid
Andreev bound states in quantum circuits
The Josephson effect in mesoscopic weak links is mediated by phase dependent subgap states, the so-called Andreev bound states (ABSs). Although their role in superconductor-normal metal-superconductor junctions was described by Kulik already in the 70's, it was not until rather recently that direct evidence of these states was obtained experimentally using different techniques. Moreover, the combination of high quality hybrid nanostructures and circuit QED techniques are allowing us to explore the physics of ABSs in novel conditions and test qubit proposals based on these states. In this presentation we give an overview of the joint efforts with our experimental colleagues to describe ABSs in semiconductor nanowire Josephson junctions and their detection/manipulation using cQED techniques . In a second stage we consider the case of ABSs immersed in superconducting quantum circuits and discuss the prospects for the design of protected qubits based on these devices .
3 June Sahel Ashhab, Advanced ICT Research Institute, Tokyo, Japan
New frontiers in quantum optics enabled by superconducting circuits
The remarkable advances in superconducting qubit and resonator technology over the past two decades have allowed the realization of a variety of quanum optics phenomena that had been studied theoretically in the preceding decades but could not be realized with other physical systems. At the same time, these advances stimulated new ideas enabled by the newly emerging experimental capabilities. I will talk about a few of our research results that belong to this category. First, I will talk about ultrastrong and deep-strong qubit-oscillator coupling in cavity QED. The deep-strong coupling regime was experimentally realized in our group in 2016. Research on deep-strong coupling led to the discovery of a hidden symmetry in the asymmetric quantum Rabi model. It also highlighted a divergence in the theoretical treatment of multimode cavity QED, in which a qubit coupled to an infinite number of modes in a transmission line resonator has its frequency effectively suppressed to zero. I will talk about our results on these topics. Finally, I will talk about some of our recent simulation results pertaining to generalized squeezing that is induced by high-order nonlinearities in quantum optics. We find that higher-order squeezing results in oscillatory dynamics, in contrast to second-order squeezing, which can in theory continue indefinitely to generate infinitely squeezed states.
2 April Alberto Rodríguez González, Universidad de Salamanca
postponed
11 February Xi Chen, ICMM, CSIC
Shortcuts to Adiabaticity: From Quantum Control to Quantum Computing."
Quantum computing offers the potential to solve complex problems beyond the reach of classical computers. However, realizing practical quantum computation requires overcoming significant challenges, particularly in ensuring the stability, robustness, and efficiency of quantum operations. In this talk, we delve into the fascinating realm of "Shortcuts to Adiabaticity" (STA) and explore how this cutting-edge approach can revolutionize quantum control and quantum computation. STA methods offer ingenious methods to achieving adiabatic-like behavior in quantum systems, bypassing the need for time-consuming adiabatic evolutions. We focus on three key aspects (i) Optimal control with machine learning: We unveil how optimal control and machine learning play a pivotal role in discovering optimal shortcuts. We show several examples, where quantum control protocols can be tailored with various errors and noise, drastically reducing time while maintaining high-fidelity quantum states. (ii) Longitudinal coupling for improved qubit readout: Accurate qubit readout is paramount for fault-tolerant quantum computing. We explore how longitudinal coupling between qubits and readout resonators empowers faster and more robust qubit readout processes. This advancement holds the potential to accelerate the generation of CZ and GHZ entangled states for quantum computing as well. (iii) Digitized counter-diabatic quantum optimization algorithms: Counter-diabatic driving has long been recognized as a pathway to faster adiabatic evolution. We delve into the realm of digitized counter-diabatic quantum optimization, where auxiliary control fields are digitized and incorporated. This novel approach mitigates non-adiabatic transitions, leading to reduced errors and enhanced quantum computation performance, with many examples in Maxcut, factorization, portfolio optimization, protein folding, quantum chemistry and so on.
Academic year 2018 - 2019
20 February Diego Porras, Institute of Fundamental Physics, CSIC
"Topological Amplification in Photonic Lattices"
Driven-dissipative lattices are quantum models where dissipation and/or decoherence are added to the unitary quantum dynamics of tight-binding models. Those models are implemented, for example, in photonic setups such as superconducting circuits or coupled photonic cavities. The same theoretical paradigm can be used to describe vibronic lattices in trapped ions or nano-mechanical systems. The presence of dissipation and gain/loss mechanisms make the description of driven-dissipative lattices very different form their unitary counterparts. For example, it is a priori not trivial at all how to extend the theory of topological phases and topological insulators to this dissipative scenario. In my talk I will introduce a theoretical formalism that allows us to classify topological phases of driven-dissipative lattices by a formal mapping between dissipative lattices and effective chiral Hamiltonians. Our theory reveals the existence of topologically non-trivial dissipative phases in which photonic lattices act as directional amplifiers. This surprising connection will allow us to use Topological Band Theory to predict the performance of quantum amplifiers and sensors based on the symmetries of the underlying photonic lattice.
D. Porras & Samuel Fernández-Lorenzo, arXiv:1812.01348
13 February Toni Ramsak, University of Ljubljana
"Stability analysis of non-adiabatic qubit manipulations"
A promising method of qubit malipulation in quantum information processing applications is the manipulation where the Rashba effect in non-adiabatic systems induces quantum phases, including the spin rotation. By the virtue of exact unitary transformations [1] we recently proved that the ratio of the non-adiabatic Anandan phase and the adiabatic Wilczek-Zee counterpart can be tuned to any real number [2].
Stability properties of qubit transformations and the corresponding fidelity can also be studied exactly and as an example we will present results for spin-orbit dynamics influenced by the Ornstein-Uhlenbeck coloured noise of driving fields [3]. We will demonstrate also how these non-adiabatic systems can be coupled to thermal baths. In particular, by the known unitary transformation [1] the system can be expressed in the Floquet basis which enables an exact derivation of dissipators and the Lindblad equation. Some typical solutions of the corresponding Lindblad equation will be presented [4].
[1] T. Cadez, J. H. Jefferson, and A. Ramsak, Phys. Rev. Lett. 112, 150402 (2014).
[2] A. Ramsak, T. Cadez, A. Kregar, and L. Ulcakar, Eur. Phys. J. ST 227, 353 (2018).
[3] L. Ulcakar and A. Ramsak, New J. Phys. 19, 093015 (2017); L. Ulcakar and A. Ramsak, Int. J. Mod. Phys. B 32, 1840028 (2018).}
[4] B. Donvil, L. Ulcakar, T. Rejec, and A. Ramsak, in preparation.
16 January Juan Carlos Cuevas, Universidad Autonoma de Madrid
"Super-Planckian radiative heat transfer"
Understanding heat exchange via thermal radiation is key for many areas of science and engineering [1]. Our knowledge about the thermal radiation is still largely based on Planck’s law for black bodies. In particular, Planck’s law establishes an upper limit for the thermal energy that can be transferred between two objects via radiation. However, this fundamental law of physics has known limitations and, in principle, it is only valid when all the dimensions involved in the problem are larger than the so-called thermal wavelength (lTh), which is around 10 microns at room temperature. In this talk, I will present an overview of our efforts devoted to explore the limits of Planck’s law in two situations in which it is no longer valid. First, I will discuss the radiative heat transfer between two objects in cases in which they are separated by a distance smaller than lTh and the thermal exchange is dominated by evanescent waves [2-4]. Then, I will discuss the radiative heat transfer between objects with some of their dimensions being smaller than lTh. In particular, I will show that in this case it is possible to overcome the blackbody limit by orders of magnitude even in the far-field regime [5], i.e., when they are separated by macroscopic distances. I will illustrate this phenomenon in the case of micron-sized dielectric devices [5,6] and 2D materials such as graphene [7].
[1] J.C. Cuevas and F.J. García-Vidal, Radiative Heat Transfer, ACS Photonics 5, 3896 (2018).
[2] B. Song, Y. Ganjeh, S. Sadat, D. Thompson, A. Fiorino, V. Fernández-Hurtado, J. Feist, F.J. Garcia-Vidal, J.C. Cuevas, P. Reddy, E. Meyhofer, Nature Nanotechnol. 10, 253 (2015).
[3] K. Kim, B. Song, V. Fernández-Hurtado, W. Lee, W. Jeong, L. Cui, D. Thompson, J. Feist, M.T.H. Reid, F.J. García-Vidal, J.C. Cuevas, E. Meyhofer, P. Reddy, Nature 528, 387 (2015).
[4] L. Cui, W. Jeong, V. Fernández-Hurtado, J. Feist, F.J. García-Vidal, J.C. Cuevas, E. Meyhofer, P. Reddy, Nature Commun. 8, 14479 (2017).
[5] V. Fernández-Hurtado, A.I. Fernández-Domínguez, J. Feist, F.J. García-Vidal, J.C. Cuevas, Phys. Rev. B 97, 045408 (2018).
[6] D. Thompson et al., Nature 561, 216 (2018).
[7] V. Fernández-Hurtado, A.I. Fernández-Domínguez, J. Feist, F.J. García-Vidal, J.C. Cuevas, ACS Photonics 5, 3082 (2018).
5 December Miguel A. F. Sanjuán, Universidad Rey Juan Carlos
"Nonlinear Dynamics, Chaos and Complex Systems: A historical perspective"
When we talk about dynamics, we do not only understand the motion of celestial bodies and solid mechanical systems, but any changes with respect to time of one or more variables. From that point of view, we can find dynamics everywhere, in any field of science. Thus, now we have a more general vision, including stock market movements and economic variables, concentration changes in chemical reactions, changes in physiological, biological and medical variables, action potentials of neurons, etc … providing a more interdisciplinary perspective.
The various interactions between the constituent parts of a physical system and their feedback mechanisms, are a source of nonlinearity and complexity, which added to the sensitivity dependence to initial conditions which is a hallmark of chaotic behavior, constitutes a change of perspective in dynamical systems with important consequences for the understanding of science.
I will give a historical perspective of Nonlinear Dynamics, Chaos Theory and Complex Systems, including some of the different sources that have contributed to the construction of the discipline as we know it today. Among them, the three-body problem in celestial mechanics, turbulence in fluid dynamics, irreversibility and fundamentals of statistical physics and the logistic map and population dynamics in biology. Many schools of mathematics and physics have played an essential role in the historical development of the subject, including the French, Russian, Japanese and American school. The knowledge of this historical perspective allows us to understand the breadth of the discipline itself and the multiple interdisciplinary applications to various fields of science.
Academic year 2017 - 2018
4 December David Pérez García, Universidad Complutense
"Undecidability in physics... and its consequences"
The pioneering work of Goedel and Turing in the 30s showed that there exist problems in mathematics and computer science that cannot be solved. They are called undecidable. Since then, several problems in physics have been shown to be undecidable too. In this talk I will show that many interesting properties of a quantum many body system are indeed undecidable. This negative result has, however, a positive side. It predicts the existence of a new effect that we name "size-driven quantum phase transition”. I will present this effect, its characteristic features, as well as our recent ideas to try to observe it.
The results presented in the talk have been done in collaboration with J. Bausch, T. Cubitt, B. Doucot, S. Iblisdir, A. Lucia and M.M. Wolf.
17 November Jesús Pérez Ríos, School of Natural Sciences and Technology, Universidad del Turabo, USA
"Ultracold Rydberg atoms in high density media: chemistry and many-body physics"
A single Rydberg atom in a Bose-Einstein condensate (BEC), where thousands of neutral atoms are within the Rydberg orbit, experiences a fast decay depending on its principal quantum number n, in comparison with the natural decay rate of Rydberg atom. The physics behind this phenomenon has remained unexplored until the present work, where it is shown that the decay mechanism of a Rydberg atom in a density medium is due to both reactive and non-reactive ultracold chemical processes: l-mixing collisions and chemi-ionization. These chemical reactions have been studied by means of a new theoretical framework including explicitly the role of the Rydberg electron on the dynamics of the Rydberg-neutral energy landscape, as well as the short-range Rydberg core-neutral potential energy curves coming from quantum chemistry calculations. On the other hand, several Rydberg atoms in a BEC may form ultra-long Rydberg molecules through light-assisted chemical reactions, where a photon provides the necessary energy for binding the Rydberg atom to a neutral one, leading to the formation of ultralong Rydberg molecules. The production and control of these homonuclear molecules with giant dipole moments may be used to design novel many-body Hamiltonians as well as to explore intriguing anisotropic blockade effects leading to novel quantum phase transitions.
Previous years: 2017-2016, 2015-2014, 2014-2013, 2013-2012, 2012-2011, 2011-2010, 2010-2009, 2009-2008
If you have any further suggestions for speakers, please email me at: charles.creffield AT gmail.com