Seminars from 2024

The recent discovery of high harmonic generation in solids [1], merging the fields of strong field and condensed matter physics, opened the door for the direct observation of Bloch oscillations [1], all-optical reconstruction of the band structure [2] and direct observation of the influence of the Berry curvature in the optical response [3]. In this work, we will focus on high harmonic generation in strongly correlated and topological materials. First, I will show how high harmonic spectroscopy can be used to induce and time resolve insulator-to-metal transitions in strongly correlated materials, using the Hubbard model [4]. I will further demonstrate how high harmonic spectroscopy can be used to identify topological phases of matter and how the Berry curvature leaves its fingerprint in the nonlinear optical response of the material [5]. Using a combination of w-2w counter-rotating strong circular fields, we demonstrate that we are able to induce valley polarization in hexagonal 2D materials and use HHG spectroscopy to read the valley polarization [6]. At last, I will show how the use of Wannier orbitals can be useful in the calculation of the nonlinear optical response of solids [7].

Last twenty years brought intensive studies of many-body localization (MBL) both theoretical and experimental. While experiments always deal with finite samples and their dynamics for a limited time, the question remains whether many-body localization exists in the thermodynamic limit as a separate phase or rather it is a transient, small systems phenomenon. Existing mathematical proofs are not widely accepted. During the seminar I will describe numerical attempts aiming at finding MBL via spectral statistics or time dynamics. As in recent review, P. Sierant et al, Many-Body Localization in the Age of Classical Computing arXiv:2403.07111 we shall not give an definite solution but rather discuss open questions.

Quantum Computation with Fermions, Bosons, and Qubits with Eleanor Crane Episode 174 Abstract: Finding a scalable and universal framework for quantum simulation of strongly correlated fermions and bosons is important in fields ranging from material science to high-energy physics. While digital qubit-only quantum computers in principle offer such universality, the overhead encountered in mapping fermions and bosons to qubits renders this endeavour extremely challenging to implement in practice. In this talk, I will explain an approach to simulating bosonic matter, fermionic matter, and Abelian gauge fields in (2+1)D, which uses hybrid digital qubit-boson (or ‘oscillator-qubit’) and qubit-fermion operations, avoiding this overhead altogether. I will show how our compilation strategies for hybrid oscillator-qubit computation can be used to study dynamics as well as ground states, and develop measurement techniques of non-local observables, and mention the influence of hardware errors in circuit QED coupled to high-Q cavities. I will also show that it is possible to implement fully fault-tolerant operations using logical fermions comprised of physical fermions as can be found in neutral atoms. Illustrating the advantage of our hybrid qubit-oscillator approach over all-qubit hardware, the end-to-end comparison of the gate complexity for the Z2 gauge-invariant bosonic hopping term finds an improvement of the asymptotic scaling from $\mathcal{O}(\log(S)^2)$ to $\mathcal{O}(1)$ in our framework, as well as a constant factor improvement of better than $10^3$, and the $U(1)$ plaquette term benefits from an improvement from $\mathcal{O}(\log(S))$ to $\mathcal{O}(1)$. Illustrating the advantage of our hybrid qubit-fermion approach over all-qubit hardware, the fermionic fast Fourier transform, a widely-used subroutine in quantum algorithms for materials, finds an improvement from $\mathcal{O}(N\log(N))$ to $\mathcal{O}(\log(N))$ in circuit depth as well as from $\mathcal{O}(N^2)$ to $\mathcal{O}(N)$ in Clifford gate complexity. Our work establishes hybrid qubit-oscillator quantum simulation and qubit-fermion fault-tolerant quantum computation as viable and advantageous methods for the study of the quantum aspects of nature.

In this talk, I will delve into correlated decay in open quantum systems composed of many-particles, discussing both fundamental principles and practical applications. I will focus primarily on our recent work on universal scaling laws for correlated decay [1], which apply broadly to a large class of Markovian quantum systems. I will also address the specific case of atomic arrays in free space and highlight the implications of these scaling laws for fault-tolerant quantum computing, metrology, and many-body dynamics. In the latter part of the talk, I will introduce the concept of collective transition quenching, an application of correlated decay where dissipative interactions in systems with multiple competing decay channels suppress all but the dominant decay path. This ‘winner takes all’ dynamic leads to the near-deterministic preparation of the dominant ground state. This mechanism provides a way to control and manipulate of open quantum systems, with applications in molecular photochemistry and other fields. [1] Wai-Keong Mok, Avishi Poddar, Eric Sierra, Cosimo C Rusconi, John Preskill, Ana Asenjo-Garcia, arXiv:2406.00722 (2024).

Precision is a virtue in science and particularly in nanotechnology where carefully fabricated nanometer-scale devices hold great promise in both classical and quantum regimes. Ground- state cooling or phonon amplification require, for example, a sideband resolved photon-phonon coupling where unavoidable imperfections often impose severe performance limits. However, imperfection and disorder are ubiquitous in Nature and emerge with a critical role in nanoscale devices. In this talk, I will explore the limits imposed by imperfection in different nanophotonic and optomechanical nanodevices, but not only. In certain cases, disorder may be invoked to enhance the light-matter interaction in different fields of nanotechnology such as quantum photonics [1] and optomechanics [2, 3]. In the last case, we explore disorder-induced Anderson-localized cavity-optomechanics in a two-dimensional platform with a full phononic gap in the GHz frequency range [4]. The dynamics of this system are strongly nonlinear and complex [5], two important ingredients for neural networks and reservoir computing. References [1] P.D. Garcia, P Lodahl. Physics of Quantum Light Emitters in Disordered Photonic Nanostructures. Annalen der Physik 529, 1600351 (2017). [2] G. Arregui, R. Cecil Ng, M. Albrechtsen, S. Stobbe, C. M. Sotomayor Torres, P. David García. Cavity optomechanics with Anderson-localized optical modes. Phys. Rev. Lett. 130, 043802 (2023). [3] G. Madiot, R. C Ng, G. Arregui, O. Florez, M. Albrechtsen, S. Stobbe, P. D Garcia, C. M Sotomayor- Torres. Optomechanical generation of coherent GHz vibrations in a phononic waveguide. Phys. Rev. Lett. 130, 106903 (2023). [4] O. Florez, G. Arregui, M. Albrechtsen, R. C. Ng, J. Gomis-Bresco, S. Stobbe, C. M. Sotomayor-Torres, P. David García. Engineering nanoscale hypersonic phonon transport. Nature Nanotechnology 17, 947 (2022). [5] G Arregui, D Navarro-Urrios, N Kehagias, CMS Torres, PD Garcia. All-optical radio-frequency modulation of Anderson-localized modes. Physical Review B 98 (18), 180202 (2018).

There are many intriguing physical phenomena that are associated with topological features — global properties that are not discernible locally. The best-known examples are the quantum Hall effects in electronic systems, where insensitivity to local properties manifests itself as robust conductance. In the talk, we first discuss how similar physics can be explored with photons; specifically, how various topological models can be simulated in various photonics systems, from ring resonators to photonic crystals and fiber loops. We then discuss how the integration of optical nonlinearity can lead to unique bosonic phenomena, such as topological frequency combs, topological sources of quantum light and chiral quantum optics. These results may enable the development of classical and quantum optical devices with built-in protection for next-generation optoelectronic and quantum technologies.

The demanding experimental access to the ultrafast dynamics of materials challenges our understanding of their electronic response to applied strong laser fields. For this purpose, trapped ultracold atoms with highly controllable potentials have become an enabling tool to describe phenomena in a scenario in which some effects are more easily accessible and 12 orders of magnitude slower [1]. In this work, we introduce a mapping between the parameters of attoscience platforms and atomic cloud simulators and propose an experimental protocol to access the emission spectrum of high-harmonic generation (HHG), a regime that has so far been elusive to cold-atom simulation [2]. We show that these platforms offer a unique opportunity to access and measure the emission spectrum of HHG through absorption measurements [3]. Furthermore, it simulates the physical response of a single-atom target. This is in contrast with real experiments, where thousands of atoms are simultaneously driven to collect enough photons to resolve the spectrum, which challenges phase-matching conditions when a large ionization occurs under strong fields. As we illustrate, the benchmark offered by these simulators can provide new insights into the conversion efficiency of extended and short nuclear potentials, as well as the response to applied elliptical polarized fields or ultrashort few-cycle pulses. [1] S. Sala, J. F¨orster, and A. Saenz, Ultracold-atom quantum simulator for attosecond science, Phys. Rev. A 95, 11403 (2017). [2] R. Senaratne, et al., Quantum simulation of ultrafast dynamics using trapped ultracold atoms, Nat. Commun. 9, 2065 (2018). [3] Javier Arg¨uello-Luengo, Javier Rivera-Dean, Philipp Stammer, Andrew S. Maxwell, David M. Weld, Marcelo F. Ciappina, and Maciej Lewenstein, PRX Quantum 5, 010328 (2024).

The study of the interaction of atoms with the electromagnetic modes of a waveguide has opened a platform where it is possible to study the propagation of pulses and their relationship with changes in atomic decay rates due to superradiance and subradiance. This collective phenomenon also modifies the propagation of electromagnetic pulses by presenting precursors and oscillations related to the coupling of atomic dipoles. This presentation will try to show the similarities and differences experimentally in the evolution in a waveguide compared to in a cavity.

This is a pedagogical review of essentially to families of works, Refs. [1, 2] on the one hand, and Refs. [3, 4] on the other hand, and it contains a few novelties. We first review the new discretization of the Dirac equation by discrete-time quantum walk (DQW) which is introduced in Ref. [1]. This discretization has the following properties. It is unitary and strictly local, as any DQW discretization, but also, and this is the new part, it is both independent of the Clifford-algebra representation used to write down the Dirac equation and very similar to usual lattice-gauge-theory (LGT) discretizations, with an on-site mass term. Moreover, we remind that this new discretization avoids fermion doubling thanks to a natural Wilson term which moreover does not break unitarity. Reference [2] is a follow-up to that work, in which we define an action functional based on that new discretization of the Dirac equation; we do not do any review of that reference in particular. All this was done in the single-particle sector, that is, at the level of classical fields (relativistic quantum mechanics). Then, we give a brief introduction to the extensions to the multi-particle sector, i.e., to quantum fields, which have been done by Arrighi et al., and which use more standard, older DQWs; upgraded in a certain way to the multi-particle sector, DQWs become so-called quantum cellular automata (QCAs), and these QCAs can be used to discretize and further quantum simulate quantum field theories (QFTs), both in 1 + 1 dimensions [3] and in 2 + 1 and 3 + 1 dimensions [4]. This brief introduction to Refs. [3, 4] only covers the fermionic part, not the gauge-field part. Where: [1] P. Arnault, “Clifford algebra from quantum automata and unitary Wilson fermions,” Phys. Rev. A 106, 012201 (2022), arXiv:2105.12314. [2] P. Arnault and C. Cedzich, “A single-particle framework for unitary lattice gauge theory in discrete time,” New J. Phys. 24, 123031 (2022). [3] P. Arrighi, C. Bény, and T. Farrelly, “A quantum cellu- lar automaton for one-dimensional QED,” Quantum Inf. Process. 19 (2020). [4] N. Eon, G. Di Molfetta, G. Magnifico, and P. Arrighi, “A relativistic discrete spacetime formulation of 3+1 QED,” Quantum 7, 1179 (2023).

In waveguide Quantum Electrodynamics, a few localized atoms send photons into a 1D waveguide. Here we present a platform that switches the roles of light and matter, with state-selective optical-lattice wells acting as quantum emitters radiating ultracold matter waves. This platform has characteristics that complement well other synthetic systems for the study of light-matter interactions. First, the heavier nature of atoms as compared to photons makes the decay dynamics several orders of magnitude slower, with the advantages of enhancing non-Markovian effects and enabling the measurement of the time evolution at the early stages of the decay. Second, the tunability of the system parameters allows for control of the initial states, adiabatic preparation of bound states and readout of the radiation momentum distribution in a time-of-flight picture.

Quantization of electrical networks à la Faddeev-Jackiw

Robust spin-spin interactions mediated by harmonic oscillator modes are central to many quantum technologies. On the other hand, dynamical decoupling (DD) is a well-established paradigm to protect spins from decoherence. In this talk, I will present existing DD methods and explain how they can be combined with oscillator-mediated spin-spin interactions. In particular, I will present two concrete proposals for experimental realisation. One for SiV centres coupled via strain in diamond and another for trapped ions coupled via static magnetic field gradients.

Quantum non-demolition measurements (QND) allow repeated evaluation of an observable without destroying the quantum system. Efficient characterization of these measurements is an essential part of certifying, improving, and scaling up quantum processors. In this thesis, we address the challenge of simulating and characterizing QND measurements through quantum tomography. First, we introduce a self-consistent tomography scheme to perform a complete physical characterization of an arbitrary QND detector, including the reconstruction of the measurement processes and the extraction of relevant quantifiers such as readout fidelity, QNDness, and destructiveness. This framework is a diagnostic tool for the dynamics of QND detectors, allowing us to identify errors, and to improve their calibration and design. We illustrate this on a realistic Jaynes-Cummings simulation of dispersive superconducting qubit readout. We characterize non-dispersive errors, quantify the backaction introduced by the readout cavity, and calibrate the optimal measurement point. Then, we show that this procedure can be efficiently parallelized when addressing single- and two-qubit readout on a multi-qubit quantum processor. We provide an experimental demonstration of the tomographic protocol on a 7-qubit IBM-Q device, characterizing the quality of conventional qubit readout and generalized measurements such as parity or measurement-reset-feedback schemes. We also show how to quantify measurement crosstalk and use it to certify the quality of simultaneous readout on multiple qubits. Our method is an efficient alternative for characterizing and understanding QND measurements in current quantum devices from theoretical and experimental points of view.

Bosonic or continuous-variable coding is a field concerned with robust quantum information processing and communication with electromagnetic signals or mechanical modes. I review bosonic quantum memories and discuss quantum spherical codes, a new class of multimode extensions of the cat codes.

In this talk I will present the latest dissipative methods being used to simulate the long-term dynamics of quantum observables in many-body setups with tensor networks, and how one can generalize them to arbitrary initial states fuerther than infinite temperature; at the end we will make contact with information spreading and operator hydrodynamics

The contraction of tensor networks is a central task in the application of tensor network methods to the study of quantum and classical many-body systems. In this talk, I will discuss the impact of gauge degrees of freedom in the virtual indices of the tensor network on the contraction process, specifically focusing on boundary matrix product state methods for contracting two-dimensional tensor networks. We show that the gauge transformation can affect the entanglement structures of the eigenstates of the transfer matrix and change how the physical information is encoded in the eigenstates, which can influence the accuracy of the numerical simulation. We illustrate this effect through a systematic analysis of local gauge transformations. Additionally, we go beyond the scope of local gauge transformations and analyze an example that incorporates non-local gauge transformations.

In this talk, I will discuss how optomechanical cavities can engineer artificial magnetic fields for nanomechanical vibrations, orchestrated by light. This leads to acoustic chirality and non-Hermitian topological states. Theoretical ideas are tested experimentally in optomechanical nanobeam photonic crystals. These optomechanical systems support telecom-frequency optical resonances linked to multiple mechanical overtones with high coherence. By controlling the phase of lasers in a cavity, we can tune strong mechanical interactions that imprint nonreciprocal Peierls phases that break time-reversal symmetry – akin to the Aharonov-Bohm effect for electrons[1].

In this talk, I will present an alternative method that circumvents this difficulty. We map the problem to the problem of finding the ground state energy of a generic spin Hamiltonian [1]. We then use several techniques to derive rigorous upper and lower bounds to the ground state energy [2,3].

The generation of non-classically correlated states of light is a crucial ingredient for quantum technologies based on quantum optics. In order to achieve this, we need interfaces that induce effective interactions between individual photons. This can be realized by chirally coupling an array of two-level atoms to a waveguide. As the atoms cannot be excited twice, photons propagating through the waveguide will build up spatial correlations as they are individually absorbed and emitted by the atoms. A fundamental limit to this mechanism is the spontaneous emission of the atoms into free space. In this thesis we show, by properly considering the vacuum emission, that wave interference phenomena of the fields can protect certain modes from decaying into free space, when the array is periodic and dense enough. This phenomenon is called subradiance. Subradiance will allow for the building of special two photon correlations with fermionic characteristics, protected against losses through the propagation. We perform numerical and analytical methods to understand the relation between fermionization and subradiance.

Increasing the density of quantum devices opens avenues to explore novel regimes of many-body quantum dynamics and enhance the performance of various quantum applications such as precise sensing. At the same time, this effort poses new challenges as densely packed systems exhibit correlated dissipation, significantly impacting the decay rate of correlated quantum states. It is thus natural to ask: What is the maximum decay rate of a system with correlated dissipation? Addressing this questions for large numbers of particle is however complicated by the exponential scaling of the Hilbert space dimension.