In recent years there have been impactful technology developments in the field of quantum sensing, relating to our ability to precisely control quantum states of atoms, molecules or artificial atoms in superconducting circuits. The potential of quantum sensors is huge, allowing for significantly improved sensitivities and precisions compared to conventional detectors.
In this set of lectures, we will get familiar with the building blocks of circuit quantum electrodynamics: superconducting qubits and microwave resonators. We will tour the obligatory stops: how to make a superconducting qubit, how to operate it with the help of a resonator, how to model and understand all of it. The aim of these introductory lectures is to lay the ground for understanding how the methods and technology belonging to the quantum information science field can be applied to enhance the sensitivity of experiments in fundamental physics.
The lectures will be given by Jens Koch, Professor of Physics at Northwestern University (US). Prof. Koch is also Deputy Director of the Superconducting Quantum Materials and Systems (SQMS) Quantum Information Science Research Center, and co-Director of CAPST (Center for Applied Physics and Superconducting Technologies).
In recent years there have been impactful technology developments in the field of quantum sensing, relating to our ability to precisely control quantum states of atoms, molecules or artificial atoms in superconducting circuits. The potential of quantum sensors is huge, allowing for significantly improved sensitivities and precisions compared to conventional detectors.
In this set of lectures, we will get familiar with the building blocks of circuit quantum electrodynamics: superconducting qubits and microwave resonators. We will tour the obligatory stops: how to make a superconducting qubit, how to operate it with the help of a resonator, how to model and understand all of it. The aim of these introductory lectures is to lay the ground for understanding how the methods and technology belonging to the quantum information science field can be applied to enhance the sensitivity of experiments in fundamental physics.
The lectures will be given by Jens Koch, Professor of Physics at Northwestern University (US). Prof. Koch is also Deputy Director of the Superconducting Quantum Materials and Systems (SQMS) Quantum Information Science Research Center, and co-Director of CAPST (Center for Applied Physics and Superconducting Technologies).
In recent years there have been impactful technology developments in the field of quantum sensing, relating to our ability to precisely control quantum states of atoms, molecules or artificial atoms in superconducting circuits. The potential of quantum sensors is huge, allowing for significantly improved sensitivities and precisions compared to conventional detectors.
In this set of lectures, we will get familiar with the building blocks of circuit quantum electrodynamics: superconducting qubits and microwave resonators. We will tour the obligatory stops: how to make a superconducting qubit, how to operate it with the help of a resonator, how to model and understand all of it. The aim of these introductory lectures is to lay the ground for understanding how the methods and technology belonging to the quantum information science field can be applied to enhance the sensitivity of experiments in fundamental physics.
The lectures will be given by Jens Koch, Professor of Physics at Northwestern University (US). Prof. Koch is also Deputy Director of the Superconducting Quantum Materials and Systems (SQMS) Quantum Information Science Research Center, and co-Director of CAPST (Center for Applied Physics and Superconducting Technologies).
The time-honoured Rayleigh criterion specifies the minimum separation between two incoherent sources that may be resolved into distinct objects. It also applies to signals in the time-frequency domain. We revisit this problem by examining the Fisher information required for resolving the two signals. The resulting Cramér–Rao bound gives the minimum error achievable for any unbiased estimator. When only the intensity in the image plane is recorded, this bound diverges as the separation between the sources tends to zero, an effect that has been dubbed the Rayleigh curse. Nonetheless, this curse can be lifted with suitable measurements. We discuss optimal strategies that confirm immunity to the Rayleigh curse and an unprecedented experimental precision.
Contrary to the standard Landau paradigm, topological phases are not distinguished by different spontaneous symmetry-breaking patterns, but rather by different non-local topological invariants. Non-trivial topology manifests itself in the presence of conducting edge states in bulk insulating phases,
quantized conductances or fractional charges, which in the case of symmetry-protected topological (SPT) phases are robust against perturbations that do not break certain protecting symmetries. Recently, this class has been enlarged to include higher-order SPT (HOSPT) phases, protected by crystalline symmetries and hosting edge states of co-dimension larger than one, such as corner states in 2D. We investigate a two-dimensional system of ultracold bosonic atoms inside an optical cavity, and show how
photon-mediated interactions give rise to a plaquette-ordered bond pattern in the atomic ground state that opens a non-trivial topological gap in 2D. The symmetry breaking results in a higher-order topological phase hosting corner states, that we characterize by means of a many-body topological invariant and through its entanglement structure. Finally, we demonstrate how this higher-order topological Peierls insulator can be readily prepared in atomic experiments through adiabatic protocols.
Monday, April 4th, h 15,00 - zoom link:
https://unipd.zoom.us/j/367589168
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Marco Ballarin - PhD student
Title:
An introduction to quantum error correction
Abstract:
Quantum computers are promising machines expected to speed up some classical computations and perform quantum simulations. However, their results are strongly affected by noise and errors. Thus, these results cannot be fully trusted. For this reason, quantum error correction techniques have been developed to reduce those errors. These techniques involve a high number of qubits, forbidding a scaling analysis using exact diagonalization methods. For this reason, using tensor networks allows us to overcome this issue and study the scaling properties of quantum error correction codes.
In this seminar, we review some of the errors that can affect quantum computers. Then, we introduce the framework of quantum error correction, focusing on the stabilizers codes. We treat in detail the 7-qubits Steane code, as an example of stabilizer codes. Finally, we introduce quantum trajectories, a method to simulate noisy evolutions with Matrix Product States.
Abstract:
Many-body systems are typically characterised via low-order correlation functions, that are directly related to response functions. In this talk, I will show how it is possible to provide a characterisation of many-body systems via a direct and assumption-free data mining of one of the pillars of both classical and quantum statistical mechanics - the partition function. The core idea of this programme is the fact that, once sampled stochastically (such as in experiments with in-situ imaging, or via Monte Carlo simulations), partitions functions can be construed as a very high dimensional manifolds. Such manifolds can be characterised via basic concepts, in particular, by their intrinsic dimension.
I will discuss theoretical results for both classical and quantum many-body spin systems that illustrate how data structures undergo structural transitions whenever the underlying physical system does, and display universal (critical) behavior in both classical and quantum mechanical cases. I will conclude with remarks on the applicability of our theoretical framework to synthetic quantum systems (quantum simulators and quantum computers), and emphasize its potential to provide a direct, scalable measure of Kolmogorov complexity of output states.
Abstract:
Laser cooled trapped ions offer unprecedented control over both internal and external degrees of freedom at the single-particle level. They are considered among the foremost candidates for realizing quantum simulation and computation platforms that can outperform classical computers in specific tasks. In this talk I will show how linear arrays of trapped 171Yb+ ions can be used as a versatile platform for studying out-of-equilibrium many-body quantum systems and to implement quantum variational algorithms. I will show how a trapped-ion quantum simulator [1] allowed the implementation of a Quantum Approximate Optimization algorithm (QAOA) used to approximate the ground state energy of a long-range transverse field Ising model of up to 40 qubits [2]. Large linear ion chains also enabled the investigation of how confinement can suppress information propagation and thermalization of meson-like quasi-particles in a many-body system [3,4]. Finally, I will report on the observation of a Stark MBL in a long-range disorder-free system [5] and I will conclude outlining the prospect of simulation of dissipative Floquet dynamics [6].
[1] G. Pagano, et al., Quantum Sci. Technol., 4, 014004 (2019)
[2] G. Pagano, et al., PNAS 117, 41, 25396 (2020)
[3] F. Liu, et al., Phys. Rev. Lett. 122, 150601 (2018)
[4] W. L. Tan, P. Becker, et al., Nature Phys. 17 (6), 742 (2021)
[5] W. Morong, et al., Nature 599, 393 (2021)
[6] P. Sierant, et al., Quantum 6, 638 (2022)
Monday, March 28th, 10,00 - zoom link:
https://unipd.zoom.us/j/367589168
Alessandro Pisana - Master's student
Title:
Entangling polyhedra in quantum gravity via uniformization constraints
Abstract:
Loop Quantum Gravity (LQG) is one of the most promising approaches to quantize gravitational interaction. Basis states in the kinematical Hilbert space of LQG, the so-called spin network states, have a dual geometrical interpretation as many-body states representing an unentangled quantum polyhedra collection. Entangled states are recovered via polyhedra uniformization constraints, minimized by tensor networks techniques. In my talk, I will present preliminary results concerning entangled polyhedra states. In particular, I will show how it is possible to recast uniformization constraints for a spin-1/2 chain of tetrahedra as an Ising model with exchange interaction. FInally, I will show the numerical results obtained by simulating the resulting Ising Hamiltonian by means of Matrix Product States.
Abstract: The classification of quantum states and operations is an area of common interest between Quantum Information Theory (QIT) and Quantum Many-Body Physics (QMBP). However, while in QIT one is typically interested in classifying states up to transformations that do not increase entanglement (LOCC), phases of matter in QMBP are defined in terms of unitary transformations that preserve locality, i.e. shallow quantum circuits. In this talk, motivated by the advent of noisy intermediate-scale quantum devices, I will introduce deterministic state-transformation protocols between many-body quantum states that can be implemented by low-depth quantum circuits followed by LOCC. I will show that this gives rise to a different classification of phases in which topologically ordered states or other paradigmatic entangled states become trivial. In this setting, I will provide a full classification of phases in 1D in the context of matrix product states. Finally, I will discuss how the set of unitary operations is enhanced by LOCC, allowing one to perform certain large-depth quantum circuits in terms of low-depth ones.
Based on: LP, G. Styliaris, J. I. Cirac, Phys. Rev. Lett. 127, 220503 (2021); arXiv: 2103.13367
Abstract:
In this talk I will discuss the role of noise in determining the
topology of a neural network, whose dynamics is governed by
biologically inspired model. I will then give an outlook on how these
concepts could be implemented for engineering quantum target dynamics
via a noisy environment.
A key prediction of the Bardeen-Cooper-Schrieffer theory of superconductivity is that the critical temperature increases with the density of states. Therefore a promising way to increase the critical temperature and eventually achieve room temperature superconductivity is to engineer electronic bands with very small bandwidth. In the limit of vanishing bandwidth, called the flat band limit, the density of states is diverging and the critical temperature of the superconducting transition is linearly proportional to the interaction strength, and thus much higher than in the case of a dispersive band.
In the first part of this talk, I will first review the theory of superconducting transport in the flat band limit, more specifically the relation between superfluid weight and quantum metric [1]. The quantum metric is a band structure invariant closely related to the Berry curvature and topological invariants such as the Chern and winding numbers. As consequence, the superfluid weight of a flat band with nonzero Chern number is always nonzero. Recent experimental results obtained with twisted bilayer graphene provide evidence that the quantum metric indeed plays an important rôle in superfluid transport [2, 3].
In the second part of the talk, I will discuss two recent works [4, 5] addressing an important conceptual problem: while it can be shown that the superfluid weight is a geometry-independent quantity, that is an observable depending only the hopping matrix elements but not on the spatial arrangements of the orbitals, the quantum metric actually does depend on the lattice geometry. This discrepancy leads to paradoxical results, which I illustrate using the Su-Schrieffer-Heeger model as an example. The paradox is resolved by using the generalize random phase approximation to show that the superfluid weight is in fact proportional to the minimal quantum metric, the integral over the Brillouin zone of the quantum metric minimized over the orbital positions. The minimal quantum metric is a novel band structure invariant with potentially interesting applications in condensed matter physics, in particular regarding the classification of topological states of matter. For instance, I will discuss how the minimal quantum metric can be used to provide a geometry independent formulation of the winding number.
References
[1] S. Peotta and P. Törmä, Nature Communications 6, 8944 (2015)
[2] Tian et al., Nature 614, 440 (2023)
[3] P. Törmä, S. Peotta and B. A. Bernevig, Nature Reviews Physics 4, 528 (2022)
[4] K. E. Huhtinen, J. Herzog-Arbeitman, A. Chew, B. A. Bernevig, P. Törmä, Physical Review B
106, 014518 (2022)
[5] M. Tam, S. Peotta, Phys. Rev. Research 6, 013256 (2024).
In textbook quantum mechanics, time is a (classical)
parameter that is not described by the theory. Famously, there is no
time observable, which means that there is no theoretical prescription
of how to perform time measurements, much to the consternation of
experimentalists who perform time measurements routinely. We describe a
(slight) extension of quantum mechanics where time is treated as any
other observable, we explore the implications, and briefly touch on the
(ongoing) effort to extend this construction to a relativistic setting.
The last decade witnessed intense efforts to realize zero-energy Majorana modes in topological superconductors. Most of the experimental observations relied on conductance measurements and spectroscopy, which, however, are not sufficient to unveil the non-local nature of these exotic quasiparticles. In this talk I will discuss a transport phenomenon, the topological Kondo effect, that can provide more direct evidence of the non-locality of Majorana modes. The study of this phenomenon beyond the renormalization group predictions is challenging. To obtain quantitative indications of its onset, I will first discuss possible physical implementations of this phenomenon and then present the matrix-product-state simulations of the dynamics of superconductor-seminductor hybrid systems aiming at realizing this peculiar transport phenomenon. Our simulations of quantum quenches, in particular, allow us to determine the topological Kondo temperature that characterizes the emergence of this effect and to discuss realistic conditions to observe its peculiar fractional conductance.
In this talk I will outline a novel approach to quantum mereology based on minimal information scrambling. Generalized quantum subsystems are defined by pairs of von Neumann algebras and their commutant and their scrambling in terms of a novel Algebraic Out of Time Order Correlation (A-OTOC) function. The short time expansion of the A-OTOC allows one to define a notion of Gaussian Scrambling rate. The latter has a simple geometrical interpretation, and its local minima provide an operational criterion for the selection of emergent subsystems. Examples of factors and maximal abelian algebras will be discussed and shown that in these key cases the formalism leads to physically meaningful connections to operator entanglement and to coherence generating power respectively.
References:
P. Zanardi, E. Dallas, S. Lloyd, Operational Quantum Mereology and Minimal Scrambling, arXiv:2212.14340
P. Zanardi, Quantum scrambling of observable algebras, Quantum 6, 666 (2022),
For more than one hundred years, the Physical Review (PR) journals have published cutting-edge research in physics from scientists all around the world. More recently, Physical Review X (PRX) was launched to become a high-profile venue to publish a small number of *landmark* high-impact papers from all areas of physics.
One of the distinguishing features of PRX and the entire PR family is its rigorous peer review process, which the editors strive to make as fair, thorough and timely as possible. In this talk, after introducing PRX and its editorial philosophy, I will explain how the review process in the PR journals is structured, what its purposes are, and how it is meant to benefit the scientific community overall. I will then provide some concrete tips and advice on how to write a good paper and prepare a submission for one of our journals, as well as on how to write a convincing response to the referees. Finally, I will describe what being a referee entails, and give some suggestions on how to write a high-quality referee report.
The formation of patterns in nature is often determined by the interplay of noise and interactions of different range. Numberless examples are encountered in everyday life and can be described classically. In the nano-world, where the laws of quantum physics are dominant, our understanding is still in its infancy. The dynamics of a quantum many-body system subject to interactions, dissipation and driving forces poses severe theoretical challenges. Understanding the quantum dynamics of these pattern formation processes is an important question of fundamental research and a crucial issue for quantum technological applications, where one aims at robust quantum coherent dynamics in systems of mesoscopic size. A promising and flourishing approach to tackle these questions is offered by the study of ultracold ensembles of atoms coupled to the light fields of high-finesse optical cavities. In these systems quantum structures of photons and atoms emerge from a quantum nonlinear interaction between scattering particles in the presence of noise and dissipation. The inter-atomic interactions are here mediated by multiple scattering of cavity photons and have a long-range character, which makes these systems a unique platform for shedding light into dynamics predicted in other fields of physics, ranging from nuclear physics, nonlinear dynamics, and astrophysics. In this talk I will discuss the basic physical mechanisms leading to crystalline structures of photons and atoms and will review insights gained by theoretical and experimental studies of these systems.
Broadband quantum noise reduction is a fundamental challenge in the development of next-generation Gravitational Wave Detectors (GWDs). In detuned signal-recycled Fabry-Pérot–Michelson interferometers, the optical spring effect introduces a second resonance, which traditionally requires an additional Filter Cavity to implement Frequency-Dependent Squeezing (FDS). While effective, this approach significantly increases financial costs and technological complexity.
Recent theoretical work proposes an alternative strategy based on Quantum Teleportation, offering a novel pathway to suppress quantum noise. This method utilizes conditional EPR entanglement to create phase rotations in optical fields frequency-shifted from the interferometer carrier frequency, ω0. The first phase rotation is introduced by single-mode squeezing offset by ΔΩB, generating an initial FDS state. A second rotation is achieved through a Frequency-Dependent entanglement link, wherein an entangled idler field at ω0+ΔΩA interacts with the FDS state. Bell-state measurement facilitates the projection of the idler’s phase properties onto the FDS state, enabling the teleportation of double-rotated squeezing to ω0.
This approach promises full-bandwidth quantum noise suppression for detuned interferometers without modifying the detector’s existing infrastructure. Beyond its practical implications, this method demonstrates how quantum communication protocols can be reinterpreted as advanced sensing tools, marking a paradigm shift in the intersection of quantum optics and gravitational wave detection.
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