Below you will find a list of seminars organised by ICTQT. For comprehensive list of quantum events in other institutions please see the KCIK website.
Speaker: Szymon Pustelny (Institute of Theoretical Physics, JU)
Abstract
The ability to generate, modify, and retrieve a quantum state is of paramount importance for quantum information. Conventional physical implementations of the schemes, enabling realization of the tasks, employ single microscopic objects (atoms, photons, superconducting circuits, etc.). However, operation with such objects presents many experimental challenges. In the seminar, an alternative approach, enabling realization of quantum-state engineering and tomography using the collective state of many atoms (10^9), will be presented.Speaker: Sander Gribling (Université de Paris)
Abstract
In this talk, I will first give an overview of recent techniques such as taking linear combinations of unitaries (LCU) and the quantum singular value transformation framework (QSVT). These techniques allow one to reduce many quantum algorithmic problems to questions about finding good / the best polynomial approximations to certain functions. We study one such function: the inverse. In other words, we consider the problem of solving linear systems of equations. Prior work has shown that an asymptotically optimal approximation to the inverse can be evaluated using LCU and/or QSVT. We show the same for the optimal approximating polynomial, thus achieving constant factor improvements. This is based on https://arxiv.org/abs/2109.04248 which is joint work with Daniel Szilagyi and Iordanis Kerenidis. About the speaker: Sander Gribling’s research focuses on the interaction between optimization and quantum information theory / quantum computing. He is also interested in the many uses of polynomials in quantum information theory: polynomial optimization, quantum query complexity, and quantum algorithms.Speaker: Shubhayan Sarkar (CTP, PAS)
Abstract
A lot of work has recently been put into finding device-independent certification schemes for composite quantum systems. Most of them are however restricted to lower-dimensional systems, in particular two-qubit states. In this talk, I will first explain the basics of device-independent certification of quantum systems. Then, I will present some of our recent results that certify entangled quantum states of arbitrary local dimension and large classes of arbitrary outcome quantum measurements. Finally, I will present a scheme to certify the optimal amount of randomness that can be generated from arbitrary dimensional quantum systems.Speaker: Felix Huber (Institute of Theoretical Physics, UJ)
Abstract
Speaker: Wen-Long Ma (Institute of Semiconductors, Chinese Academy of Sciences)
Abstract
The relation between projective measurements and generalized quantum measurements is a fundamental problem in quantum physics, and clarifying this issue is also important to quantum technologies. While it has been intuitively known that projective measurements can be constructed from sequential generalized or weak measurements, there is still lack of a proof of this hypothesis in general cases. Here we rigorously prove it from the perspective of quantum channels. We show that projective measurements naturally arise from sequential generalized measurements in the asymptotic limit. Specifically, a selective projective measurement arises from a set of typical sequences of sequential generalized measurements. We provide an explicit scheme to construct projective measurements of a quantum system with sequential generalized quantum measurements. Remarkably, a single ancilla qubit is sufficient to mediate a sequential weak measurement for constructing arbitrary projective measurements of a generic system. As an example, we present a protocol to measure the modular excitation number of a bosonic mode with an ancilla qubit.Speaker: Christoph Dittel (University of Freiburg)
Abstract
Indistinguishability is the essential ingredient for many-body interference — a purely quantum mechanical interference effect between many identical bosonic or fermionic particles that can be exploited for a variety of applications, ranging from simulations with ultracold atoms to photonic quantum information processing. In this talk I give an introduction into this fascinating many-body property. Starting from its consequences on interference effects, I show how partial particle distinguishability entails a wave-particle duality relation on the many-body level. Moreover, I discuss how partial distinguishability due to mixedness — in the particles’ internal degrees of freedom — induces many-body decoherence whose strength increases exponentially in the number of constituents.Speaker: Srijon Ghosh
Abstract
The quest for small quantum thermal machines that can supersede their classical counterparts in performance has been an important and vibrant component in the field of quantum thermodynamics. These machines are expected to not only provide a better understanding of the interplay between the concepts from quantum information theory and thermodynamics, but also lead in building efficient quantum technologies. I will present a design of quantum thermal devices (e.g., quantum battery and quantum refrigerator) based on quantum spin models in arbitrary dimensions where the ground and the thermal states are chosen as initial states. We show that their performances are robust against decoherence and impurities which are inevitably present during preparation. Going beyond the usual convention, I will also discuss a measurement-based quantum refrigerator having an arbitrary number of qubits interacted through variable range interaction. I will show that in such a machine, repeated evolution followed by a measurement on the single accessible qubit has potential to reduce the temperature in the rest of the subsystems, thereby demonstrating cooling in the device.
Speaker: Lorenzo Catani (TU Berlin)
Abstract
Uncertainty relations express limits on the extent to which the outcomes of distinct measurements on a single state can be made jointly predictable. The existence of nontrivial uncertainty relations in quantum theory is generally considered to be a way in which it entails a departure from the classical worldview. However, this view is undermined by the fact that there exist operational theories which exhibit nontrivial uncertainty relations but which are consistent with the classical worldview insofar as they admit of a generalized-noncontextual ontological model. This prompts the question of what aspects of uncertainty relations, if any, cannot be realized in this way and so constitute evidence of genuine nonclassicality. We here consider uncertainty relations describing the tradeoff between the predictability of a pair of binary-outcome measurements (e.g., measurements of Pauli X and Pauli Z observables in quantum theory). We show that, for a class of theories satisfying a particular symmetry property, the functional form of this predictability tradeoff is constrained by noncontextuality to be below a linear curve. Because qubit quantum theory has the relevant symmetry property, the fact that it has a quadratic tradeoff between these predictabilities is a violation of this noncontextual bound, and therefore constitutes an example of how the functional form of an uncertainty relation can witness contextuality. We also deduce the implications for a selected group of operational foils to quantum theory and consider the generalization to three measurements.
Based on https://arxiv.org/abs/2207.11779.
Speaker: Nicolás Gigena (University of Warsaw)
Abstract
In this work, we study a three-parameter family of Bell functionals in a bipartite scenario with 3 measurement settings per party and 2 outcomes per measurement. The members of this family can be thought of as variations of the well-known I3322 functional, the only one in this scenario corresponding to a tight Bell inequality if we take aside the CHSH inequality. An analysis of their largest value achievable by quantum realisations (quantum value) naturally splits the set into two branches, and for the first of them, we show that this value is given by a simple function of the parameters defining the functionals. In this case we completely characterise the realisations attaining the optimal value and show that these functionals can be used to self-test any partially entangled state of two qubits. The optimal measurements, however, are not unique and form a one-parameter family of qubit measurements. Within the second branch, the quantum value presents a more complex dependence on the parameters defining the functionals and is studied numerically, identifying first the regions in parameter space where two-qubit systems suffice to approach the quantum value. The remainder of the branch includes the I3322 functional, for which a particular sequence of finite-dimensional realisations introduced by K. Pál and T. Vértesi is known to numerically attain the quantum value in the limit of infinite local dimension. We study the performance of these special strategies beyond the I3322 case and analyse the optimal solutions in those cases where they succeed in approaching the quantum value.Speaker: Sagnik Chakraborty (Nicolaus Copernicus University, Torun)
Abstract
We discuss a model of a unitary evolution of two-qubits where the joint Hamiltonian is so chosen that one of the qubits acts as a bath and thermalizes the other qubit which is acting as the system. The corresponding master equation for the system, for a specific choice of parameters, takes the Gorini-Kossakowski-Lindblad-Sudarshan (GKLS) form with constant coefficients representing pumping and damping of a single qubit system. Based on this model we construct an Otto cycle connected to a single qubit bath and study its thermodynamic properties. Our analysis goes beyond the conventional weak coupling scenario and illustrates the effects of finite bath including non-Markovianity. We find closed form expressions for efficiency (coefficient of performance), power (cooling power) for heat engine regime (refrigerator regime) for different modifications of the joint Hamiltonian.Ref: arXiv:2206.14751v1