5 research outputs found
Fast quantum state discrimination with nonlinear PTP channels
We investigate models of nonlinear quantum computation based on deterministic
positive trace-preserving (PTP) channels and associated master equations. The
models are defined in any finite Hilbert space, but the main results are for
dimension . For every normalizable linear or nonlinear positive map
on bounded linear operators , there is an associated normalized PTP
channel . Normalized PTP channels include unitary
mean field theories, such as the Gross-Pitaevskii equation for interacting
bosons, as well as models of linear and nonlinear dissipation. They classify
into 4 types, yielding 3 distinct forms of nonlinearity whose computational
power we explore. In the qubit case these channels support Bloch ball torsion
and other distortions studied previously, where it has been shown that such
nonlinearity can be used to increase the separation between a pair of close
qubit states, resulting in an exponential speedup for state discrimination.
Building on this idea, we argue that this operation can be made robust to noise
by using dissipation to induce a bifurcation to a phase where a pair of stable
fixed points create an intrinisically fault-tolerant nonlinear state
discriminator
The emergence of chaos in continuously monitored open quantum systems
This thesis makes a unique contribution to the field of quantum chaos by theoretically demonstrating the effect that measurement has on the emergence of chaos from the quantum world and demonstrating a means to control the onset of chaos in the quantum system using adaptive measurements.
Here we investigate how the choice of the continuous measurement strategy for an open quantum system affects the emergence of chaos in the transition from the quantum limit to the classical limit when the system is dissipative. We consider two models in our research. The Duffing oscillator is classically chaotic and also dissipative( ie. an open quantum system), whereas the driven top is classically a closed system; adding dissipation via continuous measurement therefore changes the behaviour in from the classical limit.
The first half of this thesis presents the investigation of a dissipative system whose classical limit is chaotic. We explore the emergence of chaos from the open quantum system that is continuously monitored and investigate the dependence on the choice of monitoring by changing a single parameter in a homodyne measurement scheme, effectively changing the information gained by the measurement. We show that the emergence of chaos in the regime where quantum effects are still present can be determined solely by changing the measurement parameter. This is a result of the interplay between the quantum interference effects induced by the nonlinear dynamics and the localisation and decoherence that occurs due to the measurement. We also investigate the case where the classical limit is regular for the Duffing oscillator, and demonstrate the semiclassical effect of chaos induced by the measurement back-action. A certain choice of measurement leads to a noise which drives the system to large spread in the dimensionless position enabling a non-classical transition mechanism that is classically forbidden, inducing chaos in the system. These results are verified by the numerical calculation of the maximal Lyapunov exponent in the quantum regime.
The second part of this thesis investigates the possibility of controlling the degree of chaos with quantum control. We design an effective control scheme to control the degree of chaos using the measurement dependency of the state. We propose an adaptive measurement scheme which changes the homodyne measurement angle in real time depending on the direction of the state's interference fringes in phase space. This is done using the knowledge gained by the measurement signal. We show that this control scheme can enhance or suppress chaos. By enhancing the degree of chaos we are also able to push the onset of chaos further into the quantum regime than was possible before. By suppressing chaos we generate highly non-classical states and regular motion. The feasibility of experimentally realising this control technique is discussed in detail.
The final section of this thesis considers a chaotic system that is not dissipative in the classical limit: the driven top. We investigate the effect that opening the quantum system to decoherence has on the degree of chaos when we continuously measure the system. We demonstrate that the presence of decoherence suppresses the chaos and alters the dynamics of the quantum system. This is seen to worsen as the strength of the measurement is increased unless a particular measurement is chosen that perfectly cancels out the decoherence resulting in the Hamiltonian evolution in addition to noise from the measurement. These results are verified by the separation time between classical and quantum dynamics
Understanding Quantum Technologies 2022
Understanding Quantum Technologies 2022 is a creative-commons ebook that
provides a unique 360 degrees overview of quantum technologies from science and
technology to geopolitical and societal issues. It covers quantum physics
history, quantum physics 101, gate-based quantum computing, quantum computing
engineering (including quantum error corrections and quantum computing
energetics), quantum computing hardware (all qubit types, including quantum
annealing and quantum simulation paradigms, history, science, research,
implementation and vendors), quantum enabling technologies (cryogenics, control
electronics, photonics, components fabs, raw materials), quantum computing
algorithms, software development tools and use cases, unconventional computing
(potential alternatives to quantum and classical computing), quantum
telecommunications and cryptography, quantum sensing, quantum technologies
around the world, quantum technologies societal impact and even quantum fake
sciences. The main audience are computer science engineers, developers and IT
specialists as well as quantum scientists and students who want to acquire a
global view of how quantum technologies work, and particularly quantum
computing. This version is an extensive update to the 2021 edition published in
October 2021.Comment: 1132 pages, 920 figures, Letter forma
Online learning of physics during a pandemic: A report from an academic experience in Italy
The arrival of the Sars-Cov II has opened a new window on teaching physics in academia.
Frontal lectures have left space for online teaching, teachers have been faced with a new way
of spreading knowledge, adapting contents and modalities of their courses. Students have
faced up with a new way of learning physics, which relies on free access to materials and
their informatics knowledge. We decided to investigate how online didactics has influenced
students’ assessments, motivation, and satisfaction in learning physics during the pandemic
in 2020. The research has involved bachelor (n = 53) and master (n = 27) students of
the Physics Department at the University of Cagliari (N = 80, 47 male; 33 female). The
MANOVA supported significant mean differences about gender and university level with
higher values for girls and master students in almost all variables investigated. The path
analysis showed that student-student, student-teacher interaction, and the organization of
the courses significantly influenced satisfaction and motivation in learning physics. The
results of this study can be used to improve the standards of teaching in physics at the
University of Cagliar