674 research outputs found
Advances in Bosonic Quantum Error Correction with Gottesman-Kitaev-Preskill Codes: Theory, Engineering and Applications
Encoding quantum information into a set of harmonic oscillators is considered
a hardware efficient approach to mitigate noise for reliable quantum
information processing. Various codes have been proposed to encode a qubit into
an oscillator -- including cat codes, binomial codes and
Gottesman-Kitaev-Preskill (GKP) codes. These bosonic codes are among the first
to reach a break-even point for quantum error correction. Furthermore, GKP
states not only enable close-to-optimal quantum communication rates in bosonic
channels, but also allow for error correction of an oscillator into many
oscillators. This review focuses on the basic working mechanism, performance
characterization, and the many applications of GKP codes, with emphasis on
recent experimental progress in superconducting circuit architectures and
theoretical progress in multimode GKP qubit codes and
oscillators-to-oscillators (O2O) codes. We begin with a preliminary
continuous-variable formalism needed for bosonic codes. We then proceed to the
quantum engineering involved to physically realize GKP states. We take a deep
dive into GKP stabilization and preparation in superconducting architectures
and examine proposals for realizing GKP states in the optical domain (along
with a concise review of GKP realization in trapped-ion platforms). Finally, we
present multimode GKP qubits and GKP-O2O codes, examine code performance and
discuss applications of GKP codes in quantum information processing tasks such
as computing, communication, and sensing.Comment: 77+5 pages, 31 figures. Minor bugs fixed in v2. comments are welcome
Dynamical Systems
Complex systems are pervasive in many areas of science integrated in our daily lives. Examples include financial markets, highway transportation networks, telecommunication networks, world and country economies, social networks, immunological systems, living organisms, computational systems and electrical and mechanical structures. Complex systems are often composed of a large number of interconnected and interacting entities, exhibiting much richer global scale dynamics than the properties and behavior of individual entities. Complex systems are studied in many areas of natural sciences, social sciences, engineering and mathematical sciences. This special issue therefore intends to contribute towards the dissemination of the multifaceted concepts in accepted use by the scientific community. We hope readers enjoy this pertinent selection of papers which represents relevant examples of the state of the art in present day research. [...
Transport and nonequilibrium dynamics in 1d many-body systems with bosonic degrees of freedom
In this thesis, different wave-function based numerical methods are used to study many-body systems with bosonic degrees of freedom in 1d.
The main focus is on the Holstein model of spinless fermions, where ground-state phases, nonequilibrium dynamics, and thermalization are investigated.
Using matrix-product-state methods for the investigation of such models with electron-phonon coupling poses special numerical challenges.
Local basis optimization, projected purification, and subspace expansion are used to overcome these challenges.2021-12-1
Geometric Integrators for Schrödinger Equations
The celebrated Schrödinger equation is the key to understanding the dynamics of
quantum mechanical particles and comes in a variety of forms. Its numerical solution
poses numerous challenges, some of which are addressed in this work.
Arguably the most important problem in quantum mechanics is the so-called harmonic
oscillator due to its good approximation properties for trapping potentials. In
Chapter 2, an algebraic correspondence-technique is introduced and applied to construct
efficient splitting algorithms, based solely on fast Fourier transforms, which
solve quadratic potentials in any number of dimensions exactly - including the important
case of rotating particles and non-autonomous trappings after averaging by Magnus
expansions. The results are shown to transfer smoothly to the Gross-Pitaevskii
equation in Chapter 3. Additionally, the notion of modified nonlinear potentials is
introduced and it is shown how to efficiently compute them using Fourier transforms.
It is shown how to apply complex coefficient splittings to this nonlinear equation and
numerical results corroborate the findings.
In the semiclassical limit, the evolution operator becomes highly oscillatory and standard
splitting methods suffer from exponentially increasing complexity when raising
the order of the method. Algorithms with only quadratic order-dependence of the
computational cost are found using the Zassenhaus algorithm. In contrast to classical
splittings, special commutators are allowed to appear in the exponents. By construction,
they are rapidly decreasing in size with the semiclassical parameter and can be
exponentiated using only a few Lanczos iterations. For completeness, an alternative
technique based on Hagedorn wavepackets is revisited and interpreted in the light of
Magnus expansions and minor improvements are suggested. In the presence of explicit
time-dependencies in the semiclassical Hamiltonian, the Zassenhaus algorithm
requires a special initiation step. Distinguishing the case of smooth and fast frequencies,
it is shown how to adapt the mechanism to obtain an efficiently computable
decomposition of an effective Hamiltonian that has been obtained after Magnus expansion,
without having to resolve the oscillations by taking a prohibitively small
time-step.
Chapter 5 considers the Schrödinger eigenvalue problem which can be formulated as
an initial value problem after a Wick-rotating the Schrödinger equation to imaginary
time. The elliptic nature of the evolution operator restricts standard splittings to
low order, Âż < 3, because of the unavoidable appearance of negative fractional timesteps
that correspond to the ill-posed integration backwards in time. The inclusion
of modified potentials lifts the order barrier up to Âż < 5. Both restrictions can be
circumvented using complex fractional time-steps with positive real part and sixthorder
methods optimized for near-integrable Hamiltonians are presented.
Conclusions and pointers to further research are detailed in Chapter 6, with a special
focus on optimal quantum control.Bader, PK. (2014). Geometric Integrators for Schrödinger Equations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/38716TESISPremios Extraordinarios de tesis doctorale
Technical Design Report EuroGammaS proposal for the ELI-NP Gamma beam System
The machine described in this document is an advanced Source of up to 20 MeV
Gamma Rays based on Compton back-scattering, i.e. collision of an intense high
power laser beam and a high brightness electron beam with maximum kinetic
energy of about 720 MeV. Fully equipped with collimation and characterization
systems, in order to generate, form and fully measure the physical
characteristics of the produced Gamma Ray beam. The quality, i.e. phase space
density, of the two colliding beams will be such that the emitted Gamma ray
beam is characterized by energy tunability, spectral density, bandwidth,
polarization, divergence and brilliance compatible with the requested
performances of the ELI-NP user facility, to be built in Romania as the Nuclear
Physics oriented Pillar of the European Extreme Light Infrastructure. This
document illustrates the Technical Design finally produced by the EuroGammaS
Collaboration, after a thorough investigation of the machine expected
performances within the constraints imposed by the ELI-NP tender for the Gamma
Beam System (ELI-NP-GBS), in terms of available budget, deadlines for machine
completion and performance achievement, compatibility with lay-out and
characteristics of the planned civil engineering
On The Dynamics and Control Strategy of Time-Delayed Vibro-Impact Oscillators
Being able to control nonlinear oscillators, which are ubiquitous, has significant engineering implications in process development and product sustainability design. The fundamental characteristics of a vibro-impact oscillator, a non-autonomous time-delayed feedback oscillator, and a time-delayed vibro-impact oscillator are studied. Their being stochastic, nonstationary, non-smooth, and dynamically complex render the mitigation of their behaviors in response to linear and stationary inputs very difficult if not entirely impossible.
A novel nonlinear control concept featuring simultaneous control of vibration amplitude in the time-domain and spectral response in the frequency-domain is developed and subsequently incorporated to maintain dynamic stability in these nonlinear oscillators by denying bifurcation and route-to-chaos from coming to pass. Convergence of the controller is formulated to be inherently unconditional with the optimization step size being self-adaptive to system identification and control force input. Optimal initial filter weights are also derived to warrant fast convergence rate and short response time. These novel features impart adaptivity, intelligence, and universal applicability to the wavelet based nonlinear time-frequency control methodology. The validity of the controller design is demonstrated by evaluating its performance against PID and fuzzy logic controllers in controlling the aperiodic, broad bandwidth, discontinuous responses characteristic of the time-delayed, vibro-impact oscillator
Multi-parameter optimisation of quantum optical systems
Quantum optical systems are poised to become integral components
of technologies of the future. While there is growing commercial
interest in these systems---for applications in information
processing, secure communication and precision metrology---there
remain significant technical challenges to overcome before
widespread adoption is possible. In this thesis we consider the
general problem of optimising quantum optical systems, with a
focus on sensing and information processing applications. We
investigate four different classes of system with varying degrees
of generality and complexity, and demonstrate four corresponding
optimisation techniques.
At the most specific end of the spectrum---where behaviour is
best understood---we consider the problem of interferometric
sensitivity enhancement, specifically in the context of
long-baseline gravitational wave detectors. We investigate the
use of an auxiliary optomechanical system to generate squeezed
light exhibiting frequency-dependent quadrature rotation. Such
rotation is necessary to evade the effect of quantum back action
and achieve broadband sensitivity beyond the standard quantum
limit. We find that a cavity optomechanical system is generally
unsuitable for this purpose, since the quadrature rotation occurs
in the opposite direction to that required for broadband
sensitivity improvement.
Next we introduce a general technique to engineer arbitrary
optical spring potentials in cavity optomechanical systems. This
technique has the potential to optimise many types of sensors
relying on the optical spring effect. As an example, we show that
this technique could yield an enhancement in sensitivity by a
factor of 5 when applied to a certain gravitational sensor based
on a levitated cavity mirror.
We then consider a particular nanowire-based optomechanical
system with potential applications in force sensing. We
demonstrate a variety of ways to improve its sensitivity to
transient forces. We first apply a non-stationary feedback
cooling protocol to the system, and achieve an improvement in
peak signal-to-noise ratio by a factor of 3, corresponding to a
force resolution of 0.2fN. We then implement two non-stationary
estimation schemes, which involve post-processing data taken in
the absence of physical feedback cooling, to achieve a comparable
enhancement in performance without the need for additional
experimental complexity.
Finally, to address the most complex of systems, we present a
general-purpose machine learning algorithm capable of
automatically modelling and optimising arbitrary physical systems
without human input. To demonstrate the potential of the
algorithm we apply it to a magneto-optical trap used for a
quantum memory, and achieve an improvement in optical depth from
138 to 448.
The four techniques presented differ significantly in their style
and the types of systems to which they are applicable.
Successfully harnessing the full range of such optimisation
procedures will be vital in unlocking the potential of quantum
optical systems in the technologies of the futur
Toward simulating Superstring/M-theory on a quantum computer
We present a novel framework for simulating matrix models on a quantum
computer. Supersymmetric matrix models have natural applications to
superstring/M-theory and gravitational physics, in an appropriate limit of
parameters. Furthermore, for certain states in the Berenstein-Maldacena-Nastase
(BMN) matrix model, several supersymmetric quantum field theories dual to
superstring/M-theory can be realized on a quantum device. Our prescription
consists of four steps: regularization of the Hilbert space, adiabatic state
preparation, simulation of real-time dynamics, and measurements. Regularization
is performed for the BMN matrix model with the introduction of energy cut-off
via the truncation in the Fock space. We use the Wan-Kim algorithm for fast
digital adiabatic state preparation to prepare the low-energy eigenstates of
this model as well as thermofield double state. Then, we provide an explicit
construction for simulating real-time dynamics utilizing techniques of
block-encoding, qubitization, and quantum signal processing. Lastly, we present
a set of measurements and experiments that can be carried out on a quantum
computer to further our understanding of superstring/M-theory beyond analytic
results.Comment: 68 pages; v2: minor correction
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