183 research outputs found
Quantum control by effective counterdiabatic driving
No full textWe review a scheme for the systematic design of quantum control protocols based on shortcuts to adiabaticity in few-level quantum systems. The adiabatic dynamics is accelerated by introducing high-frequency modulations in the control Hamiltonian, which mimic a time-dependent counterdiabatic correction. We present a number of applications for the high-fidelity realization of quantum state transfers and quantum gates based on effective counterdiabatic driving, in platforms ranging from superconducting circuits to Rydberg atoms
Quantum Simulation of Three-Body Interactions in Weakly Driven Quantum Systems
No full textThe realization of effective Hamiltonians featuring many-body interactions beyond pairwise coupling would enable the quantum simulation of central models underpinning topological physics and quantum computation. We overcome crucial limitations of perturbative Floquet engineering and discuss the highly accurate realization of a purely three-body Hamiltonian in superconducting circuits and molecular nanomagnets
Dynamical enhancement of spatial entanglement in massive particles
No full textWe discuss dynamical enhancement of entanglement in a driven Bose-Hubbard model and find an enhancement of two orders of magnitude from the ground state value which is robust against fluctuations in experimental parameters
Fast adiabatic evolution by oscillating initial Hamiltonians
No full textWe propose a method to produce fast transitionless dynamics for finite-dimensional quantum systems without requiring additional Hamiltonian components not included in the initial control setup, remaining close to the true adiabatic path at all times. The strategy is based on the introduction of an effective counterdiabatic scheme: a correcting Hamiltonian is constructed which approximatively cancels nonadiabatic effects, inducing an evolution tracking the adiabatic states closely. This can be absorbed into the initial Hamiltonian by adding a fast oscillation in the control parameters. We show that a consistent speedup can be achieved without requiring strong control Hamiltonians, using it both as a stand-alone shortcut to adiabaticity and as a weak correcting field. A number of examples are treated, dealing with quantum state transfer in avoided-crossing problems and entanglement creation
Accelerating adiabatic protocols for entangling two qubits in circuit QED
No full textWe introduce a method to speed up adiabatic protocols for creating entanglement between two qubits dispersively coupled to a transmission line, while keeping fidelities high and maintaining robustness to control errors. The method takes genuinely adiabatic sweeps, ranging from a simple Landau-Zener drive to boundary cancellation methods and local adiabatic drivings, and adds fast oscillations to speed up the protocol while canceling unwanted transitions. We compare our protocol with existing adiabatic methods in a state-of-the-art parameter range and show substantial gains. Numerical simulations emphasize that this strategy is efficient also beyond the rotating-wave approximation and that the method is robust against random static biases in the control parameters and with respect to damping and decoherence effects
Measures and dynamics of entangled states
No full textIn the first part of the present thesis, we derive a theory for quantifying
entanglement of mixed bipartite quantum states.
We derive upper and lower bounds for the concurrence of quantum states in
arbitrary finite dimensions,
such as to confine its actual value to a finite interval.
We test these estimates for various sets of states with very satisfactory
results.
In particular, our lower bound detects entangled states
with positive partial transpose.
In view of the specific requirements of laboratory experiments,
we derive an approximate expression for the concurrence of almost
pure - i.e.
weakly mixed - states.
Comparison of this quasi-pure approximation with the above upper and lower
bounds shows that its range of validity even comprises states with
relatively large mixing.
Finally, we propose a generalised concurrence for multipartite mixed states.
For its quantitative characterisation, we can use the same strategies as
in our treatment of bipartite systems.
These novel techniques for a quantitative description of the entanglement of
mixed states allow to monitor the production and the decay of
entanglement under coherent and incoherent forcing - as we show by
applying our theory to a realistic scenario of ion trap experiments.
In the second part of the thesis, we introduce suitably defined quasi
probability representations
such as to quantify the entanglement of pure bipartite states, with
an immediate generalisation for pure states of multipartite systems.
It is shown that the statistical moments as well as various entropies of
these representations - characterising their localisation properties -
are non-increasing under local operations and classical
communication, hence that they are proper entanglement monotones.Im ersten Teil der vorliegenden Arbeit entwickeln wir eine Theorie zur
quantitativen Abschaetzung der Verschraenkung gemischter Quantenzustaende
von Zweiparteiensystemen.
Wir leiten obere und untere Schranken der Concurrence
fuer beliebige endliche Dimensionen her,
was es insbesondere ermoeglicht, ein endliches Intervall fuer deren
tatsaechlichen Wert anzugeben.
Wir testen diese Schranken an unterschiedlichen Typen quantenmechanischer
Zustaende und erhalten verlaessliche Beschreibungen.
Ausserdem ist unsere untere Schranke in der Lage, die nichttriviale
Verschraenkung von
Zustaenden mit positiver partieller Transponierter zu erkennen.
Im Hinblick auf konkrete experimentelle Anforderungen geben wir eine explizite
Naeherung fuer nahezu reine - d.h. schwach gemischte - Zustaende an, die
eine rein algebraische Abschaetzung ermoeglicht.
Deren Vergleich mit den allgemeiner gueltigen oberen und unteren Schranken
zeigt, dass ihr Gueltigkeitsbereich sich sogar auf
Zustaende mit relativ starkem Mischungsgrad erstreckt.
Schliesslich schlagen wir eine moegliche Verallgemeinerung der
Concurrence fuer Systeme mit beliebig vielen Untersystemen vor.
Es zeigt sich, dass diese verallgemeinerte Concurrence mit Hilfe derselben
Methoden quantitativ gefasst werden kann, die auch in unserer Behandlung von
Zweiparteiensystemen zum Ziel fuehren.
Diese neuen Methoden zur Abschaetzung des Verschraenkungsgrades beliebiger
gemischter Zustaende ermoeglichen es, die Erzeugung und den Zerfall von
Verschraenkung unter dem Einfluss kohaerenter und inkohaerenter Prozesse
mit vergleichweise geringem Aufwand zu verfolgen,
was wir mit der Anwendung unserer Theorie auf ein fuer
Ionenfallenexperimente typisches Szenario zeigen.
Der zweite Teil der Arbeit liefert eine quantitative
Charakterisierung reiner Zustaende von Zweiparteiensystemen, die sich
direkt auf Systeme mit einer beliebigen Anzahl von Unterteilungen
verallgemeinern laesst.
Hierzu benutzen wir geeignet definierte Quasiwahrscheinlichkeitsverteilungen und
zeigen, dass deren statistische Momente oder Entropien, die ihre
Lokalisierungseigenschaften beschreiben,
unter lokalen Operationen und klassischer Kommunikation nicht
anwachsen und somit Verschraenkungsmonotone sind
High fidelity quantum gates of trapped ions mediated by a dissipative bus mode
No full textWe describe entangling quantum gates for trapped ions mediated by
a dissipative bus mode and show that suitably designed, polychromatic control
pulses decrease ion-phonon entanglement substantially while maintaining the mediated
interaction. In particular for multi-qubit gates this yields a signi cant improvement in
gate performance
Regulation of the actin cytoskeleton by an interaction of IQGAP related protein GAPA with filamin and cortexillin I.
No full textFilamin and Cortexillin are F-actin crosslinking proteins in Dictyostelium discoideum allowing actin filaments to form three-dimensional networks. GAPA, an IQGAP related protein, is required for cytokinesis and localizes to the cleavage furrow during cytokinesis. Here we describe a novel interaction with Filamin which is required for cytokinesis and regulation of the F-actin content. The interaction occurs through the actin binding domain of Filamin and the GRD domain of GAPA. A similar interaction takes place with Cortexillin I. We further report that Filamin associates with Rac1a implying that filamin might act as a scaffold for small GTPases. Filamin and activated Rac associate with GAPA to regulate actin remodelling. Overexpression of filamin and GAPA in the various strains suggests that GAPA regulates the actin cytoskeleton through interaction with Filamin and that it controls cytokinesis through association with Filamin and Cortexillin
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