arXiv subjects

Sophie Schirmer

Publications and source records attributed to Sophie Schirmer.

21 records · Page 2Linked to original sources

arXiv2011

Fundamental Speed Limits on Quantum Coherence and Correlation Decay

The study and control of coherence in quantum systems is one of the most exciting recent developments in physics. Quantum coherence plays a crucial role in emerging quantum technologies as well as fundamental experiments. A major obstacle to the utilization of quantum effects is decoherence, primarily in the form of dephasing that destroys quantum coherence, and leads to effective classical behaviour. We show that there are universal relationships governing dephasing, which constrain the relative rates at which quantum correlations can disappear. These effectively lead to speed limits which become especially important in multi-partite systems.

quant-ph↗

arXiv2011

Bang-Bang Control Design for Quantum State Transfer based on Hyperspherical Coordinates and Optimal Time-energy Control

We present a constructive control scheme for solving quantum state engineering problems based on a parametrization of the state vector in terms of complex hyperspherical coordinates. Unlike many control schemes based on factorization of unitary operators the scheme gives explicit expressions for all the Euler angles in terms of the hyperspherical coordinates of the initial and final state. The factorization, when applicable, has added benefits that phase rotations can be combined and performed concurrently. The control procedure can be realized using a simple bang-bang or square-wave-function controls. Optimal time-energy control is considered to find the optimal control amplitudes. The extension of the scheme to implement unitary operators is also discussed.

quant-ph↗

arXiv2010

Global Control Methods for GHZ State Generation on 1-D Ising Chain

We discuss how to prepare an Ising chain in a GHZ state using a single global control field only. This model does not require the spins to be individually addressable and is applicable to quantum systems such as cold atoms in optical lattices, some liquid- or solid-state NMR experiments, and many nano-scale quantum structures. We show that GHZ states can always be reached asymptotically from certain easy-to-prepare initial states using adiabatic passage, and under certain conditions finite-time reachability can be ensured. To provide a reference useful for future experimental implementations three different control strategies to achieve the objective, adiabatic passage, Lyapunov control and optimal control are compared, and their advantages and disadvantages discussed, in particular in the presence of realistic imperfections such as imperfect initial state preparation, system inhomogeneity and dephasing.

quant-ph↗