05.70.+o Quantum information & quantum control

Entanglement Storage Units

Date: 
2011-08-16
Author(s): 

T. Caneva, T. Calarco, S. Montangero

Reference: 

New J. Phys. 14 093041 (2012)

We introduce a protocol to drive many body quantum systems into long-lived entangled states, protected from decoherence by big energy gaps. With this approach it is possible to implement scalable entanglement-storage units. We test the protocol in the Lipkin-Meshkov-Glick model, a prototype many-body quantum system that describes different experimental setups.

Least-squares approximation by elements from matrix orbits achieved by gradient flows on compact lie groups

Date: 
2010-12-13
Author(s): 

Chi-Kwong Li, Yiu-Tung Poon, Thomas Schulte-Herbrüggen

Reference: 

Math. Comp. 80 (2011), 1601-1621

Let $ S(A)$ denote the orbit of a complex or real matrix $ A$ under a certain equivalence relation such as unitary similarity, unitary equivalence, unitary congruences etc.

Prospects for fast Rydberg gates on an atom chip

Date: 
2011-04-21
Author(s): 

Matthias M. Müller, Harald R. Haakh, Tommaso Calarco, Christiane P. Koch, Carsten Henkel

Atom chips are a promising candidate for a scalable architecture for quantum information processing provided a universal set of gates can be implemented with high fidelity. The difficult part in achieving universality is the entangling two-qubit gate. We consider a Rydberg phase gate for two atoms trapped on a chip and employ optimal control theory to find the shortest gate that still yields a reasonable gate error. Our parameters correspond to a situation where the Rydberg blockade regime is not yet reached.

Optimizing entangling quantum gates for physical systems

Date: 
2011-09-15
Author(s): 

M. M. Müller, D. M. Reich, M. Murphy, H. Yuan, J. Vala, K. B. Whaley, T. Calarco, C. P. Koch

Reference: 

Phys. Rev. A 84, 042315 (2011).

Optimal control theory is a versatile tool that presents a route to significantly improving figures of merit for quantum information tasks. We combine it here with the geometric theory for local equivalence classes of two-qubit operations to derive an optimization algorithm that determines the best entangling two-qubit gate for a given physical setting. We demonstrate the power of this approach for trapped polar molecules and neutral atoms.

The quantum speed limit of optimal controlled phasegates for trapped neutral atoms

Date: 
2011-07-25
Author(s): 

M. H. Goerz, T. Calarco, C. P. Koch

Reference: 

J. Phys. B: At. Mol. Opt. Phys. 44, 154011 (2011)

We study controlled phasegates for ultracold atoms in an optical potential. A shaped laser pulse drives transitions between the ground and electronically excited states where the atoms are subject to a long-range 1/R3 interaction. We fully account for this interaction and use optimal control theory to calculate the pulse shapes. This allows us to determine the minimum pulse duration, respectively, gate time T that is required to obtain high fidelity.

Coherent optimal control of photosynthetic molecules

Date: 
2011-03-04
Author(s): 

F. Caruso, S. Montangero, T. Calarco, S. F. Huelga, M. B. Plenio

Reference: 

Phys. Rev. A 85, 042331 (2012)

We demonstrate theoretically that open-loop quantum optimal control techniques can provide efficient tools for the verification of various quantum coherent transport mechanisms in natural and artificial light-harvesting complexes under realistic experimental conditions.

Chopped random basis quantum optimization

Date: 
2011-08-22
Author(s): 

T. Caneva, T. Calarco, S. Montangero

Reference: 

Phys. Rev. A 84, 022326 (2011)

In this work we describe in detail the "Chopped RAndom Basis" (CRAB) optimal control technique recently introduced to optimize t-DMRG simulations [arXiv:1003.3750]. Here we study the efficiency of this control technique in optimizing different quantum processes and we show that in the considered cases we obtain results equivalent to those obtained via different optimal control methods while using less resources. We propose the CRAB optimization as a general and versatile optimal control technique.

Robustness and Errors in Quantum Optimal Control

Date: 
2010-07-14
Author(s): 

Antonio Negretti, Rosario Fazio, Tommaso Calarco

Reference: 

arXiv:1007.2405v1 [quant-ph]

We introduce a new approach to quantify the robustness of optimal control of closed quantum systems. Our theory allows to assess the degree of distortion that can be applied to a set of known optimal control parameters, which are solutions of an optimal control problem. The formalism is applied to an exactly solvable model and to the Landau-Zener model, whose optimal control problem is solvable only numerically. The presented method is of importance for any application where a high degree of controllability of the quantum system dynamics is required.

Optimal Control at the Quantum Speed Limit

Date: 
2009-12-07
Author(s): 

T. Caneva, M. Murphy, T. Calarco, R. Fazio, S. Montangero, V. Giovannetti, and G. E. Santoro

Reference: 

Phys. Rev. Lett. 103, 240501 (2009)

Optimal control theory is a promising candidate for a drastic improvement of the performance of quantum information tasks. We explore its ultimate limit in paradigmatic cases, and demonstrate that it coincides with the maximum speed limit allowed by quantum evolution.

Speeding up critical system dynamics through optimized evolution

Date: 
2011-07-12
Author(s): 

T. Caneva, T. Calarco, R. Fazio, G. E. Santoro, S. Montangero

Reference: 

Phys. Rev. A 84, 012312 (2011)

The number of defects which are generated upon crossing a quantum phase transition can be minimized by choosing properly designed time-dependent pulses. In this work we determine what are the ultimate limits of this optimization. We discuss under which conditions the production of defects across the phase transition is vanishing small.

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