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Quantum Information Theory

Robustness and Errors in Quantum Optimal Control

Mon, 2011-02-14 19:43 - Daniel Rudolph
Date: 
2010-07-14
Author(s): 

Antonio Negretti, Rosario Fazio, Tommaso Calarco

Reference: 

arXiv:1007.2405v1 [quant-ph]

URL: 

http://arxiv.org/abs/1007.2405v1

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

Mon, 2011-02-14 19:40 - Daniel Rudolph
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)

URL: 

http://link.aps.org/doi/10.1103/PhysRevLett.103.240501

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.

Quantum register based on coupled electron spins in a room-temperature solid

Mon, 2011-02-14 19:37 - Daniel Rudolph
Date: 
2010-02-28
Author(s): 

P. Neumann, R. Kolesov, B. Naydenov, J. Beck1, F. Rempp, M. Steiner, V. Jacques, G. Balasubramanian, M. L. Markham, D. J. Twitchen, S. Pezzagna, J. Meijer, J. Twamley, F. Jelezko & J. Wrachtrup

Reference: 

Nature Physics 6, 249-253 (2010)

URL: 

http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1536.html

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication. However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials, particularly diamond, however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin–orbit coupling.

Single-Shot Readout of a Single Nuclear Spin

Mon, 2011-02-14 19:33 - Daniel Rudolph
Date: 
2010-07-01
Author(s): 

Philipp Neumann, Johannes Beck, Matthias Steiner, Florian Rempp, Helmut Fedder, Philip R. Hemmer, Jörg Wrachtrup and Fedor Jelezko

Reference: 

Science 329 no. 5991 pp. 542-544

URL: 

http://www.sciencemag.org/content/329/5991/542.abstract

Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin’s behavior.

Weak continuous monitoring of a flux qubit using coplanar waveguide resonator

Mon, 2011-02-14 19:08 - Daniel Rudolph
Date: 
2010-05-26
Author(s): 

G. Oelsner, S. H. W. van der Ploeg, P. Macha, U. Hübner, D. Born, S. Anders, E. Il’ichev, and H.-G. Meyer, M. Grajcar, S. Wünsch, M. Siegel, A. N. Omelyanchouk, B. Verkin, O. Astafiev

Reference: 

Phys. Rev. B 81, 172505 (2010)

URL: 

http://link.aps.org/doi/10.1103/PhysRevB.81.172505

We study a flux qubit in a coplanar waveguide resonator by measuring transmission through the system. In our system with the flux qubit decoupled galvanically from the resonator, the intermediate coupling regime is achieved. In this regime, dispersive readout is possible with weak back action on the qubit. The detailed theoretical analysis and simulations give good agreement with the experimental data and allow us to make the qubit characterization.

Efficient quantum state tomography

Mon, 2011-02-14 17:47 - Q-Essence Coordinator
Date: 
2010-12-21
Author(s): 

Marcus Cramer, Martin B. Plenio, Steven T. Flammia, Rolando Somma, David Gross, Stephen D. Bartlett, Olivier Landon-Cardinal, David Poulin and Yi-Kai Liu

Reference: 

Nat. Commun. 1, 149 (2010)
doi:10.1038/ncomms1147

Quantum state tomography—deducing quantum states from measured data—is the gold standard for verification and benchmarking of quantum devices. It has been realized in systems with few components, but for larger systems it becomes unfeasible because the number of measurements and the amount of computation required to process them grows exponentially in the system size. Here, we present two tomography schemes that scale much more favourably than direct tomography with system size.

Tensor Rank and Stochastic Entanglement Catalysis for Multipartite Pure States

Mon, 2011-02-14 17:29 - Q-Essence Coordinator
Date: 
2010-11-08
Author(s): 

Lin Chen, Eric Chitambar, Runyao Duan, Zhengfeng Ji, and Andreas Winter

Reference: 

Phys. Rev. Lett. 105, 200501 (2010)

URL: 

http://link.aps.org/doi/10.1103/PhysRevLett.105.200501

Quantum networks reveal quantum nonlocality

Mon, 2011-02-14 17:21 - Q-Essence Coordinator
Date: 
2011-02-08
Author(s): 

Daniel Cavalcanti, Mafalda L. Almeida, Valerio Scarani and Antonio Acín

Reference: 

Nat. Commun. 2 , 184 (2011)
doi:10.1038/ncomms1193 (2011)

The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information-processing tasks, for example, quantum communication, quantum key distribution, quantum state estimation or randomness extraction. Still, deciding whether a quantum state is nonlocal remains a challenging problem.

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