Featured Video Abstract

ArXiv identifier: 
1009.2683
Speakers: 
Christoffer Wittmann
Authors: 
C. Wiechers, L. Lydersen, C. Wittmann, D. Elser, J. Skaar, C. Marquardt, V. Makarov, and G. Leuchs

We present a method to control the detection events in quantum key distribution systems that use gated single-photon detectors. We employ bright pulses as faked states, timed to arrive at the avalanche photodiodes outside the activation time. The attack can remain unnoticed, since the faked states do not increase the error rate per se. This allows for an intercept–resend attack, where an eavesdropper transfers her detection events to the legitimate receiver without causing any errors.

ArXiv identifier: 
1002.4918
Speakers: 
Anzi Hu and Ludwig Mathey
Authors: 
Anzi Hu, L. Mathey, Carl J. Williams, Charles W. Clark

We study the noise correlations of one-dimensional binary Bose mixtures, as a probe of their quantum phases. In previous work, we found a rich structure of many-body phases in such mixtures, such as paired and counterflow superfluidity. Here we investigate the signature of these phases in the noise correlations of the atomic cloud after time-of-flight expansion, using both Luttinger liquid theory and the time-evolving block decimation (TEBD) method. We find that paired and counterflow superfluidity exhibit distinctive features in the noise spectra.

ArXiv identifier: 
1004.5018
Speakers: 
Koji Maruyama
Authors: 
D. Burgarth, K. Maruyama, F. Nori

A number of many-body problems can be formulated using Hamiltonians that are quadratic in the creation and annihilation operators. Here, we show how such quadratic Hamiltonians can be efficiently estimated indirectly, employing very few resources. We find that almost all properties of the Hamiltonian are determined by its surface, and that these properties can be measured even if the system can only be initialised to a mixed state.

ArXiv identifier: 
1002.3020
Speakers: 
Neill Lambert
Authors: 
Neill Lambert, Clive Emary, Yueh-nan Chen and Franco Nori

We consider the question of how to distinguish quantum from classical transport through nanostructures. To address this issue we have derived two inequalities for temporal correlations in nonequilibrium transport in nanostructures weakly coupled to leads. The first inequality concerns local charge measurements and is of general validity; the second concerns the current flow through the device and is relevant for double quantum dots.

ArXiv identifier: 
1003.5877
Speakers: 
Filippo Caruso
Authors: 
Filippo Caruso, Susana F. Huelga, Martin B. Plenio

The unavoidable presence of noise is thought to be one of the major problems to solve in order to pave the way for implementing quantum information technologies in realistic physical platforms. However, here we show a clear example in which noise, in terms of dephasing, may enhance the capability of transmitting not only classical but also quantum information, encoded in quantum systems, through communication networks.

ArXiv identifier: 
0906.2731
Speakers: 
Masaki Owari
Authors: 
Miguel Navascues, Masaki Owari, Martin B. Plenio

In this paper, we present new progress on the study of the symmetric extension criterion for separability. First, we show that a perturbation of order O(1/N) is sufficient and, in general, necessary to destroy the entanglement of any state admitting an N Bose symmetric extension. On the other hand, the minimum amount of local noise necessary to induce separability on states arising from N Bose symmetric extensions with Positive Partial Transpose (PPT) decreases at least as fast as O(1/N^2).

ArXiv identifier: 
0903.0543
Speakers: 
Giulio Chiribella
Authors: 
A. Bisio, G. Chiribella, G. M. D'Ariano, S. Facchini, P. Perinotti

We prove that the optimal strategy to store an unknown group transformation into a quantum memory is to apply the available uses in parallel on a suitable entangled state. The optimal retrieving strategy is the incoherent, ``measure-and-rotate'' strategy, in which the quantum memory is measured and a unitary depending on the outcome is performed. The same result holds for approximate re-alignment of reference frames for quantum communication.

ArXiv identifier: 
0905.2942
Speakers: 
Cesar A. Rodriguez-Rosario
Authors: 
Cesar A. Rodriguez-Rosario, James D. Whitfield, Alan Aspuru-Guzik

We introduce the quantum stochastic walk (QSW), which determines the evolution of generalized quantum mechanical walk on a graph that obeys a quantum stochastic equation of motion. Using an axiomatic approach, we specify the rules for all possible quantum, classical and quantum-stochastic transitions of a vertex as defined from its connectivity. We show how the family of possible QSW encompasses both the classical random walk (CRW) and the quantum walks (QW) as special cases, but also includes more general probability distributions.

ArXiv identifier: 
0905.0901
Speakers: 
Steve Flammia
Authors: 
Dave Bacon and Steven T. Flammia

The difficulty in producing precisely timed and controlled quantum gates is a significant source of error in many physical implementations of quantum computers. Here we introduce a simple universal primitive, adiabatic gate teleportation, which is robust to timing errors and many control errors and maintains a constant energy gap throughout the computation above a degenerate ground state space. Notably this construction allows for geometric robustness based upon the control of two independent qubit interactions.

ArXiv identifier: 
0810.4340
Speakers: 
S. Virmani
Authors: 
M. B. Plenio and S. Virmani

We consider the possibility of adding noise to a quantum circuit to make it efficiently simulatable classically. In previous works this approach has been used to derive upper bounds to fault tolerance thresholds - usually by identifying a privileged resource, such as an entangling gate or a non-Clifford operation, and then deriving the noise levels required to make it `unprivileged'. In this work we consider extensions of this approach where noise is added to Clifford gates too, and then `commuted' around until it concentrates on attacking the non-Clifford resource.