QIC Abstracts

 Vol.5 No.7 November 1, 2005
Research Articles:
System design for large-scale ion trap quantum information processor (pp515-537)
         J. Kim,  S. Pau, Z. Ma, H.R. McLellan, J.V. Gates, A. Kornblit, R.E. Slusher, R.M. Jopson, I. Kang and M. Dinu
 We present a detailed system design and available technology choices for building a large scale ($>$ 100 qubits) ion trap quantum information processor (QIP). The system design is based on technologies that are within reach today, and utilizes single-instruction-on-multiple-data (SIMD) principles to re-use resources that cannot be duplicated easily. The system engineering principles adopted highlight various design tradeoffs in the QIP design and serve as a guideline to find design spaces for a much larger QIP.

Recasting Mermin's multi-player game into the framework of pseudo-telepathy (pp538-550)
         G. Brassard, A. Broadbent, and  A. Tapp
Entanglement is perhaps the most non-classical manifestation of quantum \mbox{mechanics}. Among its many interesting applications to information processing, it can be harnessed to \emph{reduce} the amount of communication required to process a variety of distributed computational tasks. Can it be used to eliminate communication altogether? Even though it cannot serve to signal information between remote parties, there are distributed tasks that can be performed without any need for communication, provided the parties share prior entanglement: this is the realm pseudo-telepathy.    One of the earliest uses of multi-party entanglement was presented by Mermin in 1990. Here we recast his idea in terms of pseudo-telepathy: we provide a new computer-scientist-friendly analysis of this game. We prove an upper bound on the best possible classical strategy for attempting to play this game, as well as a novel, matching lower bound. This leads us to considerations on how well imperfect quantum-mechanical apparatus must perform in order to exhibit a behaviour that would be classically impossible to explain. Our results include improved bounds that could help vanquish the infamous detection loophole.

A quantum cryptographic protocol with detection of compromised server (pp551-560)
         D. R. Kuhn 
 This paper presents a server-based hybrid cryptographic protocol, using quantum and classical resources, to generate a key for authentication and optionally for encryption in a network. A novel feature of the protocol is that it can detect a compromised server.  Additional advantages are that it avoids the requirement for timestamps used in classical protocols, guarantees that the trusted server cannot know the authentication key, can provide resistance to multiple photon attacks, and can be used with BB84 or other quantum key distribution protocols. Each resource shares a previously distributed secret key with the trusted server, and resources can communicate with the server using both classical and quantum channels.  Resources do not share secret keys with each other, so that the key distribution problem for the network is reduced from to .

Globally controlled artificial semiconducting molecules as quantum computers (pp561-572) 
         J.  Tribollet   
Quantum computers are expected to be considerably more efficient than classical computers for the execution of some specific tasks. The difficulty in the practical implementation of those computers is to build a microscopic quantum system that can be controlled at a larger macroscopic scale. Here I show that vertical lines of donor atoms embedded in an appropriate Zinc Oxide semiconductor structure can constitute artificial molecules that are as many copy of the same quantum computer. In this scalable architecture, each unit of information is encoded onto the electronic spin of a donor. Contrary to most existing practical proposals, here the logical operations only require a global control of the spins by electromagnetic pulses. Ensemble measurements simplify the readout. With appropriate improvement of its growth and doping methods, Zinc Oxide could be a good semiconductor for the next generation of computers.

A comparison of decoherence-free subsystem/subspace for partially broken symmetry (pp573-582)
         S. Siddiqui and J. Gea-Banacloche
We study the performance of the 3-qubit decoherence-free subsystem and the 4-qubit decoherence-free subspace in the presence of partially correlated noise. We characterize their performance in terms of the average and worst-case fidelity, as a function of the ratio of the interqubit distance to the correlation length of the noise, and find that, in general, more symmetric arrangements (triangles or squares) lead to better performance. Overall, we find the 3-qubit code to perform better than the 4-qubit code by about a factor of 2 in the average infidelity. We observe that this is related to the greater robustness of this code against uncorrelated (independent) errors.

Highly asymmetric quantum cloning in arbitrary dimension (pp583-592)
         J. Fiuravsek, R. Filip, and N.J. Cerf
We investigate the universal asymmetric cloning of states in a Hilbert space of arbitrary dimension. We derive the class of optimal and fully asymmetric $1\rightarrow 3$ cloners, which produce three copies, each having a different fidelity. A simple parametric expression for the maximum achievable cloning fidelity triplets is then provided. As a side-product, we also prove the optimality of the $1\rightarrow 2$ asymmetric cloning machines that have been proposed in the literature.

Quantum algorithms for subset finding (pp593-604)
         A.M. Childs and J.M. Eisenberg
Recently, Ambainis gave an $O(N^{2/3})$-query discrete-time quantum walk algorithm for the element distinctness problem, and more generally, an $O(N^{L/(L+1)})$-query algorithm for finding $L$ equal numbers. We review this algorithm and give a simplified and tightened analysis of its query complexity using techniques previously applied to the analysis of continuous-time quantum walk. We also briefly discuss applications of the algorithm and pose two open problems regarding continuous-time quantum walk and lower bounds.

On “Quantum and Classical Message Identification via Quantum Channels” (pp605-606)
         A. Winter

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