UTS OPUS
The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.
http://opus.lib.uts.edu.au:80/research
2015-03-31T18:25:23Z
2015-03-31T18:25:23Z
Five Two-Qubit Gates Are Necessary for Implementing Toffoli Gate
Yu, N
Duan, R
Ying, M
http://hdl.handle.net/10453/34602
2015-03-31T06:56:23Z
2013-01-16T00:00:00Z
Five Two-Qubit Gates Are Necessary for Implementing Toffoli Gate
Yu, N; Duan, R; Ying, M
In this paper, we settle the long-standing open problem of the minimum cost of two-qubit gates for simulating a Toffoli gate. More precisely, we show that five two-qubit gates are necessary. Before our work, it is known that five gates are sufficient and only numerical evidences have been gathered, indicating that the five-gate implementation is necessary. The idea introduced here can also be used to solve the problem of optimal simulation of three-qubit control phase introduced by Deutsch in 1989.
2013-01-16T00:00:00Z
On zero-error communication via quantum channels in the presence of noiseless feedback
Duan, R
Severini, S
Winter, A
http://hdl.handle.net/10453/34601
2015-03-31T06:45:38Z
2015-02-11T00:00:00Z
On zero-error communication via quantum channels in the presence of noiseless feedback
Duan, R; Severini, S; Winter, A
We initiate the study of zero-error communication via quantum channels assisted by noiseless feedback link of unlimited quantum capacity, generalizing Shannon's zero-error communication theory with instantaneous feedback. This capacity depends only on the linear span of Kraus operators of the channel, which generalizes the bipartite equivocation graph of a classical channel, and which we dub "non-commutative bipartite graph". We go on to show that the feedback-assisted capacity is non-zero (allowing for a constant amount of activating noiseless communication) if and only if the non-commutative bipartite graph is non-trivial, and give a number of equivalent characterizations. This result involves a far-reaching extension of the "conclusive exclusion" of quantum states [Pusey/Barrett/Rudolph, Nat Phys 8:475, 2012]. We then present an upper bound on the feedback-assisted zero-error capacity, motivated by a conjecture originally made by Shannon and proved by Ahlswede. We demonstrate that this bound is additive and given by a nice minimax formula. We also prove a coding theorem showing that this quantity is the entanglement-assisted capacity against an adversarially chosen channel from the set of all channels with the same Kraus span, which can also be interpreted as the feedback-assisted unambiguous capacity. The proof relies on a generalization of the "Postselection Lemma" (de Finetti reduction) [Christandl/Koenig/Renner, PRL 102:020503, 2009] that allows to reflect additional constraints, and which we believe to be of independent interest. This capacity is a relaxation of the feedback-assisted zero-error capacity; however, we do not know whether they coincide in general. We illustrate our ideas with a number of examples, including classical-quantum channels and Weyl diagonal channels, and close with an extensive discussion of open questions.
2015-02-11T00:00:00Z
No-Signalling Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovasz Number
Duan, R
Winter, A
http://hdl.handle.net/10453/34599
2015-03-31T06:21:15Z
2014-09-11T00:00:00Z
No-Signalling Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovasz Number
Duan, R; Winter, A
We study the one-shot zero-error classical capacity of a quantum channel assisted by quantum no-signalling correlations, and the reverse problem of exact simulation of a prescribed channel by a noiseless classical one. Quantum no-signalling correlations are viewed as two-input and two-output completely positive and trace preserving maps with linear constraints enforcing that the device cannot signal. Both problems lead to simple semidefinite programmes (SDPs) that depend only the Kraus operator space of the channel. In particular, we show that the zero-error classical simulation cost is precisely the conditional min-entropy of the Choi-Jamiolkowski matrix of the given channel. The zero-error classical capacity is given by a similar-looking but different SDP; the asymptotic zero-error classical capacity is the regularization of this SDP, and in general we do not know of any simple form. Interestingly however, for the class of classical-quantum channels, we show that the asymptotic capacity is given by a much simpler SDP, which coincides with a semidefinite generalization of the fractional packing number suggested earlier by Aram Harrow. This finally results in an operational interpretation of the celebrated Lovasz $\vartheta$ function of a graph as the zero-error classical capacity of the graph assisted by quantum no-signalling correlations, the first information theoretic interpretation of the Lovasz number.
2014-09-11T00:00:00Z
Unsynchronized fault location based on the negative-sequence voltage magnitude for double-circuit transmission lines
Mahamedi, B
Zhu, JG
http://hdl.handle.net/10453/34597
2015-03-31T06:16:55Z
2014-01-01T00:00:00Z
Unsynchronized fault location based on the negative-sequence voltage magnitude for double-circuit transmission lines
Mahamedi, B; Zhu, JG
This paper describes a new approach to fault location for double-circuit transmission lines based on only the voltage data of both ends of the faulted circuit. The ratio between the magnitudes of negative-sequence voltages measured at both ends of the faulted circuit is utilized to estimate the fault location. Since only the magnitudes are used, the data of both ends are not required to be synchronized, which removes any concern about data synchronization. Moreover, since only the voltage data are required, the errors caused by current transformers can be avoided. The proposed method can effectively locate the single-phase-to-ground, double-phase-to-ground, and phase-to-phase faults disregarding the fault resistance and prefault conditions and without any need for fault classification and phase selection. Unlike the iterative methods, the proposed method is fully analytical and does not cause much computing burden to the line relays. The accuracy and practicality of the proposed method make it an attractive function to implement in numerical relays. © 1986-2012 IEEE.
2014-01-01T00:00:00Z