- Kwek, Leong Chuan

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# Kwek, Leong Chuan

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Kwek, Leong Chuan

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leongchuan.kwek@nie.edu.sg

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Natural Sciences & Science Education (NSSE)

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7006483792

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- PublicationOpen AccessViolating Bell inequalities maximally for two d-dimensional systems(American Physical Society, 2006)
;Chen, Jing-Ling ;Wu, Chunfeng; ;Oh, Choo HiapGe, Mo-LinShow more We show the maximal violation of Bell inequalities for two d-dimensional systems by using the method of the Bell operator. The maximal violation corresponds to the maximal eigenvalue of the Bell operator matrix. The eigenvectors corresponding to these eigenvalues are described by asymmetric entangled states. We estimate the maximum value of the eigenvalue for large dimension. A family of elegant entangled states app that violate Bell inequality more strongly than the maximally entangled state but are somewhat close to these eigenvectors is presented. These approximate states can potentially be useful for quantum cryptography as well as many other important fields of quantum information.Show more WOS© Citations 36Scopus© Citations 34 177 160 - PublicationUnknownElectronic entanglement purification scheme enhanced by charge detections
Show more We present an entanglement purification scheme for the mixed entangled states of electrons with the aid of charge detections. Our scheme adopts the electronic polarizing beam splitters rather than the controlled-NOT (CNOT) operations, but the total successful probability of our scheme can reach the quantity as large as that of the CNOT-operation-based protocol and twice as large as that of linear-optics-based protocol for the purification of photonic entangled states. Thus our scheme can achieve a high successful prabability without the usage of CNOT operations.Show more - PublicationUnknownContinuous multipartite entangled state in Wigner representation and violation of the Zukowski-Brukner inequality(American Physical Society, 2005)
;Wu, Chunfeng ;Chen, Jing-Ling; ;Oh, Choo HiapXue, KangShow more We construct an explicit Wigner function for the N-mode squeezed state. Based on a previous observation that the Wigner function describes correlations in the joint measurement of the phase-space displaced parity operator, we investigate the nonlocality of the multipartite entangled state by the violation of the Żukowski-Brukner N-qubit Bell inequality. We find that quantum predictions for such a squeezed state violate these inequalities by an amount that grows with the number N.Show more WOS© Citations 9Scopus© Citations 9 327 196 - PublicationOpen AccessSecurity of quantum key distributions with entangled qudits(American Physical Society, 2004)
;Durt, Thomas ;Kaszlikowski, Dagomir ;Chen, Jing-LingShow more We consider a generalization of Ekert's entanglement-based quantum cryptographic protocol where qubits are replaced by N - or d -dimensional systems (qudits). In order to study its robustness against optimal incoherent attacks, we derive the information gained by a potential eavesdropper during a cloning-based individual attack. In doing so, we generalize Cerf’s formalism for cloning machines and establish the form of the most general cloning machine that respects all the symmetries of the problem. We obtain an upper bound on the error rate that guarantees the confidentiality of qudit generalizations of the Ekert’s protocol for qubits.Show more WOS© Citations 81Scopus© Citations 94 125 149 - PublicationOpen AccessRelation between geometric phases of entangled bipartite systems and their subsystems(American Physical Society, 2003)
;Tong, Dianmin ;Sjöqvist, Erik; ;Oh, Choo HiapEricsson, MarieShow more This paper focuses on the geometric phase of entangled states of bipartite systems under bilocal unitary evolution. We investigate the relation between the geometric phase of the system and those of the subsystems. It is shown that (1) the geometric phase of cyclic entangled states with nondegenerate eigenvalues can always be decomposed into a sum of weighted nonmodular pure state phases pertaining to the separable components of the Schmidt decomposition, although the same cannot be said in the noncyclic case, and (2) the geometric phase of the mixed state of one subsystem is generally different from that of the entangled state even if the other subsystem is kept fixed, but the two phases are the same when the evolution operator satisfies conditions where each component in the Schmidt decomposition is parallel transported.Show more WOS© Citations 42Scopus© Citations 2 112 131 - PublicationOpen AccessNoncyclic geometric phase for neutrino oscillation
Show more We provide explicit formulas for the noncyclic phases or Pancharatnam phases of neutrino oscillations. Since the Pancharatnam phase is a generalization of the Berry phase, our results generalize the previous findings for the Berry phase in a recent paper [M. Blasone, P. A. Henning, and G. Vitiello, Phys. Lett. B 466 262 (1999)]. Unlike the Berry phase, the noncyclic phase offers a distinctive advantage in terms of measurement and prediction. In particular, for three-flavor mixing, our explicit formula offers an alternative means of determining the CP-violating phase. Our results can also be extended easily to explore the noncyclic phase associated with neutron-antineutron oscillations.Show more WOS© Citations 24Scopus© Citations 24 294 151 - PublicationOpen AccessQuantum cryptography: Security criteria reexamined(American Physical Society, 2004)
;Kaszlikowski, Dagomir ;Ajay Gopinathan ;Liang, Yeong Cherng; Englert, Berthold-GeorgShow more We find that the generally accepted security criteria are flawed for a whole class of protocols for quantum cryptography. This is so because a standard assumption of the security analysis, namely that the so-called square-root measurement is optimal for eavesdropping purposes, is not true in general. There are rather large parameter regimes in which the optimal measurement extracts substantially more information than the square-root measurement.Show more WOS© Citations 10Scopus© Citations 12 178 102 - PublicationOpen AccessUnified approach for exactly solvable potentials in quantum mechanics using shift operators(American Physical Society, 2000)
;Ge, Mo-Lin; ;Liu, Yong ;Oh, Choo HiapWang, Xiang-BinShow more We present a unified approach for solving and classifying exactly solvable potentials. Our unified approach encompasses many well-known exactly solvable potentials. Moreover, the new approach can be used to search systematically for a new class of solvable potentials.Show more WOS© Citations 9Scopus© Citations 12 321 172 - PublicationOpen AccessCoherent eavesdropping attacks in quantum cryptography: Nonequivalence of quantum and classical key distillation(American Physical Society, 2005)
;Kaszlikowski, Dagomir ;Lim, Jenn Yang; Englert, Berthold-GeorgShow more The security of a cryptographic key that is generated by communication through a noisy quantum channel relies on the ability to distill a shorter secure key sequence from a longer insecure one. We show that—for protocols that use quantum channels of any dimension and completely characterize them by state tomography—the noise threshold for classical advantage distillation of a specific kind is substantially lower than the threshold for quantum entanglement distillation if the eavesdropper can perform powerful coherent attacks. In marked contrast, earlier investigations had shown that the thresholds are identical for incoherent attacks on the same classical distillation scheme. It remains an open question whether other schemes for classical advantage distillation have higher thresholds for coherent eavesdropping attacks.Show more WOS© Citations 2Scopus© Citations 3 125 121 - PublicationOpen AccessQuantum dot as a resource for teleportation and state swapping
Show more We studied the use of quantum dots as a resource for teleportation and investigated how entanglement of the resource would affect the average fidelity of such a process. We then identified a suitable magnetic field, interdot distance, and the temperature to be used to achieve an average fidelity beyond the limit of the classical communication channel. We also explored the effects of decoherence on the teleportation process. We then investigated the state transfer or swapping between two quantum dots. We found that perfect quantum swap is possible in this system. A smaller magnetic field and interdot distance shortened the perfect state transfer time. Finally, we report on the performance of the system under different decoherence models.Show more WOS© Citations 6Scopus© Citations 7 113 140