Leong Chuan Kwek
Email Address
cqtklc@nus.edu.sg
190 results
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Publication Security of quantum key distributions with entangled qudits(2004-03) Durt, T.; Kaszlikowski, D.; Chen, J.-L.; Kwek, L.C.; PHYSICSThe generality of Ekert's entanglement-based cryptographic protocol where qubits were relaced by N or d-dimensional system was presented. The information gained by a potential eavesdropper was derived, to study its robustness against optimal incoherent attacks, during a cloning -based individual attack. It was shown that the accepatable error rate of the N-DEB protocol increases with the dimension N. It was found that the maximal error rate provides a good qualitative measure of the security of the protocol.Publication The hybrid quantum computer(2007-08) Kwek, L.C.; Lim, Y.L.; Wu, C.; Chen, J.-L.; Liu, X.; Feng, X.; Tong, D.M.; Choo, K.W.; Oh, C.H.; PHYSICSIn this paper, we look at the possibility of realizing one-way quantum computing through a hybrid quantum computing architecture based on stationary qubits inside an optical cavity and flying qubits (photons). It has been shown that direct qubit-qubit interactions for two-qubit gate implementations can be replaced by the experimentally less demanding generation of single photons on demand and linear optics photon pair measurements. The outcomes of these measurements indicate either the completion of the gate or the presence of the original qubits, such that the operation can be repeated until success. © 2007 MAIK "Nauka/Interperiodica".Publication Quantum State Transmission in a Superconducting Charge Qubit-Atom Hybrid(Nature Publishing Group, 2016) Yu, D; Valado, M.M; Hufnagel, C; Kwek, L.C; Amico, L; Dumke, R; CENTRE FOR QUANTUM TECHNOLOGIESHybrids consisting of macroscopic superconducting circuits and microscopic components, such as atoms and spins, have the potential of transmitting an arbitrary state between different quantum species, leading to the prospective of high-speed operation and long-time storage of quantum information. Here we propose a novel hybrid structure, where a neutral-atom qubit directly interfaces with a superconducting charge qubit, to implement the qubit-state transmission. The highly-excited Rydberg atom located inside the gate capacitor strongly affects the behavior of Cooper pairs in the box while the atom in the ground state hardly interferes with the superconducting device. In addition, the DC Stark shift of the atomic states significantly depends on the charge-qubit states. By means of the standard spectroscopic techniques and sweeping the gate voltage bias, we show how to transfer an arbitrary quantum state from the superconducting device to the atom and vice versa. © 2016 The Author(s).Publication Strategies for positive partial transpose (Ppt) states in quantum metrologies with noise(MDPI AG, 2021-05-28) Majumder, Arunava; Shrotriya, Harshank; Kwek, Leong-Chuan; CENTRE FOR QUANTUM TECHNOLOGIESQuantum metrology overcomes standard precision limits and has the potential to play a key role in quantum sensing. Quantum mechanics, through the Heisenberg uncertainty principle, imposes limits on the precision of measurements. Conventional bounds to the measurement precision such as the shot noise limit are not as fundamental as the Heisenberg limits, and can be beaten with quantum strategies that employ ‘quantum tricks’ such as squeezing and entanglement. Bipartite entangled quantum states with a positive partial transpose (PPT), i.e., PPT entangled states, are usually considered to be too weakly entangled for applications. Since no pure entanglement can be distilled from them, they are also called bound entangled states. We provide strategies, using which multipartite quantum states that have a positive partial transpose with respect to all bi-partitions of the particles can still outperform separable states in linear interferometers. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Publication Tomographic quantum cryptography(2003-08) Liang, Y.C.; Kaszlikowski, D.; Englert, B.-G.; Kwek, L.C.; Oh, C.H.; PHYSICS; DEAN'S OFFICE (SCIENCE)A report on a protocol for quantum cryptogrphy, used for the purpose of quantum state tomography, was presented. The incoherence was found in eavesdropping attacks. The conditions under which a secure key cryptographic key was generated were also explained. The investigation was carried out for the channels, transmitting quantum systems of any finite dimension.Publication Luttinger liquid of photons and spin-charge separation in hollow-core fibers(2011-04-11) Angelakis, D.G.; Huo, M.; Kyoseva, E.; Kwek, L.C.; CENTRE FOR QUANTUM TECHNOLOGIESIn this work we show that light-matter excitations (polaritons) generated inside a hollow-core one-dimensional fiber filled with two types of atoms, can exhibit Luttinger liquid behavior. We first explain how to prepare and drive this quantum-optical system to a strongly interacting regime, described by a bosonic two-component Lieb-Liniger model. Utilizing the connection between strongly interacting bosonic and fermionic systems, we then show how spin-charge separation could be observed by probing the correlations in the polaritons. This is performed by first mapping the polaritons to propagating photon pulses and then measuring the effective photonic spin and charge densities and velocities by analyzing the correlations in the emitted photon spectrum. The necessary regime of interactions is achievable with current quantum-optical technology. © 2011 American Physical Society.Publication Effects of noisy quantum channels on one-qubit rotation gate(2012-05) Fu, X.G.; Kwek, L.C.; Min, T.D.; CENTRE FOR QUANTUM TECHNOLOGIESWe investigate the effects of noisy quantum channels on the entanglement of cluster states and one way quantum computational gates. We take a basic model, the rotational gate about x axis based on the cluster state, in order to get the most essential effects of the paradigmatic noisy quantum channels. The entanglement of cluster states in the noisy channels and the fidelity between the rotated state without noisy channel and that with noisy channel are calculated. © Science China Press and Springer-Verlag Berlin Heidelberg 2012.Publication Dynamical symmetry and geometric phase(2007-04-01) Wang, Z.S.; Kwek, L.C.; Lai, C.H.; Oh, C.H.; PHYSICSBy considering dynamical symmetry between canonically equivalent systems, we investigate the connection between the geometric phase and dynamical invariants, where the Liouville-von-Neumann equation is directly deduced. Furthermore, we show that an arbitrary shift of the Hamiltonian, where f i(t) is a real function and Xi is a generator of dynamical symmetry, leaves the geometric phase invariant. © 2007 The Royal Swedish Academy of Sciences.Publication Transitions in the quantum computational power(2014-05-13) Wei, T.-C.; Li, Y.; Kwek, L.C.; CENTRE FOR QUANTUM TECHNOLOGIESWe construct two spin models on lattices (both two and three dimensional) to study the capability of quantum computational power as a function of temperature and the system parameter. There exists a finite region in the phase diagram such that the thermal equilibrium states are capable of providing a universal fault-tolerant resource for measurement-based quantum computation. Moreover, in such a region the thermal resource states on the three-dimensional lattices can enable topological protection for quantum computation. The two models behave similarly in terms of quantum computational power. However, they have different properties in terms of the usual phase transitions. The first model has a first-order phase transition only at zero temperature whereas there is no transition at all in the second model. Interestingly, the transition in the quantum computational power does not coincide with the phase transition in the first model. © 2014 American Physical Society.Publication Corrigendum: Quantum State Transmission in a Superconducting Charge Qubit-Atom Hybrid(2017) Yu, D.; Valado, M.M.; Hufnagel, C.; Kwek, L.C.; Amico, L.; Dumke, R.; CENTRE FOR QUANTUM TECHNOLOGIESThis corrects the article DOI: 10.1038/srep38356.