Browsing by Author "Li, CM"

Now showing 1 - 11 of 11

A simplification of entanglement purification
(2004-07-26)
Hsieh, JY
;
Li, CM
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
This study proposes a modification of existing entanglement purification protocols for pairs of qubits. The proposed protocol restores a desired pure state by standard purification local operations and classical communications, without preliminarily estimating the entangled state to be purified. The proposed protocol is demonstrated to outperform the previously proposed IBM and Oxford protocols. (C) 2004 Elsevier B.V. All rights reserved.
An improved phase error tolerance in a quantum search algorithm
(2004-10-01)
Hsieh, JY
;
Li, CM
;
Chun, DS
;
電子物理學系
;
Department of Electrophysics
As the matching condition in the Grover search algorithm is transgressed due to inevitable errors in phase inversions, it causes a reduction in the maximum probability of success. With a given degree of maximum success, we have derived a generalized and improved criterion for the tolerated error and the corresponding size of the quantum database under the inevitable gate imperfections. The vanishing inaccurancy of this condition has also been shown. Moreover, a concise formula for evaluating a minimum number of iterations is also presented in this work.
Analytical technique for simplification of the encoder-decoder circuit for a perfect five-qubit error correction
(2006-05-30)
Hsieh, JY
;
Li, CM
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
Simpler encoding and decoding networks are necessary for more reliable quantum error-correcting codes (QECCs). The simplification of the encoder-decoder circuit for a perfect five-qubit QECC can be derived analytically if the QECC is converted from its equivalent one-way entanglement purification protocol. In this work, the analytical method to simplify the encoder-decoder circuit is introduced and a circuit that is as simple as the existing simplest circuits is presented as an example. The encoder-decoder circuit presented here involves nine single- and two-qubit unitary operations, only six of which are controlled-NOT gates.
Formulation of a family of sure-success quantum search algorithms
(2004-09-01)
Hsieh, JY
;
Li, CM
;
Lin, JS
;
Chuu, DS
;
電子物理學系
In this work, we consider a family of sure-success quantum algorithms, which is grouped into even and odd members for solving a generalized Grover search problem. We prove the matching conditions for both groups and give the corresponding formulae for evaluating the iterations or oracle calls required in the search computation. We also present how to adjust the phase angles in the generalized Grover operator to ensure the sure-success if minimal oracle calls axe demanded in the search.
General SU(2) formulation for quantum searching with certainty
(2002-05-01)
Hsieh, JY
;
Li, CM
;
電子物理學系
;
Department of Electrophysics
A general quantum search algorithm with arbitrary unitary transformations and an arbitrary initial state is considered in this work. To search a marked state with certainty, we have derived, using an SU(2) representation: (1) the matching condition relating the phase rotations in the algorithm, (2) a concise formula for evaluating the required number of iterations for the search, and (3) the final state after the search, with a complex phase in its amplitude. Moreover, the optimal choices and modifications of the phase angles in the Grover kernel are also studied.
Hamiltonian and measuring time for analog quantum search
(2005-11-01)
Hsieh, JY
;
Li, CM
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
In this work, an analog analogue of Grover's quantum search algorithm was studied. A generalized Hamiltonian driving the evolution of a quantum state in the analog search system was derived. Equations relating all parameters considered in the present problem were given according to the required maximal probability for finding the marked state. Using these equations, both the measuring time and the system energy gap suitable for a quantum search with or without certainty can thus be evaluated. Moreover, the reduction of the searching probability caused by a random and uncontrollable phase error for an initial state has been investigated. It was shown that in an efficient quantum search computation, the measuring time should be proportional to the square root of the size of the database.
Proposal for teleportation of charge qubits via super-radiance
(2005-08-18)
Chen, YN
;
Li, CM
;
Chuu, DS
;
Brandes, T
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電子物理學系
A scheme is proposed to teleport charge qubits via super-radiance. Reservoir-induced entanglement is generated between two semiconductor dots in a microcavity where a quantum state encoded in a third quantum dot is then tuned into collective decay with one of the entangled dots. Teleportation is achieved automatically in our scheme which we also extend to quantum wires.
Quantum secret sharing using product states
(2005-02-01)
Hsu, LY
;
Li, CM
;
電子物理學系
;
Department of Electrophysics
This study proposes quantum secret sharing protocols using product states. The first two protocols adopt the quantum key distribution protocol using product states [Guo Phys. Rev. A 64, 042301 (2001)]. In these two protocols, the sender does not reveal any information about the qutrits until confirming that each receiver has received a qutrit. This study also considers the security and some possible eavesdropping strategies. In the third proposed protocol, three-level Bell states are exploited for qutrit preparation via nonlocality swapping.
Rotational entangled states between two coupled molecules
(2006-01-28)
Liao, YY
;
Chen, YN
;
Li, CM
;
Chuu, DS
;
電子物理學系
A method is proposed to create entanglement between two coupled identical polar molecules separated at a distance of tens of nanometres. Entangled states of two coupled polar molecules controlled by half-cycle pulses are studied theoretically. The von Neumann entropy is calculated to characterize the degree of entanglement. By varying the pulse shape of the applied laser, transition from regular to irregular behaviour occurs. Moreover, the entanglement is also found to be enhanced by applying multiple laser pulses. The behaviour from quantum to classical limit is also discussed.
Shot-noise spectrum of superradiant entangled excitons
(2004-06-01)
Chen, YN
;
Brandes, T
;
Li, CM
;
Chuu, DS
;
電子物理學系
The shot noise produced by tunneling of electrons and holes into a double dot system incorporated inside a p-i-n junction is investigated theoretically. The enhancement of the shot noise is shown to originate from the entangled electron-hole pair created by superradiance. The analogy to the superconducting Cooper pair box is pointed out. A series of Zeno-like measurements is shown to destroy the entanglement, except for the case of maximum entanglement.
W state generation and effect of cavity photons on the purification of dot-like single quantum well excitons
(2005-11-01)
Li, CM
;
Chen, YN
;
Luo, CW
;
Hsieh, JY
;
Chuu, DS
A scheme of three-particle entanglement purification is presented in this work. The physical system undertaken for investigation is dot-like single quantum well excitons independently coupled through a single microcavity mode. The theoretical framework for the proposed scheme is based on the quantum jump approach for analyzing the progress of the trible-exciton entanglement as a series of conditional measurement has been taken on the cavity field state. We first investigate how cavity photon affects the purity of the double-exciton state and the purification efficiency in two-particle protocol. Then we extend the two-particle case and conclude that the three-exciton state can be purified into W state, which involves the one-photon-trapping phenomenon, with a high yield. Finally, an achievable setup for purification using only modest and presently feasible technologies is also proposed.