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arXiv:0908.4377 · Journal of Physics A Mathematical and Theoretical

Extraction of an entanglement by repetition of the resonant transmission of an ancilla qubit

Kazuya Yuasa

Published Feb 5, 2010• 9 citations
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Abstract

A scheme for the extraction of entanglement in two noninteracting qubits (spins) is proposed. The idea is to make use of the resonant transmission of an ancilla qubit through the two fixed qubits, controlled by the entanglement in the scatterers. Repetition of the resonant transmission extracts the singlet state in the target qubits from their arbitrary given state. Neither the preparation nor the post-selection of the ancilla spin is required, in contrast to the previously proposed schemes.

DOI
10.1088/1751-8113/43/9/095304

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A scheme for the extraction of entanglement in two noninteracting qubits (spins) is proposed. The idea is to make use of the resonant transmission of an ancilla qubit through the two fixed qubits, controlled by the entanglement in the scatterers. Repetition of the resonant transmission extracts the singlet state in the target qubits from their arbitrary given state. Neither the preparation nor the post-selection of the ancilla spin is required, in contrast to the previously proposed schemes.
A scheme for preparing two fixed noninteracting qubits in a maximally entangled state is presented. By repeating on- and off-resonant scattering of ancilla qubits, the target qubits are driven from an arbitrary initial state into a singlet state with probability 1 (perfect efficiency). Neither the preparation nor the post-selection of the ancilla spin state is required. The convergence from an arbitrary input state to the unique fixed point (mixing property) is proved rigorously, and its robustness is investigated by scrutinizing the effects of imperfections in the incident wave of the ancilla—such as mistuning to a resonant momentum, imperfect monochromatization, and fluctuation of the incident momentum—as well as detector efficiency.
Inspired by a recent paper [J. Phys. B 47, 055502 (2014)], we propose a simplified scheme to generate and stabilize a Bell state of two qubits coupled to a resonator. In the scheme only one qubit is needed to be driven by external classical fields, and the entanglement dynamics is independent of the phases of these fields and insensitive to their amplitude fluctuations. This is a distinct advantage as compared with the previous ones that require each qubit to be addressed by well-controlled classical fields. Numerical simulation shows that the steady singlet state with high fidelity can be obtained with currently available techniques in circuit quantum electrodynamics.
We study a protocol for two-qubit state guidance that does not rely on feedback mechanisms. In our scheme, entanglement can be concentrated by arranging the interactions of the qubits with a continuous variable ancilla. By properly post-selecting the outcomes of repeated measurements of the state of the ancilla, the qubit state is driven to have a desired amount of purity and entanglement. We stress the primary role played by the first iterations of the protocol. Inefficiencies in the detection operations can be fully taken into account.
An effcient entanglement distribution protocol was proposed using a qudit(4-dimensional space) as a separable mediating ancilla.Two qubits were used to describe a qudit in order to simplify the description.Firstly,both of the two ancilla qubits and the two distant qubits(a and b) were formed into a four-qubit unlockable bound entangled state,called the Smolin state.Then,a projective measurement in the Bell basis was performed on the two ancilla qubits and the measurement results was delivered to a and b.Based on the measurement results,the qubits a and b can be converted into a standard singlet state locally.At any stage of the protocol,the two ancilla qubits was always separable from the subsystem(a and b).This scheme provides an effective way to distribute the maximally entangled states by separable states,and presents a concise formalism to describe and understand this physical process.
We study a protocol for two-qubit-state guidance that does not rely on feedback mechanisms. In our scheme, entanglement can be concentrated by arranging the interactions of the qubits with a continuous variable ancilla. By properly post-selecting the outcomes of repeated measurements performed on the state of the ancilla, the qubit state is driven to have a desired amount of purity and entanglement. We stress the primary role played by the first iterations of the protocol. Inefficiencies in the detection operations can be fully taken into account. We also discuss the robustness of the guidance protocol to the effects of an experimentally motivated model for mixedness of the ancillary states.