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arXiv:quant-ph/0311156

Swapping and entangling qubits of single photons and atoms

Chen, T. W., C. K. Law, Paul Leung

Published Nov 23, 2003• 0 citations
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Abstract

A scheme is proposed here to achieve swapping and entangling of photonic and atomic qubits with high fidelity. The mechanism is based on the scattering of a single photon from a $Λ$-type three-level atom. The evolution of the coupled system is analyzed by projecting the quantum state onto a `bright' and a `dark' state. Quantum interference of these two states, which is determined by a frequency-dependent phase angle, can be exploited to perform various two-qubit transformations. It is remarkable that the probability of success of such transformations can approach unity in the strong coupling cavity QED regime.

DOI
10.48550/arxiv.quant-ph/0311156

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A scheme is proposed here to achieve swapping and entangling of photonic and atomic qubits with high fidelity. The mechanism is based on the scattering of a single photon from a ΛΛΛ-type three-level atom. The evolution of the coupled system is analyzed by projecting the quantum state onto a bright' and a dark' state. Quantum interference of these two states, which is determined by a frequency-dependent phase angle, can be exploited to perform various two-qubit transformations. It is remarkable that the probability of success of such transformations can approach unity in the strong coupling cavity QED regime.
We investigate a scheme of atomic quantum memory to store photonic qubits in cavity QED. This is motivated on the recent observation that the quantum-state swapping between a single-photon pulse and a Lambda-type atom trapped in a cavity is ideally realized via scattering for some specific case in the strong coupling cavity regime [T. W. Chen, C. K. Law, and P. T. Leung, Phys. Rev. A 69, 063810 (2004)]. We derive a simple formula for calculating the fidelity of this atom-photon swapping for quantum memory. We further propose a feasible method which implements conditionally the quantum memory operation with the fidelity of almost unity even if the atom-photon coupling is not so strong. This method can also be applied to store a photonic entanglement in spatially separated atomic quantum memories.
We propose a waveguide-QED system where two single photons of distinct frequency or polarization interact strongly. The system consists of a single ladder-type three level atom coupled to a waveguide. When both optical transitions are coupled strongly to the waveguide's mode, we show that a control photon tuned to the upper transition induces a π phase shift and tunneling of a probe photon tuned to the otherwise reflective lower transition. Furthermore, the system exhibits single photon scattering by a classical control beam. Waveguide-QED schemes could be an alternative to high quality cavities or dense atomic ensembles in quantum information processing.
Quantum gates are the building blocks of quantum circuits, which in turn are the cornerstones of quantum information processing. In this work, we theoretically investigate a single-step implementation of both a universal two- (cnot) and three-qubit (quantum Fredkin) gates in a cross-cavity setup coupled to a $\mathrm{\ensuremath{\Lambda}}$-type three-level atom. Within a high-cooperativity regime, the system exhibits an atomic-state-dependent $\ensuremath{\pi}$-phase gate involving the two-mode single-photon bright and dark states of the input light pulses. This allows for the controlled manipulation of light states by the atom and vice versa. Our results indicate these quantum gates can be implemented with high probability of success using the state-of-the-art parameters, either for the weak- or strong-coupling regime, where the quantum interference is due to an electromagnetically induced transparencylike phenomenon and the Autler-Townes splitting, respectively. This work not only paves the way for implementing quantum gates in a single step using simple resources, thus avoiding the need to chain basic gates together in a circuit, but it also endorses the potential of cross-cavity systems for realizing universal quantum computation.
Protocols for probabilistic entanglement-assisted quantum teleportation and for entanglement swapping of material qubits are presented. They are based on a protocol for postselective Bell- state projection which is capable of projecting two material qubits onto a Bell state with the help of ancillary coherent multiphoton states and postselection by balanced homodyne photodetection. Provided this photonic postselection is successful, we explore the theoretical possibilities of realizing unit-fidelity quantum teleportation and entanglement swapping with 2525%25 success probability. This photon-assisted Bell projection is generated by coupling almost resonantly the two material qubits to single modes of the radiation field in two separate cavities in a Ramsey-type interaction sequence and by measuring the emerged field states in a balanced homodyne detection scenario. As these quantum protocols require basic tools of quantum state engineering of coherent multiphoton states and balanced homodyne photodetection, they may offer interesting perspectives in particular for current quantum optical applications in quantum information processing.
A scheme is proposed to realize a two-qubit quantum phase gate for the intracavity modes. In the scheme, two qubits are encoded in zero- and one-photon Fock states of two intracavity modes, and the three-level $\ensuremath{\Lambda}$-type atom trapped in a high-QQQ cavity mediates the conditional phase gate within a given interaction time. The influence of cavity decay and atomic spontaneous emission on the gate fidelity is also discussed.
We propose an atomic quantum transistor based on exchange by virtual photons between two atomic systems through the control gate-atom. The quantum transistor is realized in two QED cavities coupled in nano-optical scheme. We have found novel effect in quantum dynamics of coupled three-node atomic system which provides control-SWAP(θ) processes in quantum transistor operation. New possibilities of quantum entanglement in an example of bright and dark qubit states have been demonstrated for quantum transport in the atomic chain. Potentialities of the proposed nano-optical design for quantum computing and fundamental issues of multi-atomic physics are also discussed.