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arXiv:0903.4123 · Physical Review A

Generalized Toffoli gates using qudit catalysis

Radu Ionicioiu, Timothy P. Spiller, William J. Munro

Published Jul 13, 2009• 60 citations
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Abstract

We present quantum networks for a $n$-qubit controlled gate ${C}^{n\ensuremath{-}1}(U)$ which use a higher-dimensional (qudit) ancilla as a catalyzer. In its simplest form the network has only $n$ two-particle gates (qubit-qudit)---this is the minimum number of two-body interactions needed to couple all $n+1$ subsystems ($n$ qubits plus one ancilla). This class of controlled gates includes the generalized Toffoli gate ${C}^{n\ensuremath{-}1}(X)$ on $n$ qubits, which plays an important role in several quantum algorithms and error correction. A particular example implementing this model is given by the dispersive limit of a generalized Jaynes-Cummings Hamiltonian of an effective spin $s$ interacting with a cavity mode.

DOI
10.1103/physreva.80.012312

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We present quantum networks for a nnn-qubit controlled gate Cn\ensuremath−1(U){C}^{n\ensuremath{-}1}(U)Cn\ensuremath−1(U) which use a higher-dimensional (qudit) ancilla as a catalyzer. In its simplest form the network has only nnn two-particle gates (qubit-qudit)---this is the minimum number of two-body interactions needed to couple all n+1n+1n+1 subsystems (nnn qubits plus one ancilla). This class of controlled gates includes the generalized Toffoli gate Cn\ensuremath−1(X){C}^{n\ensuremath{-}1}(X)Cn\ensuremath−1(X) on nnn qubits, which plays an important role in several quantum algorithms and error correction. A particular example implementing this model is given by the dispersive limit of a generalized Jaynes-Cummings Hamiltonian of an effective spin sss interacting with a cavity mode.
The simplest decomposition of a Toffoli gate acting on 3 qubits requires five 2-qubit gates. If we restrict ourselves to controlled-sign (or controlled-NOT) gates this number climbs to 6. We show that the number of controlled-sign gates required to implement a Toffoli gate can be reduced to just 3 if one of the three quantum systems has a third state that is accessible during the computation---i.e., is actually a qutrit. Such a requirement is not unreasonable or even atypical since we often artificially enforce a qubit structure on multilevel quantums systems (e.g., atoms, photonic polarization plus spatial modes). We explore the implementation of these techniques in optical quantum processing and show that linear optical circuits could operate with much higher probabilities of success.
Local implementation of a nonlocal d-dimensional quantum Toffoli gate is considered. We construct respectively two quantum networks for realizing this nonlocal quantum gate conclusively. The first one is based on Eisert’s insight but follow a different pathway which results in a nonlocal d-dimensional Toffoli gate design. This scheme involves a local d-dimensional three-body interaction, which does not appear naturally in physical system. The other one can improve significantly the local implementation of this nonlocal gate if the target node harnesses a qutrit as a catalyser. In its simplest form this quantum network that does the improved scheme is built entirely of local single-body and two-body elementary gates, and with only 3 two-body elementary gates at the target node. The latter is simpler but conclusive, and more efficient but with less resource, which will make it more feasible with the current experimental technology and more suitable for large-scale quantum network.
Single-step implementations of multiqubit gates are generally believed to provide a simpler design, a faster operation, and a lower decoherence. For three coupled qubits interacting with a photon field, a realizable scheme for a single-step Toffoli gate is investigated. We find that the three-qubit system can be described by four effective modified Jaynes-Cummings models in the states of two control qubits. Within the rotating-wave approximation, the modified Jaynes-Cummings models are shown to be reduced to the conventional Jaynes-Cummings models with renormalized couplings between qubits and photon fields. A single-step Toffoli gate is shown to be realizable with tuning of the four characteristic oscillation periods that satisfy a commensurate condition. Possible values of the system parameters are estimated for a single-step Toffoli gate. Further, we discuss our single-step Toffoli-gate operation errors due to imperfections in the system parameters; numerical calculations show that a Toffoli gate with high fidelity can be obtained by adjusting pairs of the photon-qubit and qubit-qubit coupling strengths. In addition, a decoherence effect on the Toffoli-gate operation due to a thermal reservoir is discussed.
We show a significant reduction of the number of quantum operations and the improvement of the circuit depth for the realization of the Toffoli gate by using qudits. This is done by establishing a general relation between the dimensionality of qudits and their topology of connections for a scalable multiqudit processor, where higher qudit levels are used for substituting ancillas. The suggested model is of importance for the realization of quantum algorithms and as a method of quantum error correction codes for single-qubit operations.
Abstract In distributed quantum computation, quantum remote-controlled gates are used frequently and applied on separate nodes or subsystems of a network. One of the universal and well-known controlled gates is the n-qubit controlled-NOT gate, especially Toffoli gate for the case of three qubits, which are frequently used to synthesize quantum circuits. In this paper, we considered a more general case, an n-qubit controlled-U gate, and present a general protocol for implementing these gates remotely with minimum required resources. Then, the proposed method is applied to implement a Toffoli gate in bipartite and tripartite systems. In this method, we considered cases in which a group of qubits belongs to one subsystem of the network. Then, we improved its consumption resources.
The Toffoli gate, which has practical applications in the implementation of practical quantum algorithms and quantum error correction, is usually constructed from concatenated two-qubit or qubit-qutrit gates. Here we show that the Toffoli gate for three qubits can be implemented through a single XY\mathit{XY}XY interaction plus two single-qubit rotations. This method greatly simplifies the experimental implementation of the Toffoli gate and promises a much higher fidelity compared to those based on elementary gate decomposition. We present an example for the implementation of the required XY\mathit{XY}XY interaction in cavity QED.