We present a way to realize a multiplex-controlled phase gate of $n\ensuremath{-}1$ control qubits simultaneously controlling one target qubit, with
n qubits distributed in
n different cavities. This multiqubit gate is implemented by using
n qutrits ( three-level natural or artificial atoms) placed in
n different cavities, which are coupled to an auxiliary qutrit. Here, the two logic states of a qubit are represented by the two lowest levels of a qutrit placed in a cavity. We show that this
n-qubit controlled phase gate can be realized using only
2n+2 basic operations, i.e., the number of required basic operations only increases linearly with the number
n of qubits. Since each basic operation employs the qutrit-cavity or qutrit-pulse resonant interaction, the gate can be fast implemented when the number of qubits is not large. Numerical simulations show that a three-qubit controlled phase gate, which is executed on three qubits distributed in three different cavities, can be high-fidelity implemented by using a circuit QED system. This proposal is quite general and can be applied to a wide range of physical systems, with atoms, NV centers, quantum dots, or various superconducting qutrits distributed in different cavities. Finally, this method can be applied to implement a multiqubit controlled phase gate with atoms using a cavity. A detailed discussion on implementing a three-qubit controlled phase gate with atoms and one cavity is presented.
We propose a scheme for implementing a remote three-qubit controlled-Z gate with three atoms separately trapped in three distant cavities coupled by optical fibres. The influences of various decoherence processes, such as spontaneous emission of the atoms and photon leakage of the cavities and the optical fibres, on the fidelity are also investigated. It is found that the gate can be implemented with high fidelity even when these decoherence processes are considered. This scheme can be extended to the implementation of a remote N -qubit controlled-Z gate with N atoms separately trapped in N distant cavities coupled by optical fibres.
We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.