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Communications in Theoretical Physics

On Effectiveness of Protocol Creating Maximum Entanglement of Two Charge-Phase Qubits by Cavity Field

Li Chong, Gao Yibo

Published Nov 1, 2007• 0 citations
Publisher

Abstract

We revisit the protocols to create maximally entangled states between two Josephson junction (JJ) charge phase qubits coupled to a microwave field in a cavity as a quantum data bus. We analyze a novel mechanism of quantum decoherence due to the adiabatic entanglement between qubits and the data bus, the off-resonance microwave field. We show that even if the variable of the data bus can be adiabatically eliminated, the entanglement between the qubits and data bus remains and can decohere the superposition of two-particle state. Fortunately we can construct a decoherence-free subspace of two-dimension to against this adiabatic decoherence. To carry out the analytic study for this decoherence problem, we develop Fröhlich transformation to re-derive the effective Hamiltonian of these systems, which is equivalent to that obtained from the adiabatic elimination approach.

DOI
10.1088/0253-6102/48/5/036

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We revisit the protocols to create maximally entangled states between two Josephson junction (JJ) charge phase qubits coupled to a microwave field in a cavity as a quantum data bus. We devote to analyze a novel mechanism of quantum decoherence due to the adiabatic entanglement between qubits and the data bus, the off-resonance microwave field. We show that even through the variable of the data bus can be adiabatically eliminated, the entanglement between the qubits and data bus remains and can decoher the superposition of two-particle state. Fortunately we can construct a decoherence-free subspace of two-dimension to against this adiabatic decoherence.To carry out the analytic study for this decoherence problem, we develop Frőhlich transformation to re-derive the effective Hamiltonian of these system, which is equivalent to that obtained from the adiabatic elimination approach .
We propose a theoretical protocol to create the entanglement of two qubits via the Born-Oppenheimer (BO) approximation. In our scheme, each qubit is coupled to a faster data bus whose frequency is much larger than the energy spacing of the qubits and thus the BO approximation is valid. Then the adiabatic separation of qubits from the data bus can induce an effective potential to couple the two qubits, which can be utilized to create a quantum logic gate. We also discuss the quantum decoherence caused by the adiabatic entanglement between the two qubits and the external field.
We propose a theoretical protocol for quantum logic gates between two Josephson junction charge-phase qubits through the control of their coupling to a large junction whose Josephson coupling energy is much larger than its Coulomb charge energy. In the low excitation limit of the large junction, it behaves effectively as a quantum data-bus mode of a harmonic oscillator. Our protocol can be fast since it does not require the data-bus to stay adiabatically in its ground state, as such it can be implemented over a wide parameter regime independent of the data-bus quantum state.
Abstract The optimal generation of entangled states is of significance to information processing and state engineering. Here, an efficient scheme is proposed for creating entangled states between two superconducting qubits in a circuit quantum electrodynamics (QED). The two qubits that resonantly interact with a common cavity mode of transmission‐line resonator (TLR) can be effectively coupled by the data bus of the microwave resonator. By the invariant‐based shortcuts to adiabaticity (STA), two types of maximally entangled states can be controllably induced only by adjusting the Rabi drivings. Based on the resonant drivings with constant rates, the shortcut strategy is capable of implementing faster operations when compared with the dispersive approach. The fidelities are highly robust against the decoherence effects and the instabilities of Rabi rates. Thus the proposal could offer a potential route toward the entanglement generations in an optimized manner.
We study analytically and numerically the problem of two qubits with fixed coupling irradiated with quantum or classical fields. In the classical case, we derive an effective Hamiltonian and describe its entangling properties. We identify a coupling/decoupling switching protocol and we construct composite pulse sequences leading to a controlled-NOT (CNOT) gate. In the quantum case, we show that qubit-qubit photon multiparticle entanglement and maximally entangled two qubit states can be obtained by driving the system at very low powers (one quantum of excitation). Our results can be applied to a variety of systems of two superconducting qubits coupled to resonators.
From the viewpoint of quasinormal modes, we describe a decoherence mechanism of charge qubit of Josephson junctions (JJ) in a lossy microcavity, which can appear in a realistic experiment for quantum computation based on a JJ qubit. We show that nonlinear coupling of a charge qubit to the quantum cavity field can result in additional dissipation of the resonant mode due to the effective interaction between those nonresonant modes and the resonant mode, which is induced by the charge qubit itself. We calculate the characteristic time of the decoherence by making use of the system plus bath method.