Preparation Of Quantum Entangled States Of Multiple Qubits Encoded With Microwave-field Quantum States In Circuit QED

The circuit QED,consisting of microwave cavities and artificial atoms,has developed fast in the past decade and has been considered as one of the leading candidates for QIP.In addition,entanglement is of fundamental interest in testing quantum mechanics against local hidden theory,but also has practical applications in quantum communication and quantum information processing.In the past decade,much efforts have been devoted to generation of entangled states.On the other hand,there is much work on the investigation of entangled states of multiple qubits.There exist two types of important multi-qubit entanglement state,that is GHZ state and W state.This paper focuses preparation of GHZ states and W states of multiple qubits encoded with microwave field quantum sates in the circuit QED.In this paper,we present two innovative works:In one work,we propose a method to create a hybrid GHZ entangled state of n particlelike optical(PO)qubits and n wavelike optical(WO)qubits,by using 2n microwave cavities coupled to a superconducting flux qutrit.The two logic states of a PO qubit here are represented by the vacuum state and the single-photon state of a cavity.while the two logic states of a WO qubit are indicated by the two coherent states of a cavity.The procedure for preparing the GHZ state consists of only a few basic operations,and the circuit resources are significantly reduced because of using only one flux qutrit as the coupler.The GHZ-state preparation time does not depend on the number of qubits,and the GHZ state is deterministically generated since no measurement is made.In addition,the intermediate higher-energy level of the qutrit during the entire operation is virtually excited and thus decoherence from this level is greatly suppressed.This proposal is quite general and can be extended to create the proposed hybrid GHZ state,by using a ?-type three-level natural or artificial atom coupled to 2n microwave or optical cavities.As an example,our numerical simulation demonstrates that within current circuit-QED technology,the hybrid GHZ state of two PO qubits and two WO qubits can be prepared with a high fidelity.In another work,cat-state qubits(qubits encoded with cat states)have recently attracted much attention because of their enhanced lifetimes with quantum error correction.We here consider a circuit QED system consisting of three superconducting qutrits,each coupled to an individual cavity but all coupled to a common cavity.We show that entangled W states of three cat-state qubits hosted by the three individual cavities can be prepared with only a few basic operations.The higher energy level of the qutrits is not occupied;thus,decoherence from this level of the qutrits is greatly suppressed.In addition,the W states can be prepared deterministically.Numerical simulations show that high-fidelity production of the W states of three cat-state qubits is feasible with current circuit QED technology.