Shortcut-based quantum gates on superconducting qubits in circuit QED

Construction of optimal gate operations is significant for quantum computation. Here an efficient scheme is proposed for performing shortcut-based quantum gates on superconducting qubits in circuit quantum electrodynamics (QED). Two four-level artificial atoms of Cooper-pair box circuits, having sufficient level anharmonicity, are placed in a common quantized field of circuit QED and are driven by individual classical microwaves. Without the effect of cross resonance, one-qubit NOT gate and phase gate in a decoupled atom can be implemented using the invariant-based shortcuts to adiabaticity. With the assistance of cavity bus, a one-step SWAP gate can be obtained within a composite qubit-photon-qubit system by inversely engineering the classical drivings. We further consider the gate realizations by adjusting the microwave fields. With the accessible decoherence rates, the shortcut-based gates have high fidelities. The present strategy could offer a promising route towards fast and robust quantum computation with superconducting circuits experimentally.     

Conditional quantum phase gate between two 3-state atoms

We propose a scheme for conditional quantum logic between two 3-state atoms that share a quantum data bus such as a single mode optical field in cavity QED systems, or a collective vibrational state of trapped ions. Making use of quantum interference, our scheme achieves successful conditional phase evolution without any real transitions of atomic internal states or populating the quantum data bus. In addition, it requires only common addressing of the two atoms by external laser fields.     

Design of many-atom cavity QED systems for strong two-photon nonlinearity

We propose a new scheme for strong two-photon nonlinearity (TPN) by cavity QED systems with many three-level atoms, although the TPN due to the saturation effect of two-level atoms generally declines with an increase in the number of atoms. The reduction in anharmonicity by the increase in the number of atoms can be overcome if the system satisfies particular criteria. The new scheme is significant for realizing TPN devices using solid-state materials because it is generally difficult to control the number of atoms, which are few, introduced in the device.     

Geometric Phase in a Cavity QED with Cold Atoms Trapped in?a?Double-Well Potential

We have studied the geometric phase in a cavity QED with cold atoms trapped in a double-well potential. It is found that the geometric phase in a cycle case is independent of the driven field.     

光学腔量子电动力学的实验进展

研究受限在微腔中的光场与原子的相互作用可以帮助我们深刻认识原子与光子作用的动力学过程，腔量子电动力学是研究光子与原子相互作用的一种有力工具，强作用腔量子电动力学的研究为量子信息提供了一种实现量子逻辑运算的途径，文章简要介绍该研究领域的背景、现状及发展动态。     

A scheme for realizing n-qubit controlled-phase gates with atoms in cavity QED

We present a new scheme for realizing a n-qubit controlled-phase gate with atoms in cavity QED. The present scheme operates essentially by exchanging a single photon between the control atoms and the cavity mode before and after a phase shift performed on the target atom. It is interesting to note that the gate can be implemented in a very simple way and by employing resonant interaction with one cavity only.     

Heisenberg-Type Quantum Steering by Continuous Weak Measurement in Circuit QED *

Quantum steering has attracted great interest in the last decade, especially in the celebrated optomechanical, cold atom, and quantum optical systems. However, there is still a lack of studies on quantum steering in circuit quantum electrodynamics (QED), which provides a useful experimental platform for revealing novel quantum phenomena. In this work, we investigate the steering of qubit by continuous weak measurement in a circuit QED system and establish a set of multiplicative steering inequalities based on the Heisenberg uncertainty principle. Different from the widely studied systems mentioned above, multiplicative steering inequalities in the circuit QED system are in various forms. We find that only a portion of them can be used to show the detection dependence of the qubit state and we also analyze the reason. Furthermore, we discuss several conditions for the violation of a typical steering inequality, including the measurement strength and methods in detecting the cavity field as well as the quantum efficiency of the detector. This preliminary work could be helpful to quantum steering experiments in circuit QED systems.     

Approximate Toffoli Gate Originated from a Single Resonant Interaction of Cavity Dissipation and Atomic Spontaneous Emission

We propose a potentially practical scheme to implement an approximate three-qubit Toffoli gate by a single resonant interaction in dissipative cavity QED in which the cavity mode decay and atomic spontaneous emission are considered. The scheme does not require two-qubit controlled-NOT gates but uses a three-qubit phase gate and two Hadamard gates, where the approximate phase gate can be implemented by only a single dissipative resonant interaction of atoms with the cavity mode. Discussions are made for the advantages and the experimental feasibility of our scheme.     

Quantum Dense Coding Based on Entangled Bell State in Cavity QED

We investigate quantum dense coding based on entangled Bell states in cavity QED. We implement a experimentally feasible new scheme in cavity QED with atomic qubits where the atoms interact with a highly detuned cavity mode with the assistance of a classical field. The scheme is insensitive to the cavity decay and the thermal field. Based on cavity QED techniques, the scheme can be realizable.     

Effects of Purity on Dynamics of Multipartite Entanglement

We study the effects of purity on entanglement dynamics of a multipartite system in cavity QED. Three two-level atoms A, B and C are initially prepared in an entangled state and locally coupled with independent cavities a, b and c, respectively. We consider the effects of purity of atomic initial state on the entanglement evolution of both the systems of atoms and cavities. It is found that depending on the purity of atomic initial state, the entanglement of atoms （cavities） may or may not exhibit the sudden death （birth） phenomenon.     

Scheme for Implementing Controlled Dense Coding with Six-Atom Cluster State in Cavity QED

We propose an experimentally feasible protocol for implementing controlled dense coding with a six-atom cluster state in cavity QED. In the scheme, we investigate that the atoms interact simultaneously with the highly detuned single-mode cavity and the strong classical driving field, and thus our scheme is not sensitive to both the cavity decay and the thermal field. In addition, the four-atom entangled states can be exactly distinguished by performing the single-atom measurements in cavity QED, therefore our scheme might be implemented in a simple way.     

Quasi-superradiant soliton state of matter in quantum metamaterials

Strong interaction of a system of quantum emitters (e.g., two-level atoms) with electromagnetic field induces specific correlations in the system accompanied by a drastic increase of emitted radiation (superradiation or superfluorescence). Despite the fact that since its prediction this phenomenon was subject to a vigorous experimental and theoretical research, there remain open question, in particular, concerning the possibility of a first order phase transition to the superradiant state from the vacuum state. In systems of natural and charge-based artificial atom this transition is prohibited by “no-go” theorems. Here we demonstrate numerically and confirm analytically a similar transition in a one-dimensional quantum metamaterial – a chain of artificial atoms (qubits) strongly interacting with classical electromagnetic fields in a transmission line. The system switches from vacuum state to the quasi-superradiant (QS) phase with one or several magnetic solitons and finite average occupation of qubit excited states along the transmission line. A quantum metamaterial in the QS phase circumvents the “no-go” restrictions by considerably decreasing its total energy relative to the vacuum state by exciting nonlinear electromagnetic solitons.     

Detecting magnetically guided atoms with an optical cavity

We show that a low-finesse cavity can be efficient for detecting neutral atoms. The low finesse can be compensated for by decreasing the mode waist of the cavity. We have used a near-concentric resonator with a beam waist of 12 microm and a finesse of only 1100 to detect magnetically guided Rb atoms with a detection sensitivity of 0.1 atom in the mode volume. For future experiments on single-atom detection and cavity QED applications, it should be beneficial to use miniaturized optical resonators integrated on atom chips.     

Unconventional Geometric Quantum Gate in Circuit QED

We propose a scheme to implement an unconventional geometric phase gate in circuit QED, i.e. two superconducting charge qubits inside a superconducting transmission line resonator. The quantum operation depends only on global geometric features, and thus is insensitive to the state of the cavity mode.     

Generation of Entangled Coherent States in Circuit-QED

We study theoretically the generation of entangled states of microwaves in a circuit quantum electrodynamics (QED) system. Our system includes a transmission-line resonator and a Cooper-pair box which acts as an artificial atom. It is shown that in the dispersive regime of the circuit-QED system, a cross-Kerr interaction can be obtained by properly preparing the initial state of the qubit. Based on this cross-Kerr interaction, we show that the coherent coupling of the two lowest-lying cavity modes through the qubit can generate a macroscopic entangled state.     

Scheme for Implementing Perfect Quantum Teleportation with Non-maximally Entangled W-Class State in Cavity QED

We propose an experimentally feasible teleportation scheme with three-atom W-class state, which was first proposed by Agrawal and Pati [P. Agrawal and A. Pati, Phys. Rev. A 74 (2006) 062320], in cavity QED. In this scheme atoms interact simultaneously with a nonresonant cavity and there is no energy exchange between the atoms and the cavity. Thus it is insensitive to the cavity decay, which is of importance in view of experiment.     

Efficient Scheme for Dense Coding in Cavity QED and Ion Trap Systems

We propose a scheme for realizing dense coding in cavity QED. The scheme is based on the resonant interaction of two atoms with a cavity mode. The scheme may be realizable with presently available techniques in the microwave cavity QED setup. The idea can also be generalized to the ion trap system.     

Scheme for generation of four-atom Greenberger-Horne-Zeilinger states in an optical cavity

We propose a scheme for generating maximally GHZ state for four atoms trapped in a two-mode optical cavity via combination of cavity QED and linear optics system. The GHZ state can be not only generated deterministically with a single resonant interaction in cavity QED, but also can be prepared probabilistically based on cavity QED and linear optics elements. The fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photon-detectors. Finally, we briefly analyze and discuss the experimental feasibility of the proposed scheme.     

在腔QED中实现最优非对称经济型1到3的相位协变克隆方案

本文提出一个在腔QED中实验上可行的方案来实现最优非对称经济型1到3相位协变克隆,这种克隆是不需要辅助粒子的。在这个的方案中,三个原子中的两个同时与腔场发生作用,并且它们受经典场的作用。并且此方案不受腔场的态和腔衰变的影响,因此,在实验上是可能实现的。