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Optimal two-qubit operation is of significance to quantum information processing. An efficient scheme is proposed for realizing the shortcut to adiabatic two-qubit state swap in a superconducting circuit quantum electrodynamics (QED) via effective drivings. Two superconducting qutrits are coupled to a common cavity field and individual classical drivings. Based on two Gaussian-type Rabi drivings, two-qubit state swap can be adiabatically implemented within a reduced three-state system. To speed up the operation, these two original Rabi drivings are modified in the framework of shortcuts to adiabaticity, instead of adding an extra counterdiabatic driving. Moreover, owing to a shorter duration time, the decoherence effects on the accelerated quantum operation can be mitigated significantly. The strategy could offer an optimized method to construct fast and robust quantum operations on superconducting qubits experimentally. [ABSTRACT FROM AUTHOR]

We propose a way for implementing a two-step NiSWAP and NSWAP gates based on the qubit-qubit interaction with N superconducting qubits, by coupling them to a resonator driven by a strong microwave field. The operation times do not increase with the growth of the qubit number. Due to the virtual excitations of the resonator, the scheme is insensitive to the decay of the resonator. Numerical analysis shows that the scheme can be implemented with high fidelity. Moreover, we propose a detailed procedure and analyze the experimental feasibility. So, our proposal can be experimentally realized in the range of current circuit QED techniques. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) microwave cavities have been extensively used for coupling and interacting with superconducting quantum bits (qubits), providing a versatile platform for quantum control experiments and for realizing hybrid quantum systems. While having high quality factors (>10) superconducting cavities do not permit magnetic field control of qubits. In contrast, cavities made of normal metals are transparent to magnetic fields, but experience lower quality factors (∼10). We have created a hybrid cavity which is primarily composed of aluminium but also contains a small copper insert reaching the internal quality factor of ≃10, an order of magnitude improvement over all previously tested normal metal cavities. In order to demonstrate precise magnetic control, we performed spectroscopy of three superconducting qubits, where individual control of each qubit's frequency was exerted with small external wire coils. An improvement in quality factor and magnetic field control makes this 3D hybrid cavity an attractive new element for circuit quantum electrodynamics experiments. [ABSTRACT FROM AUTHOR]

The transfer of quantum entanglement (or quantum coherence) is not only fundamental in quantum mechanics but also important in quantum information processing. We here propose a way to achieve the coherent transfer of W-class entangled states of qubits among different cavities. Because no photon is excited in each cavity, decoherence caused by the photon decay is suppressed during the transfer. In addition, only one coupler qubit and one operational step are needed and no classical pulses are used in this proposal; thus, the engineering complexity is much reduced and the operation is greatly simplified. We further give a numerical analysis showing that high-fidelity transfer of a three-qubit W state is feasible within the present circuit QED technique. The proposal can be applied to a wide range of physical implementations with various qubits such as quantum dots, nitrogen vacancy centers, atoms, and superconducting qubits. [ABSTRACT FROM AUTHOR]

We observe the quantum Zeno effect—where the act of measurement slows the rate of quantum state transitions—in a superconducting qubit using linear circuit quantum electrodynamics readout and a near-quantum-limited following amplifier. Under simultaneous strong measurement and qubit drive, the qubit undergoes a series of quantum jumps between states. These jumps are visible in the experimental measurement record and are analyzed using maximum likelihood estimation to determine qubit transition rates. The observed rates agree with both analytical predictions and numerical simulations. The analysis methods are suitable for processing general noisy random telegraph signals.

• An efficient scheme for optimally generating the entangled states between two superconducting qubits was developed. • Shortcut-based creation of entangled states in a controllable manner is interesting for quantum information processing. • Operation robustness can be significantly improved using the technique of invariant-based shortcuts to adiabaticity. Optimal generation of entangled states is of significance to quantum information processing. Here an efficient scheme is proposed for performing controllable and accelerated generation of entangled states between two superconducting qubits. Two three-level artificial atoms of Cooper-pair box circuits, coupling to a quantized field of transmission-line resonator, are individually driven by classical microwaves. In the two-photon resonance with a large detuning, each atom is reduced to an effective two-level qubit. Within a composite qubit-photon-qubit system, two types of entangled state can be controllably created using the technique of invariant-based shortcuts to adiabaticity. Compared with an adiabatic case, the entanglement generations in the shortcut manner need much shorter times. Numerical simulations show that the operations are robust against decoherence effects, and the interatomic cross resonance is safely negligible. The proposed scheme could pave a promising avenue towards optimized preparation of entangled states with superconducting qubits in circuit QED. [ABSTRACT FROM AUTHOR]