A Scheme for Implementing the Deutsch-Jozsa algorithm via Josephson Charge Qubits Coupled through Microwaves

Based on the Josephson charge qubits coupled through microwaves, a scheme for implementation of the Deuutsch-Jozsa algorithm is proposed under the present scalable and feasible microfabrication technique. It would be a valuable step toward complex quantum computation.     

Wide‐band phase locking of Josephson junctions in a resonator

We found that the chain of junctions acts both as the source of radiation and as a part of the superconducting resonator when the effective capacitance of the resonator is larger than the total capacitance of all junctions. At this condition junctions are synchronized in‐phase not only at the resonance steps but also in the whole hysteretic region of I –V characteristics below the resonant frequency. The maximal allowable spread of critical currents for this effect is about 5–10%. We analyzed the origin of the effect both numerically and by the method of slowly varying amplitudes.

     

Microwave Response of Bi2Sr2CaCu2O8+δ Surface Intrinsic Josephson Junctions with Bending Pancake Vortices

We study the microwave response of surface intrinsic Josephson junctions on Bi2Sr2 CaCu2O8＋δ, in which bending pancake vortex lines are introduced in a controllable way. It is found that the bending vortices can greatly influence the response. In some cases, typical Shapiro steps that lie far above the quasiparticle branch are observed, with the step interval satisfying the Josephson relation and their amplitude versus the square root of microwave power following the Bessel function behaviour. In the other cases, current steps that lie on the quasiparticle branch are observed, but only one or two steps appear at the same time under the variation of the microwave power.     

Cooper-Pair Number-Phase Quantization for a Mesoscopic LC Circuit Including a Josephson Junction

By introducing the entangled state representation and Feynman assumption that ＇electron pairs are bosons, ..., a bound pair acts as a Bose particle ＇, we construct an operator Hamiltonian for a mesoscopic inductance-capacitance （LC） circuit including a Josephson junction, then we use the Heisenberg equation of motion to derive the current equation and the voltage equation across the inductance as well as across the Josephson junction. The result manifestly shows how the junction voltage is affected by the capacitance coupling. In this way the Cooper-pair number-phase quantization for this system is completed.     

Kink properties on curved manifold

In the framework of the two-dimensional field model the influence of the curvature on kink width is discussed. Breading of the kink width in a curved region of the manifold is observed. Examples of kinks on curved manifolds are studied analytically and numerically as well. The deformation of the kink front in the form of the travelling waves propagating along the curved surfaces are found. Enlarging of the travelling wave speed in a curved regions of the manifold is predicted.     

Resonance phenomena in macroscopic quantum tunneling: The small viscosity limit

We present a theoretical study of the resonant quantum behavior for a macroscopic superconducting system interacting with an external microwave at proper frequency. Here we consider a system described by a double well potential, an rf-SQUID, in the extremely underdamped regime. Numerical simulations for resonant phenomena have been performed for this system, whose parameters belong to the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system, φx, can show three resonance peaks, in a small microwave frequency range. One peak is connected with the anticrossing and the other two with the external microwave frequency ν. The relative position and the height of the two lateral peaks depend on the microwave frequency. This behavior is studied here for the first time.     

Quantum Spontaneous Magnetization in Single-Junction Superconducting π-Rings

Based upon the calculations of energy levels and quantum states, we discuss the probability of quantum spontaneous magnetization flux as a function of the screen parameter β for the superconducting ring containing one Josephson π-junction, and compare the result with that in the classical case. Our results indicate that there is significant difference in the magnetization behaviour aroundβ = 1 between the quantum and classical situations.     

Holonomic quantum computation with superconducting charge-phase qubits in a cavity

We theoretically propose a feasible scheme to realize holonomic quantum computation with charge-phase qubits placed in a microwave cavity. By appropriately adjusting the controllable parameters, each charge-phase qubit is set as an effective four-level subsystem, based on which a universal set of holonomic quantum gates can be realized. Further analysis shows that our system is robust to the first-order fluctuation of the gate charges, and the intrinsic leakages between energy levels can be ignored.     

Quantum computing in decoherence-free subspaces with superconducting charge qubits

Taking into account the main noises in superconducting charge qubits (SCQs), we propose a feasible scheme to realize quantum computing (QC) in a specially-designed decoherence-free subspace (DFS). In our scheme two physical qubits are connected with a common inductance to form a strong coupling subsystem, which acts as a logical qubit. Benefiting from the well-designed DFS, our scheme is helpful to suppress certain decoherence effects.     

Persistent currents in superconducting quantum interference devices

Starting from the reduced dynamical model of a two-junction quantum interference device, it shown that a quantum analog of the system can be exhibited. This quantum model extends the well-known properties of the device when its characteristic dimensions are of the order of mesoscopic length scales. By finding eigenvalues of the corresponding Hamiltonian operator, the persistent currents flowing in the ring have been obtained. The resulting quantum analog of the overdamped two-junction quantum interference device can be seen as a supercurrent qubit operating in the limit of negligible capacitance and finite inductance.     

Scalable quantum computing with Josephson charge-phase qubits inside a cavity

This Letter proposes a theoretical scheme for scalable quantum computing with charge-phase qubits inside a common cavity. Individually addressing the applied gate pulses, we obtain the switchable interqubit couplings mediated by the cavity mode, from which a universal set of logic gates can be constructed. In our scheme the interqubit couplings are completely feasible to perform conditional gates, and the classical microwaves cause negligible leakage errors.