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We investigate anisotropic XXZ Heisenberg spin-1 / 2 chains with control fields acting on one of the end spins, with the aim of exploring local quantum control in arrays of interacting qubits. In this work, which uses a recent Lie-algebraic result on the local controllability of spin chains with 'always-on' interactions, we determine piecewise-constant control pulses corresponding to optimal fidelities for quantum gates such as spin-flip (NOT), controlled-NOT (CNOT), and square-root-of-SWAP (). We find the minimal times for realizing different gates depending on the anisotropy parameter Δ of the model, showing that the shortest among these gate times are achieved for particular values of Δ larger than unity. To study the influence of possible imperfections in anticipated experimental realizations of qubit arrays, we analyze the robustness of the obtained results for the gate fidelities to random variations in the control-field amplitudes and finite rise time of the pulses. Finally, we discuss the implications of our study for superconducting charge-qubit arrays. [ABSTRACT FROM AUTHOR]

We investigate the Josephson effect in S[sub F]XS[sub F] junctions, where S[sub F] is a superconducting material with a ferromagnetic exchange field, and X is a weak link. The critical current I[sub c] increases with the (antiparallel) exchange fields, if the distribution of transmission eigenvalues of the X layer has its maximum weight at small values. This exchange-field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an S[sub F]IS[sub F] junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference in the exchange fields h[sub 1] – h[sub 2] in an S[sub F]IS[sub F] junction corresponds to the potential difference V[sub 1] – V[sub 2] in an SIS junction; i.e., the singularity in I[sub c] [in an S[sub F]IS[sub F] junction] at | h[sub 1] – h[sub 2] | = Δ[sub 1] + Δ[sub 2] is the analogue of the Riedel peak. We also discuss the AC Josephson effect in S[sub F]IS[sub F] junctions. © 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We consider the visibility of the Aharonov–Bohm effect for cotunneling transport through a clean one-channel ring coupled to a fluctuating magnetic flux. We concentrate on the modification of the destructive interference at Φ 0/ 2 by the fluctuating flux, since changes in the magnitude of the current away from this point can also be caused by renormalization effects and do not necessarily indicate dephasing. For fluctuations arising from the Nyquist noise in an external coil at T= 0, the suppression of the destructive interference shows up only in a contribution proportional to V 3, and therefore does not affect the linear conductance. In this sense, the Nyquist bath does not lead to dephasing in the linear transport regime at zero temperature in our model. [ABSTRACT FROM AUTHOR]

Superconductor–ferromagnet–superconductor (SFS) Josephson junctions are known to exhibit a transition between and 0 states. In this letter, we find the π–0 phase diagram of an SFS junction depending on the transparency of an intermediate insulating layer (I). We show that, in general, the Josephson critical current is nonzero at the π–0 transition temperature. Contributions to the current from the two spin channels nearly compensate each other, and the first harmonic of the Josephson current as a function of phase difference is suppressed. However, higher harmonics give a nonzero contribution to the supercurrent. © 2001 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]