Dispersive Manipulation of Paired Superconducting Qubits

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum information processing (QIP) on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias.Comment: 5 pages, incl. 1 figur

Measurement of Two-Qubit States by a Two-Island Single Electron Transistor

We solve the master equations of two charged qubits measured by a single-electron transistor (SET) consisted of two islands. We show that in the sequential tunneling regime the SET current can be used for reading out results of quantum calculations and providing evidences of two-qubit entanglement, especially when the interaction between the two qubits is weak

Coulomb blockade electrometer based on single Cooper pair tunneling

We have studied the electrometric characteristics of the Bloch transistor, i.e. the structure comprising a double small-capacitance superconductor junction (where both the Josephson and the charging energies, EJ1,2∽EC1,2≫kBT) and an adjacent gate electrode polarizing its island. The transistor bias is realized through small-sized high-ohmic resistors (≫RQ=h/4e2≈6.5 kΩ) to ensure a high electromagnetic impedance of the transistor environment. At low bias current, a coherent flow of single Cooper pairs occurs and the average voltage across the transistor shows a 2e-periodic dependence on the polarization charge Q0. The sensitivity of such an electrometer has been evaluated and found to be comparable to that of the single-electron counterpart. The device has been fabricated with Al/AlOx/Al junctions and two miniature on-chip Cr resistors (each of 80 kΩ and 10μm long) which were located very close to the junctions. We used this device to measure 1/f noise of the background charge and found it to be about 9×10−4e/Hz1/2 at 10Hz