Dynamics of superconducting qubits with exactly solvable bias pulses

The dynamics of a quantum two-state system (qubit) with external control bias pulses of special shapes is considered. The bias pulses represent the potentials for which the Schrödinger equation can be solved exactly. The probability to register a definite direction of the current in a loop and its time-averaged values are calculated for the flux qubit; calculations are performed both analytically and numerically in the presence of relaxation and decoherence within the framework of the density matrix formalism. It is demonstrated that there exist external bias pulses for which the definite current direction probability is a monotonically increasing function of time that approaches a limiting value exceeding 1/2. The probability to find the system in the excited state is calculated, and the possibility of inverse population in a properly driven two-state system is demonstrated given that the relaxation and decoherence rates are small enough.     

Degenerate ground state in a mesoscopic YBa2Cu3O(7-x) grain boundary Josephson junction

We have measured the current-phase relationship I(varphi) of symmetric 45 degrees YBa2Cu3O7-x grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T, a local minimum at a temperature T*. (ii) At T approximately T*, the first harmonic of I(phi) changes sign. (iii) For T相似文献   

Electron-phonon coupling and phonon generation in normal-metal microbridges

The second derivative of current—voltage characteristic, $\text{d}{}^2\text{I}\text{d}\text{V}{}^2$, of a small orifice connecting two pieces of normal metals is shown to be proportional to the function $\text{G(ω) =}\text{α}\text{?}{}^2\text{(ω)F(ω)}$ at ω = eV, where F(ω) is the phonon density of states, and α̃² (ω) the square of the electron—phonon matrix element averaged over the Fermi surface and multiplied by the additional structure factor taking into account the geometry of the orifice. The constriction is shown to work, in a current-carrying state, as a source of non-equilibrium phonons emitted in the immediate vicinity of the orifice.     

Consistency of ground state and spectroscopic measurements on flux qubits

We compare the results of ground state and spectroscopic measurements carried out on superconducting flux qubits which are effective two-level quantum systems. For a single qubit and for two coupled qubits we show excellent agreement between the parameters of the pseudospin Hamiltonian found using both methods. We argue that by making use of the ground state measurements the Hamiltonian of N coupled flux qubits can be reconstructed as well at temperatures smaller than the energy level separation. Such a reconstruction of a many-qubit Hamiltonian can be useful for future quantum information processing devices.     

Macroscopic quantum interference effects in superconducting multiterminal microstructures

We review the macroscopic quantum phenomena in superconducting microstructures based on multiterminal junctions. The multiterminal Josephson junction presents a system in which the weak coupling takes place between several massive superconducting banks (terminals). Compared with the conventional (two-terminal) junctions such systems have additional degrees of freedom and a corresponding set of control parameters, preset transport currents and (or) applied magnetic fluxes. The general phenomenological theory of multiterminal Josephson junctions is presented. The specific multichannel interference effects (studied theoretically and experimentally) are described for two microstructures: the four-terminal SQUID and a system consisting of two weakly coupled superconducting rings.