Interaction of mesoscopic Josephson devices with non-classical microwaves

Mesoscopic Josephson devices, interacting with non-classical microwaves, are studied. The phase difference in Josephson's equations is a quantum mechanical operator, whose expectation value with respect to the density matrix describing the microwaves, determines the current. Dual phenomena with vortex condensates in Josephson array insulators are also considered. Dual Josephson junctions for vortices, made from two insulators separated by a weak link through which the vortices tunnel, are described by dual Josephson equations.     

Vortices in Josephson arrays interacting with non-classical microwaves: The effect of dissipation

Vortices circulating in a ring made from a Josephson array in the insulating phase are studied. The ring contains a `dual Josephson junction' through which the vortices tunnel. External non-classical microwaves are coupled to the device. The time evolution of this two-mode fully quantum mechanical system is studied, taking into account the dissipation in the system. The effect of the quantum statistics of the photons on the quantum statistics of the vortices is discussed. Entropic calculations quantify the entanglement between the two systems. Quantum phenomena in the system are also studied through Wigner functions. After a certain time (which depends on the dissipation parameters) these quantum phenomena are destroyed due to dissipation. Received 21 October 2002 / Received in final form 11 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: a.konstadopoulou@brad.ac.uk     

Quantum effects on capacitively coupled intrinsic Josephson junctions

We numerically study quantum effects in intrinsic Josephson junctions of layered high-T_c superconductors in order to explain recent experimental observations on the switching rate enhancement in the low temperature quantum regime. We pay attention to the capacitive coupling between neighboring junctions and perform simulations for the Schrödinger equation derived from the Hamiltonian describing the capacitive coupling. The simulation results reveal that the phase dynamics show synchronous behaviors when entering the quantum regime. This is qualitatively consistent with the experimental result.     

Quantum oscillations of the nonequilibrium chemical potential in Josephson junctions

The problem of electron injection in Josephson junctions is considered theoretically. The effect of quantum oscillations of the chemical potential in a nonequilibrium Josephson junction is predicted. The oscillation spectrum is presented by a single Josephson mode if the transparency of the oxide barrier is not too high.     

Quantum phase fluctuations in an array of mesoscopic Josephson junctions

The effects of macroscopic ordering in a system of mesoscopic Josephson junctions are investigated by the quantum Monte Carlo simulation technique (using path integrals). The phase diagram of the system in the T-q plane (q is the dimensionless quantum parameter , where J is the Josephson coupling constant and C ₀ is the self-capacitance of the granules) is investigated in detail. An analysis of the behavior of the relative root-mean-square phase shifts, as well as the helicity and vorticity moduli, demonstrates the need to employ these two quantities as the parameters which most completely reflect the character of the topological phase transition in the quantum system under consideration. Two methods are proposed for calculating the vorticity modulus: 1) a modification of the Gibbs-Bogolyubov variational principle for calculating the free energy change in response to alteration of the type of boundary conditions; 2) calculation of the response to the introduction of an infinitesimal magnetic flux at some point in the system. The calculations confirm the absence of reentrant melting and phase transitions of a non-Kosterlitz-Thouless type in the region of strong quantum phase fluctuations q>1. Fiz. Tverd. Tela (St. Petersburg) 39, 1513–1519 (September 1997)     

Quantum two-level systems in Josephson junctions as naturally formed qubits

The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction can be considered as a multiqubit register. It can be coupled to other junctions to allow the application of quantum gates to an arbitrary pair of qubits in the system. Our results indicate an alternative way to realize superconducting quantum information processing.     

Quantum phase transition in one-dimensional arrays of resistively shunted small Josephson junctions

We have observed a superconductor-insulator transition in one-dimensional (1D) arrays of small Josephson junctions by changing both the resistance R(S) of normal metal resistors shunting each junction and the ratio of the Josephson coupling energy E(J) to the charging energy E(C). The phase boundary lies at R(S) approximately R(Q) (R(Q) identical with h/4e(2)=6.45 kOmega) when E(J)/E(C) is smaller than about unity. We discuss the obtained phase diagram in terms of theoretical models of the dissipation-driven quantum phase transition, with particular attention to differences from 2D arrays.     

Quantum field theory with an interaction on the boundary

We consider quantum theory of fields ? defined on a D dimensional manifold (bulk) with an interaction V (?) concentrated on a d < D dimensional surface (brane). Such a quantum field theory can be less singular than the one in d dimensions with an interaction V (?). It is shown that scaling properties of fields on the brane are different from the ones in the bulk. We discuss as an example fields on de Sitter space.     

Enhancement of Josephson phase diffusion by microwaves

We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position V(top) of the peak increases with the power P of microwave radiation as V(top) proportional to sqrt[P], while its current amplitude weakly decreases with P. As the microwave frequency increases, the peak feature evolves into Shapiro steps with a finite slope. Our theoretical analysis, taking into account the enhancement of incoherent superconducting current by multiphoton absorption, is in good agreement with experimental data.     

An approximate theory for bunched solitons on finite Josephson tunnel junctions

An approximate hamiltonian theory is used to predict critical bias currents for the transition from bunched to symmetric soliton modes on Josephson junctions of finite length, recently observed experimentally and numerically.     

Superconducting phase qubits with shadow-evaporated Josephson junctions

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device(SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as 0.05 μm~2 are fabricated for the qubit, in which the number of the decoherencecausing two-level systems(TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T_1 and dephasing time T_φ on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.     

Fabrication of rhenium Josephson junctions

Rhenium is a superconductor with a relatively weak tendency to oxidize, which is advantageous in superconducting quantum circuit and qubit applications. In this work, Re/A1-A1Ox/Re Josephson tunnel junctions were fabricated using a selective film-etching process similar to that developed in Nb trilayer technology. The Re films had a superconducting transition temperature of 4.8 K and a transition width of 0.2 K. The junctions were found to be highly reproducible using the fabrication process and their characteristics had good quality with a low leakage current and showed a superconducting gap of 0.55 meV.