Why quantum engineering?

The progress in experimental techniques and theoretical modeling made possible to fabricate and test macroscopic structures, which use quantum coherent solid state qubits as building blocks. The results of such quantum engineering are likely to go far beyond the limited goals of quantum computing and quantum communication and provide a direct way to probing quantum-classical boundary. Some recent developments are discussed

Nonlinear Transport in a Quantum Point Contact due to Soft Disorder Induced Coherent Mode Mixing

We show that the coherent mixing of different transverse modes, due to forward scattering of carriers by soft impurity- or boundary potentials leads to a nonlinear, asymmetric current response of quantum point contacts (QPC). The oscillating contribution to the current is sensitive both to driving voltage and to gate voltage in direct analogy to the electrostatic Aharonov-Bohm effect. Our calculations are in a good agreement with recent experimental data showing small-scale conductivity nonlinearities and asymmetry in QPC.Comment: 4 pages, 2 figures (availiable upon request), REVTEX, Applied Physics Report 93-4

Wigner function description of a qubit-oscillator system

We derive the Bloch-type equations of motion for the Wigner function of a qubit nonlinearly coupled to a linear oscillator, and investigate their usefulness in the quantum-classical transition regime

DC SQUID based on the mesoscopic multiterminal Josephson junction

A theory is offered for a novel device, mesoscopic four-terminal SQUID. The studied system consists of a mesoscopic four-terminal junction, one pair of terminals of which is incorporated in a superconducting ring and the other one is connected with a transport circuit. The nonlocal weak coupling between the terminals leads to effects of phase dragging and magnetic flux transfer. The behaviour of a four-terminal SQUID, controlled by the external parameters, the applied magnetic flux and the transport current is investigated. The critical current and the current voltage characteristics as functions of magnetic flux are calculated. In the nonlocal mesoscopic case they depend not only on the magnitude of the applied flux but also on its sign, allowing measurement of the direction of the external magnetic field.Comment: 11 pages, 4 figures, presented at the EUCAS 2001 conferenc

Spontaneous currents in Josephson junctions between unconventional superconductors and d-wave qubits (Review Article)

The modern physics of superconductivity can be called the physics of unconventional superconductivity. The discovery of the d-wave symmetry of the order parameter in high-temperature superconductors and the triplet superconductivity in compound Sr₂RuO₄ has caused a huge stream of theoretical and experimental investigations of unconventional superconductors. In this review we discuss novel aspects of Josephson effect related to the symmetry of the order parameter. The most intriguing of them is spontaneous current generation in an unconventional weak link. The example of a Josephson junction as a grain boundary between two disorientated d-wave or f-wave superconductors, is considered in detail. Josephson current–phase relations and the phase dependences of the spontaneous current, that flows along the interface are analyzed. The spontaneous current and spontaneous phase difference are manifestations of the time-reversal symmetry (T ) breaking states in the system. We analyzed the region of appearance of T -breaking states as function of temperature and mismatch angle. A review of the basics of superconducting qubits with emphasis on specific properties of d-wave qubits is given. Recent results in the problem of decoherence in d-wave qubits, which is the major concern for any qubit realization, are presented

Quasiclassical Theory of Spontaneous Currents at Surfaces and Interfaces of d-Wave Superconductors

We investigate the properties of spontaneous currents generated at surfaces and interfaces of d-wave superconductors using the self-consistent quasiclassical Eilenberger equations. The influence of the roughness and reflectivity of the boundaries on the spontaneous current are studied. We show that these have very different effects at the surfaces compared to the interfaces, which reflects the different nature of the time reversal symmetry breaking states in these two systems. We find a signature of the ``anomalous proximity effect'' at rough d-wave interfaces. We also show that the existence of a subdominant order parameter, which is necessary for time reversal symmetry breaking at the surface, suppresses the spontaneous current generation due to proximity effect at the interface between two superconductors. We associate orbital moments to the spontaneous currents to explain the ``superscreening'' effect, which seems to be present at all ideal d-wave surfaces and interfaces, where d_{xy} is the favorite subdominant symmetry.Comment: 13 pages, 17 postscript figure

Quantum theory as a relevant framework for the statement of probabilistic and many-valued logic

Based on ideas of quantum theory of open systems we propose the consistent approach to the formulation of logic of plausible propositions. To this end we associate with every plausible proposition diagonal matrix of its likelihood and examine it as density matrix of relevant quantum system. We are showing that all logical connectives between plausible propositions can be represented as special positive valued transformations of these matrices. We demonstrate also the above transformations can be realized in relevant composite quantum systems by quantum engineering methods. The approach proposed allows one not only to reproduce and generalize results of well-known logical systems (Boolean, Lukasiewicz and so on) but also to classify and analyze from unified point of view various actual problems in psychophysics and social sciences.Comment: 7 page

Mechanisms of Spontaneous Current Generation in an Inhomogeneous d-Wave Superconductor

A boundary between two d-wave superconductors or an s-wave and a d-wave superconductor generally breaks time-reversal symmetry and can generate spontaneous currents due to proximity effect. On the other hand, surfaces and interfaces in d-wave superconductors can produce localized current-carrying states by supporting the T-breaking combination of dominant and subdominant order parameters. We investigate spontaneous currents in the presence of both mechanisms and show that at low temperature, counter-intuitively, the subdominant coupling decreases the amplitude of the spontaneous current due to proximity effect. Superscreening of spontaneous currents is demonstrated to be present in any d-d (but not s-d) junction and surface with d+id' order parameter symmetry. We show that this supercreening is the result of contributions from the local magnetic moment of the condensate to the spontaneous current.Comment: 4 pages, 5 figures, RevTe

Multi-Terminal Superconducting Phase Qubit

Mesoscopic multi-terminal Josephson junctions are novel devices that provide weak coupling between several bulk superconductors through a common normal layer. Because of the nonlocal coupling of the superconducting banks, a current flow between two of the terminals can induce a phase difference and/or current flow in the other terminals. This "phase dragging" effect is used in designing a new type of superconducting phase qubit, the basic element of a quantum computer. Time-reversal symmetry breaking can be achieved by inserting a pi-phase shifter into the flux loop. Logical operations are done by applying currents. This removes the necessity for local external magnetic fields to achieve bistability or controllable operations.Comment: 7 pages, 3 figure

Operation of universal gates in a DXD superconducting solid state quantum computer

We demonstrate that complete set of gates can be realized in a DXD superconducting solid state quantum computer (quamputer), thereby proving its universality.Comment: 4 pages, 2 figure