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arXiv:cond-mat/0701095 · Physical Review B

Magnetic flux in mesoscopic rings: Quantum Smoluchowski regime

Jerzy Dajka, Łukasz Machura, Sz. Rogoziński, J. Łuczka

Published Jul 31, 2007• 9 citations
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Abstract

Magnetic flux in mesoscopic rings in the quantum Smoluchowski regime is investigated. Quantum corrections to the dissipative current are shown to form multistable steady states and can result in statistical enhancement of the magnetic flux. The relevance of quantum correction effects is supported via the entropic criterion. A possible application for a qutrit architecture of quantum information is proposed.

DOI
10.1103/physrevb.76.045337

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Magnetic flux in mesoscopic rings in the quantum Smoluchowski regime is investigated. Quantum corrections to the dissipative current are shown to form multistable steady states and can result in statistical enhancement of the magnetic flux. The relevance of quantum correction effects is supported via the entropic criterion. A possible application for a qutrit architecture of quantum information is proposed.
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We discuss the relationship between external magnetic field and persistent currents of three mesoscopic-coupled rings with charge discreteness. Through numerical calculation and analysis, it is found that the properties of the persistent currents depend on both external magnetic flux and coupled factors. The positive coupling between the rings will decrease the persistent currents when 0< m <1 with m being the coupling coefficient. The quantum current magnification effect appearing in the rings is also investigated.
Current enhancement- a novel quantum phenomena is found to occur in a mesoscopic hybrid ring at equilibrium. The hybrid system is described by a ring with bubble which is in turn coupled to a reservoir. In the system the ring encloses a magnetic flux ΦΦΦ while the bubble does not enclose any flux. The novelty of this work lies in the fact that while earlier current enhancement was observed in non-equilibrium systems (e.g., a ring coupled to two reservoirs at different chemical potentials μ1μ_{1}μ1​ and μ2μ_{2}μ2​), herein we prove that current enhancement can also arise in equilibrium. In addition, we show that the closed system analog of our chosen open hybrid ring system violates parity effects. Finally, we bring to focus the discrepancy between the equilibrium magnetic moment (obtained via energy eigenvalues) and that calculated from the currents in the system.
The possibility of making a flux qubit on a nonsuperconducting mesoscopic ballistic quasi-one-dimensional ring is discussed. We showed that such a ring can be effectively reduced to a two-state system with two external control parameters. The two states carry opposite persistent currents and are coupled by tunneling, which leads to a quantum superposition of states. The qubit states can be manipulated by resonant microwave pulses. The flux state of the sample can be measured by a superconducting quantum interference device magnetometer. Two or more qubits can be coupled by the flux the circulating currents generate. The problem of decoherence is also discussed.
In this paper we investigate the behavior of a fully quantum mechanical system consisting of a mesoscopic SQUID ring coupled to one or two electromagnetic field modes. We show that we can use a static magnetic flux threading the SQUID ring to control the transfer of energy, the entanglement and the statistical properties of the fields coupled to the ring. We also demonstrate that at, and around, certain values of static flux the effective coupling between the components of the system is large. The position of these regions in static flux is dependent on the energy level structure of the ring and the relative field mode frequencies, In these regions we find that the entanglement of states in the coupled system, and the energy transfer between its components, is strong.
Multiple and single measurements of quantum states of mesoscopic superconducting loops are carried out in order to investigate a possibility of macroscopic quantum superposition and of creation of a superconductor quantum register. Asymmetric superconducting rings are used in order to be convinced that single measurement gives result corresponding to one of permitted states and multiple one gives an average value on these states. We have measured magnetic dependencies of resistance, rectified voltage and critical current on these rings. The observed quantum oscillations of the resistance and the rectified voltage, corresponding to multiple measurement of quantum states, give evidence of two permitted states at the magnetic flux inside the ring divisible by half of the flux quantum. But the observed quantum oscillations of the critical current, corresponding to single measurement, not only does not confirm these two quantum states, but are in a direct contradiction with the observed oscillations of the resistance. It is assumed that the observed contradiction between the results of measurements made on the same ring can testify to violation of the principle of realism on the mesoscopic level that is a necessary condition for an opportunity of creation of a quantum computer.