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In this talk I address the question: Under what conditions can we legitimately describe a quantum-mechanical system by a Schrödinger equation in its own right, and how are these conditions related to the degree of “entanglement” with its environment? As examples of systems that are often claimed to be strongly entangled with their environments but nevertheless seem to be well described by one-particle-like...
We have operated a Josephson qubit device in which a long coherence time is obtained by decoupling the qubit from the readout circuit during manipulation. The achieved quantum coherence is sufficient to allow qubit manipulation with NMR-like techniques. We report pulsed microwave experiments that demonstrate the controlled manipulation of the qubit state.
We show theoretically that the Aharonov-Casher effect can be used to modulate flux tunneling. We study a variant of an rf-SQUID in which the Josephson junction is replaced by a Bloch transistor — two junctions separated by a small superconducting island on which the charge can be induced by an external gate voltage. When the Josephson coupling energies of the junctions are equal and the induced charge...
We present the characterization of a fully integrated Josephson device consisting of an rf-SQUID coupled to a readout system based on a dc-SQUID sensor. In the classical regime, data on the decay rate from the metastable flux states of rf-SQUID are reported. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments...
An rf-SQUID under appropriate temperature and bias conditions can be used to perform tests on macroscopic quantum coherence, or as a qubit element for quantum computing. For both these purposes it is necessary a readout system allowing to discriminate stroboscopically the rfSQUID state with high fidelity. This task can be performed by means of a hysteretic dc-SQUID inductively coupled to the rf-SQUID...
We model a SQUID ring coupled to one or two electromagnetic field oscillator modes, each component being treated quantum mechanically. We show that the external flux on the SQUID ring can be used to control the exchange of energy, and level of entanglement, between the coupled systems. We then extend this model to include dissipation, and provide examples of the time evolution of these coupled, multicomponent...
This paper discusses two methods for generating quantum transitions between energy eigenstates in a model persistent current qubit. The alternatives are compared and their advantages and disadvantages are considered. Of particular concern is the backreaction of the qubit on the magnetic flux bias coils, used to control the qubit, due to the large effective currents at or near to a quantum mechanical...
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 nonclassical 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...
An arbitrary quantum state cannot be “cloned” perfectly, that is, reproduced with “fidelity” F = 1 into M > 1 states identical to the original by any conceivable physical device. The main root of this impossibility resides in the linearity of quantum mechanics. A second “quantum impossibility” process, based on the complete positivity character of any quantum operation, forbids the realization...
Several new metallic and semiconducting nanostructures may provide interesting substrates for observations involving quantum coherence. Boron-based nanotubes with beryllium atoms in the hexagonal faces should exhibit uniformly metallic properties with multiple conduction channels of disparate character. The apical dislocation in a carbon nanocone mixes the low-energy electronic states of a graphene...
In this chapter, fabrications and characterizations of nanometer-sized transistors that incorporate individual molecules and carbon nanotubes will be described. In the first part of the presentation, we describe transport measurements of single-nanotube transistors that allow the characterization of electronic shell filling and exchange coupling as well as Fabry-Perot interference between traveling...
We create a one-dimensional array of Josephson junctions by condensing 87Rb atoms in the combined potential of an optical standing wave superimposed on a harmonic magnetic trap. The array realizes at the same time an atom interferometer, which allows monitoring coherence across the system and obtaining via some simple theoretical assumptions the most relevant parameters of the ground state. We can...
Within the density matrix formalism, it is shown that a simple way to get decoherence is through the introduction of a “quantum” of time (chronon): thus replacing the differential Liouvillevon Neumann (LvN) equation with a finite-difference version of it. In this way, one is given the possibility of using a rather simple quantum equation to describe the decoherence effects due to interaction with...
We explore the feasibility of a two-step semiclassical approximation of Shor’s quantum algorithm based on coherent states. The first step is the implementation of a classical time-evolution for the spin 1/2 system making the computation. The second step is the coherent states approximation of the Shor’s algorithm itself, in particular of the quantum Fourier transform (QFT). One of the aims is to eventually...
We study the interaction of ultrafast laser radiation (<100fs pulses) with a Josephson junction. For very weak radiated power, heating of the junction is assumed to be negligible and relaxation of the order parameter is disregarded. The time evolution of the order parameter phase p(t), after the arrival of the pulse, is studied within a time-dependent Bogoliubov-de Gennes mean field theory. While...
Recently the single-electron electrometer has gained new attention as it might serve as the primary readout device of the coherent state of a charge qubit represented by a superconducting single-electron box. Already in the early 1990s it became clear by pioneering experiments of the Saclay group (e.g., Ref. [1]) that the single-electron transistor can serve as the most sensitive electrometer known...
We investigate the properties of Josephson junction triangular prism qubits coupled by mutual inductance to a resonant LC bus. We show how the symmetries of the qubit potential may be used to implement the Duan Cirac Zoller scheme for holonomic quantum computation.
We successfully distinguished flux qubit states by means of single-shot measurement with a high-resolution dc-SQUID. We studied the readout results of the qubit states for two types of sample (classical and quantum) and found that the classical sample exhibited incoherent macroscopic tunneling between two flux states. With the quantum sample, we directly observed the superposition states of the qubit...
There has recently been substantial progress in observing the superposition of charge and flux quantum states [1–5] in superconducting devices and of quantum states in a current-biased Josephson junctions [6,7]. These experiments are of considerable interest not only from a fundamental point of view but also because such devices may ultimately be used as qubits in a quantum computer [8]. Probably...
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