Superconducting qubit

To gain some insight into this problem we focus here on the analysis of the Berry phase [1] in a weakly dissipative system. It is particularly timely to address this issue now given the recent experimental activities in realization of controlled quantum two-level systems (qubits), and in particular, the interest in observing a Berry phase (BP) (see, e.g., [5]). For instance, the superconducting qubits have a coupling to their environment, which is weak but not negligible [10, 15, 4], and thus it is important to find both the conditions under which the Berry phase can be observed and the nature of that Berry phase. 

Leggett, A. J. Superconducting qubits—a major roadblock dissolved Science (Washington, D. C.) 296, 861-862 (2002). 

The fast decoherence of locally prepared entangled states in condensed media studied here is compared with decoherence (in the 10 to 10 s range) in objects studied in quantum optics in high vacuum, with the disappearance of the superposition state in NHj or ND3 molecules in dilute gases, and with the lifetime of superconducting qubits in solids (10 s) at low temperature. 

DiVincenzo, D.P. Fault-tolerant Architectures for Superconducting Qubits. Phys. Scripta 2009(T137), 014020 (2009)... 

Electron spin echo envelope modulation (ESEEM) and ENDOR have been used to study the overlap of the donor wavefunction with naturally-occurring Si. Low-field (6-110 mT) measurements with 50 MHz and 200 MHz excitation showed that the bismuth excitation energy could be tuned for future coupling with superconducting qubits,which require low magnetic fields. Si Bi has electron spin coherence times that are at least as long as Si P with natural silicon and isotopically-pure Si (ref. 76). Like Si P, Si Bi is suitable for bound exciton experiments including nuclear hyperpolarization. ... 

Such a system was independently realized a few years later in a completely different field driven quantum flux qubits [38]. Here, a superconducting loop can support a quantum unit of current in either direction around the loop. In an external dc magnetic field, the degeneracy of the two directions is lifted,... 

EMPLOYMENT OF SUBMICRON YBA2CU307 x GRAIN BOUNDARY JUNCTIONS FOR THE FABRICATION OF "QUIET" SUPERCONDUCTING FLUX-QUBITS... 

The simplest superconducting flux qubit is a superconducting loop interrupted by one Josephson junction (radiofrequency rf-SQUID - SQUID is an acronym for superconducting quantum interference device). The potential energy of such a devices is described by the equation ... 

Before going into details of the possible use of symmetric DD junctions (qi = —a.2, see Fig. 7) for the implementation of superconducting quiet qubit, in this section we describe general aspects of symmetric DD grain boundary junctions. In the case of a symmetric junction, the direction perpendicular to the interface does not point toward a node. In this sense, no low-energy exci-... 

Whereas coherence can persist up to the nanosecond range for atomic and molecular systems exposed to dilute gaseous environments, the situation is radically different in liquids and solids. Interactions with neighbouring atoms, with phonons in crystalline materials and with conduction electrons in metals, shift the coherence times down by several orders of magnitude, and local quantum superpositions are usually not observable. Intermediate cases are the electronic states used as qubits in the form of superconducting islands introduced by Y. Nakamura et al. [4]. The latest reports [5] show coherence times up to 10 s for these objects, which would allow time for operations of a quantum computer. The decoherence mechanisms in such circuits have been discussed theoretically by Burkhard et al. [6],... 

FIGURE 17.6 External electric and classical microwave fields can be used to manipulate polar molecules in EZ traps to encode information in rotational states and perform one-qubit operations. Superconducting stripwire resonators can then couple different molecules and carry two-qubit gates. The sites are selected by adjusting the EZ trap voltage appropriately. (From Andr4 A. et al., Nature Phys., 2, 636, 2006 Cote, R., News Views, Nature Phys., 2, 583, 2006. With permission.)... 

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