Superconducting qubits

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,... 

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). 

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-... 

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. 

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.)... 

The chapter provides a brief introduction to the field of quantum information processing and computation, discusses various platforms for implementing quantum computers, and makes the case for using permanent molecular dipoles as realizations of individually addressable qubits. A look into the future reveals the contours of a superconducting microwave resonator of an optical quantum computer enmeshed with an ensemble of polar molecules. 

Another interesting study is that of Yang et al. [28]. They used a NMR quantum computer to simulate the BCS superconductivity Hamiltonian. In the experiment, performed in the two-qubit chloroform dissolved in acetone- /6, they observed the energy gap between the superconductor and normal states, directly from the NMR spectrum. 

Within the past decade much progress has also been made in experimental realizations of quantum computing hardware. Many architectures have been proposed based on a variety of physical hardware. On a small scale, quantum information has been stored and manipulated in superconducting quantum bits (qubits) [4,5], trapped ions [6,7], electron spins [8-11], nuclear spins in the liquid or solid state [12], and other systems. On the theoretical side, new quantum algorithms have recently been found, exhibiting significant pol momial speedups on quantum computers for solution of sparse linear equations or differential equations [13,14], quantum Monte Carlo problems [15], and classical simulated annealing problems [16]. 

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

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