Environmentally decoupled sds-wave Josephson junctions for quantum computing

Quantum computers have the potential to outperform their classical counterp arts in a qualitative manner, as demonstrated by algorithms' which exploit the parallelism inherent in the time evolution of a quantum state. In quant um computers, the information is stored in arrays of quantum two-level syst ems (qubits), proposals for which include utilizing trapped atoms and photo ns(2-4), magnetic moments in molecules(5) and various solid-state implement ations(6-10). But the physical realization of qubits is challenging because useful quantum computers must overcome two conflicting difficulties: the c omputer must be scalable and controllable, yet remain almost completely det ached from the environment during operation, in order to maximize the phase coherence time(11). Here we report a concept for a solid-state 'quiet' qub it that can be efficiently decoupled from the environment. It is based on m acroscopic quantum coherent states in a superconducting quantum interferenc e loop. Our two-level system is naturally bistable, requiring no external b ias: the two basis states are characterized by different macroscopic phase drops across a Josephson junction, which may be switched with minimal exter nal contact.