Quantum computers have the potential to outperform their classical counterparts in a qualitative manner, as demonstrated by algorithms  which exploit the parallelism inherent in the time evolution of a quantum state.In quantum computers, the information is stored in arrays of quantum two-level systems (qubits), proposals for which include utilizing trapped atoms and photons [2-4], magnetic moments in molecules  and various solid-state implementations [6-10]. But the physical realization of qubits is challenging because useful quantum computers must overcome two conflicting difficulties: the computer must be scalable and controllable, yet remain almost completely detached from the environment during operation, in order to maximize the phase coherence time . Here we report a concept for a solid-state 'quiet' qubit that can be efficiently decoupled from the environment. It is based on macroscopic quantum coherent states in a superconducting quantum interference loop. Our two-level system is naturally bistable, requiring no external bias: the two basis states are characterized by different macroscopic phase drops across a Josephson junction, which may be switched with minimal external contact.