We have designed and operated a superconducting tunnel junction circuit that behaves as a twolevel atom: the "quantronium". An arbitrary evolution of its quantum state can be programmed with a series of microwave pulses, and a projective measurement of the state can be performed by a pulsed readout sub-circuit. The measured quality factor of quantum coherence Qϕ ≃ 25000 is sufficiently high that a solid-state quantum processor based on this type of circuit can be envisioned.Can we build machines that actively exploit the fundamental properties of quantum mechanics, such as the superposition principle or the existence of entangled states? Applications such as the transistor or the laser, often quoted as developments based on quantum mechanics, do not actually answer this question. Quantum mechanics enters into these devices only at the level of material properties but their state variables such as voltages and currents remain classical. Proposals for true quantum machines emerged in the last decades of the 20th century and are now being actively explored: quantum computers [1], quantum cryptography communication systems [2] and detectors operating below the standard quantum limit [3]. The major difficulty facing the engineer of a quantum machine is decoherence [4]. If a degree of freedom needs to be manipulated externally, as in the writing of information, its quantum coherence usually becomes very fragile. Although schemes that actively fight decoherence have recently been proposed [5,6], they need very coherent quantum systems to start with. The quality of coherence for a two-level system can be quantitatively described by the quality factor of quantum coherence Q ϕ = πν 01 T ϕ where ν 01 is its transition frequency and T ϕ is the coherence time of a superposition of the states. It is generally accepted that for active decoherence compensation mechanisms, Q ϕ 's larger than 10 4 ν 01 t op are necessary, t op being the duration of an elementary operation [7].Among all the practical realizations of quantum machines, those involving integrated electrical circuits are particularly attractive. However, unlike the electric dipoles of isolated atoms or ions, the state variables of a circuit like voltages and currents usually undergo rapid quantum decoherence because they are strongly coupled to an environment with a large number of uncontrolled * To whom correspondence should be addressed; E-mail: vion@drecam.saclay.cea.fr † Member of CNRS. ‡ Present address: Applied Physics, Yale University, New Haven, CT 6520, USA degrees of freedom [8]. Nevertheless, superconducting tunnel junction circuits [9,10,11,12,13] have displayed Q ϕ 's up to several hundred [14] and temporal coherent evolution of the quantum state has been observed on the nanosecond time scale [10,15] in the case of the single Cooper pair box [16]. We report here a new circuit built around the Cooper pair box with Q ϕ in excess of 10 4 , whose main feature is the separation of the write and readout ports [17,18]. This circuit, which behaves as a tunable ar...
