The authors have designed and operated a superconducting tunnel junction circuit that functions as a two-level atom, known as a "quantonium." This circuit can be programmed to evolve its quantum state using microwave pulses and can perform projective measurements through a pulsed readout sub-circuit. The quality factor of quantum coherence, \( Q_{\varphi} \), is measured to be approximately 25000, which is high enough to support solid-state quantum processing. The circuit's main feature is the separation of the write and readout ports, allowing for precise control and measurement of the quantum state. The authors describe the circuit's design, including the use of a Cooper pair box and a large Josephson junction, and detail the readout process, which involves entangling the system with a large Josephson junction to measure the supercurrent in the loop. They also report on spectroscopic measurements of the transition frequency and time-domain experiments demonstrating controlled manipulation of the quantum state. The coherence time \( T_{\varphi} \) is found to be around 0.50 μs, allowing for 8000 coherent free precession turns on average. The authors discuss the limitations of the coherence time, primarily due to charge and phase noises, and suggest ways to improve the quality factor \( Q_{\varphi} \).The authors have designed and operated a superconducting tunnel junction circuit that functions as a two-level atom, known as a "quantonium." This circuit can be programmed to evolve its quantum state using microwave pulses and can perform projective measurements through a pulsed readout sub-circuit. The quality factor of quantum coherence, \( Q_{\varphi} \), is measured to be approximately 25000, which is high enough to support solid-state quantum processing. The circuit's main feature is the separation of the write and readout ports, allowing for precise control and measurement of the quantum state. The authors describe the circuit's design, including the use of a Cooper pair box and a large Josephson junction, and detail the readout process, which involves entangling the system with a large Josephson junction to measure the supercurrent in the loop. They also report on spectroscopic measurements of the transition frequency and time-domain experiments demonstrating controlled manipulation of the quantum state. The coherence time \( T_{\varphi} \) is found to be around 0.50 μs, allowing for 8000 coherent free precession turns on average. The authors discuss the limitations of the coherence time, primarily due to charge and phase noises, and suggest ways to improve the quality factor \( Q_{\varphi} \).