This paper presents a superconducting tunnel junction circuit, called a "quantronium," which behaves as a two-level quantum system. The circuit is designed to manipulate and measure its quantum state using microwave pulses and a readout sub-circuit. The quantum coherence quality factor $ Q_{\varphi} $ is measured to be approximately 25,000, which is sufficiently high to support a solid-state quantum processor. The paper discusses the challenges of building quantum machines that exploit quantum mechanics, such as superposition and entanglement. While traditional devices like transistors and lasers use quantum properties, they do not fully utilize quantum mechanics. Recent proposals for quantum machines include quantum computers, quantum cryptography, and detectors below the standard quantum limit. A major challenge is decoherence, which can be mitigated by maintaining high coherence. The quantronium is an integrated electrical circuit that acts as a tunable artificial atom. It consists of a Cooper pair box with a Josephson junction and a gate capacitance. The circuit's quantum state is manipulated using microwave pulses, and its state is read out by measuring the supercurrent in a superconducting loop. The readout method involves entangling the spin of the quantronium with the phase of a large Josephson junction, allowing discrimination between the two quantum states. The quantronium's quantum state is prepared and manipulated at an optimal working point where it is immune to charge noise. The circuit's performance is tested using spectroscopic measurements of the transition frequency $ \nu_{01} $, which is found to be around 16.5 GHz at the optimal point. The coherence time $ T_{\varphi} $ is measured to be approximately 0.5 microseconds, allowing the spin to perform about 8000 coherent precession cycles. The paper also discusses the limitations of the quantronium's coherence time, which is affected by charge and flux noise. The linewidth of the resonant peak increases when deviating from the optimal point, indicating the influence of noise. The results suggest that improving the circuit's design could significantly enhance its coherence, enabling further developments in quantum computing and other quantum technologies.This paper presents a superconducting tunnel junction circuit, called a "quantronium," which behaves as a two-level quantum system. The circuit is designed to manipulate and measure its quantum state using microwave pulses and a readout sub-circuit. The quantum coherence quality factor $ Q_{\varphi} $ is measured to be approximately 25,000, which is sufficiently high to support a solid-state quantum processor. The paper discusses the challenges of building quantum machines that exploit quantum mechanics, such as superposition and entanglement. While traditional devices like transistors and lasers use quantum properties, they do not fully utilize quantum mechanics. Recent proposals for quantum machines include quantum computers, quantum cryptography, and detectors below the standard quantum limit. A major challenge is decoherence, which can be mitigated by maintaining high coherence. The quantronium is an integrated electrical circuit that acts as a tunable artificial atom. It consists of a Cooper pair box with a Josephson junction and a gate capacitance. The circuit's quantum state is manipulated using microwave pulses, and its state is read out by measuring the supercurrent in a superconducting loop. The readout method involves entangling the spin of the quantronium with the phase of a large Josephson junction, allowing discrimination between the two quantum states. The quantronium's quantum state is prepared and manipulated at an optimal working point where it is immune to charge noise. The circuit's performance is tested using spectroscopic measurements of the transition frequency $ \nu_{01} $, which is found to be around 16.5 GHz at the optimal point. The coherence time $ T_{\varphi} $ is measured to be approximately 0.5 microseconds, allowing the spin to perform about 8000 coherent precession cycles. The paper also discusses the limitations of the quantronium's coherence time, which is affected by charge and flux noise. The linewidth of the resonant peak increases when deviating from the optimal point, indicating the influence of noise. The results suggest that improving the circuit's design could significantly enhance its coherence, enabling further developments in quantum computing and other quantum technologies.