We demonstrate fast and ultrasensitive charge detection with a cavity-embedded Cooper pair transistor (cCPT) via dispersive readout of its Josephson inductance. We report a minimum charge sensitivity of $14\mu e/\sqrt{\mathrm{Hz}}$ with a detection bandwidth on the order of $1\mathrm{MHz}$ using $16$ aW of power, corresponding to the single-photon level of the cavity. In addition, our measured sensitivities are within a factor of $5$ of the quantum limit for this device. The single-photon-level sensitivity of the cCPT is comparable to that of the rf single-electron transistor, which typically operates using picowatts of power corresponding to hundreds of thousands of photons in its tank circuit. Our results support the feasibility of using the cCPT to mediate an optomechanical interaction that reaches the single-photon strong coupling regime.

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• Optoelectronics
• Optomechanics
• Quantum fluctuations & noise
• Quantum information with solid state qubits
• Quantum metrology
• Superconducting quantum optics
• Superconducting devices
• Microwave techniques