the lowest energy level of a detectable photon using the most sensitive photon detectors is typically in the range of a few femtojoules (10^-15 joules) or even lower.
Photon detectors, such as photomultiplier tubes (PMTs), avalanche photodiodes (APDs), and superconducting nanowire single-photon detectors (SNSPDs), have made significant advancements in recent years. These detectors can detect individual photons with high sensitivity and low noise levels.
The exact energy threshold for detection depends on various factors, including the specific type of detector, its design, cooling methods (in some cases), and other technical considerations. Advances in detector technology and ongoing research may further push the detection limits lower in the future.
It is worth noting that the sensitivity of photon detectors has improved over time, and it is always a good idea to consult the latest scientific literature and technical specifications of specific detectors to get the most accurate and up-to-date information on their performance.