Go to your university library’s website. Search for "Oxford Scholarship Online." Search "Loudon Quantum Theory." Download the legal PDF chapter by chapter. Then, buy a used paperback copy for $30 on AbeBooks—it’s worth every penny for the marginalia. Have you found a legitimate way to access Loudon’s PDF? Check the comments on academic forums like Physics Stack Exchange for updated library links—but always verify copyright.
Rodney Loudon’s The Quantum Theory of Light is arguably the most revered and cited textbook in the field of quantum optics. For over four decades, from its first edition in 1973 to the third edition in 2000, it has served as the gold standard for graduate students and researchers transitioning from classical electromagnetism to the quantum nature of radiation. loudon quantum theory of light pdf link
Instead, leverage your university library, request an ILL, or purchase the official ebook for $70–90. Consider it an investment in your quantum optics career—a book you will reference for 20+ years. Go to your university library’s website