  It was recently conjectured that 1/f noise is a fundamental characteristic of
spectral fluctuations in chaotic quantum systems. In this Letter we show that
the level fluctuations of experimental realizations of the Sinai billiard
exhibit {\em 1/f} noise, corroborating the conjecture. Assuming that the
statistical properties of these systems are those of the Gaussian orthogonal
ensemble (GOE), we compare the experimental results with the universal behavior
predicted in the random matrix framework, and find an excellent agreement. The
deviations from this behavior observed at low frequencies can be easily
explained with the semiclassical periodic orbit theory. In conclusion, it is
shown that the main features of the conjecture are affected neither by
non-universal properties due to the underlying classical dynamics nor by the
uncertainties of the experimental setup.
