rings to increase the quality factor Q, because it appears as 1/Q2 in the expression for noise. Definition of Q: . Since the operating frequency of the ring is given (474 THz), it remains to increase the circulating energy in the ring W and decrease the power losses dW/dt as much as possible. W is obviously proportional to the length of the ring, but must be limited to avoid multimodes. The power losses dW/dt however can be vastly decreased. The ensuing decreased signal output power is not critical, as modern silicon detectors have low noise, and for very low signals photomultipliers are used. The power loss can be minimized by increasing the reflectivity of the mirrors to as close to 1 as possible and by eliminating other, spurious, sources of power loss, for example the inaccuracy of mirror curvature. Any interfaces or apertures that would decrease the quality factor of the ring are avoided. The whole ring is filled with a HeNe mixture of suitable partial pressures (up to a few hundred Pascal), to achieve lasing and good suppression of multiple pairs of modes. (Typically, the HeNe lasing gas at 633 nm is used; attempts for an argon ring laser failed.) Further, the lasing is excited with radio frequency to easily adjust the amplitude to just below the appearance of the second pair of modes. The Rayleigh scattering of the HeNe gas is, at this time, negligible. For mirrors of proper curvature (spherical shape is acceptable) and equal reflectances r, the quality factor is . This equation gives rise to formidable quality factors. For a 4 m x 4 m ring equipped with 1 ppm mirrors (R = 1-10−6) we would get, at 474 THz, Q = 4×1013. This quality factor produces a passive resonance line of rms = 5 Hz, which is eight orders of magnitude smaller than the atomic linewidth of the Ne line (a 1:1 mixture of the two isotopes and has a gain bandwidth of about 2.2 GHz). (Note that for example in regular pendulums the Q is of the order of 103 and in wristwatch-type quartzes it is of the order of 106.) The active ring further reduces the linewidth by several orders of magnitude, and increasing the measuring time may additionally decrease the linewidth by many orders of magnitude. Measurement The integral of the definition equation for Q above is: (τ is the photon