Quenching the quantum noise in the beat note of a laser

The spontaneous emissions on two coupled transitions of a quantum system ma y become correlated, as is well known from the observation of narrow "dark lines, either by preparation of the emitter in a common upper level, or by phase correlating the two emitting states, if the common level is the lower one. The quantum noise from spontaneous emission on the two gain-providing lines of a corresponding bimodal laser oscillator turns correlated (CSE la ser) upon the application of an rf field (omega (rf)) that phase-correlates the upper states in the gain medium, such that the noise of the laser beat note at omega (beat) becomes reduced. This phenomenon has been demonstrate d for omega (rf) = omega (bent)/2 and for omega (rf) = omega (beat) with a HeNe-Zeeman laser. Phase coupling and suppression of phase diffusion have b een revealed by measurements of the phase dynamics vs, strength of the coup ling rf field and de Zeeman splitting. In the optimum, the quantum noise of the beat note vanishes, within the 2% experimental uncertainty. Michelson interferometry with the light of a CSE laser permits one to achieve precisi on that far exceeds the limit set by quantum noise, as required, e.g., for the detection of gravitational waves. The output of a single-mode laser of low technical noise phase-correlates an excited state with the correspondin g ground state: Such light has coherently excited a vibrational resonance a nd even an electronic resonance of an individual trapped ion. The operation of quantum-logic gates is feasible on these transitions.