Quantum Fluctuations in Masers with Loss

R.L. Bailey, J. H. Sanders

Kōichi Shimoda, Hidetosi Takahasi, Charles Hard Townes

Fluctuations in the amplification and absorption of waves by quantum processes are considered. Assuming for each quantum the probability (per unit time) a of producing another quantum, probability b of being absorbed, and assuming a probability c that a new quantum is introduced, a set of differential equations is obtained. By solving these equations, a complete expression for the probability of distribution of quanta is obtained, as well as expressions for the average values and fractional fluctuation. The expressions developed are applied in particular to maser-type amplifiers, and certain fluctuations in the amplification of electromagnetic waves are pointed out which are important when their quantum character becomes significant. This condition can occur in maser-type amplifiers, where thermal and extraneous noises may be very small. For such an amplifier, a is proportional to the number of excited molecules, whereas b consists of a term proportional to the number of molecules in the ground state plus terms due to certain other losses. The noise temperature of a maser-type traveling-wave amplifier is the "effective temperature" a ( a - b ) -1 h ν/ k . In superregenerative and regenerative amplifiers using resonant cavities, the noise temperature is rather similar, if losses through the input coupling hole are excluded from b . In any case, the limiting noise for an ideal amplifier corresponds to a classical noise temperature of h ν/ k .

G. Scharf

Noriyuki Hatakenaka, Susumu Kurihara

The possibility of maser action using a current-biased mesoscopic Josephson junction in a high-Q microcavity is discussed. A mesoscopic Josephson junction behaves like an artificial atom as far as the interaction with the photon field is concerned. Under a certain bias current, the Josephson ``atom'' is resonantly excited or pumped by quantum-mechanical fluctuations of the Josephson phase, i.e., macroscopic resonant tunneling. When the pumping overcomes the loss of photons from the cavity, maser action is possible. Quantum statistical properties of photons are also investigated numerically, taking into acount the microcavity effect on the basis of a Jaynes-Cummings model with pumping and cavity damping. We found that a mesoscopic Josephson junction within the high-Q microcavity acts as a thresholdless maser with sub-Poissonian photon statstics. \textcopyright{} 1996 The American Physical Society.

G. Scharf