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Physical Review Letters

Correlated Spontaneous-Emission Lasers: Quenching of Quantum Fluctuations in the Relative Phase Angle

Marlan O. Scully

Published Dec 16, 1985• 298 citations
Publisher

Abstract

In quantum-beat and Hanle-effect experiments, spontaneous-emission events from two coherently excited states are strongly correlated. A doubly resonant laser cavity driven by such atomic configurations can have vanishing diffusion coefficient for the relative phase angle.
DOI
10.1103/physrevlett.55.2802

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As discussed in the last three chapters, the fundamental source of noise in a laser is spontaneous emission. A simple pictorial model for the origin of the laser linewidth envisions it as being due to the random phase diffusion process arising from the addition of spontaneously emitted photons with random phases to the laser field. In this chapter we show that the quantum noise leading to the laser linewidth can be suppressed below the standard, i.e., Schawlow–Townes limit by preparing the atomic systems in a coherent superposition of states as in the Hanle effect and quantum beat experiments discussed in Chapter 7. In such coherently prepared atoms the spontaneous emission is said to be correlated. Lasers operating via such a phase coherent atomic ensemble are known as correlated spontaneous emission lasers (CEL). An interesting aspect of the CEL is that it is possible to eliminate the spontaneous emission quantum noise in the relative linewidths by correlating the two spontaneous emission noise events. A number of schemes exist in which quantum noise quenching below the standard limit can be achieved. In two-mode schemes a correlation between the spontaneous emisson events in two different modes of the radiation field is established via atomic coherence so that the relative phase between them does not diffuse or fluctuate. In a Hanle laser and a quantum beat laser this is achieved by pumping the atoms coherently such that every spontaneously emitting atom contributes equally to the two modes of the radiation, leading to a reduction and even vanishing of the noise in the phase difference.
An intense single-mode correlated-spontaneous-emission laser or maser can be realized by driving the active atoms coherently with an injected external field. The scheme involves single-photon transitions unlike the two-photon or quantum-beat correlated-emission lasers that utilize correlations between successive photon-cascade emissions or between simultaneous emissions into modes of the field. Quenching of both the amplitude and phase noise and, in certain cases, squeezing of the amplitude fluctuations are found.