The Quantum Zeno effect and V-scheme Lasing Without Inversion
F.B. de Jong, R.J.C. Spreeuw, and H.B. van Linden van den Heuvell
Van der Waals-Zeeman Laboratorium, Universiteit van Amsterdam
We show that the quantum Zeno effect (QZE) plays an essential role which has so far been overlooked in discussions of lasing without inversion (LWI) in a V-level scheme. We investigate this role using Monte-Carlo wavefunction simulations, which provide new insight into the physical origin of inversionless lasing. The QZE appears as a result of spontaneous emission and the corresponding collapse of the wavefunction. Whereas the usual explanation of LWI in terms of quantum interference requires coherence, the QZE explains why also incoherent processes are needed.
The interest in lasing without inversion (LWI) derives from the potential for short wavelength lasing and the interesting interplay between coherence in atoms and in light. There has been a large number of theoretical contributions (see overviews [1-4]) and amplification without inversion was demonstrated experimentally [5-9]. Recently actual lasing without inversion in atomic Rb  and Na  has been reported. In the latter experiment lasing occurred at a frequency slightly higher than that of the driving field. These experiments show that a transition between two incoherently coupled levels can be made to lase by the addition of a coherent driving field that couples one of the levels to a third level. The physical mechanism of LWI in the V scheme is usually discussed in terms of quantum interference. In this paper we show that in addition also the quantum Zeno effect is essential. We find that neither quantum interference nor the quantum Zeno effect is sufficient by itself; both are needed. Whereas for quantum interference one needs coherence, for the quantum Zeno effect one needs incoherent processes. In this paper we investigate the role of the quantum Zeno effect by means of Monte-Carlo wavefunction (MCWF) simulations of the three-level V scheme. The quantum Zeno effect is the suppression of coherent transitions between quantum states due to frequent measurements [12-13] and has been observed experimentally in a V scheme . Other MCWF simulations of the V scheme showing the quantum Zeno effect have been performed in the context of quantum measurement theory [14,15].
F.B. de Jong, R.J.C. Spreeuw, and H.B. van Linden van den Heuvell, Phys. Rev. A, 55, 3918 (1997)
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