Quantum Control of Spontaneous Emission

Heekyung Han


We have learned from the class that optically excited atomic and molecular
states inevitably decay emitting spontaneously light.  However, recent
studies have developed theoretically and experimentally the cancellation or
reduction of spontaneous emission that results from quantum interference for
a multilevel atom.  Consider an optically excited atom in four different
conditions, Fig. 1 [Scully and Zhou, Science 281, 1973 (1998)].  In (A), the
atom in a superposition of ground state |b> and excited state |a>, drops to
the ground state through emitted light.  But if we have a pair of excited
states |a1> and |a2> for our atom as in (B), we can cancel the emitted
radiation field by making the atomic radiators of the same frequencies and
180` out of phase.  Therefore the atom remains locked in the superposition
of states |a1> and |a2> because no energy is carried away.  To be more
intricate, when an atom is placed in electromagnetic cavity tuned to a
frequency midway between |a1> and |a2> as in (C), it will not radiate if the
states are coherently prepared.  Furthermore, even in free space, the driven
atom of (D) can also lead to cancellation of spontaneous emission with two
dipole currents of the same frequency and 180` out of phase.
The first experimental observation was done in sodium dimers. [Xia et al.,
Phys. Rev. Lett. 77, 1032 (1996)]  The energy-level scheme is shown in Fig.
2 and the spontaneous intensities from the upper pair to level |b> (dashed)
and to level |d> (solid) versus the detuning of the laser field is presented
in Fig. 3.  Using two-photon coupling from |c> to the upper pair avoids the
effect of the Doppler shift in one-photon coupling.  The zero of the
spontaneous emission to level |b> at the center in Fig. 3 is the
experimental evidence of spontaneous emission cancellation because the peak
of spontaneous emission to |d> at the center proves non-zero population in
the upper two levels.  Further theoretical studies have investigated the
perspectives of using the bare and dressed states [Lee et al., Phys. Rev. A
55, 4454 (1997)] and a second-order perturbation theory [Agarwal, Phys. Rev.
A 55, 2457 (1997)] to provide a physical understanding this experiment on
constructive and destructive interference effects in spontaneous emission.
These will be briefly presented.




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