By Dieter Meschede
High radiation rates are usually associ- ated with macroscopic lasers. Laser adiation is “coherent”—its amplitude and phase are well-defined—but its generation requires energy inputs to vercome loss. Excited atoms spontaneously emit in a random and incoherent
fashion, and for N such atoms, the emission
rate simply increases as N. However, if these
atoms are in close proximity and coherently
coupled by a radiation field, this microscopic
ensemble acts as a single emitter whose emission rate increases as N2 and becomes “
superradiant,” to use Dicke’s terminology (1). On
page 662 of this issue, Kim et al. (2) show
the buildup of coherent light fields through
collective emission from atomic radiators injected one by one into a resonator field. There
is only one atom ever in the cavity, but the
emission is still collective and superradiant.
These results suggest another route toward
The spontaneous emission from a single
atom can be modeled as a quantum antenna
without preferred emission direction. The
radiation field of individual atomic emitters is also considered “incoherent” because
no well-defined phase exists. Embedding an
atom into the field of a small-volume optical resonator provides some directionality
because the radiation is injected mainly into
the geometrically determined cavity field [the
Purcell effect (3)].
A rather different limit is realized in a
conventional laser oscillator (see the figure,
bottom panel). In order to maintain coherent
oscillations, all members of the microscopic
antenna ensemble (the macroscopic polarization of the laser medium) must be fully synchronized with the local phase of the laser
wave. Coherent laser light is then driven by
this phased array and emitted in a direction
determined by the mirror assembly.
Laser radiation is a threshold process. The
laser switches on when there is sufficient en-
ergy to self-organize the ensemble in such a
way that the polarization continuously drives
the field. The threshold is conditioned on the
rate of stimulated emission induced by the
field of the laser resonator that must over-
come the losses suffered by competition from
spontaneous fluorescence. Above the thresh-
old, additionally supplied energy will domi-
nantly feed the laser field and not be lost. In
conventional lasers, the number of photons
populating the laser resonator field is already
very large at threshold.
Cooperative radiation effects do not fundamentally need to overcome any threshold and
do not depend on any mirror geometry. Atom
ensembles can always show an effective interaction through the constructive or destructive
interaction of their individual contributions.
What matters is that the atoms couple phase-
coherently to a joint mode of the radiation
field. For example, such phase-coherent emit-
ters located within a single half wavelength
create a superradiant emitter whose emission
scales as N2. A commentary on superradiance
by Scully and Svidzinsky (4) noted applica-
tions such as quantum memories.
In recent years, control of atomic emitters
coupled to a radiation field has seen much
experimental progress, such as providing
the phase control required for synchronized
interaction of atoms and radiation fields.
A resonator field not only geometrically facilitates phase-controlled atom-field coupling
but also enhances the electromagnetic field
amplitude at the emitter. In this situation,
stimulated emission into the resonator field
can increase to the level that threshold conditions are rendered almost negligible. The
effective interaction of two atoms coupled
to a joint radiation field is shown in the figure, top panel (5). The atomic radiation fields
interfere constructively for full-wavelength
spacing and destructively for half-wavelength
spacing, like phased arrays of radio antennae
used to control emission patterns.
Kim et al. have now shown how to build
up coherent light fields by collective emission from atomic radiators injected one by
one into a resonator field (see the figure,
middle panel). Phase-stable lasers induce
dipole emitters with a superposition of the
atomic ground and excited state. Insertion
at selected positions supports constructive
interference. In their experiment, only a
single emitter (a barium atom) is present in
the cavity at any time. Nonetheless, the radiation field exhibits the properties of collective
emission, the N2 emission rate. The long-lived resonator field stores the radiation field
and effectively couples every atomic dipole
with its preceding and following neighbors.
The threshold for the superradiant buildup
of a coherent radiation field is again negligible, a consequence of injecting a controlled
stable polarization that drives the laser field
from the beginning. Such thresholdless lasing is of interest probably not for high-power
but for low-power applications. Attempts
have been made to reduce thresholds by geometrically optimizing the ratio of stimulated
and spontaneous emission. The present experiment may point out alternative routes. j
1. R. Dicke, Phys. Rev. 93, 99 (1954).
2. J. Kim, D. Yang, S.-h. Oh, K. An, Science359, 662 (2018).
3. E. M. Purcell, Phys. Rev. 69, 37 (1946).
4. M.O.Scully, A.A.Svidzinsky, Science 325,1510(2009).
5. R. Reimann et al. , Phys. Rev. Lett. 114, 023601 (2015).
Superradiators created atom by atom
Collective emission is observed from atoms dropping singly through a resonator field
Institut für Angewandte Physik, Universität Bonn, 53115 Bonn,
Germany. Email: firstname.lastname@example.org
Interfering atomic radiators: Two phase-coherently
driven atoms emit their radiation feld into the cavity
constructively (superradiantly) for 2π phase diference
and destructively (no emission) for π phase diference.
Cavity memory efects: Stimulated emission
causes traversing atomic dipoles to deliver their
radiation feld into the resonator feld. Subsequent
atoms show collective radiation phenomena because
the long-lived cavity feld stores the feld and thus
memorizes preceding atoms.
Lasing: Conventional devices use the macroscopic
polarization of a laser medium to drive the laser feld.
The polarization and coherent laser feld switch on in a
self-organized way at threshold.
9 FEBRUARY 2018 • VOL 359 ISSUE 6376 641
The radiation of single atoms can be focused and
amplified with superradiance or lasing. Kim et al.
now show that single atoms falling through a cavity
can also superradiate with negligible threshold.