Abstract
When a quantum system is put into an excited state, it will decay back to the ground state through a process termed spontaneous emission. It is generally assumed that spontaneous emission between different individual emitters would not be coherent with each other; to produce coherent light one would need population inversion and stimulated emission. In this work, we show that an optically-thin ensemble of 11,000 radiating atoms spontaneously organize to produce spatially coherent light. The reason for this coherence is collective-coupling of the individual emitters via Dicke superradiance and subradiance (as opposed to amplification through stimulated emission).
Highlights
Spontaneous emission occurs due to the coupling of a quantum system to a continuum of radiation modes
The emission rate can be enhanced or reduced compared to the natural rate of a single isolated atom: effects commonly referred to as superradiance and subradiance [2–4]. This enhancement and reduction of the decay rates can be understood classically as constructive and destructive interference between the radiation originating from individual emitters
Whether there is superradiance or subradiance, spatial coherence is established between the atoms, which is mapped to their emitted light
Summary
Spontaneous emission occurs due to the coupling of a quantum system (e.g., a neutral atom) to a continuum (infinite number) of radiation modes. Whether there is superradiance or subradiance, spatial coherence is established between the atoms, which is mapped to their emitted light (i.e., the individual emitters are no longer uncorrelated but, instead, have a defined phase relationship). This coherence is essential to collective decay: it is no coincidence that Dicke’s original paper is titled “Coherence in Spontaneous Radiation Processes.”. The first studies of subradiance, especially in large ensembles, have not been performed until much more recently [7–9] These effects have been experimentally observed in a large number of physical systems, including neutral atoms, ions, molecules, nitrogen-vacancy centers in diamond, and superconducting Josephson junctions [10–23]. The excited-state fraction in our experiments is about 0.3 (i.e., there is no population inversion)
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