Abstract
It is a straightforward result of electromagnetism that dipole oscillators radiate more strongly when they are synchronized, and that if there are $N$ dipoles, the overall emitted intensity scales with $N^2$. In atomic physics, such an enhanced radiative property appears when coherence among two-level identical atoms is established, and is well-known as "superradiance" \cite{Dicke:1954aa}. In superfluorescence (SF), atomic coherence develops via a self-organisation process stemming from the common radiated field, starting from a incoherently prepared population inversion \cite{Bonifacio:1975aa}. First demonstrated in a gas \cite{Skribanowitz:1973} and later in condensed matter systems \cite{Florian:1984}, its potential is currently being investigated in the fields of ultranarrow linewidth laser development for fundamental tests in physics \cite{Meiser:2009,Meiser:2010,Bohnet:2012aa,Norcia:2016, Norcia:2018}, and for the development of devices enabling entangled multi-photon quantum light sources \cite{Raino:2018aa,Angerer:2018aa}. A barely developed aspect in superradiance is related to the properties of the dipole array that generates the pulsed radiation field. In this work we establish the experimental conditions for formation of a macroscopic dipole via superfluorescence, involving the remarkable number of $4\times10^{12}$ atoms. Even though rapidly evolving in time, it represents a flexible test-bed in quantum optics. Self-driven atom dynamics, without the mediation of cavity QED nor quantum dots or quantum well structures, is observed in a cryogenically-cooled rare-earth doped material. We present clear evidence of a decay rate that is enhanced by more than 1-million times compared to that of independently emitting atoms. We thoroughly resolve the dynamics by directly measuring the intensity of the emitted radiation as a function of time.
Highlights
The paradigm effect for collective behavior in quantum optics is superradiance (SR), extensively studied both theoretically and experimentally starting from its prediction by Dicke in 1954 [1]
We have reported unambiguous evidence of superfluorescence achieved by incoherently seeding a cw population inversion on the 1.54-μm transition in erbium-doped yttrium orthosilicate (Er):YSO
The observed cleanest sech-squared light pulses differ from the complex superradiance dynamics observed by several groups, which are affected by ringing effects, asymmetry with long tails and multiple structures, showing that coherent ringing is not an intrinsic feature of SF in solid-state extended media, as has generally come to be believed [30,38]
Summary
The paradigm effect for collective behavior in quantum optics is superradiance (SR), extensively studied both theoretically and experimentally starting from its prediction by Dicke in 1954 [1]. Our work pushes into a new regime, demonstrating spontaneous formation of a macroscopic dipole composed of the remarkable number of atoms N 4 × 1012 in erbium-doped yttrium orthosilicate (Er:YSO, Er3+:Y2SiO5) This optical material exhibits the narrowest homogeneous linewidths and the longest coherence lifetimes [17] and is widely investigated for spectral hole burning applications [18,19], cavity QED [20,21], and the reversible, coherent conversion of microwave photons into the optical telecom C band around 1.54 μm [22]. The lifetime of the macroscopic quantum state we demonstrate is ∼100 ns, exceeding previously reported values in solid state This property and its high atom number might determine a nonvanishing probability of photon superabsorption [25,26] if the level that superfluoresces is probed by an intense laser field. Possibility to investigate this process would pave the way to a new method of elusive particle detection, whereby a factor N enhancement of rare events interaction rates might be accomplished in recently proposed upconversion schemes [27,28]
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