How many principles does it take to change a light bulb…into a laser?

Abstract

Abstract. Quantum optics did not, and could not, flourish without the laser. The present paper is not about the principles of laser construction, still less a history of how the laser was invented. Rather, it addresses the question: what are the fundamental features that distinguish laser light from thermal light? The obvious answer, "laser light is coherent", is, I argue, so vague that it must be put aside at the start, albeit to revisit later. A more specific, quantum theoretic, version, "laser light is in a coherent state", is simply wrong in this context: both laser light and thermal light can equally well be described by coherent states, with amplitudes that vary stochastically in space. Instead, my answer to the titular question is that four principles are needed: high directionality, monochromaticity, high brightness, and stable intensity. Combining the first three of these principles suffices to show, in a quantitative way -involving, indeed, very large dimensionless quantities (up to ∼ 10 51 ) -that a laser must be constructed very differently from a light bulb. This quantitative analysis is quite simple, and is easily relatable to "coherence", yet is not to be found in any text-books on quantum optics to my knowledge. The fourth principle is the most subtle and, perhaps surprisingly, is the only one related to coherent states in the quantum optics sense: it implies that the description in terms of coherent states is the only simple description of a laser beam. Interestingly, this leads to the (not, as it turns out, entirely new) prediction that narrowly filtered laser beams are indistinguishable from similarly filtered thermal beams. I hope that other educators find this material useful; it may contain surprises even for researchers who have been in the field longer than I have.2

Determining random lasing action

“…Evidence of increased coherence when crossing the lasing threshold has been used as a proof of lasing 43 . However, coherence is a weaker property than Poissonian photon statistics to assess lasing, because it can be added to a thermal light source by spatial and spectral filtering, at the price of reduced brightness 44 . Moreover, coherence could be the result of processes other than lasing, such as superradiance or photon condensation 33 .…”

Experimental measurement of kinetic parameters using quantum plasmonic sensing

“…Typically in the Kretschmann configuration, as described above, the source of light is a laser. This source of classical light is well represented as a coherent state in the quantum formalism [29,30]. Thus, when comparing the different sources of light for measuring the kinetics we use the coherent state as the classical benchmark.…”

Concept of information laser: from quantum theory to behavioural dynamics

“…It is interesting to observe, as mentioned before, that the problem of the quantum origin of laser light remains still today an open question. Recently, in the context of photon correlation for quantum information, an interesting discussion and auto-critics has been led by Wiseman [37] then followed by a controversial reply by Elsäβer [38]. One important conclusion from these discussions is that there exists a crucial property of laser light: a laser must have stable intensity.…”

“…These expressions are called the Schawlow-Townes laws [40] for the laser linewidth showing the laser threshold phase transition behaviour. The spectral linewidth for a laser below and above threshold can be put in a very simple form as a function of the photon number n, the cavity characteristics ∆ω c and the second-order correlation function g (2) (0) (as discussed earlier, this quantity varies from the value 2 to the value 1 when passing the threshold [33,34,37,39]):…”