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IEEE Journal of Selected Topics in Quantum Electronics

Are Nonclassical Light Sources Lasers?

Weng W. Chow

Published Apr 26, 2013• 34 citations
Publisher

Abstract

The spontaneous emission contribution to laser noise is explained and the reason for circumventing the resulting fundamental limit is discussed. A question arises concerning the classification of nonclassical light sources that result from combining cavity quantum electrodynamics and semiconductor-laser technology to control randomness in spontaneous emission.

DOI
10.1109/jstqe.2013.2257993

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The spontaneous emission contribution to laser noise is explained and the reason for circumventing the resulting fundamental limit is discussed. A question arises concerning the classification of nonclassical light sources that result from combining cavity quantum electrodynamics and semiconductor-laser technology to control randomness in spontaneous emission.
Among the novel sources of nonclassical light that have recently been studied are a number of novel lasers. Conventional laser theory describes a classical stochastic field. I show that with a change in the standard operating conditions the conventional laser model produces nonclassical fields. Deviations from the classical description in standard laser theory are small by some inverse power of the threshold photon number. Operating conditions are designed to make stimulated emission dominant; thus laser cavities are good cavities that store photons and give a high threshold photon number. Low threshold photon numbers are achieved in bad cavities and most readily in a single-atom bad-cavity system. Such a system trades phase coherence for reduced intensity noise.
As discussed in the last three chapters, the fundamental source of noise in a laser is spontaneous emission. A simple pictorial model for the origin of the laser linewidth envisions it as being due to the random phase diffusion process arising from the addition of spontaneously emitted photons with random phases to the laser field. In this chapter we show that the quantum noise leading to the laser linewidth can be suppressed below the standard, i.e., Schawlow–Townes limit by preparing the atomic systems in a coherent superposition of states as in the Hanle effect and quantum beat experiments discussed in Chapter 7. In such coherently prepared atoms the spontaneous emission is said to be correlated. Lasers operating via such a phase coherent atomic ensemble are known as correlated spontaneous emission lasers (CEL). An interesting aspect of the CEL is that it is possible to eliminate the spontaneous emission quantum noise in the relative linewidths by correlating the two spontaneous emission noise events. A number of schemes exist in which quantum noise quenching below the standard limit can be achieved. In two-mode schemes a correlation between the spontaneous emisson events in two different modes of the radiation field is established via atomic coherence so that the relative phase between them does not diffuse or fluctuate. In a Hanle laser and a quantum beat laser this is achieved by pumping the atoms coherently such that every spontaneously emitting atom contributes equally to the two modes of the radiation, leading to a reduction and even vanishing of the noise in the phase difference.
The author reviews methods for generating sub-Poissonian light and related concepts. This light has energy fluctuations reduced below the level which corresponds to a classical Poissonian process (shot-noise level). After an introduction to the concept of nonclassical light, an overview is given of the main methods of quantum-noise reduction. Sub-Poissonian processes are exemplified in different areas of optics, ranging from single-atom resonance fluorescence to nonlinear optics, laser physics, and cavity quantum electrodynamics. Emphasis is placed on the conceptual foundations, and on developments in laser theory that lead to the possibility, already demonstrated experimentally, of linewidth narrowing and sub-Poissonian light generation in lasers and masers. The sources of quantum noise in these devices are analyzed, and four noise-suppression methods are discussed in detail: regularization of the pumping, suppression of spontaneous-emission noise, nonadiabatic evolution of the atomic variables, and twin-beam generation.
The basic equations governing noise phenomena are derived from first principles and applied to examples in optical communications. Quantum noise arises from two sources, the momentum fluctuations of electrons at optical frequencies and the uncertainty-related fluctuations of the electromagnetic field. Shot noise results from the beating of the noise sources with the signal field. In high-gain amplifiers, the spontaneous-emission noise dominates shot noise and results in a noise figure of at least 3 dB. It is shown explicitly how, at high power, both the laser field and the laser noise source become classical. The increase in noise in lasers with open cavities is not due to enhanced spontaneous emission as once thought, but to single-pass amplification. The noise fields and spontaneous currents have Gaussian distributions, while nonlasing modes have exponential photon-number distributions. Low-frequency intensity fluctuations arise from the electric current driving the laser and can be sub-Poissonian, in contrast to shot noise, which has a Poissonian distribution. The calculational tools are a wave equation for the field operator and a rate equation for the carrier-number operator, each containing spontaneous current noise sources. The correlation functions of these sources are determined by the fluctuation-dissipation theorem. [S0034-6861(96)00503-X]
Squeezed states of light have been predicted to be generated by four-wave mixing in cavity configurations. Any noise fields in the cavity can degrade or modify these squeezed states. An experiment using a Na atomic beam as the nonlinear mixing medium shows that spontaneous emission from the excited atoms results in a broadband frequency spectrum of noise in the cavity used for squeezing. The limits this noise places on the observed amount of squeezing and possible techniques for avoiding this noise source are discussed.