# Synopsis: Inside and out of lasers

Is the intracavity field of a laser necessarily a coherent state?

Coherent states are superposition states of the eigenstates of a simple harmonic oscillator, and in some sense they are the “most classical-like” quantum states of the electromagnetic field. For typical laser configurations consisting of a $100%$ reflecting mirror and a partially reflecting mirror, it is not difficult to show that if the laser is to emit a field in a coherent state, the internal cavity field also needs to be in a coherent state. In this picture, the role of the partially transmitting mirror is simply to attenuate the amplitude of the coherent state as it exits the laser cavity.

However, as described in a paper appearing in Physical Review A, David Pegg of Griffith University in Brisbane, Australia, shows that this description, while useful, can mask rich and interesting physics. Pegg assumes that the intracavity field is described by a large mean photon number and a narrow photon number distribution. Since the uncertainty principle limits how well the precise number of photons in the field and their respective phases can be known, the phase of the field is poorly defined. However, Pegg demonstrates that the role of the transmitting mirror is to entangle the output field with the input field, thereby creating a well-defined phase difference between them. The end result is that the partially transmitting optic can create phase-coherent laser light, regardless of the state of the intracavity field.

The work sheds new light on the role of the output coupler in a laser cavity and may be applicable to a wide range of quantum optics experiments that involve optical cavity fields and their intrinsic phase coherence. – Frank Narducci

Correction (15 June 2009): Paragraph 1, sentence 1, “Coherent states are eigenstates of a simple harmonic oscillator…” changed to “Coherent states are superposition states of the eigenstates of a simple harmonic oscillator…”

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