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Physical Review A

Quantized mode of a leaky cavity

S. M. Dutra, Gerard Nienhuis

Published Nov 6, 2000• 72 citations
Publisher

Abstract

We use Thomson's classical concept of mode of a leaky cavity to develop a quantum theory of cavity damping. This theory generalizes the conventional system-reservoir theory of high-$Q$ cavity damping to arbitrary Q. The small system now consists of damped oscillators corresponding to the natural modes of the leaky cavity rather than undamped oscillators associated with the normal modes of a fictitious perfect cavity. The formalism unifies semiclassical Fox-Li modes and the normal modes traditionally used for quantization. It also lays the foundations for a full quantum description of excess noise. The connection with Siegman's semiclassical work is straightforward. In a wider context, this theory constitutes a radical departure from present models of dissipation in quantum mechanics: unlike conventional models, system and reservoir operators no longer commute with each other. This noncommutability is an unavoidable consequence of having to use natural cavity modes rather than normal modes of a fictitious perfect cavity.

DOI
10.1103/physreva.62.063805

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Cambridge University Press eBooks•Sep 4, 1997•2 citations
We use Thomson's classical concept of mode of a leaky cavity to develop a quantum theory of cavity damping. This theory generalizes the conventional system-reservoir theory of high-QQQ cavity damping to arbitrary Q. The small system now consists of damped oscillators corresponding to the natural modes of the leaky cavity rather than undamped oscillators associated with the normal modes of a fictitious perfect cavity. The formalism unifies semiclassical Fox-Li modes and the normal modes traditionally used for quantization. It also lays the foundations for a full quantum description of excess noise. The connection with Siegman's semiclassical work is straightforward. In a wider context, this theory constitutes a radical departure from present models of dissipation in quantum mechanics: unlike conventional models, system and reservoir operators no longer commute with each other. This noncommutability is an unavoidable consequence of having to use natural cavity modes rather than normal modes of a fictitious perfect cavity.
Summary form only given. The conventional way to treat cavity dissipation in quantum optics is by applying the system-reservoir approach. The small system consists of the quantized discrete modes of a fictitious perfect cavity. The reservoir is the external field described by continuous annihilation and creation operators for the modes of free-space. This conventional approach has been used since the early days of the laser. The Hamiltonian that describes this approach became known as the Gardiner-Collett Hamiltonian. Despite being useful in describing several phenomena in cavities with a high Q factor, it is well known that this approach breaks down when the Q decreases. We have derived the Gardiner-Collett Hamiltonian from a rigorous modes of the universe expansion, as an approximation for high-Q, and obtained the coupling strength between the discrete cavity annihilation and creation operators (small system) and the continuous annihilation and creation operators of free-space (reservoir). We have found that the coupling strength depends on the frequency even when the reflectivity is frequency independent. The physical consequence of this frequency dependence is that for short times compared to one cavity round-trip time, the reservoir (free-space) is no longer Markovian. Our explicit expression for this frequency dependent coupling strength unlocks this fast time non-Markovian regime that had been previously inaccessible within the Gardiner-Collett Hamiltonian. Finally, we also show how to generalize the Gardiner-Collett Hamiltonian to arbitrary quality factors. This generalized Hamiltonian can be applied to develop a full quantum theory of excess noise in lasers.
The inclusion of damping in the quantized radiation field in pure and mixed states is accomplished in a natural way, without using traditional loss reservoirs, through the application of appropriate boundary conditions in a simple model of an optical cavity coupled to the outside world. By using collective operators (which are suitable weighted superpositions of the usual creation and annihilation operators for modes of the continuous spectrum, extended over each Fox–Li band) and convenient initial conditions, we are able to describe the radiation loss from an optical cavity to the outside world. The physical situations corresponding to initial conditions leading to the exponential damping are also investigated.
In an earlier work [P. J. Bardroff and S. Stenholm, Phys. Rev. A 60, 2529 (1999)], we have derived a fully quantum-mechanical description of excess noise in strongly damped lasers. This theory is used here to derive the corresponding quantum Langevin equations. Taking the semiclassical limit of these we are able to regain the starting point of Siegman's treatment of excess noise [Phys. Rev. A 39, 1253 (1989)]. Our results essentially constitute a quantum derivation of his theory and allow some generalizations.
In many problems in quantum optics, damping plays an important role. These include, for example, the decay of an atom in an excited state to a lower state and the decay of the radiation field inside a cavity with partially transparent mirrors. In general, damping of a system is described by its interaction with a reservoir with a large number of degrees of freedom. We are interested, however, in the evolution of the variables associated with the system only. This requires us to obtain the equations of motion for the system of interest only after tracing over the reservoir variables. There are several different approaches to deal with this problem. In this chapter, we present a theory of damping based on the density operator in which the reservoir variables are eliminated by using the reduced density operator for the system in the Schrödinger (or interaction) picture. We also present a ‘quantum jump’ approach to damping. In the next chapter, the damping of the system will be considered using the noise operator method in the Heisenberg picture. An insight into the damping mechanism is obtained by considering the decay of an atom in an excited state inside a cavity. The atom may be considered as a single system coupled to the radiation field inside the cavity. Even in the absence of photons in the cavity, there are quantum fluctuations associated with the vacuum state. As discussed in Chapter 1, the field may be visualized as a large number of harmonic oscillators, one for each mode of the cavity.
The quantum theory of the damped harmonic oscillator has been a subject of continual investigation since the 1930s. The obstacle to quantization created by the dissipation of energy is usually dealt with by including a discrete set of additional harmonic oscillators as a reservoir. But a discrete reservoir cannot directly yield dynamics such as Ohmic damping (proportional to velocity) of the oscillator of interest. By using a continuum of oscillators as a reservoir, we canonically quantize the harmonic oscillator with Ohmic damping and also with general damping behaviour. The dynamics of a damped oscillator is determined by an arbitrary effective susceptibility that obeys the Kramers–Kronig relations. This approach offers an alternative description of nano-mechanical oscillators and opto-mechanical systems.
In the previous chapter, we developed the equation of motion for a system as it evolved under the influence of an unobserved (reservoir) system. We used the density matrix approach and worked in the interaction picture. In this chapter, we consider the same problem of the system-reservoir interaction using a quantum operator approach. We again eliminate the reservoir variables. The resulting equations for the system operators include, in addition to the damping terms, the noise operators which produce fluctuations. These equations have the form of classical Langevin equations, which describe, for example, the Brownian motion of a particle suspended in a liquid. The Heisenberg–Langevin approach discussed in this chapter is particularly suitable for the calculation of two-time correlation functions of the system operator as is, for example, required for the determination of the natural linewidth of a laser. We first consider the damping of the harmonic oscillator by an interaction with a reservoir consisting of many other simple harmonic oscillators. This system describes, for example, the damping of a single-mode field inside a cavity with lossy mirrors. The reservoir, in this case, consists of a large number of phonon-like modes in the mirrors. We also consider the decay of the field due to its interaction with an atomic reservoir. An interesting application of the theory of the system–reservoir interaction is the evolution of an atom inside a damped cavity. It is shown that the spontaneous transition rate of the atom can be substantially enhanced if it is placed in a resonant cavity.