| Fabry–Pérot interferometer | |
|---|---|
| Name | Fabry–Pérot interferometer |
| Caption | Schematic of a Fabry–Pérot cavity |
| Classification | Optical interferometer |
| Inventors | Charles Fabry; Alfred Perot |
| Introduced | 1899 |
| Used for | Spectroscopy, laser physics, precision metrology |
Fabry–Pérot interferometer
A Fabry–Pérot interferometer is an optical resonator formed by two parallel reflecting surfaces that produces interference fringes via multiple beam interference. In the context of Quantum physics, it is a foundational apparatus for studying coherence, cavity quantum electrodynamics, and frequency selectivity of quantized electromagnetic modes, and it underpins technologies from precision metrology to laser stabilization.
The Fabry–Pérot interferometer (FPI) is central to experimental quantum optics and cavity quantum electrodynamics (cavity QED) because it creates discrete mode spectra and enhances light–matter interactions through resonant field buildup between mirrors. FP cavities are employed in experiments with single-photon sources, atomic clocks and atomic physics ensembles where control of mode density and spectral linewidth informs measurements of fundamental constants and tests of quantum coherence. Institutions such as National Institute of Standards and Technology and laboratories like CERN and university groups at Massachusetts Institute of Technology and University of Oxford have exploited FPI technologies for precision frequency measurements and advances in quantum information science.
An FPI consists of two partially reflecting surfaces separated by a cavity length; incident light undergoes multiple reflections producing a series of transmitted and reflected beams whose superposition yields an interference pattern. The transmitted intensity shows sharp resonances when the round-trip optical phase equals an integer multiple of 2π, a condition linked to constructive interference and mode formation. Important practical parameters include mirror reflectivity, finesse, free spectral range, and cavity linewidth; these govern the energy storage and quality factor (Q) that determine interaction times relevant for phenomena like Rabi oscillations and Purcell enhancement in cavity QED. The device also connects to classical interferometry methods such as the Michelson interferometer in their shared reliance on coherent superposition.
Fabry–Pérot cavities are employed to engineer photonic density of states, enabling control over spontaneous emission rates (Purcell effect) and strong-coupling regimes with single atoms, ions or quantum dots. In quantum communication and quantum cryptography implementations, FP filters provide narrowband spectral selection for entangled photon pairs generated by spontaneous parametric down-conversion in nonlinear crystals like beta barium borate or in periodically poled lithium niobate. FP resonators are integral to laser linewidth narrowing and frequency stabilization schemes such as the Pound–Drever–Hall technique, which link to standards of time and frequency maintained by organizations like International Bureau of Weights and Measures and national metrology institutes. Coherence metrics—temporal coherence, spectral purity, and phase noise—are characterized using FP transmission profiles and related to quantum state fidelity in experiments at institutions including Harvard University and Caltech.
The transmission T(ν) of an ideal lossless Fabry–Pérot etalon is given by the Airy function, T = (1 - R)^2 / (1 + R^2 - 2R cos δ), where R is mirror intensity reflectivity and δ = (4πnL/λ) cos θ is the round-trip phase with refractive index n, cavity length L, wavelength λ and incidence angle θ. Resonances occur when δ = 2πm for integer m, defining longitudinal mode frequencies ν_m = m(c/2nL). The free spectral range Δν_FSR = c/2nL and finesse F ≈ π√R/(1−R) determine mode spacing and sharpness; the cavity linewidth Δν = Δν_FSR/F. These relations are essential for quantizing electromagnetic modes in a cavity and deriving Hamiltonians used in cavity QED models such as the Jaynes–Cummings model describing atom–field coupling. Loss channels, mirror dispersion and intracavity media introduce corrections treated via input–output theory and dissipation terms in master equations.
FP devices appear in multiple geometries: plane-parallel etalons, confocal and hemispherical resonators, and fiber-based microcavities. High-finesse implementations use dielectric multilayer coatings produced by companies and research facilities specializing in thin films; mirror substrates and vibration isolation are engineered at precision labs like National Physical Laboratory (United Kingdom) and NIST. FP interferometry underlies high-resolution spectroscopy of atomic and molecular transitions, Doppler lidar, and astronomical spectrographs where stable etalons serve as calibration references (e.g., in exoplanet searches with instruments at observatories such as European Southern Observatory). In metrology, FP cavities are central to optical frequency comb stabilization and measurements of the speed of light, linking to international standards and precision tests of physical laws.
The interferometer is named for Charles Fabry and Alfred Perot, who introduced the etalon in 1899 to study spectral lines. Subsequent foundational contributions include theoretical and experimental advances by researchers such as Henri Fabre (application improvements), and later developments in cavity theory and quantum optics by figures like Roy J. Glauber and H. J. Kimble, who extended the apparatus' role into quantum field and cavity QED studies. The integration of FP cavities with lasers following the invention of the laser in 1960 expanded their technological impact through work at institutions such as Bell Labs and universities worldwide. Ongoing refinements continue in collaborations among national metrology institutes, university groups and aerospace and optical industry partners to preserve and extend their utility in science and national infrastructure.
Category:Interferometers Category:Quantum optics Category:Optical cavities