Quantum-mechanical noise in an interferometer

TLDR

Gravitational‑wave interferometers measure relative positions of distant masses, and their sensitivity is limited by two quantum‑mechanical noise sources—photon‑counting error and radiation‑pressure error—with low‑power lasers making photon‑counting error dominant. This study analyzes these two noise types and introduces a squeezed‑state technique that can trade off photon‑counting and radiation‑pressure errors to improve sensitivity. The technique requires injecting a non‑vacuum squeezed state into the interferometer’s unused port, whose unequal quadrature uncertainties can be produced by nonlinear optical processes such as degenerate parametric amplification.

Abstract

The interferometers now being developed to detect gravitational waves work by measuring the relative positions of widely separated masses. Two fundamental sources of quantum-mechanical noise determine the sensitivity of such an interferometer: (i) fluctuations in number of output photons (photon-counting error) and (ii) fluctuations in radiation pressure on the masses (radiation-pressure error). Because of the low power of available continuous-wave lasers, the sensitivity of currently planned interferometers will be limited by photon-counting error. This paper presents an analysis of the two types of quantum-mechanical noise, and it proposes a new technique---the "squeezed-state" technique---that allows one to decrease the photon-counting error while increasing the radiation-pressure error, or vice versa. The key requirement of the squeezed-state technique is that the state of the light entering the interferometer's normally unused input port must be not the vacuum, as in a standard interferometer, but rather a "squeezed state"---a state whose uncertainties in the two quadrature phases are unequal. Squeezed states can be generated by a variety of nonlinear optical processes, including degenerate parametric amplification.