Conversion of conventional gravitational-wave interferometers into quantum nondemolition interferometers by modifying their input and/or output optics

The LIGO-II gravitational-wave interferometers (ca. 2006--2008) are designed to have sensitivities near the standard quantum limit (SQL) in the vicinity of 100 Hz. This paper describes and analyzes possible designs for subsequent LIGO-III interferometers that can beat the SQL. These designs are identical to a conventional broad band interferometer (without signal recycling), except for new input and/or output optics. Three designs are analyzed: (i) a squeezed-input interferometer (conceived by Unruh based on earlier work of Caves) in which squeezed vacuum with frequency-dependent (FD) squeeze angle is injected into the interferometer's dark port; (ii) a variational-output interferometer (conceived in a different form by Vyatchanin, Matsko and Zubova), in which homodyne detection with FD homodyne phase is performed on the output light; and (iii) a squeezed-variational interferometer with squeezed input and FD-homodyne output. It is shown that the FD squeezed-input light can be produced by sending ordinary squeezed light through two successive Fabry-P\'erot filter cavities before injection into the interferometer, and FD-homodyne detection can be achieved by sending the output light through two filter cavities before ordinary homodyne detection. With anticipated technology (power squeeze factor ${e}^{\ensuremath{-}2R}=0.1$ for input squeezed vacuum and net fractional loss of signal power in arm cavities and output optical train ${\ensuremath{\epsilon}}_{*}=0.01)$ and using an input laser power ${I}_{o}$ in units of that required to reach the SQL (the planned LIGO-II power, ${I}_{\mathrm{SQL}}),$ the three types of interferometer could beat the amplitude SQL at 100 Hz by the following amounts $\ensuremath{\mu}\ensuremath{\equiv}\sqrt{{S}_{h}}/\sqrt{{S}_{h}^{\mathrm{SQL}}}$ and with the following corresponding increase $\mathcal{V}=1/{\ensuremath{\mu}}^{3}$ in the volume of the universe that can be searched for a given noncosmological source: $\mathrm{Squeezed}\mathrm{input}---\ensuremath{\mu}\ensuremath{\simeq}\sqrt{{e}^{\ensuremath{-}2R}}\ensuremath{\simeq}0.3$ and $\mathcal{V}\ensuremath{\simeq}{1/0.3}^{3}\ensuremath{\simeq}30$ using ${I}_{o}{/I}_{\mathrm{SQL}}=1.$ $Variational\ensuremath{-}output---\ensuremath{\mu}\ensuremath{\simeq}{\ensuremath{\epsilon}}_{*}^{1/4}\ensuremath{\simeq}0.3$ and $\mathcal{V}\ensuremath{\simeq}30$ but only if the optics can handle a ten times larger power: ${I}_{o}{/I}_{\mathrm{SQL}}\ensuremath{\simeq}1/\sqrt{{\ensuremath{\epsilon}}_{*}}=10.$ $\mathrm{Squeezed}\mathrm{varational}---\ensuremath{\mu}{=1.3(e}^{\ensuremath{-}2R}{\ensuremath{\epsilon}}_{*}{)}^{1/4}\ensuremath{\simeq}0.24$ and $\mathcal{V}\ensuremath{\simeq}80$ using ${I}_{o}{/I}_{\mathrm{SQL}}=1;$ and $\ensuremath{\mu}\ensuremath{\simeq}{(e}^{\ensuremath{-}2R}{\ensuremath{\epsilon}}_{*}{)}^{1/4}\ensuremath{\simeq}0.18$ and $\mathcal{V}\ensuremath{\simeq}180$ using ${I}_{o}{/I}_{\mathrm{SQL}}=\sqrt{{e}^{\ensuremath{-}2R}/{\ensuremath{\epsilon}}_{*}}\ensuremath{\simeq}3.2.$