  We present the first deep polarimetric study of Galactic synchrotron emission
at low radio frequencies. Our study is based on 21 observations of the European
Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field using the
Low-Frequency Array (LOFAR) at frequencies from 114.9 to 177.4 MHz. These data
are a part of the LOFAR Two-metre Sky Survey Deep Fields Data Release 1. We
used very low-resolution ($4.3'$) Stokes QU data cubes of this release. We
applied rotation measure (RM) synthesis to decompose the distribution of
polarised structures in Faraday depth, and cross-correlation RM synthesis to
align different observations in Faraday depth. We stacked images of about 150
hours of the ELAIS-N1 observations to produce the deepest Faraday cube at low
radio frequencies to date, tailored to studies of Galactic synchrotron emission
and the intervening magneto-ionic interstellar medium. This Faraday cube covers
$\sim36~{\rm deg^{2}}$ of the sky and has a noise of $27~{\rm \mu
Jy~PSF^{-1}~RMSF^{-1}}$ in polarised intensity. This is an improvement in noise
by a factor of approximately the square root of the number of stacked data
cubes ($\sim\sqrt{20}$), as expected, compared to the one in a single data cube
based on five-to-eight-hour observations. We detect a faint component of
diffuse polarised emission in the stacked cube, which was not detected
previously. Additionally, we verify the reliability of the ionospheric Faraday
rotation corrections estimated from the satellite-based total electron content
measurements to be of $~\sim0.05~{\rm rad~m^{-2}}$. We also demonstrate that
diffuse polarised emission itself can be used to account for the relative
ionospheric Faraday rotation corrections with respect to a reference
observation.
