  The improvement of sensitivity to gravitational waves (GWs) at lower
frequencies is still challenging on account of displacement noise. One of the
solutions is the neutron displacement-noise-free interferometer (DFI). We focus
on a simplification of the detector configuration by taking advantage of the
ability to adjust the neutron speed depending on the configuration. The new
configuration consists of two beamsplitters and two mirrors, which constitute a
single Mach-Zehnder interferometer (MZI). It is simpler than the configuration
with two MZIs in previous research. All displacement noise of mirrors and
beamsplitters can be canceled in the frequency domain. This cancellation can be
explained intuitively using a phasor diagram.
