Choose Image>Mode>Greyscale. Confirm 'Discard Color Information' and the three colour channels will be deleted and replaced by one grey channel.
Resulting pictures are usually flat but can be improved by increasing the contrast either with a Brightness/Contrast adjustment layer or using Level or Curves.
Choose Image>Mode>Lab Color. Click on the Channels Palette and you will see four channels, Lab, Lightness, a & b.
The latter two hold the colour info, so grab b and drop it into the bin. This automatically removes a as well. You are left with Alpha1 and Alpha2.
You can print from this image, but no further work which requires the use of Layers can be carried out without first converting to Greyscale (Image>Mode>Greyscale).
Choose Image>Adjust>Channel Mixer.
Tick the Monochrome box and try these settings: Red;40 Green;60 Blue;0
All can be changed, but should add up to approx 100.
*Lab uses three channels. 'L' is lightness, 'a' represents the position on the green - magenta axis and 'b' on the blue - yellow axis.
The thumbnails for 'a' & 'b' on the channels palette make little sense as they do not represent output values. Lighter values in the 'a' channel indicate more magenta and less green, and darker values in the 'b' channel show more blue and less yellow.