The type of one-way mirror used in interrogation rooms is just based on differences in light levels. The mirror is half-silvered, so most of the light is reflected while some fraction is transmitted. If one room is a lot brighter than the other, the reflection in the bright room will drown out the light from the dark room, but in the dark room the reflection is drowned out by light from the bright room.
Consider liquid crystal displays. Sending a small current through the panel changes the polarization of light passing through it. In conjunction with another polarizing filter, this can be used to toggle a pane of glass between transparent and opaque. In welding masks, this effect is often connected to a small solar panel so that the visor automatically darkens in response to bright light. To show digits or pixels on an LCD, a transparent conducting film (usually indium tin oxide) lets us direct charge to different parts of the display.
A number of researchers have been investigating transparent solar cells. The rough concept is to absorb infrared and/or ultraviolet while letting visible light pass through. I'm not sure if anything is actually on the market yet but the principle is sound.
Thus we have prepared all the ingredients for our technology sandwich.
The first layer is a transparent solar cell, the second layer is half-silvered mirror. The third layer is a polarizing filter followed by a trapped layer of liquid crystal, and the final layer is another transparent solar cell. The solar cells, or Bread Layer of the sandwich, do not need to generate any real power. They are merely used as light detectors, producing a voltage proportional to the light level on that side of the panel.
A diode connects the electrodes of the solar cells, enabling charge to flow from one cell to the other, but not the other way around. When the top layer generates a voltage greater than the bottom layer, the diode allows them to equalize. In the opposite scenario, the voltage difference between the two cells can become significant. It is wired to the liquid crystal layer, so in this situation, the polarization of the material is altered, and the layer switches from opaque to transparent.
The overall effect is a conditional half-silvered mirror: if one side is much brighter than the other, the one-way mirror operates in the conventional way. If the light levels are similar, or unfavourable, the LCD panel shuts off and no light passes through. From the first side, the mirror will continue to look like a mirror, because the half-silvered surface is up against a dark background. From the other side, the panel will appear black.
One immediately obvious use for this invention could be the windows of a building. When it's sunny outside, the building looks mirrored, and the people in the building can see outside. When it's dark, and the electric lights are on indoors, no one can peer inside because the panels have become opaque. People on the inside cannot see out, but they would not have been able to see outside under those conditions anyway.
Other uses include any and all situations where you want a pane of glass to be a one-way mirror.