An Invisibility Cloak With An On-Off Switch: What makes this possible is a process known as electromagnetically induced transparency--a phenomenon in which certain materials become transparent when zapped by light from two carefully tuned lasers...
If the frequencies of the lasers are close together, they can be tuned in a way that makes them interfere destructively. And when this happens, their ability to excite electrons cancels out.
When this happens, the laser photons suddenly pass through the material unimpeded, sometimes at dramatically reduced at speeds (which is how experiments that stop light are performed)...
Their trick is to use atoms that can exist in five electronic states rather than three. This allows additional control over the refractive index called magneto-electric cross-coupling.
The bottom line is that this allows an external magnetic field to modulate the change in refractive index.
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