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What we have here is what we call an acoustic hologram - or what we have called an acoustic hologram because I think it's the first time it has been used, a traditional hologram it's made of light and you have flat surfaces. Nonetheless it creates a 3D light field. If you remember Star Wars you can see the robot R2D2 and it's broadcasting a 3D spaceship, even from a flat surface. So this is exactly the same; we have a flat surface and we are creating a 3D acoustic field that can surround the particle.