Light to entangle mirrors

Bouncing laser beams could bring quantum strangeness to the everyday world.

The quantum world of atoms and subatomic particles is full of intuition-defying phenomena such as objects existing in two different states at once. We don’t normally have to worry about such weirdness impinging on our everyday macroscopic world. But Italian physicists have worked out how to invest something we can see and touch with quantum strangeness.

Stefano Mancini, of the University of Milan, and colleagues plan to entangle two mirrors1. The fates of entangled objects are intimately entwined by the rules of quantum mechanics. If the plan works, one mirror will not exist in one state without the other being in another well-defined state.

Mancini and colleagues’ scheme makes use of the fact that when photons of light hit a mirror, they impart some momentum to it. The pressure of this radiation can make the mirror move. An intense light beam, such as a laser, bouncing back and forth between two movable mirrors can set up a standing wave that makes them oscillate.

Thus, entanglements between photons in the light beam could be translated into entanglements between these oscillating mirrors, the researchers suggest, making the mirrors’ motions interdependent.

The team calculates that entanglement should persist even at temperatures of four degrees above absolute zero – warm for the quantum world, and easy to achieve. Entangling macroscopic objects such as mirrors may provide a way to detect extremely weak forces with high precision, says Mancini2. Such weak forces have been proposed, for example, that modify Newton’s law of gravity.

Next stop teleportation?

Physicists hope to use entangled states of quantum particles, such as photons, to process information in new ways. By encoding information into the different states of atoms and photons, they are devising secure encryption methods for data transmission, to teleport quantum states from one place to another, and to produce new, ultrafast computers.

But no matter what the writers of Star Trek would have us believe, effects such as teleportation are not generally possible at the macroscopic scale, because entanglements of more than a handful of particles are extremely fragile.

Interactions between the particles and their environment typically disrupt their delicate interdependencies. The disruption is more pronounced the warmer the system gets. Even temperatures of just a degree or so above absolute zero are usually sufficient to blur out entanglements in systems that contain many particles.


  1. Mancini, S., Giovannetti, V., Vitali, D. & Tombesi, P. Entangling macroscopic oscillators exploiting radiation pressure. Physical Review Letters, 88, 120401, (2002).
  2. Mancini, S. & Tombesi, P. High-sensitivity force measurement using entangled meters. Preprint (2001).

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