Strength lies in numbers and in teamwork: tens of thousands of artificial atoms tightly packed in a nanodiamond act cooperatively, enhancing the optical trapping forces beyond the expected classical bulk polarizability contribution.

Optical dipole forces: Working together

Conservative optical dipole forces – used to levitate and confine particles and atoms in three dimensions – arise when dipoles interact with a gradient of electric field intensity such as that in a standing wave of laser light. These forces can be well described either by classical electrodynamics or by quantum physics. In the classical picture, depending on the refractive index of the material, a particle is attracted to – or repelled from – regions with large electric field gradients. Similarly, in the quantum mechanical description, light that is red- or blue-detuned with respect to an atomic transition will push an isolated atom towards or away from regions with large electric field gradients. Macroscopically, the magnitude of the optical dipole force depends on the bulk polarizability, which measures how easily internal charges in the material are displaced by an electric field. Microscopically, in the quantum case, the relevant property is the polarizability of the atom.

Writing in Nature Physics, Mathieu Juan and co-workers show that, contingent on the presence of a sufficiently high density of artificial atoms inside a solid, quantum effects need to be taken into account to explain the magnitude of the macroscopic optical dipole force acting on the bulk.

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