Toward new tests of quantum mechanics at macroscopic scale

Quantum theory is our best available description of the behavior of light and matter at the atomic scale. For light, this means that a beam cannot be attenuated continuously to arbitrarily low level, eventually the “graininess” becomes evident and one measures individual photons.

One disturbing open question remains, and is recently regaining interest among experimental physicists: Why is our experienced world “classical”? Applying the laws of quantum mechanics to our macroscopic world leads to numerous paradoxes, such as the infamous live/dead superposition of Schrödinger’s cat. Is a quantum to classical transition occurring at an intermediate scale between atoms and cats? In other words, is it fundamentally impossible to prepare arbitrarily large and massive systems in a non-classical state? If not, would it be technically feasible using state-of-the-art experimental setups?

Now scientists at the Laboratory of Photonics and Quantum Measurements (LPQM1- EPFL) and UniGe answer the last question affirmatively in a work to appear in the April 11 issue of Physical Review Letters. The scheme is based on cavity optomechanics, a rapidly developing field of research in which a mechanical oscillator is coupled to the light field of a highly reflective cavity.

One specific system being considered to carry out the experiment, under development in Prof. Tobias Kippenberg’s group at EPFL, is shown in the opening picture above. The carefully engineered holes along the suspended silicon beam confine both light and mechanical vibrations in the central region, as depicted in the inset showing numerical simulations of mechanical (upper panel) and optical (lower panel) modes. The structure therefore behaves as a tiny drum, and photons are used to both excite and sense the motion of the drum. Classically, one would expect a continuous tuning of the “loudness” of the drum to be possible. But, under special conditions, only discrete values are observed, as predicted by quantum mechanics. The article precisely details how to observe this graininess of vibrational energy – in an object of unprecedented large scale.

As lead author Christophe Galland emphasizes: “In contrast to previous proposals that required strong coupling – which is still out of experimental reach – we prove that it is possible to prepare a carefully constructed mechanical oscillator with one single packet of vibrational energy, even for weak optomechanical coupling”. Technically, this is achieved by optically exciting the vibration with a laser and detecting, one by one, the photons that have transferred precisely one quantum of energy to the oscillator. The non-classical, “grainy” nature of the vibration can then be verified by transferring the mechanical energy back to optical photons. Using a standard setup, one can check that there is never more than a single photon emitted at a time. This proves that a single packet of energy indeed existed in the oscillator.

In addition to the exciting prospects of gaining new fundamental knowledge about the laws of nature, the scheme has technological implications. For example, it suggests promising applications of optomechanical systems as on-demand single-photon sources and quantum memories needed to build quantum repeaters. As co-author Nicolas Sangouard has shown previously, quantum repeaters can efficiently extend the range of quantum key distribution, a proven and absolutely secure way to transmit secret information. Switzerland has been a pioneer in the commercialization of such systems through the creation by co-author Prof. Nicolas Gisin of the company IdQuantique. The new results once more emphasize how tightly fundamental and applied research are now entangled.

Reference
Christophe Galland, Nicolas Sangouard, Nicolas Piro, Nicolas Gisin, Tobias J. Kippenberg; Phys. Rev. Lett.. Vol. 112, 143602 (2014).