At what point does a mechanical system—governed by classical dynamics—become a quantum system, or vice-versa? In 1920, Neils Bohr argued that quantum systems became describable with classical mechanics when the correspondence (or classical) limit had been reached. He described this as occurring "when the quantum numbers describing the system are large." The exact interpretation of "large" has been left to the reader as a 90-year-old take-home exercise.

The broad implications of Bohr's correspondence principle is that (as Bohr himself argued) one cannot derive classical mechanics from quantum mechanics; classical systems in the every-day world will be governed by classical laws, and the very small quantum systems by quantum laws. There are examples of macroscopic systems that obey quantum mechanics—Bose-Einstein condensates for example—but they are inherently quantum systems. However, in March of this year, a research group at the University of California, Santa Barbara reported on the first ever classical system—a vibrating mechanical resonator—where the behavior could be described and manipulated through quantum mechanical means.

The canonical approach for coaxing a classical system into the quantum realm is to cool it down until it reaches its "ground state." However, even the definition of what happens at the ground state differs between classical and quantum mechanics. Classical dynamics says that all kinetic energy in the device will be zero and no motion will occur. Quantum mechanics, on the other hand, describes irremovable random uncertainties where the system will actually exist in some bizarre superposition of a state with no quanta of energy and one where it has a single quanta of energy.

As an example, a simple mechanical beam fixed at one end and vibrating at an audible frequency of around 1kHz at the other would need to be cooled to a temperature significantly less than 50nK to reach its ground state. The temperature at which such a system will reach its ground state will increase proportionally to the frequency at which it vibrates. A stiffer beam will have a higher frequency, resulting in a higher ground state temperature. But it will also have a much smaller amplitude, which will make detection more difficult.

In a paper published in Nature, the team described a device that used piezoelectric materials to create what they called a "microwave-frequency 'quantum drum.'" They constructed a beam-like device about 60 microns in length that consisted of a sandwich of aluminum nitride between two sheets of aluminum. The aluminum nitride would expand and contract depending on the presence of an electric field (the piezoelectric effect).

The size, shape, and material properties of the device give rise to an isolated mechanical vibration mode near 6GHz (hence the microwave-frequency part of the name). At this frequency, the macro scale classical device should enter its quantum ground state at a temperature below 0.1K.

In order to determine if this resonator truly reaches its quantum ground state, the team connected it to a Josephson phase qubit device. According to the authors, this coupling will allow "completely quantum-coherent measurement, preserving the quantum states in the coupled system." If the device were coupled to a classical measurement system, then there would be rapid decoherence of the quantum states—you'd never be able to measure its quantum behavior.

The entire device was cooled to around 25mK, a temperature where both the qubit and mechanical device should theoretically be in their quantum ground states. Once cooled, the team used the qubit to probe the nature of the mechanical oscillator, and found that the number of phonons (a vibrational quasiparticle) in the relevant mechanical mode was very small. There was a 93-plus percent chance that the resonator is in a quantum ground state.

By controlling the qubit, the researchers were able to show that they could impart a single quanta of energy—a single phonon—to the mechanical resonator and observe the exchange take place in real time. They were able to control this to such a degree that the mechanical device existed in a superposition of the ground state and single quantum state, having it actually move at two different amounts simultaneously. On the flip side, they also showed that they could use a classical excitation (microwaves) to generate a coherent response in the mechanical resonator, which excited the qubit in a manner that agreed very well with theory.

This set of experiments provided strong evidence for the first time that "quantum mechanics applies to a mechanical object large enough to be seen with the naked eye." While this tiny device won't be available in stores anytime soon, it opens the doors to a whole new range of experiments that can probe the limits of quantum mechanics and ask questions about the nature of reality itself.

A number of groups are currently working to build even larger mechanical systems that are governed by the laws of quantum mechanics, and still others are looking at ways to put this simple device to use in a whole range of applications. However, the fact that this was the first time that physics was able to build a classical machine that existed in a quantum state that got the editors at Science magazine to name it the breakthrough of the year for 2010.

Nature, 2010. DOI: 10.1038/nature08967 (About DOIs).

Listing image by Kevin Wolfe