One of the fundamental principles of quantum mechanics is that objects can be in two states at the same time. This means that an electron can, for instance, be in two places at once. However, these "superposition" states are never seen in classical, macroscopic objects – one example being Schrödinger's famous cat, who clearly could not be both dead and alive. Until now, such states have only been observed in atomic-scale objects and some larger molecules, such as a "buckyball", which is made up of 60 carbon atoms.

Scientists have long wanted to demonstrate superposition in larger objects but a significant challenge here is to eliminate all thermal vibrations in the object, which mask or destroy quantum effects. To achieve this, the object needs to be cooled down to its quantum ground state – the state where it vibrates as little as possible.

A quantum drum
Andrew Cleland and colleagues of the University of California, Santa Barbara, have now achieved this for a substantially larger object than in previously experiments – an object so large in fact that it can just about be seen with the naked eye. The object is a mechanical resonator made of aluminium and aluminium nitride, measuring about 40 µm in length and consisting of around a trillion atoms. It is a thin disc, which resonates at about six billion vibrations per second at room temperature.

In the experiment, Cleland's team reduce the amplitude of the vibrations in the resonator by cooling it down to below 0.1 K. The high frequency of the aluminium resonator was key to the experiment's success, because the temperature to which an object needs to be cooled in order to reach its ground state is proportional to its frequency. "A regular tuning fork, for example [with significantly lower frequency], would need to be cooled by another factor of a million to reach the same state," Cleland said.

Next, the team measured the quantum state of the resonator by connecting it electrically to a superconducting quantum bit or "qubit". The qubit acts, in fact, like a "quantum thermometer" that can identify just one quantum thermal excitation, or phonon. Once this has been done, the qubit can then be used to excite a single phonon in the resonator. This excitation can be transferred many times between the resonator and qubit.

Dead and alive, at once
In this way the researchers created a superposition state of the resonator where they simultaneously had an excitation in the resonator and no excitation in the resonator, such that when they measured it, the resonator has to "choose" which state it is in. "This is analogous to Schrödinger's cat being dead and alive at the same time," says Cleland.

"Unlike other measuring instruments, [the qubit] allowed us to measure the mechanical resonator while preserving all quantum effects," explained Cleland. "Most measuring instruments disturb the mechanical object by heating it up, and so destroy the very quantum effects being sought."

The experiments could have important implications for new quantum technologies, like quantum information processing, and for investigating the boundaries between the quantum and classical worlds – one of the least understood areas in physics.

"Another long-term prospect is testing the foundations of quantum physics," writes Markus Aspelmeyer of the University of Vienna in a commentary in Nature. "For example, superposition states of massive objects may be used to test possible deviations from quantum mechanics, which have been suggested to eliminate the Schrödinger's cat paradox."

This research is published in Nature.