By playing two tiny drums, physicists have provided the most direct demonstration yet that quantum entanglement — a bizarre effect normally associated with subatomic particles — works for larger objects.
The findings, described in two Science papers on 6 May1,2, could help researchers to build measuring devices of unprecedented sensitivity, as well as quantum computers that can perform certain calculations beyond the reach of any ordinary computer.
The counter-intuitive rules of quantum mechanics predict that two objects can share a common, ‘entangled’ state. Measurable properties of one object, such as its position or velocity, are then correlated to those of the other, with a degree of correlation that is stronger than what can be achieved in classical, or non-quantum, physics.
Although nothing in the laws of quantum physics limits such quantum weirdness to subatomic particles, the theory predicts that at much larger scales — say, the size of a cat — quantum effects should be so vanishingly small as to be unobservable in practice. Physicists have long debated whether this is just a limitation of our senses and instruments, or whether macroscopic objects are governed by their own set of laws that is fundamentally different from quantum mechanics. To explore this question, researchers have been pushing to observe quantum effects at ever larger scales. “One point of our research is, is there quantum in the classical world?” says Mika Sillanpää, a physicist at Aalto University in Finland.
In an experiment at the US National Institute of Standards and Technology in Boulder, Colorado, physicist Shlomi Kotler and his collaborators built a pair of vibrating aluminium membranes akin to two tiny drums, each around 10 micrometres long.
Although these structures are barely visible to the naked eye, they are enormous by quantum standards, consisting of around one trillion atoms each. When physicists discovered quantum mechanics a century ago, “people didn’t imagine you could do an experiment with something this big”, says Kotler, who is now at the Hebrew University in Jerusalem.
The team tickled the membranes with microwave photons to make them vibrate in sync, and in such a way that their motions were in a quantum-entangled state: at any given time, as the drums wobbled up and down, measuring their displacement from flat showed they were in the same exact position, and probing their velocities returned exactly opposite values. […]