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.
Two other laboratories had done similar measurements on macroscopic vibrating objects in the past, showing indirect evidence of entangled states3,4. But Kotler and his team were able to ‘see’ the entanglement more directly by amplifying the signal at the moment it came out of their devices. Kotler says this is similar to how old record players pre-amplified their signal before sending it to the amplifier, helping to reduce hiss. The team also improved upon earlier techniques, allowing the researchers to create entanglement more reliably.
Such steps will be crucial for applications such as quantum computers that could encode information in the vibrations of an array of membranes, Kotler says — a radical alternative to current popular approaches, which typically involve electrical currents or atomic systems. Amazon recently announced that it was investigating the possibility of using vibrating crystals to encode and process quantum information.
In a separate experiment with quantum drums, a group led by Sillanpää probed the limits of the Heisenberg uncertainty principle, which states that any measurement must necessarily change the state of the object which is being measured.
The team also built a pair of tiny aluminium drums, and used microwave-frequency photons both to put it in a synchronized vibrating pattern and to read out the drums’ positions.
This experiment had a different purpose to the one carried out by Kotler’s team — the researchers wanted to probe the boundary between quantum and non-quantum behaviour. They tuned the oscillating drums to move in a coordinated but not identical way, so that some of their measurable properties were identical to those of a single, virtual, oscillating drum.
In this way, the researchers were able to measure the position of the virtual drum without affecting its velocity. For a normal quantum oscillator, that would be impossible because of the Heisenberg uncertainty principle. To circumvent that fundamental limit, the researchers “use quantum mechanics to hack quantum mechanics”, says Hoi-Kwan Lau, a theoretical physicist at Simon Fraser University in Burnaby, Canada.
As in Kotler’s experiment, the two drums shared an entangled state, and the measurement technique opens the possibility of studying how entanglement of large objects spontaneously evolves. “We can measure the entangled states continuously without destroying them,” says Laure Mercier de Lépinay, a colleague of Sillanpää at Aalto and a co-author of the paper.
The quantum-drum techniques could lead to the development of instrumentation that beats the limitations that quantum mechanics imposes on measurement. “One application would be for a force sensor,” says Lau. Depending on how such a device is engineered, it could measure different types of force, such as magnetic or gravitational, he says.