One of the more irksome results of quantum mechanics is the revelation that reality is largely a persistent illusion. Quantum mechanics is not merely a theory of the microscopic: all matter is fundamentally quantum—it just so happens that weird quantum effects are hard to observe in anything bigger than a few atoms. Like the flickering silhouettes on the wall in Plato’s allegory of the cave, the existence of macroscopic, so-called “classical” objects is merely a shadow cast by their true quantum forms. This much is not news to physicists, who have been mucking around in the quantum world for more than a century and are mostly unbothered by the crumbling edifice of reality.
Two new papers published on Thursday in Science push the boundaries of the quantum effects physicists can achieve at a macroscopic scale. Both studies observed such effects in thin aluminum “drums” about the size of a red blood cell. In the first study, U.S. and Israeli researchers directly and reliably measured quantum entanglement between the drums. And the second study, led by a Finnish team, measured entangled drums while avoiding “back action,” the inevitable noise associated with the very act of trying to measure an object’s position and momentum.
In the classical world, there is no theoretical limit to the precision of such measurements. But the uncertainty principle, formulated by German physicist Werner Heisenberg in the 1920s, states that there is a fundamental limit to how well the position and momentum of an object such as a drum can be known. “The tricks described in these two papers are ways of evading what you might have thought is the limit on measuring forces coming from the Heisenberg uncertainty principle,” says Aashish Clerk, a condensed matter physicist at the University of Chicago, who was not involved with either study.
Both entanglement and back-action evasion have been previously observed in macroscopic systems but in different, and arguably more limited, ways. In 2018 another group of researchers entangled two strips of silicon. Other experiments have even entangled vibrations in diamonds. Yet the tricks demonstrated by both teams in the recent Science papers have allowed them to observe quantum effects with far fewer caveats.
“We’re not discovering anything new about quantum mechanics here,” says Yiwen Chu, a quantum researcher at the Swiss Federal Institute of Technology Zurich, who was not involved in either study. But getting these measurements still requires “very impressive technological advances,” she says.
This arcane area of research has a simple overarching goal: “get something big into a quantum state,” Clerk says. Applications range from quantum computers to problems in physics that require subatomic precision, such as the detection of dark matter or gravitational waves.
Some researchers, such as Mika Sillanpää, a physicist at Aalto University in Finland and a co-author of the second paper, wish to measure sensitive quantum effects but have been limited by the classical nature of their macroscopic measuring tools. By bringing quantum effects into the macroscopic realm—or, put another way, returning classical objects to their true quantum selves—Sillanpää hopes to investigate quantum gravity.
Advances in quantum technology are sometimes touted for their potential consumer benefit. The new developments, while exciting, are “not for mobile phones,” Sillanpää says dryly.
Drumming Up Entanglement
More analogies have been conjured to explain quantum entanglement than nearly any other phenomenon in physics. Shlomi Kotler, a physicist at the National Institute of Standards and Technology and a co-author of the first paper, offers a simple definition: objects are entangled when their positions or momenta are known more precisely than the initial uncertainty of those positions or momenta. Entanglement is simply a correlation between objects—whether they are electrons or micron-sized aluminum drums—that exceeds what is possible with just a classical relationship.
To achieve entanglement, the two teams crafted finely tuned aluminum drums, placed them on a crystal chip, supercooled the setup to near absolute zero and then hit both drums with a pulse of microwave radiation.
“These two drums don’t talk to each other at all, mechanically,” says John Teufel, a physicist at NIST and a co-author of the first paper. “The microwaves serve as the intermediary that lets them talk to each other. And the hard part is to make sure they talk to each other strongly without anybody else in the universe getting information about them.”
Struck by the microwaves, each drum vibrates, rising up and down by about the width of a proton. This minuscule motion is detectable as a change in the voltage of a circuit connected to the drums.
“Entangling the motion of two atoms is already a hard, heroic experiment,” Teufel says. In comparison, each drum has roughly one trillion atoms. Moreover, whereas single particles have discrete quantum states such as spin up or down, the drums can be in a continuous distribution of amplitudes, or distances of vibration, as they wobble.
But if the drums are sensitive enough to be entangled from the microwave pulse and relatively noise-free, their amplitudes will be strongly correlated. Measuring the amplitude of one drum tells you what the amplitude of the other is. For example, if one drum is measured to have a high amplitude, the other must have a low amplitude.
“You just need a really, really good signal-to-noise ratio for your measurements,” Clerk says. “This is maybe the first experiment on these sorts of systems that has achieved that.”
In fact, that ratio is so low that it is possible to see the effect of entanglement by simply plotting the spatial relationship between the positions of the two drums. There, in the thousands of data points, is an uncanny correlation—proof that the classical reality of two separate drums is a shadow of a deeper truth in which entanglement makes them a single quantum object.
Hiding from Heisenberg
Instead of hitting the drums repeatedly to entangle them multiple times, the second team created a long-lasting entanglement with a method that was more like a drum roll than a single stroke. By creating this stable state, the researchers were able to make many measurements of the same entanglement with the goal of “evading” the Heisenberg uncertainty principle.
That principle is often incorrectly described as stating that any measurement, no matter how small, must give an object a kick, introducing uncertainty. “The uncertainty principle says there are some things [for which] you’re not allowed to measure both perfectly,” Clerk says. “There are other things [for which] it’s totally happy for you to measure simultaneously and perfectly.
For instance, there is no limit to how precisely you can know an object’s position or momentum. The problem comes when you try to measure both at the same time. Back-action evasion is a way of getting around this limitation without actually violating Heisenberg’s diktat. Instead of measuring each individual drum’s position and momentum, Sillanpää and his colleagues essentially measured the combined sum of the drum’s momentum through its effect on the circuit voltage.
“Nothing is violating the Heisenberg uncertainty principle. You’ve just picked a particular set of questions where you’re not asking about things that are forbidden,” Chu says.
The possibilities of the precision demonstrated by these two experiments are intriguing. It is not a stretch to imagine that similar drums could someday be used to probe the minute effects of quantum gravity on a tabletop or employed as part of a relay in a quantum network.
But perhaps the most tantalizing aspect of the work, beyond any applications, is that it simply brings us closer to the true quantum nature of the world. “All you get to see on a daily basis are the shadows,” Kotler says. “But given the right techniques, you can see that entanglement is there, ready to be used for the next step.”