News Feature: Quantum effects enter the macroworld

New experiments are revealing the power of large-scale quantum devices.

Entanglement is the counterintuitive idea that particles can have an intrinsic connection—a connection that endures no matter the distance between them. The phenomenon remains one of the most bizarre and least understood consequences of quantum mechanics. Measure the quantum properties of one of a pair of entangled particles, and the other changes instantaneously.

Researchers have pioneered ways to demonstrate quantum entanglement in mechanical systems. This artist’s conception of one experiment depicts an interferometer’s light field, which “carries” the entangled state. Image credit: Kavli Institute of Nanoscience, Delft University of Technology/Moritz Forsch.

Such strange phenomena typically have been relegated to the subatomic. But recently, physicists have taken entanglement and other quantum effects to new extremes by observing them in large systems including clouds of atoms, quantum drums, wires, and etched silicon chips. Device by device, they are bringing the quantum world into new territory—the macroscopic world.

This work is driving new applications. Some experimental quantum computers use loops of superconducting wire as qubits, storing quantum information. Large quantum objects have already been used to help detect gravitational waves; they could appear in next-generation devices such as ultrasensitive sensors and encryption systems. These innovations, though, reach beyond cutting-edge tech. Building bigger and bigger quantum objects even raises the possibility of exploring some of the most pervasive unsolved mysteries at the intersection between quantum and classical worlds—and between quantum mechanics and gravity.

Two Worlds

Ever since Austrian physicist Erwin Schrödinger first described wave–particle duality 90 years ago (1), physicists have been probing the boundary between the observable, predictable macroscopic world and the one where probabilistic quantum rules dominate. In the quantum world, a particle exists as a wave representing the probabilities of its location. Once measured, however, the particle is found at a point in space.

Furthermore, a quantum particle can be in a superposition of two quantum states, with some probability of being in either one. An electron might be in a superposition of high and low energy levels, for example. When someone makes a measurement, this state collapses; the electron is observed to have one level or the other. According to quantum mechanics, the act of measurement changes the system.

So it goes with entangled pairs. Measure the properties of one, and the properties of the other, no matter how far away, are set. This all runs counter to the reliable Newtonian rules that govern our macroscopic world, in which an object can be found reliably in one place.

Quantum rules apply to single atoms and other denizens of the smallest orders of magnitude known to science, and tiny atoms make up everything, so it stands to reason that these effects should scale up. But how far?

“I think in general people think that there is no hard boundary where everything all of a sudden changes and quantum mechanics becomes incorrect,” says physicist Simon Gröblacher at Delft University, in The Netherlands. “But if you do run into a hard limit,” notes John Teufel at the National Institutes of Standards and Technology (NIST) in Boulder, CO, “that’d be one of the most exciting finds in the field.”

Some people have suggested such a limit. According to what are known as wave-collapse theories, undiscovered natural laws explain these quantum mysteries without invoking the idea that observation changes a system. English mathematical physicist Roger Penrose has hypothesized that wavefunction collapse is a consequence of the force of gravity, so systems above a certain mass should never show quantum behavior (2). The Ghirardi–Rimini–Weber theory, published in 1986, says that a particle’s wavefunction can simply collapse spontaneously (3). It would happen rarely to each individual particle, but in a large system made of billions or more entangled particles, the collapse of one would soon cause the collapse of all. “Since the collapse theories are so ad-hoc, one can only guess what the size is,” says Mika Sillanpää at Lahti University of Technology in Mikkeli, Finland. “It could be a milligram, or size of the Earth.”

Quantum Unbound

Physicists continue to debate at what size the microscopic world transitions into the macroscopic one and how to quantify that change. “It’s a thorny issue, and you’ll get a different answer from just about everyone you talk to,” says physicist Jonathan Friedman at Amherst College in Massachusetts.

Even so, experimental physicists are already demonstrating quantum effects in ever more complex domains. It’s not easy. Quantum effects are fleeting, delicate, and fragile, drowned out by even the slightest vibration or thermodynamic fluctuations. To observe them at all requires experimental setups that isolate the system from the heat and noise of the outer world.

For years, researchers have been able to successfully muffle that noise to observe individual subatomic particles and even large atoms in entangled states. Isolating quantum effects is less a matter of size than of complexity. Observing any system from individual atoms up to microscopic drums means quieting the noise of all the moving parts so that the quantum effects can come out to play. The noisiest variable is usually heat: “Temperature is a form of noise, and it will mask some of those effects and signals you’re looking for,” says Teufel.

He says that many recent demonstrations of large-scale quantum effects take their cues from previous methods used to freeze out the noise in individual atoms: “We’re exploiting the same exquisite techniques, not for single atoms but for quadrillions of atoms, to make things that are closer to engineered devices that we can observe and exploit.”

One approach uses a loop of superconducting wire, usually about a micrometer in diameter, interrupted by junctions of nonsuperconducting material. Superconductivity means electrons flow around the loop without resistance, and the current can be measured at those junctions.

Physicists can use magnetic fields to induce current to flow in both directions around the ring at the same time. That doesn’t mean half go one way and half go the other; all the electrons act as one and simultaneously stream clockwise and counterclockwise. In 2000, Friedman was part of a group that put a large amount of magnetic flux in superposition. “I think I still hold the world record, believe it or not,” he says.

More recently, physicists have used superconducting loops to make flux qubits, which store quantum information in the magnetic flux. When tuned in the right way, flux qubits can show quantum effects up to surprising levels. In 2016, an international group of physicists used flux qubits to rule out theories that would predict wave collapse at certain currents and timescales (4). In particular, quantum laws were present in their flux qubit at a current of 170 nanoamperes, for at least 10 nanoseconds. This kind of measurement disproves theories that require wave collapse to happen at these—or lower—scales.

Mechanical systems also make appealing quantum targets. At Delft, Gröblacher achieved entanglement at scales approaching the macro world using membranes 1 millimeter in diameter that can vibrate for minutes with a single nudge, which makes them appealing as a playground for testing mechanical entanglement. He envisions using such a system, in the future, to put a living organism into a quantum superposition. That experiment would require an extremely small creature, like a tardigrade. “It would show that even such complex systems can behave according to quantum physics,” he says.

More recently, Gröblacher’s group has been conducting experiments on a device made up of two silicon chips, cooled to near absolute zero. Each chip has a tiny channel etched into it only about 10 micrometers long. Those channels act as mechanical oscillators that can translate light into motion. They expand and contract at an almost perfectly matched frequency when struck by light. They also translate motion back into light: the oscillator produces a photon that’s emitted at the same point where it entered but going in the opposite direction.

After placing the chips 20 centimeters (8 inches) apart, the researchers sent laser pulses through a beam splitter, which sent light to the oscillators. That light made one of the two oscillators vibrate. It then produced a photon, which went through another beam splitter and on to a detector. The experiment was set up so that a single excitation was shared between the oscillators. “We know for sure that one is excited, but we can’t say in principle which one until we measure them,” says Gröblacher. The researchers can’t tell which oscillator was excited without destroying the entangled state (5).

Entanglement has been demonstrated before, but in much smaller systems—from individual atoms up to clouds of cold gases made of thousands of atoms. By contrast, these oscillators contain about 10¹⁰ atoms.

In follow-up results, published last November, physicists used the same setup to prove that the entanglement satisfied Bell’s theorem, named after Irish physicist John Stewart Bell (6). Roughly speaking, the theorem says that entangled quantum states are more highly correlated than anything possible with the classical laws of physics. The study was the first to satisfy Bell’s theorem for such a large system. “This is one way mechanical systems are pushing the boundary further,” says Gröblacher. “The idea is to test quantum mechanics on a large scale, to have many particles together in a quantum state.”

Gröblacher thinks this points to a way to use fiber optics and silicon chips to build a quantum network. This year, his group showed how this experimental setup could work with radiation in the band of frequencies used for telecommunication (7).

But there are hurdles. Mitigating heat is a major challenge when it comes to such demonstrations; Gröblacher’s system, for example, requires the temperature to be near absolute zero. Temperature is a measure of how fast individual atoms are moving around, and the more motion, the harder it is to observe quantum effects. Physicists have developed tools to reduce this, including laser cooling, in which laser beams trap atoms and trade high-energy electrons for lower-energy ones, and evaporative cooling, which siphons off the highest energy atoms, similar to how escaping steam cools a cup of tea.

By cooling this aluminum drum—which has a diameter of just 20 micrometers and a thickness of 100 nanometers—to near absolute zero, researchers reduced the noise from heat, allowing quantum effects to emerge. The technique might be useful in applications such as quantum sensors or quantum computers. Image credit: National Institute of Standards and Technology/John Teufel.

Beating a Quantum Drum

In April of last year, a group led by Sillanpää at Lahti University of Technology introduced another way to show entanglement with mechanical systems. His group used tiny drums: aluminum membranes made of about 10¹² atoms and about 15 micrometers in diameter that could vibrate up and down. These were cooled close to absolute zero. The physicists dampened the mechanical vibrations using microwaves so that only the quantum fluctuations remained (8).

By analyzing the microwaves and the vibrations, the physicists determined that the instantaneous positions of the drums were entangled. If one drum was measured at position 1, the other immediately assumed position 2. The entanglement persisted as long as the drums remained exposed to the microwaves.

The effect wasn’t obvious at first, says Sillanpää. “It wasn’t something we could see right away. We didn’t understand the data, so we got some good theorists on our team to explain things and sort out the explanations,” he says. After more than a year, he says, the analyses finally showed that the vibration couldn’t be measured without affecting the other.

Already, these large quantum systems have helped build confidence in the strangeness of quantum rules. They indicate that the divide between quantum and classical worlds is less a boundary, as in objective collapse theories, and more of a disguise. “People have tried to find some fundamental principles which would limit quantum mechanics,” says condensed matter physicist Alexey Bezryadin at the University of Illinois at Urbana-Champaign. “But so far, not a single experiment can find this fundamental limit.” Tests such as these bolster the idea that there is no limit for quantum coherence—or if there is, it’s beyond existing experiments.

The other thing these experiments can’t do is let the human eye observe directly what a system looks like when it’s in two quantum states at once. For example, no matter how big these mechanical systems get and no matter how strong the quantum effects the human eye will never actually see an object in two places at once.

“People have tried to find some fundamental principles which would limit quantum mechanics. But so far, not a single experiment can find this fundamental limit.”

—Alexey Bezryadin

“It’s the conundrum of quantum mechanics,” says Teufel. “Take the drum, for example.If you want to describe what it’s doing when you’re not looking at it, it really is in two places at once, up and down. It’s both here and there, and you’re not allowed to say it’s in one or the other. But when you go in and measure, it’s in one of those two places.” There's no clear limit, he says, on how big quantum systems could be built.

The Right Stuff

Sillanpää says his goal now is to work up to showing entanglement at the millimeter scale. A big challenge is finding the right materials.

“Although many things work easily on paper, things change when you bring them into the lab,” says Sillanpää. Until a material is identified, purified, probed, cooled, and shot with lasers, physicists don’t know if it has a structure that can be used in large quantum devices. At the University of Illinois, Bezryadin has been developing materials and techniques to create quantum devices such as superconducting nanowires (9).

He says that the most useful materials that probe quantum effects are ones that can preserve coherence—that is, those that sustain quantum effects for as long as possible. Even when cooled to within a sliver of absolute zero, some materials will still have too much noise, arising from the interaction of atoms or other subtle contamination, to allow the quantum behaviors to be harnessed.

Building large quantum devices hinges on an important balance. On one hand, atoms have to be insulated for the quantum effects to be useful. On the other, they have to be sensitive to commands and data. In a quantum computer, for example, a qubit has to “talk” to other qubits and at the same time interact with the outside world.

“If it’s completely inaccessible to its environment, it’s impossible to use it,” says Bezryadin. Finding materials that fit these criteria, he says, is the bottleneck of the field. But physicists are using computational tools and models to predict and develop materials that can overcome that hurdle, and Bezryadin is optimistic that, like bottlenecks in the past, physicists will squeeze through.

In addition to finding the right materials, physicists have to perfect their recipes for building devices and large systems that show quantum effects. At NIST, Teufel, for example, has spent years developing a way to coax quantum effects from an aluminum drum that, at least for a quantum system, is giant—it contains about 10¹⁵ atoms (10). His group has found ways to cool that system to lower and lower temperatures, approaching absolute zero. Their approach is to slowly draw out noisy fluctuations that might obscure quantum behavior. And because the drum is embedded in a circuit, it may be useful in applications such as computers that combine quantum and classical parts.

Harnessing Big Quantum

Quantum computers might be the application most likely to benefit first from exploiting quantum effects in the macroworld. Flux qubits are being entangled to create experimental quantum computers that can do what classical computers can't.

Then there are photon detectors, such as the superconducting nanowire created in 2017 by engineers at Duke University in Durham, NC. The nanowire shows a drop in current when a photon zooms by. Bezryadin has pioneered ways to engineer similar superconducting nanowires using carbon nanotubes. A future quantum communications network, meanwhile, will require repeaters, and these devices could use large-scale quantum effects to relay entangled states. Atomic clocks gain their precision from clouds of entangled particles, and the more particles that are entangled, the more stable the clock.

A tantalizing possibility is cracking open what might be the most important question in fundamental physics: how is quantum mechanics related to gravity, the least understood of all the forces in nature? Superconducting loops aren’t likely to help answer that question because current doesn't have enough heft for gravity to become significant. Larger mechanical devices, however, won’t be able to avoid the influence of nature’s least understood force. “If you put a massive object in two places at once,” says Teufel, “gravity has to play a role. And when you have gravity and quantum effects together, people get excited.”

Physicists around the world are proposing experiments using mechanical entanglement at the limit where gravity has an effect. Two recent efforts along those lines were published in 2017 (11, 12) when two independent teams of theorists—one from the University of Oxford and the other led by researchers at University College London, both in the United Kingdom—proposed table-top experiments that will use mechanically entangled systems to reveal whether gravity is a quantum phenomenon. The idea behind those experiments: if you take two objects that interact only through gravity—and otherwise aren’t connected—and can produce an entangled state, then gravity itself must be a quantum behavior.

University of Oxford physicist Vlatko Vedral, who developed one of the proposals, sees two main challenges: controlling ways the system might collapse into a classical state and differentiating between gravitational and electromagnetic effects. “I believe this is not an insurmountable problem,” says Vedral, noting that researchers at the University of Vienna are moving forward with experimental designs.

Theorist Sougato Bose at University College London, who worked on the other proposal, says he’s collaborating with other groups on building an interferometer that can gravitationally entangle two masses. Their plan, Bose says, is to start with nanoparticles and gradually increase the size, up to the order of hundreds of microns, or a fraction of a millimeter. He notes that groups in Montana and France are beginning similar experiments.

Even so, not everyone is convinced. Some physicists argue that these proposed experimental designs use too many assumptions to finally settle the issue of whether gravity is a quantum phenomenon.

The mysteries of quantum gravity notwithstanding, physicists are intrigued by the implications of studying entanglement in the observable, measurable, predictable world. That a mechanical system could be in two places at once, or that measuring one vibrating system had an observable effect on another, hints that the boundary between the classical and quantum worlds isn’t only a theoretical construct but also something to be observed and understood.