Einstein’s ‘spooky action at a distance’ spotted in objects almost big enough to see
One of the strangest aspects of quantum physics is entanglement: If you observe a particle in one place, another particle—even one light-years away—will instantly change its properties, as if the two are connected by a mysterious communication channel. Scientists have observed this phenomenon in tiny objects such as atoms and electrons. But in two new studies, researchers report seeing entanglement in devices nearly visible to the naked eye.
“There really is an interesting open question, which is: ‘How far can you go up in scale?’” says Andrew Armour, a physicist at the University of Nottingham in the United Kingdom who wasn’t involved in the work. The advance could also pave the way for ultrasensitive measurements of gravity and a hack-proof quantum internet.
Albert Einstein colorfully dismissed quantum entanglement—the ability of separated objects to share a condition or state—as “spooky action at a distance.” Over the past few decades, however, physicists have demonstrated the reality of spooky action over ever greater distances—even from Earth to a satellite in space. But the entangled particles have typically been tiny, which makes it easier to shield their delicate quantum states from the noisy world.
Two research groups have now scaled up entanglement to engineered objects barely visible to the naked eye. Simon Gröblacher, a physicist at Delft University of Technology in the Netherlands, and his colleagues etched beams about 10 micrometers long into silicon chips. The beams, roughly the size of a bacterium, could oscillate up and down like a plucked guitar string. The researchers connected the chips with an optical fiber and cooled the whole setup close to absolute zero to damp out vibrations. Then, using cleverly controlled laser pulses, the team added just enough energy to get one beam vibrating a bit more strongly than the other. By measuring light coming out of the apparatus, the researchers verified that the energy boost occurred but did not learn which beam got the energy, meaning that the added energy was shared by both beams—the hallmark of quantum entanglement. The delicate entangled state lasted just a fraction of a second, the group reports today in Nature.
Mika Sillanpää, a physicist at Aalto University in Finland, and his colleagues took a different approach, manufacturing pairs of aluminum drum heads, or vibrating disks, about the width of a human hair onto a silicon chip. After cooling the setup, the researchers used microwaves to nudge the drum heads into correlated motions—as one throbbed up and down, the other did the opposite. A second set of microwave pulses probed the motions, and an analysis of the signals showed the drum heads shared a single quantum state, the team reports in a second Nature paper. “When we took the data, we had no idea if we were entangled or not,” Sillanpää says. “It turns out the answer was ‘yes.’” The entanglement can last indefinitely, he says—as long as the drum heads stay immersed in their microwave bath.
The two setups have different potential applications. Gröblacher designed his beams to vibrate at the same rate as light sent through fiber-optics, to make them compatible with existing telecommunications systems. The setup is “completely engineerable,” Gröblacher says. If he can get the entangled states to last longer and increase the distance between chips, he envisions such devices serving as nodes in an eventual quantum internet that could transmit ultrasecure information between future quantum-enabled computers.
Sillanpää says his drumheads may be better suited to precision measurement. Because quantum sensors are so sensitive, they excel at picking up extremely weak signals such as gravitational waves, the space-time ripples that were recently detected for the first time. As the devices get larger, they could also test theories of gravity that extend Einstein’s general theory of relativity into the quantum realm, connecting two areas of physics that have remained stubbornly separate.
Both experiments have pros and cons, says John Teufel, a physicist at the National Institute of Standards and Technology in Boulder, Colorado. The entanglement of Gröblacher’s beams was short-lived, but it was detected with certainty. Sillanpää’s entanglement was longer-lasting, but his team needed a complicated chain of theoretical reasoning to infer that the drum heads’ motions were truly entangled. “Ideally what you’d want … is a little bit of both,” Teufel says. Regardless, he says, the results are “very exciting first steps.”