The discovery that truth is essentially a permanent phenomenon is one of quantum mechanics’ most annoying consequences. Quantum mechanics is not just a microscopic theory: all matter is essentially quantum—just it’s that strange quantum phenomenon is difficult to find in something more significant than a few atoms. The life of macroscopic, so-called “classical” objects is merely a shadow thrown by their accurate quantum shapes, just like the flickering silhouettes on the cave wall in Plato’s allegory. This isn’t surprising to physicists, who have been tinkering with quantum mechanics for more than a century and are largely unconcerned about reality’s collapsing edifice.
Two recent articles presented in Science on Thursday push the limits of what physicists can do at a macroscopic scale with quantum effects. Such results were found in thin aluminum “drums” about a red blood cell size in both experiments. In the first study, researchers from the United States and Israel tested quantum entanglement between the drums in a precise and reliable manner. The second research, led by a Finnish team, measured entangled drums while ignoring “back action,” the noise that comes with attempting to measure the direction and momentum of an object.
There is no physical limit to the accuracy of such measurements in the classical world. However, the uncertainty theorem, coined by German physicist Werner Heisenberg in the 1920s, states a theoretical limit on how well an object’s direction and momentum can be determined. “The tricks outlined in these two articles are ways of getting around what you would think is the Heisenberg uncertainty principle’s constraint on measuring forces,” says Aashish Clerk, a condensed matter physicist at University of Chicago who was not interested in either research.
Both entanglement and back-action avoidance have been used in macroscopic processes since, albeit in separate and arguably more constrained forms. Another team of scientists intertwined two silicon strips in 2018. Diamond signals have also been intertwined in other studies. Nonetheless, the techniques used by both teams in their recent Science papers allowed them to detect quantum effects with far fewer caveats.
“We aren’t learning something new about quantum mechanics here,” says Yiwen Chu, a quantum expert at the Federal Institute of Technology who was not interested in either study. However, she claims that having these figures also necessitates “fascinating technical advances.”
According to Clerk, this enigmatic field of study has a straightforward aim: “get something large into a quantum state.” Quantum computers to problems in physics that involve subatomic accuracy, such as the discovery of dark matter or gravitational waves, are only a few examples.
Some researchers, including Mika Sillanpää, a physicist at Aalto University in Finland and a co-author of the second paper, want to quantify acute quantum effects but are constrained by their macroscopic measuring tools’ classical existence. Silanpää aims to explore quantum gravity by taking quantum phenomena into the macroscopic realm or putting it another way, restoring classical particles to their own quantum selves.
Quantum technological advancements are often lauded for their future market benefits. Sillanpää says dryly that the latest inventions, while thrilling, are “not for cell phones.”
GETTING RID OF ENTANGLEMENT
Quantum entanglement has been the subject of more analogies than almost any other physics theory. The first paper’s co-author, Shlomi Kotler, a physicist at the National Institute of Standards and Technology, gives a basic definition: phenomena are entangled when their positions or momenta are determined more accurately than the original ambiguity of such situations or momenta. Entanglement is essentially an interaction between objects—whether electrons or micron-sized aluminum drums—that goes beyond what is conceivable in a conventional relationship.
The two teams created precisely tuned aluminum drums, mounted them on a crystal chip, supercooled the rig to almost absolute zero, and then struck both drums with a blast of microwave radiation to achieve entanglement.
“Mechanically, these two drums don’t speak to each other at all,” says John Teufel, a physicist at NIST and one of the paper’s co-authors. “The microwaves function as a conduit, allowing them to communicate with one another. And the difficult part is ensuring that they communicate strongly with one another without anyone else in the universe learning about them.”
Each drum vibrates as a result of the microwaves, rising and falling by about the width of a proton. A variation in the voltage of a circuit attached to the drums detects this minuscule motion.
“It’s still a difficult, admirable experiment to entangle the motion of two atoms,” Teufel says. Each drum, by contrast, contains approximately one trillion atoms. Furthermore, while single particles have discrete quantum states such as spinning up or down, the wobbling drums may constantly distribute amplitudes or vibration distances.
The amplitudes of the drums would be closely correlated if they are sensitive enough to be entangled by the microwave pulse and comparatively noise-free. The amplitude of one drum will be used to determine the amplitude of the other. E.g., if one drum has a significant amplitude, the other must have a small amplitude.
“All you need for your calculations is a fine signal-to-noise ratio,” Clerk says. “It’s possible that this is the first work on these kinds of networks to do that.”
In truth, the ratio is so minimal that the effect of entanglement can be seen clearly by plotting the spatial relationship between the two drum positions. There is an eerie similarity among the thousands of data points, proving that the classical existence of two different drums is merely a shadow of a more profound truth in which entanglement unites them as a single quantum entity.
GETTING Further FROM HEISENBERG
Instead of constantly striking the drums to entangle them, the second team used a more like a drum roll technique than a single strike to produce a long-lasting entanglement. The researchers were able to make several calculations of the same web by establishing this stable state with the intention of “eluding” the Heisenberg uncertainty theorem.
This theory is sometimes misrepresented as meaning that every calculation, no matter how small, would cause an object to kick, thus adding confusion. “The complexity theory states that you are not able to calculate both perfectly for certain things,” Clerk explains. “There is specific stuff [about which] it’s entirely acceptable for you to calculate simultaneously and precisely.
For example, there isn’t a limit on how specifically you can determine the direction or momentum of an object. When you try to calculate them at the same time, you get into problems. Back-action avoidance is a way to get around this restriction without breaking Heisenberg’s rule. Rather than measuring the position and momentum of each individual drum, Sillanpää and his collaborators effectively calculated the amount of the drums’ momentum by their effect on the circuit voltage.
“The Heisenberg uncertainty theory is not violated in any way. You’ve only chosen a series of questions in which you don’t talk for topics that are forbidden,” Chu explains.
The accuracy shown by these two studies has exciting possibilities. Related drums could be used to probe the minute impact of quantum gravity on a tabletop or as one of a quantum network’s relays.
Still, despite all uses, perhaps the most enticing part of the work is that it actually gets one closer to the actual quantum essence of the universe. “On an everyday basis, all you see are shadows,” Kotler says. “However, with the right methods, you can see that entanglement is present and waiting to be included in the next step.”