Entangled quantum circuits further refute Einstein’s concept of local causality

A group of researchers led by Andreas Wallraff, professor of solid state physics at ETH Zurich, conducted a loophole-free Bell test to disprove the concept of “local causation” coined by Albert Einstein in response to quantum mechanics.

Researchers have provided further confirmation of quantum mechanics by showing that quantum mechanical objects far apart can correlate more strongly with each other than is possible in conventional systems. What makes this experiment unique is that researchers were able to perform it for the first time using superconducting circuits, which are considered promising candidates for building powerful quantum computers.

An old controversy

The Bell test is based on an experimental setup that was initially devised as a thought experiment by British physicist John Bell in the 1960s. Bell wanted to settle a question already debated by physicists in the 1930s: were the predictions of quantum mechanics, which ran counter to everyday intuition, correct, or were the traditional concepts also applicable in atomic microscopy, as Albert Einstein believed?

To answer this question, Bell proposed to perform a random measurement on two entangled particles at the same time and test it against Bell’s inequality. If Einstein’s concept of local causality is true, then these experiments always satisfy Bell’s inequality. In contrast, quantum mechanics predicts that they violate it.

Last doubts were dispelled

In the early 1970s, John Francis Klauser and Stuart Friedman, who received the Nobel Prize in Physics last year, performed the first experimental Bell test. In their experiments, the two researchers were able to prove that Bell’s inequality was indeed violated. But they had to make some assumptions in their experiments to be able to conduct them in the first place. So, in theory, Einstein may have been right to be skeptical of quantum mechanics.

However, with time, most of these loopholes can be closed. Finally, in 2015, various groups succeeded in conducting the first truly loophole-free Bell tests, thus finally settling the age-old controversy.

Promise applications

Wallraff’s group can now confirm these results with a novel experiment. The work of ETH researchers published in the nature Despite the initial confirmation seven years ago, research on the matter shows that it is inconclusive. There are several reasons for this.

For one thing, the ETH researchers’ experiment confirms that superconducting circuits operate according to the laws of quantum mechanics, even if they are much larger than microscopic quantum objects such as photons or ions. Electronic circuits several hundred micrometers in size made of superconducting materials and operating at microwave frequencies are called macroscopic quantum objects.

For another thing, Bell tests have practical significance. “Modified Bell tests can be used in cryptography, for example, to demonstrate that information is actually transmitted in encrypted form,” explains Simon Storz, a doctoral student in Wallraff’s group. “With our method, we can prove that Bell’s inequality is violated more efficiently than is possible in other experimental setups. That is particularly interesting for practical applications.”

Finding a compromise

However, researchers need sophisticated testing facilities for this. Because for the Bell test to be truly error-free, they must ensure that no information is exchanged between the two entangled circuits before the quantum measurements are complete. Because information can be transmitted at the speed of light, the measurement must take less time than it takes for a particle of light to travel from one circuit to another.

Therefore, when setting up an experiment, it is important to strike a balance: the greater the distance between two superconducting circuits, the more time available for measurement—and the more complex the experimental setup becomes. This is because the entire experiment must be conducted in a vacuum near absolute zero.

The ETH researchers determined that the shortest distance to perform a successful error-free Bell test is about 33 meters, since a light particle takes about 110 nanoseconds to travel this distance in a vacuum. This is a few nanoseconds longer than it took the researchers to perform the experiment.

Thirty meters vacuum

Wallraff’s team built the impressive facility in the underground passageways of the ETH campus. At its two ends is a cryostat with a superconducting circuit. These two cooling devices are connected by a 30-meter long tube whose interior is cooled to a temperature below absolute zero (–273.15°C).

Before the start of each measurement, a microwave photon is passed from one of the two superconducting circuits to the other so that the two circuits become entangled. Random number generators then decide which measurements to make on the two circuits as part of the Bell test. Next, the measurement results on both sides are compared.

A huge complication

After evaluating more than a million measurements, the researchers showed with high statistical certainty that Bell’s inequality was violated in this experimental setup. In other words, they confirmed that quantum mechanics also allows for nonlocal correlations in macroscopic electrical circuits, and as a result superconducting circuits can be entangled over large distances. This opens interesting potential applications in the field of distributed quantum computing and quantum cryptography.

Building the facility and conducting the testing was a challenge, Walroff says. Cooling the entire experimental setup to temperatures close to absolute zero requires considerable effort.

“Our machine has 1.3 tons of copper and 14,000 screws, plus a lot of physics knowledge and engineering knowledge,” says Walroff. He believes that in principle it should be possible to build facilities that cover even greater distances in the same way. For example, this technology could be used to connect superconducting quantum computers over great distances.