- ETH Zurich researchers engaged quantum mechanical objects to show that they can remain strongly correlated even when far apart.
- The gap-free Bell test offers further confirmation for quantum mechanics.
- The researchers say there are practical applications to the work, including implications for quantum cryptography and linking superconducting quantum computers over great distances.
PRESS RELEASE — ETH Zurich researchers have successfully demonstrated that distant quantum mechanical objects can correlate much more strongly with each other than is possible in conventional systems. For this experiment, they used a superconducting circuit for the first time.
A group of researchers led by Andreas Wallraff, Professor of Solid State Physics at ETH Zurich, have performed the Gapless Bell test to refute the concept of “local causality” formulated by Albert Einstein in response to quantum mechanics. By showing that distant quantum mechanical objects can correlate much more strongly with each other than is possible in conventional systems, researchers have provided further confirmation for quantum mechanics. What’s special about this experiment is that for the first time researchers can do it using superconducting circuits, which are considered a promising candidate for building powerful quantum computers.
The Bell test is based on an experimental setup originally devised as a thought experiment by British physicist John Bell in the 1960s. Bell wanted to settle a question that had been debated by physicists in the 1930s: Are the predictions of quantum mechanics, completely counterintuitive, or do conventional concepts of causality also apply in the atomic microcosm, as Albert Einstein had believed?
To answer this question, Bell proposed taking random measurements of two entangled particles at the same time and comparing them with the Bell inequality. If Einstein’s concept of local causality is correct, this experiment will always satisfy Bell’s inequality. Instead, quantum mechanics predicts that they will break it.
The last doubts were dispelled
In the early 1970s, John Francis Clauser, who was awarded last year’s Nobel Prize in Physics, and Stuart Freedman conducted the first practical Bell test. In their experiments, the two researchers were able to prove that Bell’s inequality was indeed violated. But they had to make certain assumptions in their experiments to be able to do it in the first place. So, theoretically, maybe Einstein was right to be skeptical of quantum mechanics.
However, over time, more and more of these gaps can be closed. Finally, in 2015, various groups successfully conducted the first completely gap-free Bell test, thereby finally settling a long-standing dispute.
Wallraff’s group can now confirm these results with a new experiment. The work of ETH researchers is published in well-known scientific journals Natural indicates that research on this topic has not yet been completed, despite initial confirmation seven years ago. There are several reasons for this. For one thing, the ETH researchers’ experiments confirmed that superconducting circuits also operate according to the laws of quantum mechanics, even though they are much more massive than microscopic quantum objects like photons or ions. Electronic circuits a few hundred micrometers in size made of superconducting materials and operated at microwave frequencies are called macroscopic quantum objects.
For another thing, Bell’s test also has practical meaning. “A modified Bell test can be used in cryptography, for example, to show that information is actually transmitted in encrypted form,” explains Simon Storz, a doctoral student in Wallraff’s group. “With our approach, we were able to prove much more efficiently than is possible in other experimental settings that the Bell inequality is violated. That makes it very attractive for practical applications.
However, researchers need sophisticated testing facilities for this. Because for the Bell test to be truly gap-free, they have to ensure that no information can be exchanged between the two entangled circuits before the quantum measurements are complete. Since the fastest information can be transmitted is at the speed of light, the measurement should take less time than it takes for a light particle to travel from one circuit to another.
So when setting up an experiment, it’s important to strike a balance: the farther the distance between two superconducting circuits, the more time is available for measurement – and the more complex the setup of the experiment becomes. This is because the entire experiment must be carried out in a vacuum close to absolute zero.
ETH researchers have determined the shortest distance to perform a successful gap-free Bell test is about 33 meters, because it takes light particles about 110 nanoseconds to travel this distance in a vacuum. That’s a few nanoseconds longer than it took the researchers to carry out the experiment.
Thirty meter vacuum
The Wallraff team has built an impressive facility in the underground passages of the ETH campus. At both ends is a cryostat containing a superconducting circuit. The two refrigeration devices are connected by a 30 meter long tube whose inside is cooled to a temperature just above absolute zero (–273.15°C).
Prior to the start of each measurement, microwave photons are transmitted from one of the two superconducting circuits to the other so that the two circuits become entangled. The random number generator then decides which measurements are made on the two circuits as part of the Bell test. Next, the measurement results on both sides are compared.
After evaluating more than a million measurements, researchers have shown with very high statistical certainty that Bell’s inequality is violated in this experimental setting. In other words, they have confirmed that quantum mechanics also allows non-local correlations in macroscopic electric circuits and that consequently superconducting circuits can be entangled over long distances. This opens up exciting application possibilities in the field of distributed quantum computing and quantum cryptography.
Building facilities and conducting tests was a challenge, said Wallraff. “We were able to finance the project for six years with funding from the ERC Advanced Grant.” Merely cooling the entire experimental setup to a temperature close to absolute zero requires enormous effort. “There are 1.3 tons of copper and 14,000 screws in our machine, as well as a lot of physics and engineering know-how,” says Wallraff. He believed that it was possible in principle to build facilities that would overcome greater distances in the same way. This technology, for example, could be used to link superconducting quantum computers over vast distances.