May 12, 2023 A group of researchers led by Andreas Wallraff, Professor of Solid State Physics at ETH Zurich, has performed a loophole-free Bell test to disprove the concept of local causality formulated by Albert Einstein in response to quantum mechanics.

By showing that quantum mechanical objects that are far apart can be much more strongly correlated with each other than is possible in conventional systems, the researchers have provided further confirmation for quantum mechanics. Whats special about this experiment is that the researchers were able for the first time to perform it using superconducting circuits, which are considered to be promising candidates for building powerful quantum computers.

An Old Dispute

A 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 that the greats of physics had already argued about in the 1930s: Are the predictions of quantum mechanics, which run completely counter to everyday intuition, correct, or do the conventional concepts of causality also apply in the atomic microcosm, as Albert Einstein believed?

To answer this question, Bell proposed to perform a random measurement on two entangled particles at the same time and check it against Bells inequality. If Einsteins concept of local causality is true, these experiments will always satisfy Bells inequality. By contrast, quantum mechanics predicts that they will violate it.

The Last Doubts Dispelled

In the early 1970s, John Francis Clauser and Stuart Freedman carried out the first practical Bell test. In their experiments, the two researchers were able to prove that Bells inequality is indeed violated. But they had to make certain assumptions in their experiments to be able to conduct them in the first place. So, theoretically, it might still have been the case that Einstein was correct to be skeptical of quantum mechanics.

Over time, however, more and more of these loopholes could be closed. Finally in 2015, various groups succeeded in conducting the first truly loophole-free Bell tests.

Promising Applications

Wallraffs group can now confirm these results with a novel experiment. The work by the ETH researchers published in the scientific journal Nature demonstrates that research on this topic has not concluded despite the initial confirmation seven years ago.

A number of factors contribute to this outcome. The experiment conducted by the ETH researchers establishes that, despite their larger size compared to microscopic quantum objects, superconducting circuits still abide by the principles of quantum mechanics. These electronic circuits, which are several hundred micrometers in size and made from superconducting materials, function at microwave frequencies and are known as macroscopic quantum objects.

In addition, Bell tests also have a practical significance. Modified Bell tests can be used in cryptography, for example, to demonstrate that information is actually transmitted in encrypted form, explained Simon Storz, a doctoral student in Wallraffs group. With our approach, we can prove much more efficiently than is possible in other experimental setups that Bells inequality is violated. That makes it particularly interesting for practical applications.

The Search for a Compromise

To carry out their research, the team required an advanced testing facility. A critical aspect of a loophole-free Bell test is ensuring that no information exchange occurs between the two entangled circuits before the completion of the quantum measurements. As information can only travel as fast as the speed of light, the measurement process must be faster than the time taken for a light particle to travel from one circuit to another.

When designing the experiment, striking the right balance is crucial. Increasing the distance between the two superconducting circuits allows for more time to conduct the measurement, but it also complicates the experimental setup. This is due to the need for the entire experiment to be carried out in a vacuum near absolute zero.

The ETH researchers determined that the minimum distance needed for a successful loophole-free Bell test is approximately 33 meters. A light particle takes about 110 nanoseconds to travel this distance in a vacuum, which is slightly longer than the time it took for the researchers to complete the experiment.

Thirty-meter Vacuum

Wallraffs team has built an impressive facility in the underground passageways of the ETH campus. At each of its two ends is a cryostat containing a superconducting circuit. These two cooling apparatuses are connected by a 30-meter-long tube with interiors cooled to a temperature just above absolute zero (273.15C).

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

Large-scale Entanglement

After evaluating more than one million measurements, the researchers have shown with very high statistical certainty that Bells inequality is violated in this experimental setup. In other words, they have confirmed that quantum mechanics also allows for non-local correlations in macroscopic electrical circuits and consequently that superconducting circuits can be entangled over a large distance. This opens up interesting possible applications in the field of distributed quantum computing and quantum cryptography.

Building the facility and carrying out the test was a challenge, Wallraff said. We were able to finance the project over a period of six years with funding from an ERC Advanced Grant. Just cooling the entire experimental setup to a temperature close to absolute zero takes considerable effort.

He explained, There are 1.3 tons of copper and 14,000 screws in our machine, as well as a great deal of physics knowledge and engineering know-how. Wallraff further believes that, in principle, it is possible to construct facilities capable of overcoming even greater distances using the same approach. Such technology could potentially be employed to connect superconducting quantum computers across vast distances.

Source: Felix Wrsten, ETH Zrich

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