Distant quantum objects can be much more strongly correlated with each other

The first-​ever loophole-free Bell test with superconducting circuits.

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Superposition, entanglement, and non-locality constitute fundamental features of quantum physics. The fact that quantum physics does not follow the principle of local causality can be experimentally demonstrated in Bell tests performed on pairs of spatially separated, entangled quantum systems. Although Bell tests have been explored using a broad range of quantum systems over the past 50 years, only relatively recently have experiments free of so-called loopholes succeeded.

Albert Einstein’s theory of “local causality,” developed in reaction to quantum physics, has been refuted by a team of researchers led by Andreas Wallraff, Professor of Solid State Physics at ETH Zurich. The researchers have further supported quantum mechanics by demonstrating that distant quantum mechanical objects can be far more tightly connected than is feasible in traditional systems.

A Bell test is based on an experimental setting that British physicist John Bell first developed as a thought experiment in the 1960s. Bell sought to answer a dispute that the pioneers of physics had already engaged in during the 1930s: Are the predictions of quantum mechanics, which are wholly at odds with common sense, accurate, or do they also hold in the atomic microcosm, as Albert Einstein believed?

Bell suggested making a random measurement on two entangled particles simultaneously and comparing it to Bell’s inequality to find the answer to this query. These experiments always meet Bell’s inequality if Einstein’s theory of local causation is correct. On the other hand, Quantum mechanics predicts that they will go against it.

The specialty of this novel experiment is that scientists were able, for the first time to perform it using superconducting circuits, which are considered promising candidates for building powerful quantum computers.

For starters, despite being far larger than minuscule quantum particles like photons or ions, the ETH researchers’ experiment proves that superconducting circuits follow the rules of quantum mechanics as well. The term “macroscopic quantum objects” refers to microwave-operated, superconducting electronic circuits that are several hundred micrometers in size.

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

A sophisticated test facility was required for this. Because they need to ensure that no information may be shared between the two entangled circuits before the quantum measurements are finished for the Bell test to be bug-free. Since light is the only speed at which information can be sent, the measurement must be completed in less time than it takes a light particle to move from one circuit to another.

It is mandatory to maintain the balance while setting up the experiment: The more space between the two superconducting circuits, the longer the measuring window is, and the more complex the experimental setup is. This is due to the fact that the entire experiment must be carried out in a vacuum at shallow temperatures.

According to ETH researchers, the shortest distance to complete a loophole-free Bell test is around 33 meters because it takes a light particle roughly 110 nanoseconds to cover this distance in a vacuum. That’s a few nanoseconds longer than the scientists’ experiment time.

Scientists evaluated more than one million measurements. With extreme statistical certainty, scientists have shown that Bell’s inequality is violated in this experimental setup.

In other words, they have demonstrated that superconducting circuits can be entangled across a great distance and that quantum mechanics permits non-local correlations in macroscopic electrical circuits. This offers up some intriguing potential uses for quantum cryptography and distributed quantum computing.

Thirty-meter vacuum

Scientists have built an impressive facility in the underground passageways of the ETH campus. There is a cryostat at each of its two ends. The cryostat contains a superconducting circuit. These two cooling apparatuses are connected by a 30-​meter-long tube whose interior is cooled to a temperature just above absolute zero (–273.15°C).

Each measurement begins with the transmission of a microwave photon from one of the two superconducting circuits to the other, entailing the circuits. The Bell test then uses random number generators to determine which measurements are done on the two circuits. The results of the measures on both sides are then compared.

Wallraff says. “We were able to finance the project over six years with funding from an ERC Advanced Grant.” Cooling the entire experimental setup to a temperature close to zero takes considerable effort. “There are 1.3 tonnes of copper and 14,000 screws in our machine, as well as a great deal of physics knowledge and engineering know-how.”

Journal Reference:

  1. Storz S et al.: Loophole-free Bell inequality violation with superconducting circuits. Nature, 10 May 2023. DOI: 10.1038/s41586-​023-05885-0
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