Entangled quantum circuits further disprove Einstein's concept of local causality

The "local causality" theory, developed by Albert Einstein 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 may be far more tightly connected with one another than is feasible in traditional systems. This experiment is unique in that it was carried out for the first time utilizing superconducting circuits, which are thought to be viable prospects for creating potent quantum computers.

Old disagreement

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 true 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 get the answer to this query. These experiments always meet Bell's inequality if Einstein's theory of local causation is correct. Quantum mechanics, on the other hand, predicts that they will go against it.

The last skepticism was removed

John Francis Clauser, who received the Physics Nobel Prize last year, and Stuart Freedman performed the first real Bell test in the early 1970s. The two scientists were able to demonstrate through their tests that Bell's inequality is definitely broken. But in order to perform their tests in the first place, they had to make a number of presumptions. Therefore, hypothetically, Einstein's skepticism of quantum mechanics may still have been justified.

But as time goes on, more and more of these gaps could be filled. Finally, in 2015, many groups achieved the first genuinely error-free Bell tests, putting an end to the long-running argument.

Applications that show promise Wallraff's team may now use a brand-new experiment to confirm these findings. Despite the original confirmation being seven years ago, the study by the ETH researchers published in Nature demonstrates that more research has to be done on this issue. This is due to a number of factors.

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 physics as well. The term "macroscopic quantum objects" refers to the few hundred micrometer-sized superconducting-based microwave-operated electronic circuits.

Bell tests also have a practical importance, to mention another issue. According to Simon Storz, a doctorate student in Wallraff's lab, "Modified Bell tests can be used in cryptography, for example, to show that information is actually transmitted in encrypted form." "With our method, we can demonstrate that Bell's inequality is broken considerably more quickly than we could in prior experimental configurations. Because of this, it is especially intriguing for practical applications.

Trying to reach a compromise

But for this, the researchers require a high-tech testing laboratory. Because they need to make sure that no data can be transferred between the two entangled circuits before the quantum measurements are finished for the Bell test to be really 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 go from one circuit to another.

Therefore, it's crucial to find a balance while setting up the experiment: the farther apart the two superconducting circuits are, the more time is available for the measurement, but the more complicated the experimental setup is. This is due to the fact that the entire experiment must be carried out in a vacuum at very low temperatures.

The ETH researchers have found that the smallest distance over which to carry out a successful loophole-free Bell test is roughly 33 meters, since a light particle takes this distance in a vacuum about 110 nanoseconds. That's a few of nanoseconds longer than the researchers' experiment time.

30 meters of vacuum

The ETH campus's subterranean tunnels have been home to an outstanding facility constructed by Wallraff's team. It has a cryostat with a superconducting circuit at each of its two ends. A 30-meter-long tube that is internally chilled at a temperature slightly above absolute zero (-273.15°C) connects these two cooling devices.

One of the two superconducting circuits transmits a microwave photon to the other before the beginning of each measurement, entailing the two circuits. The Bell test then uses random number generators to determine which measurements are done on the two circuits. The results of the measurements on both sides are then compared.

A significant entanglement

The researchers have demonstrated that Bell's inequality is broken in this experimental design with extremely high statistical certainty after analyzing more than one million observations. In other words, they have demonstrated that superconducting circuits may be entangled across a great distance and that quantum theory also permits non-local correlations in macroscopic electrical circuits. This offers up some intriguing potential uses for quantum cryptography and distributed quantum computing.

It was difficult to construct the laboratory and conduct the test, according to Wallraff. It takes a lot of work just to chill the complete experimental apparatus to a temperature near absolute zero.

Our machine has 14,000 screws, 1.3 tons of copper, and a ton of physics and technical expertise, according to Wallraff. He thinks that it would theoretically be conceivable to create infrastructure that could traverse much bigger distances. For example, using this technique to connect superconducting quantum computers across vast distances.

Provided by ETH Zurich