Loophole-free bell test confirms quantum mechanics and advances quantum computing with superconducting circuits

Andreas Wallraff, a Professor of Solid State Physics at ETH Zurich, has led a team of researchers in conducting a groundbreaking experiment that challenges Albert Einstein’s concept of “local causality” as a response to quantum mechanics. Their work, known as a loophole-free Bell test, has provided additional evidence supporting the principles of quantum mechanics. What makes this experiment particularly noteworthy is the use of superconducting circuits, which are highly promising for the development of advanced quantum computers. The researchers demonstrated that quantum mechanical objects separated by long distances can exhibit significantly stronger correlations than conventional systems, further bolstering our understanding of quantum phenomena.

An old dispute

The concept of a Bell test originated from a thought experiment conceived by John Bell, a British physicist, in the 1960s. Bell aimed to address a longstanding debate that emerged in the 1930s among physics luminaries, including Albert Einstein. The question at hand was whether the counterintuitive predictions of quantum mechanics held true or if the conventional principles of causality applied to the microscopic realm.

To tackle this conundrum, Bell proposed an experimental setup involving the simultaneous random measurement of two entangled particles. The key criterion for assessment was Bell’s inequality. If Einstein’s notion of local causality held, the experimental outcomes would consistently align with Bell’s inequality. On the other hand, quantum mechanics predicted a violation of this inequality.

Hence, Bell tests serve as a means to experimentally determine if the principles of quantum mechanics or classical causality govern the behavior of entangled particles, providing insight into the nature of reality at the quantum level.

Promising applications

The recent experiment conducted by Andreas Wallraff’s team, as published in Nature, signifies that the investigation into this subject is far from concluded, despite an initial confirmation of results seven years ago. This advancement is driven by several factors.

Firstly, the experiment conducted by ETH researchers verifies that the laws of quantum mechanics extend to superconducting circuits, which are significantly larger than microscopic quantum entities like photons or ions. These macroscopic quantum objects consist of electronic circuits, several hundred micrometers in size, composed of superconducting materials and operating at microwave frequencies.

Furthermore, Bell tests hold practical implications. For instance, modified Bell tests can be employed in cryptography to demonstrate secure information transmission. Simon Storz, a doctoral student in Wallraff’s group, highlights that their approach enables more efficient verification of the violation of Bell’s inequality compared to other experimental setups. This characteristic makes it particularly appealing for practical applications.

Therefore, the research conducted by Wallraff’s group not only expands our understanding of quantum phenomena in larger systems but also opens up possibilities for advancements in areas such as cryptography, leveraging the violation of Bell’s inequality.

The search for a compromise

Executing a loophole-free Bell test requires a sophisticated testing facility. The primary challenge lies in ensuring that no information can be exchanged between the entangled circuits until quantum measurements are completed. To achieve this, the measurement process must occur in a timeframe shorter than the time it takes for a light particle to travel from one circuit to the other, as information transmission speed is limited to the speed of light.

Setting up the experiment necessitates finding a balance. Increasing the distance between the superconducting circuits provides more time for the measurement but also intensifies the complexity of the experimental setup. This complexity arises due to the requirement of conducting the entire experiment in a vacuum environment at temperatures close to absolute zero.

The researchers at ETH have determined that a successful loophole-free Bell test can be conducted over a minimum distance of approximately 33 meters. In a vacuum, it takes approximately 110 nanoseconds for a light particle to traverse this distance. Notably, the researchers were able to complete the experiment in slightly less time than this, making it feasible within the determined constraints.

Thirty-meter vacuum

In the underground passageways of the ETH campus, Wallraff’s team has established an impressive facility dedicated to their research. The facility comprises two cryostats, situated at opposite ends, housing superconducting circuits. These cryostats are interconnected by a 30-meter-long tube, which maintains an extremely low temperature slightly above absolute zero (-273.15°C).

Prior to each measurement, a microwave photon is transmitted from one of the superconducting circuits to the other, entangling the two circuits. Random number generators are employed to determine which specific measurements will be performed on both circuits as part of the Bell test. Subsequently, the obtained measurement results from both sides are compared for analysis and evaluation.

Large-scale entanglement

Through the meticulous evaluation of over one million measurements, the researchers have achieved a significant outcome. They have demonstrated with a high degree of statistical certainty that Bell’s inequality is violated within their experimental setup. This confirmation reinforces the fact that quantum mechanics allows for non-local correlations even in macroscopic electrical circuits, highlighting the possibility of entangling superconducting circuits over substantial distances. This breakthrough opens intriguing prospects in the realms of distributed quantum computing and quantum cryptography.

Wallraff acknowledges the challenges involved in constructing the facility and conducting the test. The cooling process alone, which aims to bring the entire experimental setup to temperatures near absolute zero, demands considerable effort. The machine itself consists of 1.3 tons of copper, 14,000 screws, and a wealth of physics knowledge and engineering expertise.

According to Wallraff, extending this technology to overcome even greater distances appears feasible in principle. This advancement could potentially enable the connection of superconducting quantum computers over extensive geographical separations, offering new avenues for quantum computing advancements.

Source: ETH Zurich

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