Old argument
In the 60s of the 20th century, British physicist John Bell proposed a famous thought experiment. He wanted to solve a question that had been debated since the 30s: Are the predictions of quantum mechanics, which completely contradict everyday intuition, really correct? Or, as Einstein believed, the traditional concept of causality should also apply to the atomic microscopic world?
In the experiment Bell envisions, a common source S would distribute two particles to Alice and Bob. After receiving the particles, Alice and Bob can each choose one of several measurement settings to measure the received particle and record the measurement. After multiple replicates, the experiment produces a series of data results. The measurement results are then tested against Bell's inequality.
Bell's inequality was proposed by Bell in 1964, and in simple terms, if Einstein's view of localized causality is correct, then the experimental result will always satisfy Bell's inequality; Instead, quantum mechanics predicts that certain types of experimental results will violate Bell's inequality.
Clear the last doubts
In the past, scientists have conducted several Bell experiments, all of which proved the correctness of quantum mechanics. Back in the early '70s, physicists John Clause and Stuart Freedman made the first actual measurements of the Bell test. They experimentally proved that Bell's inequality was indeed violated.
However, their experiment was able to proceed because they made some assumptions in the experiment. That is, their experiment contains some loopholes. So, in theory, Einstein's questioning of quantum mechanics could still be correct.
The goal of physicists is to close these loopholes. Over time, more and more loopholes were filled, and eventually, in 2015, scientists successfully conducted the first bug-free Bell test, bringing an end to this age-old dispute.
A recent study published in the journal Nature shows that seven years ago, scientists only preliminarily settled the dispute, and research on the topic is not over. The new study provides further confirmation of the correctness of quantum mechanics by showing that objects that are far apart in quantum mechanics can be more closely interconnected than classical systems.
Seek balance
What's special about this experiment is that the researchers used superconducting circuits for the first time to conduct experiments. For the Bell test to be truly loophole-free, they say, they must ensure that no information can be exchanged between two entangled quantum circuits until the quantum measurement is complete. Since the fastest speed at which information can be transmitted is the speed of light, this means that the time used for measurement must be less than the time it takes for photons to travel from one circuit to another.
As a result, researchers need to put in a lot of effort to build a complex, low-temperature experimental setup. And when setting up the experiment, they had to find a "balance": the greater the distance between the two superconducting circuits, the more time was devoted to measurement, and at the same time the more complex the experimental setup.
The researchers created a cryostat to efficiently cool the 30-meter-long quantum connection
They found that the shortest distance for a successful hole-free Bell test was about 33 meters, and it took about 110 nanoseconds for photons to travel that distance in a vacuum, which is several nanoseconds longer than the researchers took to measure.
So they built an amazing experimental device in an underground passage. At each end of the unit there is a cryostat containing a superconducting circuit, and the two cooling units are connected by a 30-meter-long pipe inside which is cooled to a temperature just above absolute zero (-273.15°C).
Part of a 30-meter-long quantum connection: an aluminum waveguide cooled to almost absolute zero, connecting two quantum circuits
Before each measurement begins, a microwave photon is transmitted from one superconducting circuit to the other, so that the two circuits become entangled. The random number generator then decides which measurements will be made on both circuits as part of the Bell test. Next, the measurements on both sides are compared.
Large-scale entanglement
After taking more than a million measurements, the researchers showed with very high statistical certainty that Bell's inequality was violated in this experimental setup. They found that superconducting circuits made of superconducting materials that operate at microwave frequencies of several hundred micrometers also operate according to the laws of quantum mechanics, even though these macroscopic quantum objects are much larger than microscopic quantum particles such as photons or ions.
In other words, they have shown that quantum mechanics also allows for non-localized correlation in macroscopic circuits, so superconducting circuits can become entangled over great distances. This opens up interesting potential applications in both distributed quantum computing and quantum cryptography.
The researchers say the Bell test they conducted has great practical implications. For example, the improved Bell test in the new experiment could be used in cryptography to prove that information is actually transmitted in encrypted form. By using this new method, the researchers can prove that Bell's inequality is violated more effectively than in other experimental setups.
Building this experimental facility and testing it was a huge challenge. It took considerable effort to simply cool the entire experimental setup to temperatures close to absolute zero. In their equipment, there are 1.3 tons of copper and 14,000 screws, as well as a lot of physical knowledge and engineering know-how. The researchers believe that in the same way, it may be possible to build experimental devices that can overcome greater distances.
The article is reproduced in [Principle], non-commercial use, only as popular science communication materials. If there is any infringement, please contact: Jo0729, promise to delete within three days.