Physicist James Trefil once said that quantum mechanics is a “place where the human brain will simply never feel comfortable.” This discomfort happens because nature, at a microscopic scale, obeys laws at odds with our perception of macroscopic reality. These laws include superposition (a particle can simultaneously be in different states, like Erwin Schrödinger’s live and dead cat), and quantum entanglement at a distance. Albert Einstein described the latter as “spooky action at a distance,” a principle allowing particles separated by distance to respond instantaneously and behave as a single system. A spectacular experiment that defies the speed of light was recently published in Nature by an international team of scientists led by the Swiss Federal Institute of Technology (ETH) in Zurich, collaborating with Spain’s Institute of Photonic Sciences (ICFO) and Quside, a quantum computing company. The study demonstrated for the first time super quick quantum random number generators that enable “spooky action at a distance” between superconducting quantum bits.
This experiment’s results contradict Einstein, who once considered quantum entanglement impossible. The physicist believed in the principle of locality, which states that an object is influenced directly only by its immediate surroundings. But advances in quantum physics have shown that two entangled particles can share a single unified state, even if they are 30 meters apart, as in the Zurich experiment.
Einstein could not accept that an action in one place could have an instantaneous effect elsewhere. But John Bell proved in 1964 that quantum entanglement exists. Subsequent experiments with this property by John Clauser, Alain Aspect and Anton Zeilinger earned them a Nobel Prize in 2022.
A major achievement of the study published in Nature is that it experimentally demonstrated in Bell tests performed on pairs of spatially separated, entangled quantum systems that quantum physics does not follow the principle of local causality with no so-called loopholes. The absence of loopholes means everything happens exactly as predicted by quantum physics – no communication between particles.
A similar experiment was conducted a year ago by Spanish physicist Adán Cabello of the University of Seville (Spain) with ytterbium and barium ions (Science Advances). But the Nature study raised the complexity level by using two superconducting qubits entangled at temperatures close to absolute zero (-273.15°C or -459.67°F) and 30 meters apart.
Defying the speed of light
Simultaneous measurements of the two qubits showed synchronized responses consistent with spooky action or entanglement at a distance. To demonstrate the absence of loopholes (that the coordination of states did not come from signals sent between qubits), the scientists made random 17-nanosecond measurements, which is the time it takes light to travel five meters. A full measurement required another 62 nanoseconds, the time for light to travel 21 meters. Because the systems were 30 meters apart, communication between the two was impossible.
The new study is significant because it has practical applications beyond the theoretical proof. Morgan W. Mitchell, a professor at the Catalan Institution for Research and Advanced Studies (ICREA) and a co-author of the study, said, “With ordinary computing, your home device communicates constantly through the internet with a server. But to do something equivalent with quantum computers, we need to communicate them somehow, but not using classical bits. We have to use quantum bits and entanglement is the most efficient way to do this.”
Mitchell said, “This study shows that experiments like this can be done with the same superconductors used by Google and IBM. Other experiments used systems with a single pair of particles, but ours created entanglement between many electrons at both sites. And we achieved this for the first time without loopholes.”
According to Mitchell, their experiment “made progress toward distributed quantum computing with multiple computers at multiple sites… It’s a long-term goal that we’re not going to achieve immediately. But this experiment demonstrated its feasibility.”
Carlos Abellán an expert in photonics and Quside’s co-founder and CEO, said the experiment “created a spectacular and unique technology that synchronized two particles with unprecedented speed.” This required generating quantum random numbers and extracting them at extraordinarily fast speeds (17 nanoseconds) to eliminate any possibility of communication between the qubits. “We had to engineer new ways of generating and extracting the random numbers before the information reached the other side. We needed to double the speed of earlier systems,” said Abellán. “Instead of using one device for calculations, we connected eight devices in parallel, and then synchronized and combined the signals. This gave us 16 random number generators with double the speed. If we had taken 19 nanoseconds instead of 17, the experiment would have been invalidated.”
The experiment proved that quantum information can be transmitted between separate superconducting circuits housed in cryogenic systems. In other words, it works with currently available quantum computing systems. But why two separate systems can behave as one is still unexplained. “It’s a question for the philosophers, and a very difficult one at that. You can ask 10 different physicists and you’re going to get 10 different answers. It’s a mystery for new generations to solve. But these experiments prove that it really exists,” said Mitchell.
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