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## Defining ‘Interaction’ in Quantum Mechanics

**The key to establishing ever more powerful quantum computers, such as Google’s Sycamore, lies in understanding entanglement. New research shows that this phenomenon, once described as ‘spooky action at a distance’ by Einstein, is not a factor of contact, but of identity. **

As physics is essentially the study of the interactions between bodies such as planets or particles, the question of exactly when and how interaction occurs has always been a vital one.

While interaction may appear to be very clear in the macroscopic world, occurring via physical contact, the question of interaction in the microverse , governed by the rules of quantum mechanics , has always been a much thornier issue.

Interaction between particles is known to occur as a result of the exchange of force-carrying particles (bosons) . This interaction mechanism cannot explain the interaction that occurs between entangled particles, which could be separated by a distance as vast as the Universe itself.

Entangled particles are two or more particles that are unable to be described by separate equations. The mathematics that underpins quantum mechanics prevents them from being ‘unsealed’ until the values of one of the particle’s characteristics is measured — or in common parlance — until it is observed.

The instantaneous change in the state of one entangled particle as a result of a measurement made on its counterpart disturbed Einstein so much he called it “spooky action at a distance”. This, along with the non-deterministic nature of quantum physics, led him to believe that quantum physics was an incomplete theory.

Einstein proposed that there must be ‘hidden variables’ somewhere within quantum systems that explained this seemingly instantaneous change. Almost 100 years of experimental evidence has proved this idea incorrect, and the thought experiments that Einstein devised before his death has lead quantum physicists to rule out hidden variables.

We can also rule out contact at the moment of creation for these particles. Their birth can take place far apart , beyond the reach of possible ‘touch’, and yet the correlation between them still exists.

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From this phenomenon, we can deduce that quantum rules allow the interaction of particles without ‘contact’ — so how else could this interaction arise? Could these particles come into contact at some point in their history and become entangled in that way? What if the particles are prevented from ever coming within touching distance?

In a new paper published in the journal *Scientific Reports,* part of the *Nature *stable of journals, authors Paweł Błasiak from the Institute of Nuclear Physics of the Polish Academy of Sciences in Kraków and Marcin Markiewicz from the University of Gdańsk, aim to answer these questions. In the process, they determine that ‘interaction’ in quantum mechanics may arise from the identity of particles alone.

To unravel the mysteries of entanglement, the team must navigate the abstract mathematical language of quantum physics, notorious for its interpretational problems, and describe entanglement as a more ‘reality-based’ phenomenon.

The team, led by Pawel Blasiak of Department of Mathematical Physics (NZ43), Institute of Nuclear Physics Polish Academy of Sciences, did not want to see particles as mere statistical outcomes and detector clicks — they wanted to know the reality behind the mathematics of quantum theory.

By doing this, the team hopes to discover how the mathematical and theoretical underpinnings of quantum mechanics are reflected in reality — a vital key to unlock the potential of quantum computers.

## Thinking Big: The Future of Quantum Computing

The team began its experiment to test the origins of entanglement with the principle that particles are inherently entangled as a result of identity and regardless of how distant their creation.

The researchers state an entangled state can mathematically describe all identical particles, but this correlation can often be ‘masked’ by the difficulty in addressing individual particles.

This problem is compounded by the fact that all experimental assessments of entanglement require measurement to unlock the phenomenon, blurring the source of its origin. The team aimed to prove that entanglement due to indistinguishability is not merely a factor of the mathematics used to describe quantum mechanics and the formalism that arises from it.

To conduct their experiments, they chose to use three entangled qubits as test subjects. This choice cuts straight to the practical and commercial applications of quantum mechanics , namely quantum computing.

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Qubits are to quantum computers — such as Google’s quantum computer Sycamore — what bits are to traditional computers, basically an elementary unit of information. Whereas a conventional bit can exist in only two states ( 1s or 0s ), a qubit can exist in a multitude of states. This is what gives quantum computers their incredible computing power. Entanglement is a resource that ‘speeds up’ quantum computations even further.

Google’s Sycamore quantum computer uses 54 entangled qubits to achieve ‘quantum supremacy’, which is the ability to complete complex calculations 1000s of times faster than even the most sophisticated supercomputer.

If we can understand entanglement better, especially in qubits, it not only opens up the possibility of larger and faster quantum processors, but also the possibility of a quantum network of processors, and eventually, a quantum internet.

Maintaining a quantum network involves isolating entangled qubits from environmental effects that would threaten to collapse their entangled state. The larger a quantum system becomes, the more challenging establishing this environmental protection is.

The team generated qubits in an entangled tripartite state , with no contact between the particles at the point of generation. It ensured that the entangled particles did not meet at any point across their paths, and discovered that the qubits remained correlated.

This ‘interaction without touching’ protocol offers a powerful tool for the implementation of quantum processing tasks . The team’s experiment verified this not just as an aspect of the formalism of quantum mechanics, but something that occurs in reality. In the process, they determined that it is the indistinguishability of particles that gives rise to entanglement by treating the qubits as individual and localized particles and ensuring their paths never crossed.

The next step for this team and other research, such as the Google Sycamore team, is clear. Can this ‘no-touch’ method of inducing entanglement be replicated with four qubits, and, if so, what about five qubits, or six?

With quantum computing, the adage “less is more” most certainly does not hold true, and one of the critical developments in quantum computing could be creating an entangled system that is as large as possible.

## References and Further Reading

Pawel Blasiak & Marcin Markiewicz. (2019) Entangling three qubits without ever touching. *Scientific Reports* 9, 20131. Available at: https://www.nature.com/articles/s41598-019-55137-3

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