Entanglement: Interaction is Contactless

Entanglement is a mysterious element of nature with a sound mathematical description, yet an abstract reality. New research explores its nature, origins and the very foundation of quantum physics.

Common sense tells us that interaction arises in the world around us via physical contact, physics tells us that contact arises through the action of forces, and particle physics that these forces are mitigated by the exchange of force-carrying particles. Yet, there is a hindrance to this view of the physical world.

Within quantum physics, there exists a phenomenon for which interaction can occur without any possibility of contact. Entanglement is the description of two or more particles that are mathematically bound together by their qualities and characteristics. A measurement on one forces these characteristics to resolve, and thus causes an instant change in the other — even if they are at opposite sides of the Universe.

Einstein was so troubled by this aspect of nature — which he felt violates the universal speed limit of the speed of light, set by his theory of special relativity — that he referred to it as “spooky action at a distance.” The great man, arguably the most famous scientist who ever lived, spent the last few years of his life in a back and forth with another physicist Niels Bohr in an attempt to prove that quantum theory was incomplete.

Einstein’s determination to expose that quantum theory held some “hidden variables” which could explain this troubling non-locality and save so-called “local realism” eventually had the opposite effect. The debate between Einstein and Bohr led John Bell to determine that there were no hidden variables in quantum theory — to preserve the causal nature of reality, one had to accept its non-local caveat. “Spooky” though it may be, the non-local action of entanglement is real.

All well and good, but as entanglement is essentially a mathematical description of reality, the idea that two or more particles can not be described by separate equations of state. Confirming the value of one particle’s variable resolves the value of the other and thus results in an equation that can then be separated into distant parts, each representing a particle in the system.

This mathematical underpinning is open to interpretation, it can leave this element of quantum theory feeling somewhat abstract and “unreal.” Particles are represented in quantum experiments by statistical predictions and the clicks of detectors. Yet, despite its abstract nature, quantum mechanics is still the most accurate and sophisticated theory used by physicists to describe the world around us.

Enter Pawel Blasiak from the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakow and Marcin Markiewicz from the University of Gdansk, they aim to superimpose this mathematical nature to physical reality. In a paper published in the journal Scientific Reports, part of the Nature stable of journals, the duo attempt to present a more “reality-based” picture of entanglement.

“Regarding the description of measurements, you can see quantum formalism as a recipe to calculate the statistics of outcomes — detector clicks — in experiments made in a lab. It is the best theory, in terms of accuracy and applicability, that we have ever had,” Blasiak says. “The problem begins if you ask what is the reality behind all those mathematical objects. The ‘real world’ does not consists of clicks in the apparatus — it should consist of ‘stuff’ that we are supposed to observe. However, quantum theory remains silent about it.”

In the process of conducting their study, the team delved deep into the foundations of quantum theory and seem to have confirmed that the interaction that forms the basis of entanglement arises from the identity of the particles in question rather than contact between them.

“Because we are not allowed to ask, or the answer that one hears is that those objects do not have any properties before the measurement is made. That is a bit disappointing. Hence, the research in quantum foundations is so interesting and important at the same time.”

No touching! Non-locality, Identity and Interaction

The idea of two particles created separately yet being interconnected leaves quantum theorists to speculate that all identical particles are somehow inherently correlated, no matter how distant their mutual creation.

In order to demonstrate that entanglement arises as a factor of identity, the team had to keep their test particles separated.

The team focused on the idea that all particles of the same type are identical, a foundational principle of quantum physics, regardless of their distance apart or the location of their origin. Blasiak explains that this postulate requires that all particles of the same type are entangled. The problem is that the delicacy of this entangled state prevents it from being used practically or experimented with. It is, effectively, masked.

“We have developed a method, based on seminal observations of Yurke-Stoler, in which this inherent entanglement can be activated for ‘free’ — without any interaction,” he continues.

In order for this scheme to work, it was crucial that the team went further than researchers had gone before, not only ensuring that the particles used were created separately, but that they never crossed paths at any point. In this way, they could ensure that interaction couldn’t be passed by another particle or field and thus, truly arose from identity.

“The usual approach starts with systems which are treated as not entangled — in a separable state. Then, by means of some non-trivial interaction entanglement is created.

“In our approach we reach for entanglement due to particle indistinguishability. It is given by nature for free and applies to all particle from the very moment of their creation.

“Our approach can be seen as a way of unlocking this ubiquitous resource.”

“Only in this way we can justify that indistinguishability of particles is a useful resource of entanglement accessible for practical applications,” the researcher tells me. “We have shown how to generate an entangled state for two and three particles, and proposed a general “no-touching” scheme for entanglement generation for an arbitrary number of particles.”

Exploiting Entanglement: Qubits and quantum computing

In order to conduct their study, Blasiak and Markiewicz, used three entangled qubits, the particle that forms the fundamental unit of information in quantum computers. Qubits are analogous to bits in regular computers, but whereas bits can only exist in one of two states — normally described as 0 or 1 — qubits can exist in a multitude of states, thus giving quantum computers their incredible computational powers.

The team used a system of three entangled qubits to conduct their investigation

The team chose qubits to use in their experiment because their entanglement is well understood, existing in two classes of state. “As the proof-of-concept, we set out to study each class separately,” Blasiak says. “We have shown how to produce each of them which proves that any entanglement of two and three-qubit type can be extracted from indistinguishability.

“This makes us think that it will be the case in general, but it is still a conjecture.”

Whilst qubits are vital for storing and accessing information in quantum computers, entanglement plays an equally important role and a better understanding of the phenomenon could be vital if researchers are ever to develop a quantum network or even, a quantum internet.

“Entanglement is the resource used in quantum computation to speed up the computations. Without entanglement, those algorithms lose their leverage,” Blasiak explains. “Another field where entanglement plays an important role is quantum communication, where it can be used to guarantee the security of communication channels, due to entanglement listening in on private communication leaves the trace of the eavesdropper.”

The problem faced by researchers is that the larger a quantum system, the more qubits that comprise it, the greater its computational power. But the larger an entangled system is, the more difficult it is to protect from environmental influences that would collapse the mutual state.

The team’s experiments with a tripartite entangled state could be replicated with increasing numbers of entangled particles helping to establish these larger systems. For Blasiak though, the next question to answer is to see if this “no-touching” entanglement can be replicated in other identical particles.

Blasiak, stresses that for him, researching entanglement has an importance far beyond its practical applications. “Well … the lesson from our research is that entanglement is everywhere. The mere fact of particle indistinguishability can be used to extract and see entanglement in real experiments.

“That is a profuse resource, which I am not sure we fully appreciate. It would be exciting to harness it for technological purposes … time will show,” He says with a smile.

Blasiak explains that Quantum mechanics has puzzled him from the beginning of his studies at the Jagiellonian University. “How is it possible that the best theory that we have tells so little about the reality around us?” he muses. “Entanglement is at the heart of the quantum mystery. What could be more puzzling than hearing form physicists talking about spooky-action-at-a-distance or problems with understanding how cause-and-effect works in nature?