Nav: Home

Entanglement by identity, or interaction without ever touching

March 24, 2020

What is interaction and when does it occur? Intuition suggests that the necessary condition for the interaction of independently created particles is their direct touch or contact through physical force carriers. In quantum mechanics, the result of the interaction is entanglement--the appearance of non-classical correlations in the system. It seems that quantum theory allows entanglement of independent particles without any contact. The fundamental identity of particles of the same kind is responsible for this phenomenon.

"The whole is other than the sum of its parts." - Aristotle (Metaphysics, Book VIII)

Quantum mechanics is currently the best and the most accurate and sophisticated theory used by physicists to describe the world around us. Its characteristic feature, however, is the abstract mathematical language notoriously leading to serious interpretational problems. The view of reality proposed by this theory is still a subject of scientific dispute, which, instead of expiring over time, is becoming hotter and more interesting. The new motivation and intriguing questions are brought forth by a fresh perspective resulting from the standpoint of quantum information and the enormous progress of experimental techniques. This allows verification of the conclusions drawn from subtle thought experiments directly related to the problem of interpretation. Moreover, we are now witnessing enormous progress in the field of quantum communication and quantum computer technology, which significantly draws on non-classical resources offered by quantum mechanics.

The work by Pawel Blasiak from the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakow and Marcin Markiewicz from the University of Gdansk focus on analyzing widely accepted paradigms and theoretical concepts regarding the basics and interpretation of quantum mechanics. The researchers are trying to answer the question to what extent the intuitions used to describe quantum mechanical processes are justified in a realistic view of the world. For this purpose, they try to clarify specific theoretical ideas, often functioning in the form of vague intuitions, using the language of mathematics. This approach often results in the appearance of inspiring paradoxes. Of course, the more basic the concept to which a given paradox relates, the better, because it opens up new doors to deeper understanding a given problem.

In this spirit, both scientists decided to ponder the fundamental question: what is interaction and when does it occur? In quantum mechanics, the result of the interaction is entanglement, which is the appearance of non-classical correlations in the system. Imagine two particles created independently in distant galaxies. It would seem that a necessary condition for the emergence of entanglement is the requirement that at some point in their evolution the particles touch one another or, at least, indirect contact should take place through another particle or physical field to convey the interaction. How else can they establish this mysterious bond, which is quantum entanglement? Paradoxically, however, it turns out that this is possible. Quantum mechanics allows entanglement to occur without the need for any, even indirect, contact.

To justify such a surprising conclusion, a scheme should be presented in which the particles will show non-local correlations at a distance (in a Bell-type experiment). The subtlety of this approach is to exclude the possibility of an interaction understood as some form of contact along the way. Such a scheme should also be very economical, so it must exclude the presence of force carriers which could mediate this interaction (physical field or intermediate particles). Blasiak and Markiewicz showed how this can be done by starting from the original considerations of Yurke and Stoler, which they reinterpreted as a permutation of paths traversed by the particles from different sources. This new perspective allows generating any entangled states of two and three particles, avoiding any contact. The proposed approach can be easily extended to more particles.

How is it possible to entangle independent particles at a distance without their interaction? The hint is given by quantum mechanics itself, in which the identity - the fundamental indistinguishability of all particles of the same kind - is postulated. This means, for example, that all photons (as well as other families of elementary particles) in the entire Universe are the same, regardless of their distance. From a formal perspective, this boils down to symmetrization of the wave function for bosons or its antisymmetrization for fermions. Effects of particle identity are usually associated with their statistics having consequences for a description of interacting multi-particle systems (such as Bose-Einstein condensate or solid-state band theory). In the case of simpler systems, the direct result of particle identity is the Pauli exclusion principle for fermions or bunching in quantum optics for bosons. The common feature of all these effects is the contact of particles at one point in space, which follows the simple intuition of interaction (for example, in particle theory, this comes down to interaction vertices). Hence the belief that the consequences of symmetrization can only be observed in this way. However, interaction by its very nature causes entanglement. Therefore, it is unclear what causes the observed effects and non-classical correlations: is it an interaction in itself, or is it the inherent indistinguishability of particles? The scheme proposed by both scientists bypasses this difficulty, eliminating interaction that could occur through any contact. Hence the conclusion that non-classical correlations are a direct consequence of the postulate of particle identity. It follows that a way was found to purely activate entanglement from their fundamental indistinguishability.

This type of view, starting from questions about the basics of quantum mechanics, can be practically used to generate entangled states for quantum technologies. The article shows how to create any entangled state of two and three qubits, and these ideas are already implemented experimentally. It seems that the considered schemes can be successfully extended to create any entangled many-particle states. As part of further research, both scientists intend to analyze in detail the postulate of identical particles, both from the standpoint of theoretical interpretation and practical applications.

A big surprise may be the fact that the postulate of indistinguishability of particles is not only a formal mathematical procedure but in its pure form leads to the consequences observed in laboratories. Is nonlocality inherent in all identical particles in the Universe? The photon emitted by the monitor screen and the photon from the distant galaxy at the depths of the Universe seem to be entangled only by their identical nature. This is a great secret that science will soon face.

The Henryk Niewodniczanski Institute of Nuclear Physics (IFJ PAN) is currently the largest research institute of the Polish Academy of Sciences. The broad range of studies and activities of IFJ PAN includes basic and applied research, ranging from particle physics and astrophysics, through hadron physics, high-, medium-, and low-energy nuclear physics, condensed matter physics (including materials engineering), to various applications of methods of nuclear physics in interdisciplinary research, covering medical physics, dosimetry, radiation and environmental biology, environmental protection, and other related disciplines. The average yearly yield of the IFJ PAN encompasses more than 600 scientific papers in the Journal Citation Reports published by the Clarivate Analytics. The part of the Institute is the Cyclotron Centre Bronowice (CCB) which is an infrastructure, unique in Central Europe, to serve as a clinical and research centre in the area of medical and nuclear physics. IFJ PAN is a member of the Marian Smoluchowski Kraków Research Consortium: "Matter-Energy-Future" which possesses the status of a Leading National Research Centre (KNOW) in physics for the years 2012-2017. In 2017 the European Commission granted to the Institute the HR Excellence in Research award. The Institute is of A+ Category (leading level in Poland) in the field of sciences and engineering.

Assoc. Prof. Pawel Blasiak
Institute of Nuclear Physics of Polish Academy of Sciences
Tel: +48 12 662 8270



Pawel Blasiak, Marcin Markiewicz
"Entangling three qubits without ever touching"
Scientific Reports 9, 20131 (2019)
DOI: 10.1038/s41598-019-55137-3

LINKS: The website of the Institute of Nuclear Physics of the Polish Academy of Sciences

Press releases of the Institute of Nuclear Physics of the Polish Academy of Sciences

The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences

Related Quantum Mechanics Articles:

New evidence for quantum fluctuations near a quantum critical point in a superconductor
A study has found evidence for quantum fluctuations near a quantum critical point in a superconductor.
Simulating quantum 'time travel' disproves butterfly effect in quantum realm
Using a quantum computer to simulate time travel, researchers have demonstrated that, in the quantum realm, there is no 'butterfly effect.' In the research, information--qubits, or quantum bits--'time travel' into the simulated past.
Orbital engineering of quantum confinement in high-Al-content AlGaN quantum well
Recently, professor Kang's group focus on the limitation of quantum confine band offset model, the hole states delocalization in high-Al-content AlGaN quantum well are understood in terms of orbital intercoupling.
A Metal-like Quantum Gas: A pathbreaking platform for quantum simulation
Coherent and ultrafast laser excitation creates an exotic matter phase with spatially overlapping electronic wave-functions under nanometric control in an artificial micro-crystal of ultracold atoms.
Fluid mechanics mystery solved
An environmental engineering professor has solved a decades-old mystery regarding the behavior of fluids, a field of study with widespread medical, industrial and environmental applications.
Quantum leap: Photon discovery is a major step toward at-scale quantum technologies
A team of physicists at the University of Bristol has developed the first integrated photon source with the potential to deliver large-scale quantum photonics.
Understanding mechanics and materials though evolution and biomaterials
Studying the evolution of bodily processes millions of years ago as well as the properties of today's biomaterials could improve soft robotics design and inform materials science research.
USTC realizes the first quantum-entangling-measurements-enhanced quantum orienteering
Researchers enhanced the performance of quantum orienteering with entangling measurements via photonic quantum walks.
A convex-optimization-based quantum process tomography method for reconstructing quantum channels
Researchers from SJTU have developed a convex-optimization-based quantum process tomography method for reconstructing quantum channels, and have shown the validity to seawater channels and general channels, enabling a more precise and robust estimation of the elements of the process matrix with less demands on preliminary resources.
What a pair! Coupled quantum dots may offer a new way to store quantum information
Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have for the first time created and imaged a novel pair of quantum dots -- tiny islands of confined electric charge that act like interacting artificial atoms.
More Quantum Mechanics News and Quantum Mechanics Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Sound And Silence
Sound surrounds us, from cacophony even to silence. But depending on how we hear, the world can be a different auditory experience for each of us. This hour, TED speakers explore the science of sound. Guests on the show include NPR All Things Considered host Mary Louise Kelly, neuroscientist Jim Hudspeth, writer Rebecca Knill, and sound designer Dallas Taylor.
Now Playing: Science for the People

#576 Science Communication in Creative Places
When you think of science communication, you might think of TED talks or museum talks or video talks, or... people giving lectures. It's a lot of people talking. But there's more to sci comm than that. This week host Bethany Brookshire talks to three people who have looked at science communication in places you might not expect it. We'll speak with Mauna Dasari, a graduate student at Notre Dame, about making mammals into a March Madness match. We'll talk with Sarah Garner, director of the Pathologists Assistant Program at Tulane University School of Medicine, who takes pathology instruction out of...
Now Playing: Radiolab

Kittens Kick The Giggly Blue Robot All Summer
With the recent passing of Ruth Bader Ginsburg, there's been a lot of debate about how much power the Supreme Court should really have. We think of the Supreme Court justices as all-powerful beings, issuing momentous rulings from on high. But they haven't always been so, you know, supreme. On this episode, we go all the way back to the case that, in a lot of ways, started it all.  Support Radiolab by becoming a member today at