Researchers discover a new type of quantum entanglement

A study from Technion unveils a newly discovered form of quantum entanglement in the total angular momentum of photons confined in nanoscale structures. This discovery could play a key role in the future miniaturization of quantum communication and computing components.

Quantum physics sometimes leads to very unconventional predictions. This is what happened when Albert Einstein and his colleagues, Boris Podolsky and Nathan Rosen (who later founded the Faculty of Physics at Technion), found a scenario in which knowing the state of one particle immediately affects the state of the other particle, no matter how great the distance between them. Their historic 1935 paper was nicknamed EPR after its three authors (Einstein–Podolsky–Rosen).

The idea that knowing the state of one particle will affect another particle located at a huge distance from it, without physical interaction and information transfer, seemed absurd to Einstein, who called it “spooky action at a distance.”

But groundbreaking work by another Technion researcher, Research Prof. Asher Peres from the Faculty of Physics, showed that this property can be used to transmit information in a hidden way—quantum teleportation, which is the basis for quantum communication. This discovery was made by Prof. Peres with his colleagues Charles Bennett and Gilles Brassard.

The phenomenon later received the scientific name quantum entanglement, and for its measurement and implications, which include the possibility of quantum computing and quantum communication, the 2022 Nobel Prize in Physics was awarded to Profs. Alain Aspect and Anton Zeilinger, who previously received honorary doctorates from the Technion, and their colleague Prof. John Clauser.

Quantum entanglement has been demonstrated so far for a wide variety of particles and for their various properties. For photons, particles of light, entanglement can exist for their direction of travel, frequency (color), or the direction in which their electric field points. It can also exist for properties that are harder to imagine, such as angular momentum.

This property is divided into spin, which is related to the photon’s rotation of the electric field, and orbit, which is related to the photon’s rotational motion in space. This is intuitively similar to Earth, which rotates on its axis and also orbits the sun in a circular path.

It is easy for us to imagine these two rotational properties as separate quantities, and indeed, photons bound in a beam of light much wider than their wavelength. However, when we try to put photons into structures smaller than the photonic wavelength—which is the endeavor of the field of nanophotonics—we discover that it is impossible to separate the different rotational properties, and the photon is characterized by a single quantity, the total angular momentum.

So why would we even want to put photons into such small structures? There are two main reasons for this. One is obvious—it will help us to miniaturize devices that use light and thus squeeze more operations into a small area cell, similar to the miniaturization of electronic circuits.

The other reason is even more important: this miniaturization increases the interaction between the photon and the material through which the photon is traveling (or is near), thus allowing us to produce phenomena and uses that are not possible with photons in their “normal” dimensions.

In a study published in the journal Nature, the Technion researchers, led by Ph.D. student Amit Kam and Dr. Shai Tsesses, discovered that it is possible to entangle photons in nanoscale systems that are a thousandth the size of a hair, but the entanglement is not carried out by the conventional properties of the photon, such as spin or trajectory, but only by the total angular momentum.

The Technion researchers revealed the process that photons undergo from the stage in which they are introduced into the nanoscale system until they exit the measurement system, and found that this transition enriches the space of states that the photons can reside in.

In a series of measurements, the researchers mapped those states, entangled them with the same property unique to nanoscale systems, and confirmed the correspondence between photon pairs that indicates quantum entanglement.

This is the first discovery of a new quantum entanglement in more than 20 years, and it may lead in the future to the development of new tools for the design of photon-based quantum communication and computing components, as well as to their significant miniaturization.