Tracking down the mystery of entangled particles of light

June 14, 2018, University of Bern
Dr. Stefan Lerch is adjusting the source of energy entangled photons, which was used in an experiment demonstrating a transition from quantum to classical energy correlations. Credit: André Stefanov, University of Bern

Bernese researchers have taken an important step towards new measurement methods such as quantum spectroscopy. In an experiment, they succeeded in uncovering part of the mystery surrounding the so-called "entangled photons" and gaining fine control on the measured correlations.

Quantum technologies hold the promise to go beyond the capabilities of classical present technologies by making use of pure phenomenon, such as "." Quantum technologies are used in various applications, for example in quantum computers or in quantum sensing and metrology, which allows for imaging with higher resolution or determine more accurately properties of atoms and molecules.

Entangled particles

Entanglement is one of the most impressive quantum physical phenomena. It describes the property of two particles not behaving like two independent objects, but like a single physical object. The entanglement is not to be understood spatially: Entangled correlate with each other in terms of their properties. This means that if you change the properties of one particle, the other particle changes at the same time, no matter where it is. Particles of light (photons) can be entangled by splitting a single particle into two photons in a laser arrangement with a special crystal. In optics, are a major component in the development of new quantum measurement methods. They can be used because the measurement capacity of an entangled photon pair is larger than the one of two individual photons. However, quantum entanglement leads to the observation of relationships between measurements at the photon pairs, which can only be explained quantum-mechanically and not with concepts of classical physics.

Until now there has been no method to produce photon pairs that do not show quantum mechanical, but only classical energy correlations. In an experiment, a research team of the Institute of Applied Physics at the University of Bern has now succeeded in transforming the observed correlations of photon pairs from purely quantum-mechanical to completely classical. This transition represents a novelty, since quantum and classical correlations are difficult to reconcile. The researchers were able to demonstrate the transition in an experiment with a new method in which they were able to control the of the energies of two photons. The results were published in the journal Nature Communications Physics.

Shaking the photons

The entanglement of the photons is a so-called "energy-time entanglement," since the photons correlate with respect to both the emission time and the energy. Both correlations can be observed experimentally and allow conclusions to be drawn about each other. But since the researchers wanted to detect only the correlations in time of the , they had to grab into their bag of tricks: "In order to form such pairs, we randomly shook the photons, so to speak," explains Dr. Stefan Lerch, lead author of the study. By doing that, the researchers induced a perturbation. "The more perturbation was added, the less did the photons behave in a quantum way."

To change the quantum state of the photons, the researchers made use of techniques which are usually applied for the shaping of ultrashort laser pulses. "The know-how, that was developed at the University of Bern within the frame of the NCCR MUST was essential to achieve the precise control needed," notes study co-author Prof. Dr. André Stefanov.

The most promising potential application of energy-time entangled photons is spectroscopy, a physical method to investigate properties of molecules with light. "I expect entangled spectroscopy to be a ground-breaking new way of performing optical spectroscopy," says André Stefanov. It remains however to be experimentally demonstrated. The findings of the Bernese researchers are an important step on this path. "I am convinced that such a setup will be an essential component of future quantum spectroscopy experiments," adds André Stefanov.

Explore further: New technique can capture images of ultrafast energy-time entangled photon pairs

More information: Stefan Lerch et al. Observing the transition from quantum to classical energy correlations with photon pairs, Communications Physics (2018). DOI: 10.1038/s42005-018-0027-2

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5 / 5 (1) Jun 14, 2018
There is better troll bait in the link "New technique can capture images of ultrafast energy-time entangled photon pairs" right above the comment section. The artist's depiction explains it all quite simply. The problem is with the paradigm, in which the photon is a particle, and the inference to our understanding is that the crystal splits the particle into two particles which then become entangled, no matter how far apart they are.

In fact, photons should not be construed as particles, but as wave fronts. No matter how far apart the photon detectors are, they are analyzing the same wave front, ergo the same photon. Therefore, a wave front of light arriving here from a star billions of light years away is the same wave front arriving in the other direction from us away from that star also billions of light years away. It is the same photon, and it can be observed to be "entangled", if two instruments far apart detect it at the same time.

Where's the mystery?
1 / 5 (1) Jun 14, 2018
It is puzzling to me that mainstream physicists still regard photons as particles of light instead of wave fronts. That same radiating wave from that star described in my comment above spreads ever outward, but where are all those so-called photon particles coming from? The photon particles would need to be infinitely replicating as that wave spreads ever outward, but that is a violation of the first law of thermodynamics. So, not a particle.
not rated yet Jun 14, 2018
The author starts at "entanglement with correlation in time" (I thought entanglement was instant with no time invovled), then pivots to using lasers, then again pivots to spectroscopy without any explanation of what it means.
This all seems like a lot of smoke and mirrors.
not rated yet Jun 14, 2018
I under stand what you are saying about the same wavefront. It makes sense. So there is really no entanglement.

But in these experiments , Bob and Alice would need to be exactly the same distance from the photon source. I would doubt that the two receiver instruments would be exactly the same radius away from the source.

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