Spectroscopy with Entangled Photons

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. (Source: A. Stefanov, U. Bern)

Quantum techno­logies hold the promise to go beyond the capa­bilities of classical present techno­logies by making use of pure quantum pheno­menon, such as entangled particles. 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 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. Photons can be entangled by splitting a single particle into two photons in a laser arrange­ment with a special crystal. In optics, entangled photons 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 corre­lation of the energies of two photons.

With the energy-time entangle­ment 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 photon pairs, they had to grab into their bag of tricks: “In order to form such pairs, we randomly shook the photons, so to speak,” explains Stefan Lerch. 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 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 photon 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. (Source: Univ. Bern)

Reference: S. Lerch et al.: Observing the transition from quantum to classical energy correlations with photon pairs, Commun. Phys. 1, 26 (2018); DOI: 10.1038/s42005-018-0027-2

Link: Quantum Optics Lab, Institute of Applied Physics, University of Bern, Bern, Switzerland

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