Light Source Producing Two Entangled Light Beams Developed
Published: 2023-01-03 - Updated: 2023-01-17
Author: Fundação de Amparo à Pesquisa do Estado de São Paulo | Contact: https://fapesp.br/en
Peer-Reviewed Publication: Yes | DOI: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.163601
Additional References: Medical Research News Publications
Synopsis: A new light source producing two entangled light beams could be used to enhance the sensitivity of atomic magnetometers used to measure alpha waves emitted by the brain. We produced the first OPO based on rubidium atoms, in which two beams were intensely quantum-correlated, and obtained a source that could interact with other systems with the potential to serve as quantum memory, such as cold atoms. We repeated the experiment but added new detection steps that enabled us to measure the quantum correlations in the amplitudes and phases of the fields generated. As a result, we were able to show they were entangled. Furthermore, the detection technique enabled us to observe that the entanglement structure was richer than would typically be characterized. Instead of two adjacent spectrum bands being entangled, we produced a system comprising four entangled spectral bands.
- Quantum Entanglement
Quantum entanglement is the phenomenon that occurs when a group of particles is generated, interacts, or shares spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when a considerable distance separates the particles.
Continuous Variable Entanglement in an Optical Parametric Oscillator Based on a Nondegenerate Four Wave Mixing Process in Hot Alkali Atoms.
Scientists are increasingly seeking to discover more about quantum entanglement, which occurs when two or more systems are created or interact in such a manner that the quantum states of some cannot be described independently of the quantum states of others. The systems are correlated, even when a large distance separates them. Interest in studying this phenomenon is due to the significant potential for encryption, communications, and quantum computing applications. When the systems interact with their surroundings, they almost immediately become disentangled.
In the latest study by the Laboratory for Coherent Manipulation of Atoms and Light (LMCAL) at the University of São Paulo's Physics Institute (IF-USP) in Brazil, the researchers succeeded in developing a light source that produced two entangled light beams.
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships, and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil.
An article on the study is published in Physical Review Letters.
"This light source was an optical parametric oscillator, or OPO, which is typically made up of a non-linear optical response crystal between two mirrors forming an optical cavity. When a bright green beam shines on the apparatus, the crystal-mirror dynamics produce two light beams with quantum correlations," said physicist Hans Marin Florez, the last author of the article.
The problem is that light emitted by crystal-based OPOs cannot interact with other systems of interest in the context of quantum information, such as cold atoms, ions, or chips, since its wavelength is not the same as those of the systems in question.
"Our group showed in previous work that atoms themselves could be used as a medium instead of a crystal. We, therefore, produced the first OPO based on rubidium atoms, in which two beams were intensely quantum-correlated, and obtained a source that could interact with other systems with the potential to serve as quantum memory, such as cold atoms," Florez said.
This was insufficient to show the beams were entangled. In addition to the intensity, the beams' phases, which have to do with lightwave synchronization, also needed to display quantum correlations.
"That's precisely what we achieved in the new study reported in Physical Review Letters," he said. "We repeated the same experiment but added new detection steps that enabled us to measure the quantum correlations in the amplitudes and phases of the fields generated. As a result, we were able to show they were entangled. Furthermore, the detection technique enabled us to observe that the entanglement structure was richer than would typically be characterized. Instead of two adjacent spectrum bands being entangled, what we had produced was a system comprising four entangled spectral bands."
In this case, the amplitudes and phases of the waves were entangled. This is fundamental in many protocols to process and transmit quantum-coded information. Besides these possible applications, this light source could also be used in metrology.
"Quantum correlations of intensity result in a considerable reduction of intensity fluctuations, which can enhance the sensitivity of optical sensors," Florez said. "Imagine a party where everyone is talking, and you can't hear someone on the other side of the room. If the noise decreases sufficiently, if everyone stops talking, you can hear what someone says from a good distance away."
Enhancing the sensitivity of atomic magnetometers used to measure the alpha waves emitted by the human brain is one of the potential applications, he added.
About the Study
The study was supported by FAPESP through a Thematic Project coordinated by IF-USP Professor Marcelo Martinelli, one postdoctoral scholarship granted to Florez, and two Ph.D. scholarships - one granted to the article's first author Álvaro Montaña Gerreiro and the other to Raul Leonardo Rincon Celis.
The article also notes an additional advantage of rubidium OPOs over crystal OPOs.
"Crystal OPOs have to have mirrors that keep the light inside the cavity for longer so that the interaction produces correlated quantum beams, whereas the use of an atomic medium in which the two beams are produced more efficiently than with crystals avoids the need for mirrors to imprison the light for such a long time," Florez said.
Before his group conducted this study, other groups had tried to make OPOs with atoms but failed to demonstrate quantum correlations in the light beams produced. The new experiment showed no intrinsic limit in the system to prevent this from happening.
"We discovered that the temperature of the atoms is key to the observation of quantum correlations. The other studies used higher temperatures that prevented the researchers from observing correlations," he said.
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