The light source produces two intertwined beams of light

Quantum mechanics summary

Utilizing an optical parametric oscillator mild supply, the researchers succeeded in producing two entangled beams of sunshine. (Idea artist.)

One potential utility: enhancing the sensitivity of atomic magnetometers used to measure alpha waves emitted by the human mind.

Scientists are more and more striving to find extra about quantum entanglement, which happens when two or extra techniques are created or work together in such a method that the quantum states of some can’t be described independently of the quantum states of others. Techniques are interdependent, even when separated by an incredible distance. The curiosity in finding out any such phenomenon is because of the nice potential for functions in cryptography, communications and[{” attribute=””>quantum computing. The difficulty is that 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. An article on the study was published recently in the journal 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, last author of the article.

Optical Parametric Oscillator

The optical parametric oscillator (OPO) used in the study. Credit: Alvaro Montaña Guerrero

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.

However, this was not sufficient 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 bands of the spectrum being entangled, what we had actually 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 kind of 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.

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 quantum correlated 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 there was no intrinsic limit in the system to prevent this from happening. “We discovered that the temperature of the atoms is key to observation of quantum correlations. Apparently, the other studies used higher temperatures that prevented the researchers from observing correlations,” he said.

Reference: “Continuous Variable Entanglement in an Optical Parametric Oscillator Based on a Nondegenerate Four Wave Mixing Process in Hot Alkali Atoms” by A. Montaña Guerrero, R. L. Rincón Celis, P. Nussenzveig, M. Martinelli, A. M. Marino and H. M. Florez, 11 October 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.129.163601

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 PhD scholarships – one granted to the article’s first author Álvaro Montaña Gerreiro and the other to Raul Leonardo Rincon Celis.

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