Scientists are increasingly studying quantum entanglement, which occurs when two or more systems are created or interact in such a way that the quantum states of some cannot be described independently of the quantum states of the others. The systems are correlated even when separated by a large distance. The significant potential for applications in encryption, communications and quantum computing is spurring research. The difficulty is that when the systems interact with their environment, they become untangled almost instantly.
In the latest study from the Laboratory for Coherent Manipulation of Atoms and Light (LMCAL) at the University of São Paulo’s Physics Institute (IF-USP) in Brazil, researchers succeeded in developing a light source that produced two entangled light beams. Their work has been published in Physical review letters.
“This light source was an optical parametric oscillator, or OPO, which typically consists of a nonlinear optical response crystal between two mirrors that form an optical cavity. When a bright green beam shines on the device, the crystal-mirror dynamics produce two beams of light with quantum correlations,” said physicist Hans Marin Florez, last author of the paper.
The problem is that light emitted by crystal-based OPOs cannot interact with other systems of interest in quantum information, such as cold atoms, ions, or chips, since its wavelength is not the same as that 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, where 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 that the beams were entangled. In addition to the intensity, the phases of the beams, which have to do with light wave synchronization, should also show quantum correlations. “This is exactly 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 generated fields. As a result, we were able to show that they were entangled. Furthermore, the detection technique did we were able 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 consisting of four entangled spectral bands.”
In this case, the amplitudes and phases of the waves were entangled. This is fundamental to many protocols for processing and transmitting quantum encoded information. Besides these possible applications, this kind of light source can also be used in metrology. “Quantum correlations of intensity result in a significant reduction of intensity fluctuations, which can increase the sensitivity of optical sensors,” said Florez. “Imagine a party where everyone is talking and you can’t hear anyone across the room. If the noise dies down enough, if everyone stops talking, you can hear what someone is saying from a good distance away.”
Improving the sensitivity of atomic magnetometers used to measure alpha waves emitted by the human brain is one of the potential applications, he added.
The paper also notes an additional advantage of rubidium OPOs over crystal OPOs. “Crystal OPOs must have mirrors that keep the light inside the cavity for a longer time so that the interaction produces quantum-correlated beams, whereas the use of an atomic medium, where the two beams are produced more efficiently than with crystals, avoids the need for mirrors. to imprison the light for so long,” Florez said.
Before his group performed this study, other groups had tried to make OPOs with atoms, but failed to detect quantum correlations in the light beams produced. The new experiment showed that there was no inherent limit in the system to prevent this from happening. “We discovered that the temperature of the atoms is the key to observing quantum correlations. Apparently, the other studies used higher temperatures that prevented the researchers from observing correlations,” he said.
More information:
A. Montaña Guerrero et al., Continuously variable entanglement in an optical parametric oscillator based on a non-degenerate four-wave mixing process in hot alkali atoms, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.163601
Citation: Scientists develop light source that produces two entangled light beams (2023, January 3) Retrieved January 4, 2023, from https://phys.org/news/2023-01-source-entangled.html
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