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 one cannot be described independently of the quantum states of others. Systems are correlated even when they are separated by a large distance. The significant potential for applications in encryption, communications, and quantum computing is driving research. The difficulty is that when systems interact with their environment, they unravel almost immediately.
In the latest study by the Laboratory for Coherent Manipulation of Atoms and Light (LMCAL) of the Institute of Physics of the University of São Paulo (IF-USP) in Brazil, researchers have succeeded in developing a light source that generates 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 crystal with a non-linear optical response between two mirrors forming an optical cavity. When a bright green beam hits the device, the crystal-mirror dynamics creates two light beams with quantum correlations,” said physicist Hans Marin Flores, the paper’s final author.
The problem is that the 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, because its wavelength does not match the wavelength of the systems under consideration. “Our group has shown in previous work that the atoms themselves can be used as a medium instead of a crystal. Therefore, we created the first OPO based on rubidium atoms, in which the two beams were strongly quantum-correlated, and received a source that could interact with other systems that can serve as quantum memory, such as cold atoms,” Flores said.
However, this was not enough to show that the rays were entangled. In addition to the intensity of the phase of the beams, which are related to the timing of the light waves, should also display quantum correlations. “This is exactly what we have achieved in a new study published in Physical Review Letters,” he said.
“We repeated the same experiment but added new detection steps that allowed us to measure quantum correlations in the amplitudes and phases of the generated fields. As a result, we were able to show that they are entangled. In addition, the detection method allowed us to notice that the structure of entanglement was richer than one would normally characterize. Instead of two entangled adjacent spectral bands, we actually created a system consisting of four entangled spectral bands.”
In this case, the amplitudes and phases of the waves were mixed up. This is fundamental in many protocols for processing and transmitting quantum-encoded information. Apart from these possible applications, this type of light source can also be used in metrology. “Quantum intensity correlations lead to a significant reduction in intensity fluctuations, which can increase the sensitivity of optical sensors,” said Flores. “Imagine a party where everyone is talking and you can’t hear anyone on the other side of the room. If the noise decreases enough, if everyone stops talking, you can hear someone talking at a decent distance.”
He added that increasing the sensitivity of the atomic magnetometers used to measure alpha waves emitted by the human brain is one potential application.
The article also notes an additional advantage of rubidium OPOs over crystalline OPOs. “Crystal OPOs should have mirrors that keep the light inside the resonator longer so that the interaction creates quantum-correlated beams, while using an atomic medium that produces two beams more efficiently than with crystals avoids the need for mirrors. to imprison light for such a long time,” Flores said.
Before his group did this research, other groups tried to create OPOs with atoms, but were unable to demonstrate quantum correlations in the light beams they created. The new experiment showed that there is no internal limitation in the system that could prevent this. “We found that the temperature of the atoms is the key to observing quantum correlations. Apparently, other studies used higher temperatures, which made it difficult for researchers to observe correlations,” he said.
Additional Information:
A. Montagna Guerrero et al., Continuous variable entanglement in an optical parametric oscillator based on a non-degenerate four-wave mixing process in hot alkali metal atoms, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.163601
Quote: Researchers develop a light source that produces two entangled light beams (January 3, 2023), retrieved January 3, 2023, from https://phys.org/news/2023-01-source-entangled.html.