Researchers develop a light source that produces two intertwined light beams

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 others. The systems are correlated, even when they are separated by a great distance. The significant potential for applications in encryption, communications, and quantum computing stimulates research. The difficulty is that when systems interact with their environment, they almost immediately unravel.

In the latest study from the Laboratory for the Coherent Manipulation of Atoms and Light (LMCAL) at the Physics Institute of the University of São Paulo (IF-USP) in Brazil, researchers succeeded in developing a light source that produced two intertwined light beams. His work is published in Physical Review Letters.

“This light source was an optical parametric oscillator, or OPO, which is usually composed of a nonlinear optical response crystal between two mirrors that form an optical cavity. When a bright green beam shines into the apparatus, the dynamics of the crystal mirror produces two beams of light with quantum correlations”, said the physicist Hans Marin Florez, last author of the article.

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, since their wavelength is not the same as that of the systems in question. “Our group demonstrated in previous work that the atoms themselves could be used as a medium rather than a crystal. We therefore produced the first OPO based on rubidium atoms, in which two beams were strongly 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 enough to show that the rays were entangled. In addition to intensity, the phases of the beams, which have to do with the timing of the light waves, also needed to show 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 allowed 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 allowed us to observe that the structure of “Interlacing was richer than would normally be characterized. Instead of interlacing two adjacent bands of the spectrum, what we had actually produced was a system comprising four interlaced spectral bands.”

In this case, the amplitudes and phases of the waves were intertwined. This is essential in many protocols for processing and transmitting quantum-encoded information. In addition to these possible applications, this type of light source It can also be used in metrology. “Quantum intensity correlations result in a considerable reduction of intensity fluctuations, which can improve the sensitivity of optical sensors,” Florez said. “Imagine a party where everyone is talking and you can’t hear anyone across the room. If the noise goes 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 potential application, he added.

The article also points out an additional advantage of rubidium OPOs over crystal OPOs. “Crystal OPOs have to have mirrors that keep light inside the cavity longer, so that the interaction produces quantum correlated beams, while using an atomic medium in which the two beams are produced more efficiently that with crystals avoids the need for mirrors. imprison the light for so long,” Flórez said.

Before his group conducted this study, other groups had tried to make OPOs with atoms but were unable to demonstrate the quantum correlations in the beams of light produced. The new experiment showed that there was 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. Apparently the other studies used higher temperatures that prevented the researchers from looking at the correlations,” he said.