Scientists are increasingly interested in learning more about the quantum entanglement that occurs when two or more systems are formed or interact in such a way that the quantum states of one cannot be described independently of the quantum states of the others. Systems are related even when separated by great distances. The interest in studying such a phenomenon stems from the significant potential for applications in encryption, communications and quantum computing. The difficulty is that systems intermingle almost instantly when they interact with their environment.

In a recent study conducted by 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 were able to develop a light source that produces two entangled beams. light. An article about the research was recently published in a journal. Physical Review Letters.

“This light source was an optical parametric oscillator, or OPO, consisting of a crystal with a nonlinear optical response between two mirrors, typically forming an optical cavity. Physicist Hans Marin Florez, the paper’s last author, said, “When a bright green beam shines on the device, the crystal mirror “It produces two beams of light whose dynamics have quantum correlations,” he said.

The problem is that the light emitted by the crystal-based OPO cannot interact with other related systems in the context of quantum information, such as cold atoms, ions or chips, because the wavelength is not the same as that of the corresponding systems. “Our group has shown in previous work that the atoms themselves can be used as a medium rather than a crystal. Thus, we have produced the first OPO based on rubidium atoms where the two rays are heavily quantum correlated, and we have a source that can interact with other systems, such as cold atoms, that can act as a quantum memory.” said.

However, this was not enough to show that the bundles were entangled. In addition to intensities, the representation of quantum relations also requires the phases of the rays, which are related to the synchronization of light waves.

“This is exactly what we got in a new reported study. Physical Review Letterssaid. “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 confused. Also, the detection technique allowed us to see that the entanglement structure was richer than what is usually characterized. Instead of two adjacent spectral bands entanglement , we actually created a system of four entangled spectral bands.”

In this case, the amplitudes and phases of the waves are mixed. This is essential 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 density correlations lead to a significant reduction in intensity fluctuations that can improve the sensitivity of optical sensors,” Florez said. “Imagine a party where everyone is talking and you can’t hear anyone from across the room. If the noise is low enough, if everyone stops talking, you can hear someone speaking from far enough away.”

He added that increasing the sensitivity of atomic magnetometers used to measure alpha waves emitted by the human brain is a potential application.