Researchers develop a light source that produces two interlocking beams of light

Researchers have succeeded in developing a light source that produces two interlocking beams of light

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

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 connected to each other, even when they are separated by a great distance. The great potential for applications in cryptography, communications, and quantum computing is stimulating research. The difficulty is that when systems interact with their surroundings, they dissociate almost instantly.

In the latest study conducted by the Laboratory for Coherent Processing of Atoms and Light (LMCAL) at the Institute of Physics of the University of São Paulo (IF-USP) in Brazil, researchers have succeeded in developing a light source that produces two entangled beams of light. 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 dynamics of the crystal mirror produces two beams of light with quantitative correlationssaid physicist Hans Marin Florez, author of the latest article.

The problem is that the light emitted by a crystal-based OPO cannot interact with other systems of interest in the context of quantum information, such as cool atomsOr ions or flakes, because their wavelength is different from the wavelengths of the respective systems. “Our group has shown in previous work that atoms themselves can be used as a medium rather than a crystal. So we produced the first OPO based on rubidium atoms, in which two bundles were tightly interconnected, and obtained a source that could interact with other systems that have the ability to act as quantum memory, such as atoms. cold.”

However, this was not enough to show the entanglement of the beams. In addition to the intensity, the phases of the beams, which have to do with the synchronization of the light wave, also needed to display 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 quantum correlations in the amplitudes and phases of the generated fields. As a result, we were able to show that they are entangled. Moreover, the detection technique enabled us to notice that the entanglement structure was much richer than can usually be described. From the entanglement of the two adjacent spectral bands, what we actually produced was a system with four entangled spectral bands.”

In this case, the amplitudes and phases of the waves were intertwined. This is fundamental in many protocols for processing and transmitting quantum encoded information. Besides these possible applications, this kind of light source It can also be used in metrology. “Quantum intensity correlations lead to a significant reduction in 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 gets low enough, if everyone stops talking, you can hear what someone is saying from a distance.”

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

The article also indicates an additional advantage of Rubidium OPOs over crystalline OPOs. “Crystal OPOs should have mirrors that keep the light inside the cavity longer, so that the interaction produces quantum coherent beams, while using an atomic medium in which the two beams are produced more efficiently than the crystals avoids the need for mirrors to imprison 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 prove quantum correlations in rays of light produced. The new experience showed that there is no intrinsic limit in the system to prevent this from happening. “We discovered that the temperature of the atoms is the key to observing quantum correlations,” he said. “It is clear that other studies used higher temperatures that prevented the researchers from noticing the correlations.”

more information:
Montana Guerrero et al., Continuously variable entanglement in an optical parametric oscillator based on a non-generative four-wave mixing process in hot alkali atoms, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.163601

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