Quantum Breakthrough: Light Source Produces Two Entangled Light Beams

Quantum Mechanics Abstract

Researchers using an optical parametric oscillator light source succeeded in producing two entangled light beams. (Artist’s concept.)

One potential application: Enhancing the sensitivity of atomic magnetometers used to measure the alpha waves emitted by the human brain.

Scientists are increasingly seeking to discover more about quantum entanglement, which occurs when two or more systems are created or interact in such a manner that the quantum states of some cannot be described independently of the quantum states of the others. The systems are correlated, even when they are separated by a large distance. Interest in studying this kind of phenomenon is due to the significant potential for applications in encryption, communications, and quantum computing. The difficulty is that when the systems interact with their surroundings, they almost immediately become disentangled.

In the latest study by the Laboratory for Coherent Manipulation of Atoms and Light (LMCAL) at the University of São Paulo’s Physics Institute (IF-USP) in Brazil, the researchers succeeded in developing a light source that produced two entangled light beams. An article on the study was published recently in the journal Physical Review Letters.

“This light source was an optical parametric oscillator, or OPO, which is typically made up of a non-linear optical response crystal between two mirrors forming an optical cavity. When a bright green beam shines on the apparatus, the crystal-mirror dynamics produce two light beams with quantum correlations,” said physicist Hans Marin Florez, last author of the article.

Optical Parametric Oscillator

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

The problem is that 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 its wavelength is not the same as those 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, in which 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 the beams were entangled. In addition to the intensity, the beams’ phases, which have to do with lightwave synchronization, also needed to display 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 enabled us to measure the quantum correlations in the amplitudes and phases of the fields generated. As a result, we were able to show they were entangled. Furthermore, the detection technique enabled us 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 comprising four entangled spectral bands.”

In this case, the amplitudes and phases of the waves were entangled. This is fundamental in many protocols to process and transmit quantum-coded information. Besides these possible applications, this kind of light source could also be used in metrology. “Quantum correlations of intensity result in a considerable reduction of 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 decreases sufficiently, if everyone stops talking, you can hear what someone says from a good distance away.”

Enhancing the sensitivity of atomic magnetometers used to measure the alpha waves emitted by the human brain is one of the potential applications, he added.

The article also notes an additional advantage of rubidium OPOs over crystal OPOs. “Crystal OPOs have to have mirrors that keep the light inside the cavity for longer, so that the interaction produces quantum correlated beams, whereas the use of an atomic medium in which the two beams are produced more efficiently than with crystals avoids the need for mirrors to imprison the 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 demonstrate quantum correlations in the light beams produced. The new experiment showed there was no intrinsic limit in the system to prevent this from happening. “We discovered that the temperature of the atoms is key to observation of quantum correlations. Apparently, the other studies used higher temperatures that prevented the researchers from observing correlations,” he said.

Reference: “Continuous Variable Entanglement in an Optical Parametric Oscillator Based on a Nondegenerate Four Wave Mixing Process in Hot Alkali Atoms” by A. Montaña Guerrero, R. L. Rincón Celis, P. Nussenzveig, M. Martinelli, A. M. Marino and H. M. Florez, 11 October 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.129.163601

The study was supported by FAPESP through a Thematic Project coordinated by IF-USP Professor Marcelo Martinelli, one postdoctoral scholarship granted to Florez, and two PhD scholarships – one granted to the article’s first author Álvaro Montaña Gerreiro and the other to Raul Leonardo Rincon Celis.

7 Comments on "Quantum Breakthrough: Light Source Produces Two Entangled Light Beams"

  1. So, it was no real breakthrough as usual when that word is used?

    • If it was published in the Physical Review Letters, it’s probably safe to classify it as a breakthrough, yes.

  2. This is an awesome place to find all the best and latest news about amazing topics.

  3. I strongly feel that ” biological-entanglement exists from the time im-memorial, but without any scientific basis.
    We give it different nomenclatures as we fail to support it experimentally, but it exists.Different from encephalographic waves, because these waves don’t care the distance between the two subjects miles apart. Very weird to even imagine the tele-portation.


  4. It can be used in coding of data flowing in optical fibers. Two signals at a time.

  5. I can make these at he whenever I wish don’t know who to contact on about how I can manipulate making them rings

  6. Fixed gravity for you. | January 15, 2023 at 11:35 pm | Reply

    Suppose you have three identical light clocks and a rocket ship. You leave one light clock on earth and take two of the clocks with you to the moon. As you travel to the moon the frequency of the light clocks you carry never changes, but the light clock at home, seen through a telescope, appears to be gradually dropping in frequency.

    When you get to the moon you leave one light clock there and head back home. On the way back, light from the clock you left on the moon appears to be gradually increasing in frequency. Of course at the same time the light from the clock you left at home appears gradually to begin returning to its original frequency.

    Two possible ways to explain it. One way uses constant light speed plus a variable time rate, a variable distance measure and a variable wavelength. The other way uses constant time rate, constant distance measures, constant wavelength and variable light speed. Both ways require light clocks that create a frequency that doesn’t depend on gravity, in both cases a common property of light particles after they are created is that their frequency is responsive to changes in gravity.

    Someone who doesn’t have access to the moon could only guess which way is correct.

    Suppose light clock frequencies eventually large enough that you can detect a frequency change coming from a light clock on the ceiling. Now you don’t need to go to the moon to decide which way light must behave under a gravity change. You can put one light clock on the ceiling and another on the floor, each with its own wave-cycle counter, you can send entangled light pulses to both clocks multiple times and register the time delay between the pulses with both clocks. If the rate of time is truly different between the floor and ceiling, the wave-cycle counters should disagree, otherwise they will agree.

    Consider that such clocks have not been around for very long, the spacing between pulses would have to be large enough to register on the counts and it likely takes a while to check the results using as many runs as needed to get a reliable count difference. Then there is a delay wherein gatekeepers have to decide if people should begin to be told that simultaneity in entanglement effects has shown that Einstein was wrong and that the concept of absolute simultaneity is valid. That would be what began to happen late last year, if you ask me.

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