• PT
  • Gabriela Barreto Lemos, física e professora da UFRJ

    The adventures of light and its quantum phenomena

    September, 1st 2023

    Gabriela Barreto Lemos is an Assistant Professor at the Institute of Physics of the Universidade Federal of Rio de Janeiro (UFRJ), member of the Justice, Equity, Diversity, and Inclusion Commission of the Brazilian Society of Physics (SBF) and of the editorial board of Quantum Science and Technology (IOP Publishing).

    During her undergraduate course in Physics at Universidade Federal of Minas Gerais (UFMG), between 2000 and 2004, Gabriela had her first contact with quantum physics. Since then, she has built an extensive and passionate background in theoretical and experimental physics, with an emphasis on optics and quantum information. All this work has led the scientist to develop new experiments and image generation techniques and unprecedented research focuses, such as the Quantum Vision project – which converges neuroscience and physics to study whether and how our visual system perceives quantum phenomena.

    In this interview, Gabriela talks about light, photons, entanglement, and wave-particle duality. She also talks about the importance of asking tough questions and keeping a curious eye to build science; and how opening to new perspectives and dialogue with other areas – from art and philosophy to the other natural sciences – is key to breaking barriers and making revolutionary discoveries.

    Pioneer Science: What is a photon?

    Gabriela: There is an ancient question that we have been asking since the Egyptians: what is light and what is it made of? Throughout the history of science, different people have given different answers to this question, and it has evolved. In the 20th century, we came to understand light as something formed by minimal, indivisible energy elements, which are photons. While they behave like corpuscules, they also have a certain wave-like character – in the sense that, when experimenting with light, it manifests phenomena characteristic of experiments that we do with other types of waves, such as water, and with other types of matter, like a soccer ball. So, we say that the photon is neither a particle nor a wave, it is a quantum, a minimal element of something. In fact, it’s something very complex, because we want to think of the photon as a ball of light, but it’s not. It’s something that we really have a hard time understanding with our current way of thinking.

    What do photons have to do with quantum optics?

    Optics is the study of light and luminous effects, and it was created in Persia, just over a thousand years ago, with the book Optics. Then, it arrived in Europe, was studied by Newton, by Descartes and several models emerged on how light propagates and how it interacts with materials.

    In science, we don’t have absolute truths, only models. And the best thing we have today as a model in optics is the photon.

    Thus, quantum optics was born with the photon and studies how light interacts with atoms, with detectors (including the eye), how it spreads, how it is directed – all this, through the modeling of light as a photon. Quantum optics then explains, through photons, the adventures of light in the universe and how it interacts with matter.

    To study quantum phenomena, how important is the context of observation? What allows us to observe quantum phenomena?

    When we walk down the street, we are not aware of the quantum phenomena that are happening there, all the time. We can only access quantum phenomena more directly when we work with very controlled experiments and very sensitive detectors. And when we do that, we need to create a context of observation. I said that photons have a corpuscular character and sometimes a wave character: they are neither a corpuscule nor a wave. This means that depending on the context in which I observe photons, from the question that I pose through experimental elements, I will have different answers. If I ask “wavy” questions, the light will give me “wavy” answers. And this is different from influencing the result of the experiment, because what I define is the question: we create the context that asks the question, which has a range of possible answers, but we don’t know and do not control the answer that nature will give us.

    Of course, this, in a way, is true of all science. But in classical physics I can, for example, test what the velocity of a body is and at the same time ask what its position is at a given instant. In quantum physics, you can’t ask these two questions simultaneously: you must ask one and then the other, and then you can’t create a very clear narrative with the answers. This is what makes quantum physics very different from other natural sciences.

    In 2014, you were at the Institute for Quantum Optics and Quantum Information in Vienna, Austria, and you led an experiment published in the journal Nature in which a “ghost photograph” in the shape of a cat was produced. In this experiment, what question was asked to create this context?

    It was a very simple but complicated question. First, what can we gain by trying, at the same time, to test wave and corpuscular phenomena of light? For this, we ask questions along the way – for which we do not expect a fully corpuscular answer or a fully wave-like answer. There were famous, beautiful experiments that asked these questions in the 1990s. To this first question, another one was added: can we associate image formation with this? How can we form a photograph? So, the idea of the experiment was to use questions that mix the wave and corpuscular characters of light to evaluate the possibilities of creating images, differently from the usual paradigm of image creation.

    In the end, we produced an image in the light beam that did not interact with the object. And that won people’s imagination, a lot because it looked like a ghost.

    Since then, how has this experiment impacted your work? What have you been working with?

    One line of research I started to work on was to improve the imaging system that we had created to think about more practical applications, such as in the field of biology and material analysis. Another area I was investigating was what can I do with this system to inquire about quantum entanglement.

    I did some work on this that interested me a lot, because the answers were surprising: I discovered ways to manipulate information and share information through entanglement, something I didn’t think was possible.

    Today I am working here at UFRJ in ​​quantum metrics, which is a new area for me. I’m learning a lot about the limits of measurement accuracy, and we started a very nice project that asks the following question: if quantum properties are not predefined before we make measurements, what does an accurate measurement mean? If that only comes into being at that moment, then how can you say whether the measurement was accurate? It is a project that involves physicists and philosophers, and we have the role of making these philosophical questions appear in the laboratory as experimental questions.

    What is the potential of turning to other areas, which seem so far away, such as philosophy and the arts to think about quantum physics?

    I think it’s huge because different people have different views. I have been participating in interdisciplinary conferences, including quantum music, and I was criticized by my colleagues in physics, but I learned a lot! The works are beautiful.

    Many of the things that seem difficult to understand in quantum physics become more understandable when I look at them through art. Art is generally less dogmatic and allows for more imagination. At the same time, where art gets more dogmatic, physics pulls the rug and lets the imagination fly. Therefore, in this interaction, everyone wins!

    I learn a lot and so do my art colleagues. But modern science doesn’t give much space to these questions because a distinction between what is serious and what is fake is needed. And sometimes it’s hard, it’s not so clear. But this also makes us learn to have a critical view of ourselves, of our own work.

    Is interdisciplinarity necessary to do frontier science?

    Frontier science requires imagination. Science fiction, for example, anticipated so much science! This imagination is what takes us beyond our comfort zone, which is sometimes a dangerous place, but it is only there that you will discover new things. When the black hole came up as a solution to Einstein’s equations, he didn’t like it, it made him uncomfortable. And today we know that more than just existing, black holes have a fundamental role in the universe, they are the center of most galaxies, including ours. Fifty years have passed, and we see all this that challenges our minds, as gravitational waves – and we are here living it, how amazing! What a leap of imagination science requires from us to see how far the solution can go and what that means. If you’re too stuck in your place, you’ll sometimes dismiss these possibilities. The Quantum Vision and all these projects that I’m interested in push that limit, that frontier, a little bit. And at the same time, there are those who are going to work more within the confines of physics, which is also very important.

    In science, we need both people who want to push the boundaries and people who want to solidify what has been found.

    How did your work in quantum optics lead you to the Quantum Vision project?

    I was working in the United States when Jorgito Mol and Marcelo França wrote me, asking if I was interested in researching the interaction of neuroscience with quantum physics. And I said “let’s do it”! The idea was to study vision and our direct perception of quantum phenomena.

    So, we started researching how we can test if the eye can perceive entanglement; whether the eye directly sees the corpuscular and wave character of light; if the eye distinguishes that light is formed by photons and not by traditional waves; if we have any internal response about a quantum perception of light, even if we are not aware of these effects.

    When I returned to Brazil, during the pandemic, we started to think more seriously about performing this project. We invited more people to the group, such as Ado Jorio and Carlos Monken, from UFMG, who work with beautiful systems, Bruss Lima, a neurobiologist from UFRJ [Universidade Federal do Rio de Janeiro], and Gustavo Rohenkohl, an amazing guy who works with the psychophysics of the visual system. And it’s been essential to work with Gustavo and Bruss because they have experience with exactly the kind of experiment we’re starting now. They’re learning a lot of physics and we’re learning a lot about human perception, about the eye and how the brain processes information.

    In what experiments are you working on currently and intend to work on?

    We are rethinking quantum optics experiments that gave us light paradigm shifts, which made us understand light as a photon, in the following way: instead of using detectors to inquire about light, let’s use light to inquire about detectors. The questions change, but the basis is the same. So, let’s use the quantum character of light to ask how we detect that light, from the retina to the visual cortex and in relation to perception and behavior. Now we’re starting psychophysics experiments and then we’re going to move on to functional magnetic resonance experiments, to ask these questions at different levels. But there are so many possibilities since we are exploring something very new: we know a lot about the visual system, but very little about how it interacts with the quantum character of light.

    There are people who have already done some experiments to test the direct perception of quantum phenomena, but we are taking a slightly different path. Many experiments were done either by the neuro people or by the physics people, separately, and that generates inconsistencies and unanswered questions. We are joining the two fields, and this is a differential.

    As you said, it’s all very new: what are the expectations of dealing with a research like this, with many questions, paths that are still unknown, but with revolutionary potential?

    Magnetic Resonance Imaging (MRI) itself is almost a miracle because technology like this is not discovered overnight. First came the discovery of the fundamental phenomenon of the interaction of the spin of molecules or atoms with external magnetic fields. So then, we thought: these atoms are inside us, so how can they respond to this interaction? And from that, putting a person inside a tube, creating a magnetic field, and making an image of how the tissue responds at the molecular level is amazing! And that technology is already far beyond the fundamental physics that created it. So even when it appears to go nowhere, science goes somewhere; even when the answer to a question is negative, we learn something.

    It may be that an experiment that we do now does not show that human beings perceive the corpuscular effect of light, but we will learn something, and that will lead us to another question, and this other question may lead us to another answer. So, every answer is interesting because it’s going to take us down different paths.

    See this infographic for a timeline showing how we view light throughout history.


    See also