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Interview

The pleasure of doing science

With his eyes open to the future of Physics, award-winning researcher says that the necessary social responsibility of the scientist does not obscure the playful nature of the practice of science

Last March, the 55-year old, Rio de Janeiro Born physicist Luiz Davidovich, received by fax and “with surprise”, the 2001 Physics Prize of the Third World Academy of Sciences. Before him, only two other Brazilians had won it: Cesar Lattes and Jayme Tiomno. It is natural that Davidovich should feel himself “honored to be in this company”. In material terms, the honor consists of a commemorative plaque and of US$ 10,000 that will be handed over at a special ceremony, during the Academy’s General Conference, in New Delhi, next October. The reason for being awarded the prize is the set of works carried out by Davidovich on quantum optics, which, according to his peers, represents an important contribution towards the development of Physics.

In this interview granted to Pesquisa Fapesp, this Brazilian talks about his work, in particular about his theoretical propositions on the transition of phenomena from the quantum world to the classic world, which involve such sophisticated and complicated concepts for a laymen as the superposition of two states of a particle, and of interference, going on to teletransportation, which, for less accustomed ears, borders the fantasies of science fiction.

But Davidovich, picking up again ideas that he presented in an article for Notícias FAPESP (the publication that originated Pesquisa Fapesp ), in September 1999, also comments in this interview the trends in development and the challenges that Physics faces today. He talks about the fantastic possibilities for its practical applications. He addresses the advances and the difficulties of Physics in Brazil, he has a critical stance regarding Brazilian scientific policy, and, finally, produces a highly elucidative phrase on the pleasure that is involved in doing scientific research: “For those involved with it, science is before anything else an activity of play”.

Davidovich was a professor at the Pontifical Catholic University of Rio de Janeiro (PUC-Rio) from 1977 to 1994 and at the Institute of Physics of the Federal University of Rio de Janeiro (UFRJ) since then. He has a doctorate from the University of Rochester, in New York State, and post-doctorate qualifications from the Federal Institute of Technology (ETH) in Zurich, Switzerland, focused on the same work about the dynamics of the decaying of an atom that he has started with his doctorate. Davidovich is married to psychologist Solange Cantanhede, and has two sons and two stepdaughters (“but it is actually like having four children”, he says). One of the sons is an engineer and the other a lawyer, and one of the daughters is a psychologist and the other a model (Isabel Ibsen). The main excerpts from the interview with the award winning physicist now follow (the complete version can be found in our website, www.revistapesquisa2.fapesp.br)

What was the reason for the prize?
– The prize was given for a set of works on the theory of the laser and for proposals for experiments in the area of the foundations of quantum mechanics that I have been working on for 17 years.

In a recent review of your area, quantum optics, you show some interesting things about the relationship between the macroscopic physical world and the microscopic world of quantum physics. Could you talk to us about it?
– This is one of the lines of research that has taken up our time in recent years. The idea is the following: in the microscopic world, there are quantum phenomena, difficult to grasp with our intuition. One of these is interference, when, for example, we do an experiment… as matter of fact, it is a classic experiment carried out in the 19th century, the (Thomas) Young experiment[RJS3]. He was an English physicist who showed that when light is passed through a screen that has two slits close to each other, a figure is produced on another screen with light and dark fringes.

In other words, light plus light – because the light is passing through two slits – can result in shade. This figure is the so-called interference fringe, a typically wave motion phenomenon, which we can observe in a tank of water, when waves are produced by two oscillating pins, close to each other. These waves interfere with each other. There are regions where the crest of one of the waves is added to the wave produced by the other, there is a reinforcement of the amplitude of the wave. In other regions, the crest of one superposes the trough of the other, and in this case the waves cancel themselves out. In a tank of water, you can see that there is a flat line at these points where the amplitudes cancel themselves out. This phenomenon can also be seen with waves of light. It so happens that in the 20th century one learned that light is made up of corpuscles, and from that point to reconcile this wave motion behavior with the make up of corpuscles has become a great challenge for physicists.

In a classic view, there would be no way of understanding the superposition of the two data.
– Yes, because people expected the corpuscle to pass through one slit or the other. And in this case it is never going to produce an interference fringe, characteristic of a wave, which passes through both slits at the same time. The reconciliation of this complementary nature of light came about with quantum mechanics. It was through quantum mechanics that we know that when we do an experiment with light we can actually do complementary experiments. For example, we can ask ourselves “which slit did the corpuscle pass through?” If we place apparatuses behind the slits, we will discover that it went through one or the other.

But by doing this we eliminate the interference fringe. It is not possible to discover which slit the corpuscle passed through and, at the same time, to keep the interference fringe. Then came the concept of complementarity in the 20th century, which put in evidence complementary aspects of nature. Light can behave like a wave or like a corpuscle, depending on the experiment that is done. The arrangement of the experiment came to be an important part of the phenomenon observed, and this was really a conceptual revolution in physics, in the 1920’s.

We are talking about a time right after Einstein.
– That’s right. It was the contributions from Max Born, Niels Bohr and others that made it possible to understand very well quantum mechanics and the new concepts it stood for. However, when we look at the macroscopic world around us, we do not see one person in a superposition of two localized states.

Passing through one door and…
– …through the other, at the same time. Or a superposition of states in which it is simultaneously located in one door and in another contiguous one. And for a long time people asked the question of how we can explain this, how do we explain the transition from the quantum world to the classic world. Indeed, Einstein, in 1954, wrote a letter to Born, saying that he found it odd that the classic world apparently prohibited the majority of states permitted by the quantum world, which are these superpositions. The question he posed was fundamental. “Why is there this selection of states in the classic world?” There have been various explanations in the last few years, and one that has been asserted is that necessarily the macroscopic bodies are interacting with the rest of the universe. This interaction destroys the possibility of carrying out experiments on interference that evidence superposition.

This means that the possibility of superposition exists even in the macroscopic world, it is just not observable.
– Precisely. This question was put in an extreme dramatic manner by physician Erwin Schrödinger, who, in an article in 1935, drew up the famous cat paradox. He would say the follow: if we put a cat in a hermetically sealed cage, together with an atom that may decay, we may suppose that this atom, by decaying, sets into motion a mechanism, a little motor that breaks a bottle containing cyanide, which kills the cat. On the other hand, if the atom does not decay, the cat stays alive. Now, the atom is, at a given moment, in a superposition of two states, precisely like the particle that passes through the slit in Young’s experiment.

This atom can then be described as the superposition of the states of the atom that decayed and of the atom that did not decay. Of course, if we wait a long time, the component that represents the decayed atom will become much more important than the other, but at an intermediate instant, the two components are there… This atom is in a superposition. The state of the cat depends on the state of the atom. So, if the atom is in a superposition, the cat too ought to be in a superposition. Dead and not dead. Schrödinger says: “This is not what we see”. How do we explain this question? We know today that a cat cannot be isolated, nor an atom. The atom-cat system cannot be isolated from the rest of the universe. Interaction with the rest of the universe quickly destroys this quality of superposition.

What is your focus of research in this universe?
– Years ago, we proposed an experiment, in collaboration with physicists from the École Normale Supérieure, in Paris, to test the idea that the quantum world is rapidly transforming itself into a classic world. The experiment was done in Paris, in 1996, and it confirmed the theory. In a cavity formed by two parallel mirrors – which is not quite a box, since it is open, and it manages to store photons, for a very long time – an electromagnetic field is produced, very close to a classic field, as if it were a classic light, but in the region of microwaves. Next, an atom is passed through the cavity, and by manipulating the atom this field is successfully put into a superposition of two states, which corresponds to two classic fields that differ in phase. We can say that they are two fields that one can differentiate classically.

And you managed to confirm this observing only the light in the frequency of microwaves.
– Yes. But the question that arose afterwards is: “How can this be measured?” And the idea was to send a second atom, sensitive to this state of the field, inside the cavity. In the case of Schrödinger’s cat, it would be analogous to sending along a mouse that is sensitive to the state of the cat and characterizes that there is a superposition of the two states that originates the interference. And there is more to it than that: if I delay a bit sending the mouse in, I give time for the quantum superposition to transform itself into a classic alternative, such as the cat dead or alive.

That is to say, sending an atom at different times, I can follow the process by which the quantum superposition is transformed into a classic alternative. We suggested the step with a second atom in 1993, in an article with the people from the École Normale. In 1996, we published a more detailed article about the proposal for an experiment, and they did it and published it in the Physical Review Letters. My work was drawing up the theoretical side.

And afterwards?
– We are beginning to think how to measure completely the state of the field in the cavity. For a classic particle, the state is given by its position and speed, and this characterizes it completely. For quantum systems, the situation is more complicated, but it is possible. And our proposal was implemented at the École Normale in July 2001, by the group coordinated by Serge Haroche. Now they have new results and they are sending them for publication.

This is the most recent developments from the theoretical proposals…
– There are two developments. The other was published last year in Physical Newsletters. It is a work done by our group, with the participation of two students, André Carvalho and Pérola Milman, and one post-doctorate scholar, Ruynet Matos Filho, which proposes a way of protecting states from this interaction with the rest of the universe. The idea is for them to be able to keep the quantum character for a longer time, that is, to prevent this state from transforming itself into a classic alternative or from disappearing. Our proposal refers to trapping charged atoms, and at the moment there is no group in Brazil that does this. Another development is the idea of measuring quantum states of molecules, working in collaboration with Professor Nicim Zagury, from our group. Once again, we want to manage a complete characterization of a vibrational quantum state of a molecule. It is more or less this that has been done on this line of the classic-quantum limit.

And the teletransportation experiments?
– We published a work proposing an experiment that reminds us a bit of science fiction, to produce teletransportation of the state of an atom to another atom. This idea of teletransportation had originally been proposed in June 1993, by a group of researchers that included Charles Bennett, from IBM. They showed that it was possible, in principle, to send a sort of quantum fax, transferring information from one system to another.

But, in real terms, you transfer this using what impulse?
– You need to have a pair of particles that is shared by the person who wants to transfer the information and the one who wants to receive it. This pair has a special property, in fact it has already been regarded as the strangest property of quantum physics: the two particles are in what is called an entangled state, which means that their properties are correlated in a much stronger way than any classical theory could foresee. It is a quantum correlation. By measuring one of the particles, this determines the result of the measuring on the other particle. And this system of these two particles in an entangled state, one with one person, and the other with a second person, is what we could call a teletransportation machine.

If this person, let’s call her Alice, wants to send the information to Bob, the other, she makes her particle, whose state she wants to inform Bob about, interact with the other particle of the pair that is with her. Next, she does measurements about these two particles and informs Bob of the results of these two measurements. And with only the result of these two measurements, Bob can reconstruct the original state of Alice’s particle. The article that we published was the first one with a proposal for carrying out this teletransportation experimentally, which has to date not been put into effect. It is a difficult experiment.

We published an article of yours in the September 1999 issue of Notícias FAPESP, with the provocative title “Is Physics worn out?”, in which you commented on the prospects for Physics. Going back to it: are we still without a theory to make it possible to understand the spectrum of masses and charges of elementary particles?
– Yes, and this is a big challenge for the 21st century. It would be good to find a unifying theory to allow the values of the masses and charges to be extracted. This is an issue that is possibly connected to another, the unification of gravitation with quantum physics.

What are your current expectations as to the practical applications of Physics?
– When we look at the development of science and technology in the 20s, we can see that quantum physics started with a group of enthusiastic youngsters, who were twenty and something (Schrödinger and Max Born were a little bit older), whose only concern was understanding nature. They could never imagine that the discoveries would produce a conceptual revolution in the way of understanding nature. Much less that they would change people’s daily lives, with the invention of the transistor, which made information technology feasible. Quantum physics resulted in the laser, which also brought a revolution, in medicine and in communications – and even in beauty treatment. Science is carried out by people working, driven by the emotion of deepening their knowledge of nature, and we know today that this emotion and this interest serve society.

And what is the situation of Physics in Brazil?
– In the last 30 years, it has undergone an extraordinary development. In its origins, it was very theoretical, if only for the lack of opportunity for buying equipment. Afterwards, other areas made headway, like physics of condensed matter, and some extremely competent experimental groups appeared. In the regions where physics was supported in a continuous way, there was a technological ramification in the research carried out at the universities. This is the case of the high technology centers in São Carlos and Campinas, made possible thanks to the support given to science in the state of São Paulo. Now, we ought to look at this too with a critical spirit, not just hip, hip, hooray and asking what can be done to improve Brazilian physics.

Where are the holes?
– A serious problem is the need for federalizing physics in Brazil. What has been done in the state of São Paulo is an example for the rest of the country. While it is not followed, there is going to be a very great discrepancy between what is done in São Paulo and what is done in the rest of the country. This is not healthy for Brazilian physics and, in particular, for physics in São Paulo, because it reduces the opportunities of work for trained students and possible interactions.

This implies the need for permanent support in the ambit of the state.
– Yes, as well. The state foundations should be encouraged. Faperj has experienced important development over recent years, this has to be recognized. On the other hand, there are states that have no research foundation, and they are not going to make them from one day to the next. There is no point in the federal government keeping on insisting with the governors, because they do not have a political interest in giving priority to the area of science. This support for research is, then, also an obligation of the federal government, which has mechanisms to do this. There are programs that are being proposed, like the sectorial funds, but one has to be able to count on funds for spontaneous research. Without this, there is no creative development, because new ideas frequently come from the researcher working on his own.

And the shades of science?
– Amongst groups of the population, there has always been a sentiment, shall we say, anti-science, which stresses the evils done by science to humanity. For example: the Hiroshima and Nagasaki explosions, the nuclear perils, contamination from radiation, and now the effects of biological warfare. The anti-science sentiment involves a great confusion between the scientific activity and democratic control over the use of science. Obviously, the use of science is not a subject just for scientists, it should be the subject of a wide discussion in society. To do so, society has to be informed, and on this point the dissemination of science has an extremely important role.

Do you have an impassioned relationship with science?
– Certainly. Science, for those who are involved with it, is before anything else an activity of play. And the scientist who does not experience this characteristic of research will not be a good scientist. Every day, on the way to the university, we ought to remember: “I do research in a country that is one of the world champions in lack of equality in society. I have this privilege and this responsibility”. I think that it is very important, then, for us to work properly.

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