Quantum mechanics is a branch of physics that most people find surprising. Superposition of states, particle entanglement, system decoherence—these concepts are generally beyond the understanding of those who, like everyone else on Earth, live in the world of classical physics. Still, man now lives with inventions that operate using quantum effects, such as lasers and magnetic resonance imaging machines. The Nobel Prize in Physics last year was shared by two researchers working independently, using different approaches, in an area on the frontier of the field. For their “innovative experimental methods to measure and manipulate individual quantum systems,” American David Wineland, of the National Institute of Standards and Technology (NIST) and the University of Colorado, Boulder, and Frenchman Serge Haroche, of the Ecole Normale Supérieure and the Collège de France, shared the honor in 2012.
In late February and early March of this year, Wineland and Haroche attended a symposium sponsored by the Institute of Physics at the University of São Paulo, São Carlos, along with three other Nobel laureates. They granted exclusive interviews to Pesquisa FAPESP (see the story on the symposium in issue no. 205). Both have been visiting Brazil for at least two decades and Haroche, who often spends holidays on a beach in Bahia, recently made another visit to the country after the event in the state of São Paulo. They talk about their research, which is complementary in some respects, and the possible paths down which their studies may lead. Wineland conjectures, for example, that more precise atomic clocks could be useful in predicting earthquakes. And the long dreamed of quantum computer? “I don’t like the idea of presenting myself as an engineer who will build a quantum computer. Nobody knows which route we should take to achieve this. We’re making progress in small steps,” said Haroche. The interviews were granted separately. But, as the themes addressed were sometimes similar, they are published here side by side.
Was winning the Nobel last year a surprise?
Wineland – I think that most people who win the Nobel Prize have heard, in some way or another, that their name was being considered for the award. I heard my name mentioned a couple of times. In this respect, the award was not totally unexpected. But, in recent years, I had not been thinking about it. Some people wake up and go directly to the news to see who won the Nobel. But I was asleep when the announcement was made. It was my wife who woke me up. When you win a Nobel Prize, it is implied that you become a sort of spokesperson for your field of research. This happened to me and Serge. You become an inspiration for students, who have so many distractions today, to follow a scientific career. A simple way to do this is to tell my story. When I was young, in high school, I was most interested in cars and motorcycles, not in science. Certainly I was not one of the best students. But I have to say that I always liked math and physics when I was in school.
Haroche — This type of award is not something you expect. Of course, I knew I was doing work that was garnering attention in the scientific community. But our area is vast, there are many themes and sub-areas that are producing interesting results. So it was impossible to predict who would win the prize. You also need to understand—and David agrees with me on this point—that the award is for our area as a whole. Not for him and me. Research is a group effort. In my studies, I relied on the work of two senior researchers. I’m sure it was the same with David. But, due to the nature of the Nobel prize, they have to associate the award with specific people.
Were your teams rivals?
Wineland – Our approaches were distinct enough that we did not feel like competitors in the same area. Rainer Blatt [a physicist at the University of Innsbruck], who also attended the conference in São Carlos, has a very strong group and is one of my greatest competitors. Although we are competitors, we have been good friends for many years. My research and Serge’s work are somewhat complementary. To explain it in a simple way, I would say that I use light to control some properties of atoms, and Serge uses atoms to investigate and control some properties of light. Although one approach is described as being the opposite of the other, we use the same quantum physics to describe interactions between atoms and light.
Haroche — There is a beautiful symmetry in our research. In fact, we both work with the interactions between light and atoms at the most fundamental quantum level. I look at things from one perspective and he from another. Perhaps this symmetry led the Nobel committee to think it would be a good thing to reward both of us.
Does entanglement indicate that there is something of a quantum nature in a system?
Wineland – Before entanglement, you have the idea of superposition of states. One of the experiments we did in our lab was to show that a particle, an ion or an atom, can be in two places at once before entanglement occurs. Superposition is the hallmark that we have entered the strange world of quantum mechanics. For me and for others, a fundamental, still unanswered question is where the classical world ends and the quantum world begins. If there is a dividing line, we need to know where it is. This gives rise to concepts such as the existence of multiple [parallel] worlds or universes. At the moment, I believe that the hypothesis of many worlds is disturbing, but as valid as any other idea of what is really happening. As far as I know, it is a valid solution to the problem. As an experimental physicist, I feel that there is something to be discovered in this direction. But we do not have an experiment that can be done to answer this question. Perhaps there is some mechanism, some new physics, that we have not yet seen that defines this dividing line (between quantum and classical). This field is very speculative. I do not think anyone has the answer to this question. But I feel that there is something very profound to be discovered.
Haroche — The central notion [of quantum physics] is superposition, the fact that a system can be in different states simultaneously. Entanglement is a consequence of this. Two systems are entangled when they interact and can be in a superposition of states. This means that what happens to one system immediately produces an effect on the other—even if they are separated by a great distance. This is called non-locality. This is a very well established property of matter and radiation. Superficially, you can think of this property as violating the notion of causality, because information cannot propagate at a speed greater than the speed of light. Still, there are some kinds of correlations that are instantaneous. But these correlations cannot be used to propagate information. So, there is no contradiction. There are a lot of groups working on this question, especially with photons propagating in open space and in optical fibers. One of the pioneers in this field is my colleague, Alain Aspect [at the Institute of Optics and the École Polytechnique in Paris, who also attended the symposium], who did an experiment in 1982 showing this type of entanglement for the first time.
What can we expect in terms of new applications derived from quantum physics?
Wineland – Most physicists believe that applications in the field of quantum computing have to deal with simulations. For example, factorization algorithms greatly stimulated (quantum) computing. But creating a useful factorization is a very difficult challenge, because it uses a lot of resources that we have not yet mastered. With a small number of qubits [quantum bits] we can do interesting things. We might be able to simulate a system with 50 or 100 qubits. For me, this question will become interesting when we can learn something new from this simulation. For now, they are using quantum physics for simulations that we can already perform on a normal computer. It’s hard to make predictions, but perhaps in the next 10 years we will be able to run a simulation that actually teaches us something new.
Haroche — Some things are already a reality, such as quantum cryptography. But the question we must ask is whether it is useful and competitive with respect to classical cryptography. There is also metrology, in which you use quantum physics to improve measurement accuracy. A good example is David’s work with atomic clocks. It uses entanglement to measure what happens in an ion, which would be the most accurate atomic clock ever built. The quantum computer could work in a superposition state in such a way that some calculations could be made more quickly and efficiently than using a classical computer. This is our dream. The concepts needed for this are established, but they work on small systems. For a quantum computer, we would need to control hundreds of particles. There are technical and practical problems, and we do not know if they will be resolved. “I don’t like the idea of presenting myself as an engineer who will build a quantum computer. Nobody knows which route we should take achieve this. We are making progress bit-by-bit. For example, we are learning to correct minor errors that occur in quantum systems, which we call decoherence. Quantum superposition is a very fragile state, which disappears easily. In Paris, we are carrying out experiments that we call corrective feedback, in which we maintain, on average, a certain number of photons for a very long time. We have been able to correct perturbations as they occur. This is good for some things, but not enough for quantum computing. It is very difficult to predict what will happen. If we look at history, at all the technologies derived from quantum physics, such as lasers and nuclear magnetic resonance imaging for medical purposes, they all emerged from basic research not designed for these specific purposes. I’m almost certain that applications will appear. But no one can guess what will happen.
Is progress much more visible in the area of atomic clocks?
Wineland – This field is much more developed. We make atomic clocks that serve a purpose, both with atoms and with ions. A good example is GPS-based navigation systems. For decades, atomic clocks have been made for these systems. And this has been the case for centuries: whenever someone develops a more accurate clock, a new use appears, usually in the area of navigation. This is still the case. There are some communication applications that also use atomic clocks. For example, if we had an atomic clock for a GPS-type system with a precision in the millimeter range, we could measure the deformation of the earth. Potentially, such a system could predict earthquakes. Of course, other tools would be needed as well, but a clock with this precision could be useful in this regard.