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On cats, photons and strange worlds

To understand the transition from the quantum world to the classical, Brazilian physicists measure the exchange of information between light particles and the environment

018-025_Fotons_202-6CATARINA BESSELLCATARINA BESSELLAsk a physicist what the universe is made of and you will probably hear that everything, from the stars to living beings, consists of atomic particles that behave in quite an exotic manner, described to perfection by the laws of quantum mechanics. In daily life we do not notice the strange properties of particles—such as the ability to be in more than one location in space simultaneously—because they interact with their environment. The medium in which the particles are immersed, precisely due to its complexity, absorbs these quantum characteristics and dissipates them so that they cannot be recovered. Without these properties, the most elementary components of matter appear to behave like any object visible to the naked eye. However, in an experiment performed with light particles some months ago at the Federal University of Rio de Janeiro (UFRJ), a group of Brazilian physicists showed that the quantum information that reaches the environment is not always lost forever. Or at least not immediately. Under special conditions, part of the information is retained and might even be recoverable. “It’s as if the particle’s interaction with the surroundings leaves a fingerprint,” explains physicist Luiz Davidovich, who, along with Paulo Henrique Souto Ribeiro and Stephen Walborn, coordinated the team that conducted the tests.

Published in the October 12, 2012 edition of the journal Physical Review Letters, the finding that this information is not completely lost could arouse the interest of physicists and specialists in information theory, for two reasons. The first is practical. Because information does not fade completely, nor suddenly, it could become a little easier to build more stable systems, which can be used to perform calculations using quantum computers, or to pass information safely via quantum cryptography. Because the operation of these systems depends directly on the quantum properties of particles, all prototypes produced so far—even what appears to be the first commercial quantum computer, built by the Canadian company D-Wave Systems (see Pesquisa FAPESP nº 193)—must be kept at very low temperatures and isolated as much as possible from the influence of the surrounding environment.

The second reason is of a theoretical, and even philosophical, nature. A greater understanding of how atomic particles interact with the environment can help establish the limits (size, mass or energy) that define the difference between the classical and quantum worlds. In other words, to understand at what point the laws of classical mechanics break down. This question, incidentally, is still as perplexing as ever. According to physicists, nothing in quantum theory, which began to be formulated a little more than a century ago, indicates that this limit exists. Thus, if individual particles exhibit quantum characteristics, as proven by experiment, everything that is made of particles (plants, animals, planets and stars) should also demonstrate quantum behavior, such as the cat—simultaneously dead and alive—in Erwin Schrödinger’s thought experiment.

In 1926, Schrödinger, an Austrian physicist, formulated an equation in which particles were treated as waves. According to his German colleague Max Born, the waves represented the probability of a particle being found in a specific space-time region. Uncomfortable with certain interpretations often associated with this distribution of possibilities—which attributed the uncertainty about the position of a particle to the ignorance of the observer, for example, rather than to an objective property of the particle—Schrödinger tried to show the absurd consequences that could result from this type of interpretation. To illustrate the strangeness of the results, in 1935 Schrödinger suggested a thought experiment in which a cat is placed in a hermetically sealed box containing a handful of radioactive material, a radiation detector, a hammer and a glass container filled with a lethal gas. When the radioactive material decays, it releases radiation, triggering the detector, which in turn activates the mechanism that causes the hammer to break the container of poison. As a result, the cat dies.

018-025_Fotons_202-1_novoDaniel das NevesHowever, there are complications. Assuming there is a 50% chance of a particle decaying every hour, there would be a 50% probability of the cat being alive or dead 60 minutes after the start of the experiment. According to Schrödinger, the probabilistic nature of quantum physics would give rise to an interpretation that, at the end of the test, the cat would be neither alive nor dead, but a combination of the two conditions (dead and alive) at the same time—physicists call this counterintuitive situation “superposition of states,” something only possible in the quantum world. With this absurd situation, Schrödinger intended to show that one must carefully interpret the quantum mechanics he had helped formulate.

Over the course of these almost 80 years, no one has discovered holes in the theory that would eliminate this apparent paradox. Quantum mechanics is considered one of the best-tested and successful theories in physics, capable of predicting phenomena with a never before seen precision. Together, it and the theory of general relativity, formulated by Einstein, are the cornerstones of modern physics. “There is a consensus among physicists that the world is quantum,” says George Matsas, theoretical physicist at São Paulo State University (Unesp). “But we do not know how to recover the classical world from a purely quantum description.” At least, not in such a way that the solution does not appear to be magic to the eyes of a layman.

As the sophistication of quantum mechanics blurred the connection between the world of particles and everyday reality, several attempts at reconciliation have been proposed. As soon as the thought experiment was suggested, Born himself asserted that the paradox would disappear upon opening the box: the mere act of observation would eliminate the superposition of states and the cat would be revealed to be dead or alive. Other ideas followed. The most accepted explanation of why quantum properties are not observed in macroscopic objects was suggested by the German physicist Heinz-Dieter Zeh in the early 1970s. He explained that the macroscopic systems that make up the classical world, governed by the laws of Newtonian physics, are never isolated from the environment with which they continuously interact. Thus, these systems could not be described by Schrödinger’s equations, which are applicable only to closed systems. The consequence of this conclusion was verified later by Wojciech Zurek, a Polish physicist at the Los Alamos National Laboratory (LANL) in the United States. In this interaction, the quantum system’s information escapes to the environment through a phenomenon Zurek called decoherence.

018-025_Fotons_202-4CATARINA BESSELLTo understand what “loss of coherence” means, one must first know what coherence is: a property of waves, like those that spread when a stone is thrown into a lake or a rope is wriggled. A classic physics experiment—the double slit experiment that the Englishman Thomas Young used over 200 years ago to investigate whether light consists of waves or particles (quantum mechanics shows that light is both, simultaneously)—may aid understanding. One way to carry out the experiment is to turn on a monochromatic light near two plates. Two parallel slits are made in the first plate, the one nearest the lamp. They allow some light through to illuminate the second plate, a little farther away. Due to its wavelike nature—similar to waves on the surface of a lake—when the light passes through the first plate it recombines as if each slit were a light source. When the crest of one wave meets the other, they add up, resulting in a higher crest. The same happens when two valleys meet. But when a crest coincides with a valley, there is a destructive effect and they cancel each other out. The combination of crests and valleys produces a pattern of bright and dark bands on the second plate. This is what physicists call an interference pattern. “When systems produce this interference pattern, they are called coherent,” said Davidovich.

In the past century, however, physicists discovered that what happens to waves also occurs with atoms or atomic particles such as electrons. Shot one by one at random against the first plate, atoms produce an interference pattern similar to that of light. According to quantum mechanics, this can only be explained if each atom passes through both slits simultaneously. When you want to observe the interference pattern formed on the second plate, the experiment works like the sealed box containing Schrödinger’s cat. Several experiments have shown that, when using any type of detector to try to discover through which of the two slits the particle actually passed, the answer is always unique: the particle passed through the slot on the right or on the left. When this type of measurement is made, however, the interference pattern on the second plate disappears—coherence is lost. In an analogy with the cat experiment, the use of a detector for the slits corresponds to opening the box.

Physicists understand this type of measurement—such as opening the box to look at the cat—as an interaction between the system and the environment. When isolated, the system’s behavior was quantum. In this state, the photon or electron, for example, could pass through both slits simultaneously. When coherence breaks down, this capacity is lost and some particles exhibit classic behavior (pass through one of the two slits). In this transition to the classical world, quantum information is lost, such as that which allowed the particle to be in two places at once, or Schrödinger’s cat to be dead and alive at the same time. “There is no way to reproduce the classical world without losing information from the quantum world,” said Matsas.

According to Zurek, decoherence occurs because the environment performs measurements on quantum systems all the time. Like trying to figure out through which slit the electron passed, these measurements eliminate information or more fragile quantum states and leave only the most stable, which are those we perceive in the classical world. Zurek called this selective destruction of information “quantum Darwinism.”

018-025_Physics_202-2Daniel das NevesIn a 2002 article in Los Alamos Science, a journal for the general public addressing leading-edge science published by LANL, Zurek wrote: “One way to understand the objective existence induced by the environment is to recognize that observers—especially humans—do not measure anything directly. Instead, most of the data we collect about the universe is obtained when information about the systems that interest us is intercepted by the environment.”

Complicated? Many physicists think so too. Einstein himself did not feel comfortable with many of the interpretations of quantum mechanics. Once, walking through the gardens at Princeton University with his biographer, the physicist and science historian Abraham Pais, Einstein asked something along the lines of: “Do you really believe that the moon is only there when we look at it?” In the book Introducing Quantum Theory: A graphic guide, writer Joseph P. McEvoy reports that in December 2000 the American physicist John Wheeler, a scholar of quantum mechanics who worked with one of the experts in the area, the Dane Niels Bohr, and helped develop the atomic and hydrogen bombs, wrote to him on the occasion of the 100th anniversary of the discovery of quanta. In 1900, German physicist Max Planck arrived at a conclusion that would lead to the development of quantum mechanics. Planck found that, in nature, the energy exchanged between atoms was in discrete amounts (packages) and he called this radiation quanta, the plural of quantum. In the letter to McEvoy, Wheeler said: “To conclude, I proposed a title: ‘Quantum: the Glory and the Shame’. Why glory? Because no area of ​​physics has not been illuminated by quanta. And shame because we do not know why.”

In the macroscopic world, photons like those coming from the stars—and there are many photons, for example, that reach Earth—are colliding with objects all the time. “It’s as if they were measurements destroying quantum information, allowing us to see the world as classic,” says Davidovich, who for nearly three decades has been studying the complex phenomena of quantum mechanics, including the loss of coherence, which defines the transition from the quantum to the classical world.

Until now, no size, mass or energy limit has been observed that could serve as a boundary between one world and the other. In a meeting that brought together the great physicists of the world in 1927, Niels Bohr proposed that this boundary might vary from one system to another. Many years ago in Austria, the team led by  physicist Anton Zeillinger showed that fullerene molecules, which consist of 60 carbon atoms and have a structure similar to that of a soccer ball, show quantum (both wave and particle) behavior in the double slit test. The group has said it plans to repeat the test with viruses, which are much larger.

018-025_Fotons_202-6CATARINA BESSELLCATARINA BESSELLAlthough these limits are not yet known, physicists today have a more precise idea of ​​the factors that influence this transition. When he visited the laboratory of French physicist Serge Haroche at the École Normale Supériere in Paris in 1986, Luiz Davidovich began studying this issue. With the team in France, he and Brazilian colleague Nicim Zagury, also from UFRJ, began planning a system to simulate the measurement carried out by the environment on quantum systems. Ten years later, Davidovich and his French colleagues published a paper in Physical Review A describing how a system could be built to measure the quantum system information and monitor its transformation into a classical system due to interactions with the environment. The idea was to trap photons from very low-energy radiation (microwave frequency) that are in a superposition of states in a cavity made with special mirrors—this superposition is analogous to having a cavity “lit” with photons and “off” without photons at the same time—and then make an atom cross it. When passing through the cavity, the atom alters the photons’ energy, which, in turn, alters the energy level of the atom. In analyzing the atom leaving the cavity, the researchers are able to discover the characteristics of the trapped photons—whether they were or were not in a superposition of states. According to Davidovich, in this experiment, which was conducted the same year the article was published in Physical Review A, the atom, transparent to the trapped radiation, acts as a sort of “quantum mouse” that researchers send into the box containing Schrödinger’s cat. “It is a way to check on the cat without opening the box,” said Davidovich. “Depending on how the quantum mouse exits the box, we can know whether or not the cat was in a superposition of two states—dead and alive,” he explains.

This experiment demonstrated that the time it takes for quantum information to be lost—the decoherence time—is inversely proportional to the number of photons trapped in the cavity. This work was one of a series of studies that led to Haroche being awarded the Nobel Prize in Physics in 2012 (an honor shared with the American David Wineland, of the University of Colorado, also a researcher in this area). This relationship explains why they did not observe macroscopic objects in more than one place at the same time. Since they are made of a very large number of particles, these objects lose their quantum features in an absurdly short period of time.

Many years ago, Wojciech Zurek demonstrated that as the quantum system interacts with the environment that surrounds it and loses information—or in other words, undergoes decoherence—records of this information remain in the environment. Now, in the Physical Review Letters paper, Davidovich and the physicists Souto Ribeiro, Walborn, Osvaldo Jimenez Farias, Gabriel Aguilar and Andrea Valdéz-Hernández have shown in an experiment with photons that the same occurs with a property fundamental to computing and quantum cryptography, called entanglement. Entanglement is a quantum bond between particles, even when distances separate them. This connection, as intense as it is fragile, is such that the modifications undergone by some of the particles can be perceived in the others (see Pesquisa FAPESP nº 102, 123 and 136).

Using a laser beam passing through a series of crystals and filters, the UFRJ group managed see what happens with entanglement in a very simple environment—much simpler than that in which we live—over which they had complete control and could take measurements and know how much information was lost due to decoherence. “This is perhaps the only physical system for which one can fully measure the state of the environment,” says Souto Ribeiro.

018-025_Physics_202-3Daniel das NevesPassing through the first crystal, the laser beam containing trillions and trillions of photons generates only one pair of entangled photons—in this case, the researchers entangled the light’s plane of vibration, its polarization, which could be vertical or horizontal. After this first stage, each of the photons follows a distinct path towards the detector. Before the polarization is measured at the end of the trajectory, one of the photons passes through another series of crystals and filters and creates another kind of information, encoded in the path that it would take next (right or left). It’s as if the photons had interacted with the environment outside the system and transmitted part of the information to it. In the analogy with Schrödinger’s cat, this transfer of information would be the equivalent of odor molecules escaping from the box indicating whether the cat is dead or still alive.

The physicists observed that the initial entanglement between the polarization of the two photons began to disappear after they interacted with the environment. However, in some cases, at the end a distinct form of entanglement arose in which the two photons were entangled with the environment. According to the researchers, by knowing the bit of information that is lost to the environment, its recovery may be possible. “We haven’t done this yet, but we have seen that it is possible,” said Davidovich.

“Our idea is to try to understand entanglement as a physical quantity, such as energy or velocity, to try to establish its laws of evolution,” explains Souto Ribeiro who, together with colleague Walborn and colleagues Amir Caldeira and Marcos Oliveira from Unicamp, coordinated another study published in November in Physical Review Letters showing that the most stable states predicted by Zurek may become evident even before the system becomes classical.

To Souto Ribeiro, the fact that this worked in a simple environment also indicates that it should work in more complex environments, since the equations that describe the interactions with simple environments are exactly the same as those for complex ones, for which measurements are difficult. Davidovich believes that he and his colleagues have just begun to forge a new path. “Our experiment gives us only partial information about what happens because the object is far from macroscopic,” he explains. “I would like to study the fingerprints that macroscopic objects leave in the environment.” The next step would be to explore, theoretically, what would occur in this case. “To design an experiment to observe this would be extremely difficult,” he says.

Scientific Articles
FARIAS, O.J. et al. Observation of the emergence of multipartite entanglement between a bipartite system and its environment. Physical Review Letters. 12 out. 2012.
CORNELIO, M.F. et al. Emergence of the pointer basis through the dynamics of correlations. Physical Review Letters. 9 nov. 2012.