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Home , Quantum Darwinism

could explain one of the biggest conundrums of : The emergence of objective reality. by ANIL ANANTHASWAMY News

FQXi Awardee: Wojciech Zurek November 7, 2008

In 1859, published On quantum fuzziness in everyday life? And ers. "The time when I realized I could of Species in which he laid forth what triggers the transition between the really contribute, rather than just stand by his theory of natural selection, revolutio- quantum and classical? They are questions and be amazed, was when I was with nizing our understanding of biology in the that he claims his new theory of quantum Johnny," says Zurek. process. At first glance, Wojciech Zurek Darwinism can answer. That transformation had as much to do shares little in common with Darwin. Born Growing up in Bielsko-Biala in Poland in with Wheeler's approach to problem- almost a century and a half later, he is a the 1960s, Zurek was no ordinary tee- solving, as it did with his formidable physicist who is far more interested in the nager. Even before he finished high achievements in quantum theory. Making spread of quantum information than the school, he cajoled his father into buying mistakes in search for truth was unders- dispersal of biological traits. But Zurek is him a set of The Feynman Lectures in Phys- tood to be an integral part of research. now channeling the biologist to explain ics, which had just been translated into "Not being afraid of saying ‘I don't know,’ one of the deepest mysteries of physics: Polish. "The first few volumes had the air and, in fact, using that admission of ignor- just how the quantum realm transitions of amazement and surprise and astonish- ance as a springboard to make progress into the classical world we see around us. ment at how the universe works, and at was something I hadn't realized, at least It’s all down to the survival of the fittest. how much we can understand about how not with that clarity," says Zurek. "You Zurek, at the Los Alamos National it works," says Zurek. It was the beginning really need to be amazed and surprised Laboratory in New Mexico, is tackling of an obsession. and puzzled before you get somewhere fundamental issues that standard interpre- That obsession received a major boost really interesting." tations of quantum mechanics have strug- when Zurek moved to Austin, Texas, to gled with: Does the objective world dis- do his Ph.D. There he worked with the Emerging Reality appear when nobody is looking? How venerable John Wheeler—a fundamental Above all, Zurek and Wheeler were puz- does it even become “objective” in the contributor to the fields of quantum me- zled by one of the central questions that first place? Why don’t we experience chanics and general relativity, among oth- haunts quantum mechanics: How does an objective reality emerge from the quan- tum world? One of the key distinctions between a quantum object and a classical object is that the former can exist in a You need to be amazed superposition of different states. For in- and surprised before stance, an electron can be in two places at once. But that superposition only lasts as you get somewhere re- long as we don’t look at the object, or ally interesting. measure it. Once we do, it snaps into one state. - Wojciech Zurek How does such a quantum object be- come classical, choosing just one definitive state? The question has troubled physicists since the dawn of quantum mechanics, with wavefunction collapse and parallel universes being invoked as alternative ex- planations (see “Copenhagen vs. Many Worlds: And the Winner Is…”). But nei- ther has been able to explain the actual mechanism through which classical reality emerges, or how and why the transforma- tion is triggered by observations. It may seem like an esoteric problem, 1

 The Foundational Questions Institute | November 7, 2008

ticular environment than others. The act of information transfer from the quantum system to the environment serves two purposes. First, it destroys fragile quantum states that aren’t so well suited to their surroundings. This is — environment induced super-selection— the process that selects these preferred states and spreads the information about

Copenhagen vs. Many Worlds: And the Winner Is... The conundrum over the quantum to classical transition appeared on the horizon soon after the discovery of quantum mechanics. In the 1920s, quantum behemoths and QUANTUM INTERACTIONS, COLOSSAL LEGACY , and others, met The late John Wheeler (center) inspired (left to right) Bill Wooters, Kip in Copenhagen and arrived upon the Thorne, Wojciech Zurek & Bill Unruh standard interpretation of quantum mechanics: Prior to observation, a but the growing interest in quantum This is where Zurek comes in. He has quantum object exists as a wavefunc- computing has lent it immediacy. Super- focused his efforts on the notion of deco- tion that accommodates the superposi- position lies at the heart of the promise of herence, first mooted by Dieter Zeh of tion of multiple states. The act of ob- . The idea is that be- the University of Heidelberg in 1970. Ac- serving the system "collapses" the wa- cause quantum systems, such as electrons, cording to decoherence theory, a quan- vefunction and we end up with a clas- can exist in a superposition of states, they tum system can only remain in a superpo- sical state. can encode a superposition of possibilities, sition of states (that is, remain coherent) as The problem with the Copenhagen and, therefore, carry out a superposition long as it is isolated from its environment. Interpretation is that it doesn’t describe of computations simultaneously. In a clas- As soon as the environment—which in- how this collapse occurs. It also raises sical computer, the absence or presence cludes everything from air molecules to the uncomfortable philosophical ques- of an electron can encode either a '0'or a photons—enters the frame, tion of whether an objective reality '1'; in a quantum computer, superposition flies out of the window. exists in the absence of observers. could represent both simultaneously. This Decoherence argues that the reason Then in the 1950s, Hugh Everett means that, in theory, quantum comput- we don't see macroscopic objects in a III—a student of John Wheeler’s— ers can perform exponentially greater superposition of quantum states is be- came up with an even more radical numbers of calculations using the same cause they cannot be isolated from their solution that did away with collapse number of electrons as a classical com- environment. It doesn’t take a conscious entirely. He suggested that the entire puter. observer carrying out a measurement to universe is a quantum object, with snap an object from a quantum to a clas- every possible outcome of a mea- sical state, just a nudge from its surround- surement realized—just in different Zurek has a great talent ings. realities. This leads to a mind-boggling for raising important scenario, in which every possible quan- tum state exists in its own world, a points and putting them Survival of the Fittest scenario rather simplistically called the But that still doesn’t explain which quan- "many worlds" interpretation. front and center. tum states become, in effect, classical. It's So which alternative does Zurek here that Zurek has made his most im- - Frank Wilhelm back? He favors decoherence theory, portant contribution, in papers published which describes how interaction with from 1981 onwards. His argument centers the environment gradually destroys a For this to happen in practice, a quan- on the role of the environment, which . But Zurek is quick to tum computer must maintain its superpo- doesn’t just passively disturb the quantum point out that decoherence is compat- sition of states long enough to perform object; instead, it actively determines ible with both interpretations. significant calculations. But that’s easier which classical reality will be selected: "Collapse has metaphysical connota- said than done; it's clear that the world "The environment is monitoring the sys- tions. It begins to touch on whether we perceive doesn't exist in a superposi- tem. It is not just perturbing the system, it you think like Bohr or Everett," says tion of quantum states. While those build- is sucking out information.” Zurek. "What I am trying to do, among ing quantum computers fret about not They key point is that not all quantum other things, is to try and stay away destroying the superposition, theorists superposition states are created equal— from taking a side." wonder why it gets destroyed in the first some are more apt to survive in one par- 2 place.

 The Foundational Questions Institute | November 7, 2008 them throughout the environment. velop models that make concrete predic- But this does not yet explain the emer- tions that experimenters can test. He’s Natural Selection in Action gence of an objective reality. After all, got good reason to be optimistic as com- Darwin could back up his ideas on these einselected quantum states could puter simulations are already showing natural selection by looking at the still be perturbed by a direct measure- signs of quantum Darwinism at play (see characteristics of birds and animals. ment carried out by an observer. “Natural Selection in Action”). But how can we catch “quantum nat- Zurek’s next step is to make additional ural selection” in the act? testable predictions. For example, what In 2007, Wojciech Zurek of the Quantum Photography Los Alamos National Laboratory in Zurek’s current research centers on the fraction of the environment is needed to New Mexico, and his former student, idea that the environment also becomes a extract a certain amount of information Robin Blume-Kohout of the California “witness” to the state of the system. about the quantum system? This requires Institute of Technology in Pasadena, What this means is that multiple copies of extending his work to include impure developed a computer model in the states that survive interaction with environments. So far, theoretical models which a “quantum pendulum” was set their surroundings are disseminated have involved idealized pure environ- oscillating according to the laws of throughout the environment—as though ments, or ”clean slates,” on which only quantum mechanics. Such an oscillator the environment is taking photographs of the information about the system will be can exist in a superposition of states, the preferred states and passing them on. “written.” But in realistic cases, the slate is simultaneously present in all possible This survival-of-the-fittest-states and their not clean—the environment could already locations along its path. The duo proliferation throughout the environment be partially occupied with information modeled the environment by including has been dubbed quantum Darwinism. about other entities. thousands of other pendulums nearby, Quantum Darwinism ensures that dif- each “swinging” at a different frequen- ferent observers will see the same objec- It’s like a billboard cy. tive reality because they are most likely to If quantum Darwinism is correct, intercept a copy of the same stable state. which floats multiple the environmental pendulums should "The main idea of quantum Darwinism is copies of the information record a copy of the quantum pendu- that we almost never do any direct mea- lum’s position. And that is what it did. surement on anything," says Zurek. In- about our universe all Multiple copies of the same quantum stead, the environment acts as a witness, over the place. state were replicated throughout the or as a communication channel. “It is like a environment. The model showed big advertising billboard, which floats mul- - Wojciech Zurek quantum Darwinism in action. tiple copies of the information about our In addition, early this year, Roland universe all over the place." Of course, there is always a danger that Brunner of the University of Leoben in "Zurek has a great talent for raising a lifetime of work can be undermined by Austria and his colleagues showed important points and putting things— experiments; Zurek has been working on quantum Darwinism, in theory, in an which other people may have been ob- theories of quantum information for close array of quantum dots—nanoscale serving on the side in their calculations— to 30 years. Many of his theories have bits of semiconducting materials, with front and center,” says Frank Wilhelm of been confirmed by experiment, but quan- electrons that can exist in a superposi- the Institute for Quantum Computing in tum Darwinism is still new and awaiting tion of states. Waterloo, Canada. Wilhelm admires tests. Zurek, however, is unperturbed. The team studied what happens Zurek’s work for provoking a debate on After all, standing up to experimental when multiple dots are arrayed in the foundations of quantum mechanics. testing is essential for any physics theory series and also interact with the envi- Bill Unruh, a quantum theorist at the that wants to be taken seriously. In a ronment. They found that two adja- University of British Columbia in Vancou- world full of alternative ideas, only the cent quantum dots gave rise to a spe- ver also commends Zurek for wrestling fittest will survive. cial quantum state that was peculiar to with issues that most ignore. “It is a prob- their interaction, and not merely a lem that most people leave on the side of combination of states of individual their table, to think about while they are quantum dots. sitting in the bath and not to take too More importantly, they showed that seriously," says Unruh. "He's been one of they could find out about these spe- the very few people in the world who has cial states by looking at individual been willing to take it seriously.” quantum dots. It's as if stable states However, Unruh adds that it is too had been copied and imprinted on all early to say whether Zurek’s ideas can the individual dots. All observers provide the final answer to the quantum would see the same objective reali- conundrum. ty—thanks to quantum Darwinism. If Zurek wants to win everyone over to his way of thinking, he must address one big question: Can we verify quantum Darwinism experimentally? Zurek has WILL QUANTUM DOTS prove been awarded a $75,000 grant from the quantum Darwinism? (Image: Lin-Wang Wang, Lawrence Berkeley 3 Foundational Questions Institute to de- National Laboratory)

 The Foundational Questions Institute | November 7, 2008