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An important quantum algorithm may actually be a property of nature


Evidence that quantum searches are an ordinary feature of electron behavior may explain the genetic code, one of the greatest puzzles in biology.


Back in 1996, a quantum physicist at Bell Labs in New Jersey published a new recipe for searching through a database of N entries. Computer scientists have long known that this process takes around N steps because in the worst case, the last item on the list could be the one of interest.


However, this physicist, Lov Grover, showed how the strange rules of quantum mechanics allowed the search to be done in a number of steps equal to the square root of N.


[...] Today Stéphane Guillet and colleagues at the University of Toulon in France say this may be easier than anybody expected. They say they have evidence that Grover’s search algorithm is a naturally occurring phenomenon.


[...] That has obvious implications for quantum computing, but its real import may be much more profound. For some time, theorists have debated whether quantum search could explain one of the greatest mysteries about the origin of life. The idea that Grover searches occur in nature could finally solve the conundrum.


[...] The work also has implications for our thinking about the genetic code and the origin of life. Every living creature on Earth uses the same code, in which DNA stores information using four nucleotide bases. The sequences of nucleotides encode information for constructing proteins from an alphabet of 20 amino acids.


But why these numbers—four and 20—and not some others? Back in 2000, just a few years after Grover published his work, Apoorva Patel at the Indian Institute of Science in Bangalore showed how Grover’s algorithm could explain these numbers.


[...] Patel’s idea is related to the way DNA is assembled inside cells. In this situation, the molecular machinery inside a cell must search through the molecular soup of nucleotide bases to find the right one. If there are four choices, a classical search takes four steps on average. So the machinery would have to try four different bases during each assembly step.


But a quantum search using Grover’s algorithm is much quicker: Patel showed that when there are four choices, a quantum search can distinguish between four alternatives in a single step. Indeed, four is optimal number.


[...] In other words, if the search processes involved in assembling DNA and proteins is to be as efficient as possible, the number of bases should be four and the number of amino acids should to be 20—exactly as is found. The only caveat is that the searches must be quantum in nature.


[...] Since then, an increasing body of evidence has emerged that quantum processes play an important role in a number of biological mechanisms. Photosynthesis, for example, is now thought to be an essentially quantum process.


The work of Guillet and co throws a new perspective on all this. It suggests that Grover’s algorithm is not only possible in certain materials; it seems to be a property of nature. And if that’s true, then the objections to Patel’s ideas start to crumble.



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Even Huge Molecules Follow the Quantum World's Bizarre Rules


A record-breaking experiment shows an enormous molecule is also both a particle and a wave—and that quantum effects don't only apply at tiny scales.

Magnify a speck of dirt a thousand times, and suddenly it no longer seems to play by the same rules. Its outline, for example, won’t look well-defined most of the time and will resemble a diffuse, sprawling cloud. That’s the bizarre realm of quantum mechanics. “In some books, you’ll find they say a particle is in various places at once,” says physicist Markus Arndt of the University of Vienna in Austria. “Whether that really happens is a matter of interpretation.”


Another way of putting it: Quantum particles sometimes act like waves, spread out in space. They can slosh into each other and even back onto themselves. But if you poke at this wave-like object with certain instruments, or if the object interacts in specific ways with nearby particles, it loses its wavelike properties and starts acting like a discrete point—a particle. Physicists have observed atoms, electrons, and other minutiae transitioning between wave-like and particle-like states for decades.

But at what size do quantum effects no longer apply? How big can something be and still behave like both a particle and a wave? Physicists have struggled to answer that question because the experiments have been nearly impossible to design.


Now, Arndt and his team have circumvented those challenges and observed quantum wave-like properties in the largest objects to date—molecules composed of 2,000 atoms, the size of some proteins. The size of these molecules beats the previous record by two and a half times. To see this, they injected the molecules into a 5-meter-long tube. When the particles hit a target at the end, they didn’t just land as randomly scattered points. Instead, they formed an interference pattern, a striped pattern of dark and light stripes that suggests waves colliding and combining with each other. They published the work today in Nature Physics.


researchers touching machine

Physicists at the University of Vienna keep the inside of their instrument in vacuum and stabilize its exterior so it never moves more than about 10 nanometers.

Photograph: Barbara Mair/Universität Wien

“It’s surprising that this works in the first place,” says Timothy Kovachy of Northwestern University, who was not involved in the experiment. It’s an extremely difficult experiment to pull off, he says, because quantum objects are delicate, transitioning suddenly from their wavelike state to their particle-like one via interactions with their environment. The larger the object, the more likely it is to knock into something, heat up, or even break apart, which triggers these transitions. To maintain the molecules in a wave-like state, the team clears a narrow path for them through the tube, like police cordoning off a parade route. They keep the tube in a vacuum and prevent the entire instrument from wobbling even the slightest bit using a system of springs and brakes. The physicists then had to carefully control the molecules’ speed, so they don’t heat up too much. “It’s really impressive,” says Kovachy.


One possibility physicists are exploring is that quantum mechanics might in fact apply at all scales. “You and I, while we sit and talk, do not feel quantum,” says Arndt. We seem to have distinct outlines and do not crash and combine with each other like waves in a pond. “The question is, why does the world look so normal when quantum mechanics is so weird?”


green lit laser beam

The researchers propel the molecules through the interferometer using green laser beams.

Photograph: Quantum Nanophysics Group at University of Vienna

By looking for wavelike behavior in progressively larger objects, Arndt wants to understand how quantum mechanics transitions into the world we normally perceive. To that end, some physicists propose theories such as the continuous spontaneous localization model, which modifies the math of standard quantum mechanics to suggest that larger objects stay in a wavelike state for shorter times. The results of this experiment restricts the likelihood of some of these theories, says Arndt.

To perform the experiment, Arndt’s team used a green laser to launch the molecules into the tube. The molecules absorbed the energy from the light to propel them forward. Then, the molecules passed through a sequence of metal grates containing thin, nanometers-wide slits. The grates effectively divide a single molecule into multiple waves traveling in different directions and recombine them in the end to form the interference pattern. It’s a dressed-up version of the famous double-slit experiment, “one of the hallmark demonstrations of the wave nature of matter,” says Kovachy.


They also took great pains to design the optimal type of molecule for in the experiment. Eventually, they settled on a synthetic behemoth with the chemical formula, C707H260F908N16S53Zn4. Its structure was sturdy enough so that its peripheral atoms wouldn’t fall off during launch. It also contains a core assortment of atoms called a porphyrin, which absorbs green light to act as the molecule’s motor.


Now, Arndt’s team plans to run this experiment for even more massive objects. They want to test whether they can observe wave-like properties in metal nanoparticles ten times heavier than their bespoke molecule. Eventually, researchers are working toward creating wave-like interference in objects even closer to the macroscopic realm. “Can we do this for a virus? A bacteria? You can keep scaling up,” says Kovachy. Quantum mechanics has inserted a tiny alien world into ours. By doing these experiments, physicists hope to find the seam where the two places meet.


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