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    Quantum entanglement makes quantum communication even more secure

    Stealthy communication just got more secure, thanks to quantum entanglement.

    Quantum physics provides a way to share secret information that’s mathematically proven to be safe from the prying eyes of spies. But until now, demonstrations of the technique, called quantum key distribution, rested on an assumption: The devices used to create and measure quantum particles have to be known to be flawless. Hidden defects could allow a stealthy snoop to penetrate the security unnoticed.

    Now, three teams of researchers have demonstrated the ability to perform secure quantum communication without prior confirmation that the devices are foolproof. Called device-independent quantum key distribution, the method is based on quantum entanglement, a mysterious relationship between particles that links their properties even when separated over long distances.

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    In everyday communication, such as the transmission of credit card numbers over the internet, a secret code, or key, is used to garble the information, so that it can be read only by someone else with the key. But there’s a quandary: How can a distant sender and receiver share that key with one another while ensuring that no one else has intercepted it along the way?

    Quantum physics provides a way to share keys by transmitting a series of quantum particles, such as particles of light called photons, and performing measurements on them. By comparing notes, the users can be sure that no one else has intercepted the key. Those secret keys, once established, can then be used to encrypt the sensitive intel (SN: 12/13/17). By comparison, standard internet security rests on a relatively shaky foundation of math problems that are difficult for today’s computers to solve, which could be vulnerable to new technology, namely quantum computers (SN: 6/29/17).

    But quantum communication typically has a catch. “There cannot be any glitch that is unforeseen,” says quantum physicist Valerio Scarani of the National University of Singapore. For example, he says, imagine that your device is supposed to emit one photon but unknown to you, it emits two photons. Any such flaws would mean that the mathematical proof of security no longer holds up. A hacker could sniff out your secret key, even though the transmission seems secure.

    Device-independent quantum key distribution can rule out such flaws. The method builds off of a quantum technique known as a Bell test, which involves measurements of entangled particles. Such tests can prove that quantum mechanics really does have “spooky” properties, namely nonlocality, the idea that measurements of one particle can be correlated with those of a distant particle. In 2015, researchers performed the first “loophole-free” Bell tests, which certified beyond a doubt that quantum physics’ counterintuitive nature is real (SN: 12/15/15).

    “The Bell test basically acts as a guarantee,” says Jean-Daniel Bancal of CEA Saclay in France. A faulty device would fail the test, so “we can infer that the device is working properly.”

    In their study, Bancal and colleagues used entangled, electrically charged strontium atoms separated by about two meters. Measurements of those ions certified that their devices were behaving properly, and the researchers generated a secret key, the team reports in the July 28 Nature.

    Typically, quantum communication is meant for long-distance dispatches. (To share a secret with someone two meters away, it would be easier to simply walk across the room.) So Scarani and colleagues studied entangled rubidium atoms 400 meters apart. The setup had what it took to produce a secret key, the researchers report in the same issue of Nature. But the team didn’t follow the process all the way through: The extra distance meant that producing a key would have taken months.

    In the third study, published in the July 29 Physical Review Letters, researchers wrangled entangled photons rather than atoms or ions. Physicist Wen-Zhao Liu of the University of Science and Technology of China in Hefei and colleagues also demonstrated the capability to generate keys, at distances up to 220 meters. This is particularly challenging to do with photons, Liu says, because photons are often lost in the process of transmission and detection.

    Loophole-free Bell tests are already no easy feat, and these techniques are even more challenging, says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. “The requirements for this experiment are so absurdly high that it’s just an impressive achievement to be able to demonstrate some of these capabilities,” says Shalm, who wrote a perspective in the same issue of Nature.

    That means that the technique won’t see practical use anytime soon, says physicist Nicolas Gisin of the University of Geneva, who was not involved with the research.

    Still, device-independent quantum key distribution is “a totally fascinating idea,” Gisin says. Bell tests were designed to answer a philosophical question about the nature of reality — whether quantum physics really is as weird as it seems. “To see that this now becomes a tool that enables something else,” he says, “this is the beauty.” More

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    Aliens could send quantum messages to Earth, calculations suggest

    An intelligent alien civilization could beam quantum messages to Earth.

    Particles of light, or photons, could be transmitted over vast, interstellar distances without losing their quantum nature, researchers report June 28 in Physical Review D. That means scientists searching for extraterrestrial signals could also look for quantum messages (SN: 1/28/19).

    Scientists are currently developing Earth-based quantum communication, a technology that uses quantum particles to send information and has the potential to be more secure than standard, or classical, communication (SN: 6/15/17). Intelligent extraterrestrials, if they’re out there, may have also adopted quantum communication, says theoretical physicist Arjun Berera.

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    A major obstacle to quantum communication is decoherence, in which a quantum particle loses its quantumness as it interacts with its surroundings. “Quantum states you generally think of as very delicate, and if there’s any kind of external interaction, you kind of destroy that state,” Berera says.

    Since the average density of matter in space is much less than on Earth, particles could be expected to travel farther before succumbing to decoherence. So Berera and theoretical physicist Jaime Calderón Figueroa, both of the University of Edinburgh, calculated how far light — in particular, X-rays — could travel unscathed through interstellar space.

    X-ray photons could more than traverse the Milky Way, potentially traveling hundreds of thousands of light-years or even more, the researchers found.

    Based on the findings, Berera and Calderón Figueroa considered strategies to search for E.T.’s quantum dispatches. One potential type of communication to search for is quantum teleportation, in which the properties of a distant particle can be transferred to another (SN: 7/7/17). Since the technology requires both quantum and classical signals, scientists could look for such simultaneous signals to identify any alien quantum missives. More

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    Quantum physics exponentially improves some types of machine learning

    Machine learning can get a boost from quantum physics.

    On certain types of machine learning tasks, quantum computers have an exponential advantage over standard computation, scientists report in the June 10 Science. The researchers proved that, according to quantum math, the advantage applies when using machine learning to understand quantum systems. And the team showed that the advantage holds up in real-world tests.

    “People are very excited about the potential of using quantum technology to improve our learning ability,” says theoretical physicist and computer scientist Hsin-Yuan Huang of Caltech. But it wasn’t entirely clear if machine learning could benefit from quantum physics in practice.

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    In certain machine learning tasks, scientists attempt to glean information about a quantum system — say a molecule or a group of particles — by performing repeated experiments, and analyzing data from those experiments to learn about the system.

    Huang and colleagues studied several such tasks. In one, scientists aim to discern properties of the quantum system, such as the position and momentum of particles within. Quantum data from multiple experiments could be input into a quantum computer’s memory, and the computer would process the data jointly to learn the quantum system’s characteristics.

    The researchers proved theoretically that doing the same characterization with standard, or classical, techniques would require exponentially more experiments in order to learn the same information. Unlike a classical computer, a quantum computer can exploit entanglement — ethereal quantum linkages — to better analyze the results of multiple experiments.

    But the new work goes beyond just the theoretical. “It’s crucial to understand if this is realistic, if this is something we could see in the lab or if this is just theoretical,” says Dorit Aharonov of Hebrew University in Jerusalem, who was not involved with the research.

    So the researchers tested machine learning tasks with Google’s quantum computer, Sycamore (SN: 10/23/19). Rather than measuring a real quantum system, the team used simulated quantum data, and analyzed it using either quantum or classical techniques.

    Quantum machine learning won out there, too, even though Google’s quantum computer is noisy, meaning errors can slip into calculations. Eventually, scientists plan to build quantum computers that can correct their own errors (SN: 6/22/20). But for now, even without that error correction, quantum machine learning prevailed. More

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    A new gravity sensor used atoms’ weird quantum behavior to peer underground

    The best way to find buried treasure may be with a quantum gravity sensor.

    In these devices, free-falling atoms reveal subtle variations in Earth’s gravitational pull at different places. Those variations reflect differences in the density of material beneath the sensor — effectively letting the instrument peer underground. In a new experiment, one of these machines teased out the tiny gravitational signature of an underground tunnel, researchers report in the Feb. 24 Nature.

    “Instruments like this would find many, many applications,” says Nicola Poli, an experimental physicist at the University of Florence, who coauthored a commentary on the study in the same issue of Nature.

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    Poli imagines using quantum gravity sensors to monitor groundwater or magma beneath volcanoes, or to help archaeologists uncover hidden tombs or other artifacts without having to dig them up (SN: 11/2/17). These devices could also help farmers check soil quality or help engineers inspect potential construction sites for unstable ground.

    “There are many tools to measure gravity,” says Xuejian Wu, an atomic physicist at Rutgers University in Newark, N.J., who wasn’t involved in the study. Some devices measure how far gravity pulls down a mass hanging from a spring. Other tools use lasers to clock how fast an object tumbles down a vacuum chamber. But free-falling atoms, like those in quantum gravity sensors, are the most pristine, reliable test masses out there, Wu says. As a result, quantum sensors promise to be more accurate and stable in the long run than other gravity probes.

    Inside a quantum gravity sensor, a cloud of supercooled atoms is dropped down a chute. A pulse of light then splits each of the falling atoms into a superposition state — a quantum limbo where each atom exists in two places at once (SN: 11/7/19). Due to their slightly different positions in Earth’s gravitational field, the two versions of each atom feel a different downward tug as they fall. Another light pulse then recombines the split atoms.

    Thanks to the atoms’ wave-particle duality — a strange rule of quantum physics that says atoms can act like waves — the reunited atoms interfere with each other (SN: 1/13/22). That is, as the atom waves overlap, their crests and troughs can reinforce or cancel each other out, creating an interference pattern. That pattern reflects the slightly different downward pulls that the split versions of each atom felt as they fell — revealing the gravity field at the atom cloud’s location.

    Extremely precise measurements made by such atom-based devices have helped test Einstein’s theory of gravity (SN: 10/28/20) and measure fundamental constants, such as Newton’s gravitational constant (SN: 4/12/18). But atom-based gravity sensors are highly sensitive to vibrations from seismic activity, traffic and other sources.

    “Even very, very small vibrations create enough noise that you have to measure for a long time” at any location to weed out background tremors, says Michael Holynski, a physicist at the University of Birmingham in England. That has made quantum gravity sensing impractical for many uses outside the lab.  

    Holynski’s team solved that problem by building a gravity sensor with not one but two falling clouds of rubidium atoms. With one cloud suspended a meter above the other, the instrument could gauge the strength of gravity at two different heights in a single location. Comparing those measurements allowed the researchers to cancel out the effects of background noise.

    Holynski and colleagues tested whether their sensor — a 2-meter-tall chute on wheels tethered to a rolling cart of equipment — could detect an underground passageway on the University of Birmingham campus. The 2-by-2-meter concrete tunnel lay beneath a road between two multistory buildings. The quantum sensor measured the local gravitational field every 0.5 meters along an 8.5-meter line that crossed over the tunnel. Those readouts matched the predictions of a computer simulation, which had estimated the gravitational signal of the tunnel based on its structure and other factors that could influence the local gravitational field, such as nearby buildings.

    Based on the machine’s sensitivity in this experiment, it could probably provide a reliable gravity measurement at each location in less than two minutes, the researchers estimate. That’s about one-tenth the time needed for other types of gravity sensors.

    The team has since built a downsized version of the gravity sensor used in the tunnel-detecting experiment. The new machine weighs about 15 kilograms, compared with the 300-kilogram beast used for the tunnel test. Other upgrades could also boost the gravity sensor’s speed.

    In the future, engineer Nicole Metje envisions building a quantum gravity sensor that could be pushed from place to place like a lawn mower. But portability isn’t the only challenge for making these tools more user-friendly, says Metje, a coauthor on the study who is also at the University of Birmingham. “At the moment, we still need someone with a physics degree to operate the sensor.”

    So hopeful beachcombers may be waiting a long time to trade in their metal detectors for quantum gravity sensors. More

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    ‘From Data to Quanta’ defends Niels Bohr’s view of quantum mechanics

    From Data to QuantaSlobodan PerovićUniv. of Chicago, $45

    Ever since Max Planck introduced the idea of the quantum to the world, physicists have argued about whether reality is more like sand or water.

    Planck’s famous 1900 discovery that energy is grainy — at least when absorbed or emitted — moved him to label those smallest bits of energy grains “quanta.” But he believed that once emitted, as in light from a fire, those grains merged into smooth, continuous waves, just as water seems a smooth liquid to human perception. Einstein, on the other hand, insisted that light quanta traveled through space on their own, behaving like particles later called photons.

    By the mid-1920s, both the wave and particle views of light had gained experimental support, with the additional paradox that electrons — supposedly particles — could sometimes disguise themselves as waves.

    Into this arena of controversy stepped the famed Danish physicist Niels Bohr, the pioneer of exploring the architecture of the atom. Bohr announced that resolving the wave-particle paradox required a new view of reality, in which both notions shared a role in explaining experimental phenomena. In experiments designed to observe waves, waves you would find, whether electrons or light. In experiments designed to detect particles, you’d see particles. But in no experiment could you demonstrate both at once. Bohr called this viewpoint the principle of complementarity, and it successfully guided the pursuit of quantum mechanics during the following decades.

    More recently, as philosopher Slobodan Perović recounts in From Data to Quanta, Bohr’s success has been questioned by some physicists and philosophers and even popular science writers (SN: 1/19/19, p. 26). Complementarity has been derided as an incoherent application of vague philosophy expressed in incomprehensible language. But as Perović’s investigations reveal, such criticisms are rarely rooted in any deep understanding of Bohr’s methods. Rather than Bohr’s philosophy contaminating his science, Perović argues, it is his opponents’ philosophical prejudices that have led to misstatements, misunderstandings and misrepresentations of Bohr’s physics. And Bohr can’t be understood by attempting to understand his philosophy, Perović asserts, because philosophy did not guide him — experiments did.

    In fact, Bohr’s drive to understand the wave-particle paradox was fueled by a deep devotion to comprehending the experimental evidence in its totality. It was the same approach the younger Bohr took when developing his model of the atom in 1913 (SN: 7/13/13, p. 20). Various experiments suggested properties of the atom that seemed irreconcilable. But Bohr forged those experimental clues into a “master hypothesis” that produced a thoroughly novel understanding of the atom and its structure.

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    Perović describes how Bohr’s process began with lower-level hypotheses stemming from features directly given by experiment. Spectral lines — different specific colors of light emitted by atoms — led to basic hypotheses that some vibratory process, of an atom itself or its constituents, produced electromagnetic radiation exhibiting precise patterns. Intermediate hypotheses about the structure of the atom did not explain such lines, though. And then Ernest Rutherford, on the basis of experiments in his lab, inferred that an atom was mostly empty space. It contained a dense, tiny central nucleus encompassing most of the mass, while lightweight electrons orbited at a distance. But that hypothesis didn’t mesh with the precise patterns of spectral lines. And such an atom would be unstable, persisting for less than a millisecond. From all these disparate experiment-based hypotheses, Bohr applied Planck’s quantum idea to construct a master hypothesis. He reconciled the spectral lines and Rutherford’s nuclear atom with a new atomic model, in which electrons maintained stability of the atom but jumped from one orbit to another, emitting specific patterns of spectral lines in the process.

    As Perović demonstrates, Bohr followed a similar course in arriving at complementarity. While numerous experiments showed that light was a wave, by the early 1920s other experiments established that X-rays, highly energetic light, collided with electrons just as though both were particles (momentum and energy were conserved in the collisions just as the particle view required). Bohr’s master hypothesis, complementarity, seemed the only way forward.

    Throughout the book, Perović relates how Bohr has been misinterpreted, his views misleadingly conflated with those of others (like John von Neumann and Werner Heisenberg), and his philosophy incorrectly portrayed as antirealist — suggesting that only observations brought reality into existence. Bohr never said any such thing, and in fact cautioned against using language so loosely.

    Perović’s account offers a thorough survey of other historical investigations into Bohr’s work and draws liberally from Bohr’s own writings. It’s a nuanced and insightful presentation of the interplay of experiment and theory in the scientific process. This book is not easy reading, though. It’s not the place to seek clear explanations of quantum physics and Bohr’s interpretation of it. Perović opts for scholarly thoroughness and careful reasoning with a propensity for long sentences. But then again, Bohr’s writings were no breeze, either. In fact, a major complaint against Bohr has been expressed by authors who say his writings are very difficult to understand. It’s unfortunate that so many seem to think that because they can’t understand Bohr, he must have been wrong. Perović’s book provides a useful antidote to that attitude.

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    The quantum ‘boomerang’ effect has been seen for the first time

    Some quantum particles gotta get right back to where they started from.

    Physicists have confirmed a theoretically predicted phenomenon called the quantum boomerang effect. An experiment reveals that, after being given a nudge, particles in certain materials return to their starting points, on average, researchers report in a paper accepted in Physical Review X.

    Particles can boomerang if they’re in a material that has lots of disorder. Instead of a pristine material made up of orderly arranged atoms, the material must have many defects, such as atoms that are missing or misaligned, or other types of atoms sprinkled throughout.

    In 1958, physicist Philip Anderson realized that with enough disorder, electrons in a material become localized: They get stuck in place, unable to travel very far from where they started. The pinned-down electrons prevent the material from conducting electricity, thereby turning what might otherwise be a metal into an insulator. That localization is also necessary for the boomerang effect.

    To picture the boomerang in action, physicist David Weld of the University of California, Santa Barbara imagines shrinking himself down and slipping inside a disordered material. If he tries to fling away an electron, he says, “it will not only turn around and come straight back to me, it’ll come right back to me and stop.” (Actually, he says, in this sense the electron is “more like a dog than a boomerang.” The boomerang will keep going past you if you don’t catch it, but a well-trained dog will sit by your side.)

    Weld and colleagues demonstrated this effect using ultracold lithium atoms as stand-ins for the electrons. Instead of looking for atoms returning to their original position, the team studied the analogous situation for momentum, because that was relatively straightforward to create in the lab. The atoms were initially stationary, but after being given kicks from lasers to give them momenta, the atoms returned, on average, to their original standstill states, making a momentum boomerang.

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    The team also determined what’s needed to break the boomerang. To work, the boomerang effect requires time-reversal symmetry, meaning that the particles should behave the same when time runs forward as they would on rewind. By changing the timing of the first kick from the lasers so that the kicking pattern was off-kilter, the researchers broke time-reversal symmetry, and the boomerang effect disappeared, as predicted.

    “I was so happy,” says Patrizia Vignolo, a coauthor of the study. “It was perfect agreement” with their theoretical calculations, says Vignolo, a theoretical physicist at Université Côte d’Azur based in Valbonne, France.

    Even though Anderson made his discovery about localized particles more than 60 years ago, the quantum boomerang effect is a recent newcomer to physics. “Nobody thought about it, apparently, probably because it’s very counterintuitive,” says physicist Dominique Delande of CNRS and Kastler Brossel Laboratory in Paris, who predicted the effect with colleagues in 2019.

    The weird effect is the result of quantum physics. Quantum particles act like waves, with ripples that can add and subtract in complicated ways (SN: 5/3/19). Those waves combine to enhance the trajectory that returns a particle to its origin and cancel out paths that go off in other directions. “This is a pure quantum effect,” Delande says, “so it has no equivalent in classical physics.” More

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    Quantum particles can feel the influence of gravitational fields they never touch

    If you’re superstitious, a black cat in your path is bad luck, even if you keep your distance. Likewise, in quantum physics, particles can feel the influence of magnetic fields that they never come into direct contact with. Now scientists have shown that this eerie quantum effect holds not just for magnetic fields, but for gravity too — and it’s no superstition.

    Usually, to feel the influence of a magnetic field, a particle would have to pass through it. But in 1959, physicists Yakir Aharonov and David Bohm predicted that, in a specific scenario, the conventional wisdom would fail. A magnetic field contained within a cylindrical region can affect particles — electrons, in their example — that never enter the cylinder. In this scenario, the electrons don’t have well-defined locations, but are in “superpositions,” quantum states described by the odds of a particle materializing in two different places. Each fractured particle simultaneously takes two different paths around the magnetic cylinder. Despite never touching the electrons, and hence exerting no force on them, the magnetic field shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed (SN: 3/1/86).

    In the new experiment, the same uncanny physics is at play for gravitational fields, physicists report in the Jan. 14 Science. “Every time I look at this experiment, I’m like, ‘It’s amazing that nature is that way,’” says physicist Mark Kasevich of Stanford University.

    Kasevich and colleagues launched rubidium atoms inside a 10-meter-tall vacuum chamber, hit them with lasers to put them in quantum superpositions tracing two different paths, and watched how the atoms fell. Notably, the particles weren’t in a gravitational field–free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, laying bare the eerie Aharonov-Bohm influence.

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    The study not only reveals a famed physics effect in a new context, but also showcases the potential to study subtle effects in gravitational systems. For example, researchers aim to use this type of technique to better measure Newton’s gravitational constant, G, which reveals the strength of gravity, and is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).

    A phenomenon called interference is key to this experiment. In quantum physics, atoms and other particles behave like waves that can add and subtract, just as two swells merging in the ocean make a larger wave. At the end of the atoms’ flight, the scientists recombined the atoms’ two paths so their waves would interfere, then measured where the atoms arrived. The arrival locations are highly sensitive to tweaks that alter where the peaks and troughs of the waves land, known as phase shifts.

    At the top of the vacuum chamber, the researchers placed a hunk of tungsten with a mass of 1.25 kilograms. To isolate the Aharonov-Bohm effect, the scientists performed the same experiment with and without this mass, and for two different sets of launched atoms, one which flew close to the mass, and the other lower. Each of those two sets of atoms were split into superpositions, with one path traveling closer to the mass than the other, separated by about 25 centimeters. Other sets of atoms, with superpositions split across smaller distances, rounded out the crew. Comparing how the various sets of atoms interfered, both with and without the tungsten mass, teased out a phase shift that was not due to the gravitational force. Instead, that tweak was from time dilation, a feature of Einstein’s theory of gravity, general relativity, which causes time to pass more slowly close to a massive object.

    The two theories that underlie this experiment, general relativity and quantum mechanics, don’t work well together. Scientists don’t know how to combine them to describe reality. So, for physicists, says Guglielmo Tino of the University of Florence, who was not involved with the new study, “probing gravity with a quantum sensor, I think it’s really one of … the most important challenges at the moment.” More

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    A century of quantum mechanics questions the fundamental nature of reality

    Scientists are like prospectors, excavating the natural world seeking gems of knowledge about physical reality. And in the century just past, scientists have dug deep enough to discover that reality’s foundations do not mirror the world of everyday appearances. At its roots, reality is described by the mysterious set of mathematical rules known as quantum mechanics.

    Conceived at the turn of the 20th century and then emerging in its full form in the mid-1920s, quantum mechanics is the math that explains matter. It’s the theory for describing the physics of the microworld, where atoms and molecules interact to generate the world of human experience. And it’s at the heart of everything that made the century just past so dramatically unlike the century preceding it. From cell phones to supercomputers, DVDs to pdfs, quantum physics fueled the present-day electronics-based economy, transforming commerce, communication and entertainment.

    But quantum theory taught scientists much more than how to make computer chips. It taught that reality isn’t what it seems.

    “The fundamental nature of reality could be radically different from our familiar world of objects moving around in space and interacting with each other,” physicist Sean Carroll suggested in a recent tweet. “We shouldn’t fool ourselves into mistaking the world as we experience it for the world as it really is.”

    In a technical paper backing up his tweet, Carroll notes that quantum theory consists of equations that describe mathematical entities roaming through an abstract realm of possible natural events. It’s plausible, Carroll argues, that this quantum realm of mathematical possibilities represents the true, fundamental nature of reality. If so, all the physical phenomena we perceive are just a “higher-level emergent description” of what’s really going on.

    “Emergent” events in ordinary space are real in their own way, just not fundamental, Carroll allows. Belief that the “spatial arena” is fundamental “is more a matter of convenience and convention than one of principle,” he says.

    Carroll’s perspective is not the only way of viewing the meaning of quantum math, he acknowledges, and it is not fully shared by most physicists. But everybody does agree that quantum physics has drastically remodeled humankind’s understanding of nature. In fact, a fair reading of history suggests that quantum theory is the most dramatic shift in science’s conception of reality since the ancient Greeks deposed mythological explanations of natural phenomena in favor of logic and reason. After all, quantum physics itself seems to defy logic and reason.

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    It doesn’t, of course. Quantum theory represents the ultimate outcome of superior logical reasoning, arriving at truths that could never be discovered merely by observing the visible world.

    It turns out that in the microworld — beyond the reach of the senses — phenomena play a game with fantastical rules. Matter’s basic particles are not tiny rocks, but more like ghostly waves that maintain multiple possible futures until forced to assume the subatomic equivalent of substance. As a result, quantum math does not describe a relentless cause-and-effect sequence of events as Newtonian science had insisted. Instead science morphs from dictator to oddsmaker; quantum math tells only probabilities for different possible outcomes. Some uncertainty always remains.

    Quantum mechanics says that whether an electron behaves as particle or wave depends on how it is observed.Max Löffler

    The quantum revolution

    The discovery of quantum uncertainty was what first impressed the world with the depth of the quantum revolution. German physicist Werner Heisenberg, in 1927, astounded the scientific community with the revelation that deterministic cause-and-effect physics failed when applied to atoms. It was impossible, Heisenberg deduced, to measure both the location and velocity of a subatomic particle at the same time. If you measured one precisely, some uncertainty remained for the other.

    “A particle may have an exact place or an exact speed, but it can not have both,” as Science News Letter, the predecessor of Science News, reported in 1929. “Crudely stated, the new theory holds that chance rules the physical world.” Heisenberg’s uncertainty principle “is destined to revolutionize the ideas of the universe held by scientists and laymen to an even greater extent than Einstein’s relativity.”

    Werner Heisenberg, shown in 1936, declared with his uncertainty principle that a particle’s position and velocity couldn’t both be precisely measured at the same time.AIP Emilio Segrè Visual Archives

    Heisenberg’s breakthrough was the culmination of a series of quantum surprises. First came German physicist Max Planck’s discovery, in 1900, that light and other forms of radiation could be absorbed or emitted only in discrete packets, which Planck called quanta. A few years later Albert Einstein argued that light also traveled through space as packets, or particles, later called photons. Many physicists dismissed such early quantum clues as inconsequential. But in 1913, the Danish physicist Niels Bohr used quantum theory to explain the structure of the atom. Soon the world realized that reality needed reexamining.

    By 1921, awareness of the quantum revolution had begun to expand beyond the confines of physics conferences. In that year, Science News Bulletin, the first iteration of Science News, distributed what was “believed to be the first popular explanation” of the quantum theory of radiation, provided by American physical chemist William D. Harkins. He proclaimed that the quantum theory “is of much more practical importance” than the theory of relativity.

    “Since it concerns itself with the relations between matter and radiation,” Harkins wrote, quantum theory “is of fundamental significance in connection with almost all processes which we know.” Electricity, chemical reactions and how matter responds to heat all require quantum-theoretic explanations.

    As for atoms, traditional physics asserts that atoms and their parts can move about “in a large number of different ways,” Harkins stated. But quantum theory maintains that “of all the states of motion (or ways of moving) prescribed by the older theory, only a certain number actually do occur.” Therefore, events previously believed “to occur as continuous processes, actually do occur in steps.”

    Quantum theory “is of fundamental significance in connection with almost all processes which we know.”William Harkins

    But in 1921 quantum physics remained embryonic. Some of its implications had been discerned, but its full form remained undeveloped in detail. It was Heisenberg, in 1925, who first transformed the puzzling jumble of clues into a coherent mathematical picture. His decisive advance was developing a way to represent the energies of electrons in atoms using matrix algebra. With aid from German physicists Max Born and Pascual Jordan, Heisenberg’s math became known as matrix mechanics. Shortly thereafter, Austrian physicist Erwin Schrödinger developed a competing equation for electron energies, viewing the supposed particles as waves described by a mathematical wave function. Schrödinger’s “wave mechanics” turned out to be mathematically equivalent to Heisenberg’s particle-based approach, and “quantum mechanics” became the general term for the math describing all subatomic systems.

    Still, some confusion remained. It wasn’t clear how an approach picturing electrons as particles could be equivalent to one supposing electrons to be waves. Bohr, by then regarded as the foremost of the world’s atomic physicists, pondered the question deeply and by 1927 arrived at a novel viewpoint he called complementarity.

    Bohr argued that the particle and wave views were complementary; both were necessary for a full description of subatomic phenomena. Whether a “particle” — say, an electron — exhibited its wave or particle nature depended on the experimental setup observing it. An apparatus designed to find a particle would find a particle; an apparatus geared to detect wave behavior would find a wave.

    At about the same time, Heisenberg derived his uncertainty principle. Just as wave and particle could not be observed in the same experiment, position and velocity could not both be precisely measured at the same time. As physicist Wolfgang Pauli commented, “Now it becomes day in quantum theory.”

    But the quantum adventure was really just beginning.

    In the many worlds interpretation of quantum mechanics, all possible realities exist, but humans perceive just one.Max Löffler

    A great debate

    Many physicists, Einstein among them, deplored the implications of Heisenberg’s uncertainty principle. Its introduction in 1927 eliminated the possibility of precisely predicting the outcomes of atomic observations. As Born had shown, you could merely predict the probabilities for the various possible outcomes, using calculations informed by the wave function that Schrödinger had introduced. Einstein famously retorted that he could not believe that God would play dice with the universe. Even worse, in Einstein’s view, the wave-particle duality described by Bohr implied that a physicist could affect reality by deciding what kind of measurement to make. Surely, Einstein believed, reality existed independently of human observations.

    On that point, Bohr engaged Einstein in a series of discussions that came to be known as the Bohr-Einstein debate, a continuing dialog that came to a head in 1935. In that year, Einstein, with collaborators Nathan Rosen and Boris Podolsky, described a thought experiment supposedly showing that quantum mechanics could not be a complete theory of reality.

    In a brief summary in Science News Letter in May 1935, Podolsky explained that a complete theory must include a mathematical “counterpart for every element of the physical world.” In other words, there should be a quantum wave function for the properties of every physical system. Yet if two physical systems, each described by a wave function, interact and then fly apart, “quantum mechanics … does not enable us to calculate the wave function of each physical system after the separation.” (In technical terms, the two systems become “entangled,” a term coined by Schrödinger.) So quantum math cannot describe all elements of reality and is therefore incomplete.

    Bohr soon responded, as reported in Science News Letter in August 1935. He declared that Einstein and colleagues’ criterion for physical reality was ambiguous in quantum systems. Einstein, Podolsky and Rosen assumed that a system (say an electron) possessed definite values for certain properties (such as its momentum) before those values were measured. Quantum mechanics, Bohr explained, preserved different possible values for a particle’s properties until one of them was measured. You could not assume the existence of an “element of reality” without specifying an experiment to measure it.

    Niels Bohr and Albert Einstein disagreed over the nature of reality.Photograph by Paul Ehrenfest, courtesy of AIP Emilio Segrè Visual Archives, Gamow Collection

    Einstein did not relent. He acknowledged that the uncertainty principle was correct with respect to what was observable in nature, but insisted that some invisible aspect of reality nevertheless determined the course of physical events. In the early 1950s physicist David Bohm developed such a theory of “hidden variables” that restored determinism to quantum physics, but made no predictions that differed from the standard quantum mechanics math. Einstein was not impressed with Bohm’s effort. “That way seems too cheap to me,” Einstein wrote to Born, a lifelong friend.

    Einstein died in 1955, Bohr in 1962, neither conceding to the other. In any case it seemed like an irresolvable dispute, since experiments would give the same results either way. But in 1964, physicist John Stewart Bell deduced a clever theorem about entangled particles, enabling experiments to probe the possibility of hidden variables. Beginning in the 1970s, and continuing to today, experiment after experiment confirmed the standard quantum mechanical predictions. Einstein’s objection was overruled by the court of nature.

    Still, many physicists expressed discomfort with Bohr’s view (commonly referred to as the Copenhagen interpretation of quantum mechanics). One particularly dramatic challenge came from the physicist Hugh Everett III in 1957. He insisted that an experiment did not create one reality from the many quantum possibilities, but rather identified only one branch of reality. All the other experimental possibilities existed on other branches, all equally real. Humans perceive only their own particular branch, unaware of the others just as they are unaware of the rotation of the Earth. This “many worlds interpretation” was widely ignored at first but became popular decades later, with many adherents today.

    Since Everett’s work, numerous other interpretations of quantum theory have been offered. Some emphasize the “reality” of the wave function, the mathematical expression used for predicting the odds of different possibilities. Others emphasize the role of the math as describing the knowledge about reality accessible to experimenters.

    Some interpretations attempt to reconcile the many worlds view with the fact that humans perceive only one reality. In the 1980s, physicists including H. Dieter Zeh and Wojciech Zurek identified the importance of a quantum system’s interaction with its external environment, a process called quantum decoherence. Some of a particle’s many possible realities rapidly evaporate as it encounters matter and radiation in its vicinity. Soon only one of the possible realities remains consistent with all the environmental interactions, explaining why on the human scale of time and size only one such reality is perceived.

    This insight spawned the “consistent histories” interpretation, pioneered by Robert Griffiths and developed in more elaborate form by Murray Gell-Mann and James Hartle. It is widely known among physicists but has received little wider popularity and has not deterred the pursuit of other interpretations. Scientists continue to grapple with what quantum math means for the very nature of reality.

    Using principles of quantum information theory, a particle’s quantum state can be replicated at a distant location, a feat known as quantum teleportation.Max Löffler

    It from quantum bit

    In the 1990s, the quest for quantum clarity took a new turn with the rise of quantum information theory. Physicist John Archibald Wheeler, a disciple of Bohr, had long emphasized that specific realities emerged from the fog of quantum possibilities by irreversible amplifications — such as an electron definitely establishing its location by leaving a mark after hitting a detector. Wheeler suggested that reality as a whole could be built up from such processes, which he compared to yes or no questions — is the electron here? Answers corresponded to bits of information, the 1s and 0s used by computers. Wheeler coined the slogan “it from bit” to describe the link between existence and information.

    Taking the analogy further, one of Wheeler’s former students, Benjamin Schumacher, devised the notion of a quantum version of the classical bit of information. He introduced the quantum bit, or qubit, at a conference in Dallas in 1992.

    Schumacher’s qubit provided a basis for building computers that could process quantum information. Such “quantum computers” had previously been envisioned, in different ways, by physicists Paul Benioff, Richard Feynman and David Deutsch. In 1994, mathematician Peter Shor showed how a quantum computer manipulating qubits could crack the toughest secret codes, launching a quest to design and build quantum computers capable of that and other clever computing feats. By the early 21st century, rudimentary quantum computers had been built; the latest versions can perform some computing tasks but are not powerful enough yet to make current cryptography methods obsolete. For certain types of problems, though, quantum computing may soon achieve superiority over standard computers.

    Quantum computing’s realization has not resolved the debate over quantum interpretations. Deutsch believed that quantum computers would support the many worlds view. Hardly anyone else agrees, though. And decades of quantum experiments have not provided any support for novel interpretations — all the results comply with the traditional quantum mechanics expectations. Quantum systems preserve different values for certain properties until one is measured, just as Bohr insisted. But nobody is completely satisfied, perhaps because the 20th century’s other pillar of fundamental physics, Einstein’s theory of gravity (general relativity), does not fit in quantum theory’s framework.

    For decades now, the quest for a quantum theory of gravity has fallen short of success, despite many promising ideas. Most recently a new approach suggests that the geometry of spacetime, the source of gravity in Einstein’s theory, may in some way be built from the entanglement of quantum entities. If so, the mysterious behavior of the quantum world defies understanding in terms of ordinary events in space and time because quantum reality creates spacetime, rather than occupying it. If so, human observers witness an artificial, emergent reality that gives the impression of events happening in space and time while the true, inaccessible reality doesn’t have to play by the spacetime rules.

    In a crude way this view echoes that of Parmenides, the ancient Greek philosopher who taught that all change is an illusion. Our senses show us the “way of seeming,” Parmenides declared; only logic and reason can reveal “the way of truth.” Parmenides didn’t reach that insight by doing the math, of course (he said it was explained to him by a goddess). But he was a crucial figure in the history of science, initiating the use of rigorous deductive reasoning and relying on it even when it led to conclusions that defied sensory experience.

    Yet as some of the other ancient Greeks realized, the world of the senses does offer clues about the reality we can’t see. “Phenomena are a sight of the unseen,” Anaxagoras said. As Carroll puts it, in modern terms, “the world as we experience it” is certainly related to “the world as it really is.”

    “But the relationship is complicated,” he says, “and it’s real work to figure it out.”

    In fact, it took two millennia of hard work for the Greek revolution in explaining nature to mature into Newtonian science’s mechanistic understanding of reality. Three centuries later quantum physics revolutionized science’s grasp of reality to a comparable extent. Yet the lack of agreement on what it all means suggests that perhaps science needs to dig a little deeper still. More