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If you are privileged enough to live with a source of income in a first world country, it’s possible to maintain this illusion that violence is something that happens far away—in time, space, class: it happens to other type of…

Physicists in the Netherlands and Germany have shown that the theories of thermodynamics and quantum mechanics are both valid ways of describing the behaviour of photons in a quantum processor. The results, obtained by researchers at the University of Twente and the Freie Universität Berlin, open the door to a deeper understanding of how to reconcile these two great theories.

Thermodynamics and quantum mechanics are cornerstones of modern physics, but in one specific, important way, they don’t get along well. The point of contention revolves around the second law of thermodynamics, which states that a closed system will move towards maximum entropy (a measure of the system’s disorder, or chaos) in an irreversible way. The theory of quantum mechanics, in contrast, allows previous states of particles to be calculated back, meaning that the flow of information and time are both reversible.

In recent years, there have been several attempts to explore this conflict using entangled quantum systems such as ultracold atoms or superconducting quantum bits (qubits). By observing what happens when these systems thermalize and equilibrate, it should be possible to measure their entropy and quantum states at the same time, and thus resolve the paradox.

The problem is that quantum systems are very sensitive to interactions with their environment. This makes it hard to create a system that is truly closed. They are also prone to losing their quantum nature, a process known as decoherence, which makes time reversal difficult to implement.

Photonics to the rescue

To get around these challenges, the team chose to study thermalization and equilibration in systems of entangled photons. Photons have several advantages over quantum systems composed of (for example) atoms. Their intrinsically quantum nature means they do not suffer from decoherence. They can be studied at room temperature, in contrast to the ultralow temperatures necessary for atoms, and are easy to manipulate with interference. Most importantly, they allow for time reversibility: any mixing of the photons can be reversed by performing the inverse operation, meaning that entangled photons can, in effect, be “disentangled”.

In the experiment, the researchers begin by injecting single photons into waveguide channels on a chip. These photons interfere where the photonic channels on the chip meet and cross. This interference, which the team controlled with thermo-optic Mach-Zehnder interferometers, creates a superposition of photons in the waveguides and allows entanglement to build up. The photons are then detected with single-photon detectors.

Simultaneously true

To determine the system’s local and total increases in entropy, the researchers performed a series of protocols. Time reversibility, for example, was implemented by disentangling the photons, which was possible due to the full control the processor gives over the experiment.

Once these protocols were complete, measurements in the experiment’s individual output channels showed that photon numbers could no longer be precisely defined. This is because the photons were in an entangled state together and no longer individually localized in a single channel as they were at the input. However, the photon statistics the researchers measured in each channel did show that entropy increased locally in all the channels, consistent with the second law of thermodynamics. At the same time, the entanglement that built up between photons is not visible in the individual channels: only when considering the entire system does it become clear that the overall quantum state is in a pure form, consistent with quantum mechanics.

As a final check, the physicists performed operations to return the processor to its original state (time reversal). The success of these operations proved that the processes of thermalization and equilibration were due to entanglement between the quantum particles, rather than interactions with the environment. Hence, the experiment showed that thermodynamics and quantum mechanics can both be true at the same time.

High-quality data

According to Pepijn Pinkse, a quantum optics expert at the University of Twente, the team’s biggest challenge was to get enough high quality data to perform the measurements. Low losses in the photonic processor helped, he says, and more photons and larger processors should enable them to simulate more systems. The weakest element in the chain, he adds, seems to be the photon source: “We have at least 12 input channels, but only three photons at the same time to experiment with, so there’s room for improvement there,” he tells Physics World.

Nicole Yunger Halpern, an expert in quantum thermodynamics at the US National Institute of Standards and Technology (NIST) who was not involved in the research, says the experiment is important because it extends to photons previous work that involved ultracold atoms, trapped ions and superconducting qubits. This change of platform, she says, enabled the experimentalists to undo the process that led the system to equilibrate internally, making it possible to conclude that the system had retained its quantum nature while equilibrating. Doing this requires an “excellent amount of control”, she notes, adding that the challenge of achieving this control has caused groups using other platforms significant anxiety over the past several years.

The research is published in Nature Communications.

The post Quantum mechanics and thermodynamics can both be true, say physicists appeared first on Physics World.

Insider Brief

• Scientists have long tried to understand what makes quantum computers more powerful than classical computers.
• A University of Chicago-led team found a computational problem that showed entanglement is responsible for quantum speedup over any classical algorithm.
• This research is part of the first steps in the broader context of pinpointing quantum speedups.

PRESS RELEASE — For decades, scientists have been trying to solve the mystery of what makes quantum computers more powerful than classical computers. The origins of this quest can be traced all the way to Albert Einstein who famously called quantum mechanical entanglement “spooky action at a distance”. Now in a paper published in the Physical Review Letters, a team of scientists led by Assistant Professor William Fefferman from the University of Chicago’s Department of Computer Science have found a computational problem in which entanglement is directly responsible for a dramatic quantum computational speedup over any efficient classical algorithm.

Fefferman, along with lead Ph.D. student Soumik Ghosh, IBM researcher Abhinav Deshpande (who Fefferman co-advised at the University of Maryland), University of Maryland postdoc Dominik Hangleiter and University of Maryland/NIST researcher Alexey Gorshkov, debuted a problem in their paper titled “Complexity phase transitions generated by entanglement” that pinpoints two things: there is a provable quantum speedup over any classical computer, and entanglement is causing the speedup in this particular problem.

Since the early 90’s, we have had theoretical evidence that quantum computers can solve problems that are too difficult for today’s classical computers. One specific example that scientists continue to look at is Shor’s algorithm, which says quantum computers can take incredibly large numbers (think ten billion) and quickly break them into their prime factors. The foundations of modern cryptography that we use on the Internet is based on this being a hard problem to solve; so if large scale quantum computers are built, then the basis of cryptography as we know it would be compromised.

However, Shor’s algorithm is still a theoretical result because large enough and perfect enough quantum computers have not yet been built.

“Right now we are in the era of NISQ — which stands for noisy intermediate scale quantum computing,” said Ghosh. “Some companies have designed certain types of quantum computers, but one defining feature is that they are a bit noisy. Today’s quantum computers are believed to be just slightly more powerful than our best classical computers, so it’s becoming more significant to sharpen that boundary between the two.”

In the same way that classical computers are made up of bits, quantum computers are made of individual components called qubits. As Ghosh explained, today’s qubits are noisy, making them too imperfect to be efficient. A quantum computer would need hundreds of thousands of noiseless qubits to solve the near-impossible problems facing modern computers. While places like UChicago are making strides toward building large scale quantum computers that can test these theories, we don’t currently have devices capable of doing so.

There is still plenty that scientists don’t understand about the basic foundations of quantum computing that make it hard to move forward in the field. From a first principle standpoint, certain questions need to be answered: Why is quantum computing so powerful? Why does Shor’s algorithm work? What quantum properties is it using that causes these speedups? After years of research attempting to better understand these issues, this work gives an example of a quantum system for which entanglement can be identified as the clearcut answer.

“Entanglement is a fundamental property of quantum systems, and it’s a property that we think is very different from anything that happens in the classical world,” Fefferman explained. “Furthermore, there’s always been an intuition that entanglement is one of the root causes of these quantum speedups. It’s an important contributor to the power of quantum computers, but it wasn’t totally clear that entanglement was the sole cause. That’s what our paper is trying to address.”

Entanglement is a complex and largely misunderstood phenomenon that scientists have been trying to understand for the last hundred years. Einstein, for instance, was troubled by entanglement and died trying to give a classical explanation. In essence, if you have two entangled quantum particles that are separated by a distance, no matter how far, what happens to one particle can simultaneously affect the behavior of the other particle. Abstractly, if you have a large number of particles– or qubits as the basic unit of quantum information– and you want to understand the state of this entire system, the idea of entanglement implies you won’t get any real information by looking at just one qubit; you have to look at the interactions between all of the qubits to understand the state of subsets within the system.

The problem the team presented in the paper is not useful in the same sense that Shor’s algorithm is, but it can be mathematically described and is meaningful to quantum theory. The key point is that entanglement can be seen to be the root cause of the computational speedup.

“We can talk about the same computational problem with a little bit of entanglement, and then a little bit more, and so on,” said Fefferman. “The exciting part is that when this entanglement reaches a certain threshold, we go from an easy problem for a classical computer to a provably hard problem. Entanglement seems to be causing the increased difficulty and quantum speedup. We’ve never been able to show that in a problem like Shor’s algorithm.”

This research is part of the first steps in the broader context of pinpointing quantum speedups.

“The next step is trying to generalize this toy model to more practical systems of quantum computation,” said Ghosh. “We want to be able to understand what is causing speedups for the types of quantum computers that people are designing in real life and the type of processes that will be run using those computers.”

The whole universe is humming. Actually, the whole universe is Mongolian throat singing. Every star, every planet, every continent, every building, every person is vibrating along to the slow cosmic beat.

That’s the takeaway from yesterday’s remarkable announcement that scientists have detected a “cosmic background” of ripples in the structure of space and time. If the result bears up as more data are gathered, it’s a discovery that promises to open new windows on everything from the evolution of galaxies to the origin of the universe.

Scientists had been awaiting such a discovery for decades. More than 100 years ago, Einstein introduced his radical general theory of relativity. For Einstein, space and time were a single entity, “space-time,” comprising a flexible fabric that could be stretched and compressed, bent and warped. In general relativity, matter makes space-time bend, and space-time, in turn, guides how unconstrained matter will move. Because space-time is flexible, you can make it wave. Just like snapping a bedsheet, if you move enough matter around fast enough, a wave of distorted space-time will ripple outward into the universe.

Scientists predicted the existence of these ripples, called gravitational waves, as far back as the 1910s. But because the distortions they create in space-time are so minute, they weren’t detected until eight years ago. That’s when physicists used mile-long lasers to catch distortions in space-time from two black holes colliding in a distant galaxy. With that first epochal discovery, the doors to gravitational-wave science were thrown open. Since then, scientists have found many other merging black-hole pairs. But an even fainter signal carrying a profound cosmic significance still lay out of their reach.

Along with signals from discrete black-hole mergers, experts believed that a background of gravitational waves should also be washing through the universe, the space-time equivalent of car horns, jackhammers, and shouts all combining into the diffuse cacophony of city life. Plenty of cosmic phenomena could produce such a gravitational-wave background, and astronomers are now busily debating the most likely explanation. Perhaps the culprits are the zillions of supermassive black holes, some billions of times heavier than the sun, that reside at the center of every galaxy. Over cosmic timescales, galaxies collide and merge—and so do their black holes. These are near-apocalyptic events in terms of their effect on space-time, like a wall of speakers at a heavy-metal concert blasting against so many eardrums. Untold numbers of galaxies have merged across the 13.8-billion-year life of the universe, and those blasts should still be echoing in the background of space-time today. And so, perhaps, should the gravitational waves from the birth of the universe itself. The Big Bang was, well, a big bang. Initiating the expansion of everything required so much energy, and did so much violence, that it should have flooded space-time with gravitational waves that continue to ricochet around the universe to this day.

If scientists could find and analyze this background, they’d have a direct look all the way back to the first slivers of time after the moment of creation. First, however, they’d have to prove it exists. And now it seems they might have. A team of astronomers from around the world, working together as the North American Nanohertz Observatory for Gravitational Waves (a.k.a. NANOGrav), made the detection using the rapidly spinning cinders of once-massive stars called pulsars. Pulsars emit bursts of radio waves so perfectly timed that they serve as one of the universe’s most accurate natural clocks. Through 15 long years of sweat and perseverance, the NANOGrav scientists patiently tracked tiny changes in the burst patterns of 67 pulsars scattered across the Milky Way.

They found that a small change in the period of any one pulsar’s signal was linked to changes in the others’. These linked anomalies, they concluded, were reflections of changes in the distance between Earth and the pulsars as passing space-time ripples caused those distances to continually grow and shrink. Putting it all together, the NANOGrav scientists could see that these ripples were not from one discrete source but from a din, a hum, the overlapping echoes of disturbances scattered across the universe.

Over the course of a decade and a half, the NANOGrav team pored over their machines, their numbers, and their mathematical theory to bring us proof that something miraculous—something wonderful—is happening right under our noses. Actually, it’s happening to our noses, and the rest of our bodies as well. Every gravitational wave in that background the NANOGrav team found is humming through the very constitution of the space you inhabit right now. Every proton and neutron in every atom from the tip of your toes to the top of your head is shifting, shuttling, and vibrating in a collective purr within which the entire history of the universe is implicated. And if you put your hand down on a chair or table or anything else nearby, that object, too, is dancing that slow waltz.

The gravitational-wave background is huge news for the cosmos, yes, but it’s also huge news for you. The nature of reality has not changed—you will not suddenly be able to detect vibrations in your morning coffee that you couldn’t see before. And yet, moments like these can and should change how each of us sees our world. All of a sudden, we know that we are humming in tune with the entire universe, that each of us contains the signature of everything that has ever been. It’s all within us, around us, pushing us to and fro as we hurtle through the cosmos.

As an astronomer, I am often asked about UFOs. I’m pretty skeptical about them having anything to do with alien life, but I believe the questions represent something ancient and innate in us all. As children, each of us had a deep and easily triggered sense that the world is full of wonder, that everything is strange and amazing. Stepping out into the backyard, we’d get entranced staring at an anthill or watching leaves pirouette as they fell. As a toddler, my daughter would purposely tip her cup over just to be delighted by how the water spilled across the table.

Today, gifted with a new understanding of the architecture of the universe, each of us has an opportunity to revisit that wonder. After you finish reading this, take a look around you. Ponder how the solid-seeming ground beneath your feet is quietly shaking with the force of billions of years of cosmic collisions. Go outside, if you can, and watch the wind blow through the trees. Perhaps the experience will be different now that you know how the rhythm of giant black holes in distant galaxies also beats out a time in the trees’ gentle swaying.

The universe is an impossibly vast symphony of cause and effect. The endless comings and goings of galaxies, stars, and planets create a melding of songs that you are part of too. The NANOGrav discovery exposes the intricacy and gracefulness of that melding. It’s a reminder that the world always has, and always will be, worthy of wonder. But of course, you already knew that. You always have.

Several times in recent weeks I’ve heard people suggest that Mother Nature has been speaking to us through that smoke endlessly drifting south from the still-raging Canadian wildfires. She’s saying that she wants the coal, oil, and gas left in the ground, but I fear her message will have little more influence on climate policy than her previous ones did. After all, we essentially hit the “snooze”…

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