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Quantum Computing Breakthrough: Silicon Qubits Interact at Long-Distance – SciTechDaily

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Researchers at Princeton University have made an important step forward in the quest to build a quantum computer using silicon components, which are prized for their low cost and versatility compared to the hardware in today’s quantum computers. The team showed that a silicon-spin quantum bit (shown in the box) can communicate with another quantum bit located a significant distance away on a computer chip. The feat could enable connections between multiple quantum bits to perform complex calculations. Credit: Felix Borjans, Princeton University

Princeton scientists demonstrate that two silicon quantum bits can communicate across relatively long distances in a turning point for the technology.

Imagine a world where people could only talk to their next-door neighbor, and messages must be passed house to house to reach far destinations.

Until now, this has been the situation for the bits of hardware that make up a silicon quantum computer, a type of quantum computer with the potential to be cheaper and more versatile than today’s versions.

Now a team based at Princeton University has overcome this limitation and demonstrated that two quantum-computing components, known as silicon “spin” qubits, can interact even when spaced relatively far apart on a computer chip. The study was published today (December 25, 2019) in the journal Nature.

“The ability to transmit messages across this distance on a silicon chip unlocks new capabilities for our quantum hardware,” said Jason Petta, the Eugene Higgins Professor of Physics at Princeton and leader of the study. “The eventual goal is to have multiple quantum bits arranged in a two-dimensional grid that can perform even more complex calculations. The study should help in the long term to improve communication of qubits on a chip as well as from one chip to another.”

Quantum computers have the potential to tackle challenges beyond the capabilities of everyday computers, such as factoring large numbers. A quantum bit, or qubit, can process far more information than an everyday computer bit because, whereas each classical computer bit can have a value of 0 or 1, a quantum bit can represent a range of values between 0 and 1 simultaneously.

To realize quantum computing’s promise, these futuristic computers will require tens of thousands of qubits that can communicate with each other. Today’s prototype quantum computers from Google, IBM and other companies contain tens of qubits made from a technology involving superconducting circuits, but many technologists view silicon-based qubits as more promising in the long run.

Silicon spin qubits have several advantages over superconducting qubits. The silicon spin qubits retain their quantum state longer than competing qubit technologies. The widespread use of silicon for everyday computers means that silicon-based qubits could be manufactured at low cost.

The challenge stems in part from the fact that silicon spin qubits are made from single electrons and are extremely small.

“The wiring or ‘interconnects’ between multiple qubits is the biggest challenge towards a large scale quantum computer,” said James Clarke, director of quantum hardware at Intel, whose team is building silicon qubits using using Intel’s advanced manufacturing line, and who was not involved in the study. “Jason Petta’s team has done great work toward proving that spin qubits can be coupled at long distances.”

To accomplish this, the Princeton team connected the qubits via a “wire” that carries light in a manner analogous to the fiber optic wires that deliver internet signals to homes. In this case, however, the wire is actually a narrow cavity containing a single particle of light, or photon, that picks up the message from one qubit and transmits it to the next qubit.

The two qubits were located about half a centimeter, or about the length of a grain of rice, apart. To put that in perspective, if each qubit were the size of a house, the qubit would be able to send a message to another qubit located 750 miles away.

The key step forward was finding a way to get the qubits and the photon to speak the same language by tuning all three to vibrate at the same frequency. The team succeeded in tuning both qubits independently of each other while still coupling them to the photon. Previously the device’s architecture permitted coupling of only one qubit to the photon at a time.

“You have to balance the qubit energies on both sides of the chip with the photon energy to make all three elements talk to each other,” said Felix Borjans, a graduate student and first author on the study. “This was the really challenging part of the work.”

Each qubit is composed of a single electron trapped in a tiny chamber called a double quantum dot. Electrons possess a property known as spin, which can point up or down in a manner analogous to a compass needle that points north or south. By zapping the electron with a microwave field, the researchers can flip the spin up or down to assign the qubit a quantum state of 1 or 0.

“This is the first demonstration of entangling electron spins in silicon separated by distances much larger than the devices housing those spins,” said Thaddeus Ladd, senior scientist at HRL Laboratories and a collaborator on the project. “Not too long ago, there was doubt as to whether this was possible, due to the conflicting requirements of coupling spins to microwaves and avoiding the effects of noisy charges moving in silicon-based devices. This is an important proof-of-possibility for silicon qubits because it adds substantial flexibility in how to wire those qubits and how to lay them out geometrically in future silicon-based ‘quantum microchips.’”

The communication between two distant silicon-based qubits devices builds on previous work by the Petta research team. In a 2010 paper in the journal Science, the team showed it is possible to trap single electrons in quantum wells. In the journal Nature in 2012, the team reported the transfer of quantum information from electron spins in nanowires to microwave-frequency photons, and in 2016 in Science they demonstrated the ability to transmit information from a silicon-based charge qubit to a photon. They demonstrated nearest-neighbor trading of information in qubits in 2017 in Science. And the team showed in 2018 in Nature that a silicon spin qubit could exchange information with a photon.

Jelena Vuckovic, professor of electrical engineering and the Jensen Huang Professor in Global Leadership at Stanford University, who was not involved in the study, commented: “Demonstration of long-range interactions between qubits is crucial for further development of quantum technologies such as modular quantum computers and quantum networks. This exciting result from Jason Petta’s team is an important milestone towards this goal, as it demonstrates non-local interaction between two electron spins separated by more than 4 millimeters, mediated by a microwave photon. Moreover, to build this quantum circuit, the team employed silicon and germanium – materials heavily used in the semiconductor industry.”

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Reference: “Resonant microwave-mediated interactions between distant electron spins” by F. Borjans, X. G. Croot, X. Mi, M. J. Gullans and J. R. Petta, 25 December 2019, Nature.
DOI: 10.1038/s41586-019-1867-y

In addition to Borjans and Petta, the following contributed to the study: Xanthe Croot, a Dicke postdoctoral fellow; associate research scholar Michael Gullans; and Xiao Mi, who earned his Ph.D. at Princeton in Petta’s group and is now a research scientist at Google.

The study was funded by Army Research Office (grant W911NF-15-1-0149) and the Gordon and Betty Moore Foundation’s EPiQS Initiative (grant GBMF4535).

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'NOVA Universe Revealed' on PBS brings the cosmos down to Earth tonight – Space.com

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PBS’s newest science series, NOVA Universe Revealed, premieres tonight, Oct. 27, at 9 p.m. EDT (0100 GMT Oct. 28).

Space.com got a sneak peek at the five-episode series, which takes viewers on an epic journey through the cosmos. The first hour-long episode focuses on our sun and stars like it. Titled “Age of Stars,” it explores the life and death cycle of a star, including stunning archival footage from the ESA, Hubble Space Telescope and NASA, among other space organizations. Imagery from NASA’s Solar Dynamics Observatory makes for a gorgeous look at our sun. Scientists interviewed in the episode explain how a star is born and how it explodes into a supernova, and speculate about an ultimate age of darkness. 

Future episodes will explore other parts of the universe, including our Milky Way galaxy, the search for extraterrestrial life on other planets, black holes and the Big Bang. All episodes feature photorealistic animations, stunning photos, archival footage from space missions, and commentary from a diverse set of scientists. 

Related: The Universe: From the Big Bang to now in 10 easy steps

When deciding on the five topics NOVA Universe Revealed would tackle, there was only one that NOVA executive producer Chris Schmidt worried about pulling off: the Big Bang. 

“Stars, black holes, alien worlds, and galaxies — those are all objects with edges,” he told Space.com. “How do you do the story of everything?”

In the end, the episode worked beautifully. It opens with images of many of the universe’s most spectacular sights, from pulsars to enormous black holes, and humanity’s mission to explore the universe’s biggest mysteries. From there, the episode looks at the universe in reverse from a human perspective: we backtrack from the beginning of human life on Earth to the beginning of our galaxy, all the way to the beginning of the universe and even speculate about the moments before the Big Bang. 

Related: The strangest black holes in the universe

Related stories:

This episode, as well as the others, works so well because it tells the story of the universe in a way that NOVA’s viewers can connect to. 

“There’s a moment at the end [of the Big Bang episode] where Jim Gates acknowledges the idea that all of the stuff we’re made of is just the matter that was created with the Big Bang,” Schmidt said. “And to that extent, we’re part of the universe.”

Schmidt hopes this poetic idea, that all humans are inherently part of the universe, will enthrall viewers. “It’s a great opportunity to take a moment and look up from your feet,” he said. While we’re all preoccupied with our daily lives, it can be interesting and inspiring to consider the vast universe that exists beyond our sky. 

NOVA Universe Revealed was created in collaboration with BBC Studios Science Unit. New episodes will air every Wednesday at 9 p.m. ET/8CT, with the last episode airing November 24. All five episodes are currently available for free streaming on pbs.org/nova and on the PBS video app.

Follow Kasandra Brabaw on Twitter @KassieBrabaw. Follow us on Twitter @Spacedotcom and on Facebook. 

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New Measurement Rules Out Sterile Neutrinos – Forbes

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The history of science is a tumultuous one, with fascinating discoveries that either validate a theory or consign it to the trash heap of history. A recent measurement of the properties of ghostlike subatomic particles called neutrinos appears to have signed the death warrant of a theory that was thought by many to be a solution to many cosmological mysteries. A hypothetical particle called a sterile neutrino now appears to have been ruled out. Sterile neutrinos are different from ordinary neutrinos, which have been observed. Sterile neutrinos have also been considered to be a candidate for dark matter, although this is only speculation. This measurement was performed at Fermi National Accelerator Laboratory (Fermilab), America’s flagship particle physics facility, located just outside Chicago.

Ordinary neutrinos are the lightest and least interactive of the known subatomic particles. They are created in great quantities in nuclear reactors, and they interact so little that they can pass through the entire Earth with little chance of interacting during their passage. There are three known categories of ordinary neutrinos – the electron type, the muon type, and the tau type.

Ordinary neutrinos are unique in the subatomic world in that they can change their identity as they travel in a process called “neutrino oscillation.” The first hints that this was true was observed when researchers in the 1960s tried to detect neutrinos coming from the biggest nuclear reactor around – the Sun. The Sun emits only electron type neutrinos and physicists can calculate how many of them are emitted and should be detected here on Earth. Measurements of the number of solar neutrinos hitting the detectors of the 1960s and 1970s only found a third as many neutrinos as expected. This was called the “solar neutrino problem.”

The solution to the mystery turned out to be simple. These early detectors could only see the passage of electron type neutrinos. In the journey from the Sun to Earth, some of the electron type neutrinos had morphed into the other two varieties. Since those varieties didn’t interact in the detectors, the predictions and measurements disagreed. Definitive studies proving that this oscillation phenomenon was the answer to the mystery were announced in 1998 and 2001.

While the study of nuclear reactions is one method for investigating the properties of ordinary neutrinos, it isn’t the only method available to researchers. Scientists can also use particle accelerators to make beams of neutrinos that they can then direct towards waiting detectors. When physicists make beams of ordinary neutrinos using this technique, the beams consist essentially entirely of muon type neutrinos.

In the 1990s, scientists at the Los Alamos National Laboratory in New Mexico were using beams of muon type neutrinos to try to understand the solar neutrino problem. While their beam was mostly muon type neutrinos, their detector also flagged the passage of electron type neutrinos. Although the phenomenon of neutrino oscillation had not yet been definitively demonstrated, several earlier measurements suggested how quickly muon type neutrinos should oscillate into the other two varieties. However, the Los Alamos measurements suggested that they were oscillating more quickly than was expected.

One possible solution that was proposed to explain the Los Alamos measurement was that there existed a fourth type of neutrino, which is called a sterile neutrino. If it existed, this type of neutrino was even more insubstantial than the ordinary types of neutrinos. Unlike the ordinary type, sterile neutrinos wouldn’t be emitted by nuclear reactors or particle beams and would only make their presence known by speeding up neutrino oscillations.

In 2002, an experiment at Fermilab was performed to reproduce the Los Alamos measurement. The Fermilab experiment was called MiniBooNE. MiniBooNE also saw more electron type neutrinos than expected and this was considered to be additional evidence that sterile neutrinos were real. However, MiniBooNE had a significant limitation. It couldn’t distinguish between an electron or photon produced when a neutrino interacted in the detector. This meant that the MiniBooNE measurement was not definitive.

To finally determine whether the idea of sterile neutrinos was right or not, a third experiment was proposed, called MicroBooNE. MicroBooNE does not suffer from the limitations of the previous experiments. In a seminar presented on October 27, the MicroBooNE collaboration announced that four different searches for an excess of electron type neutrinos found nothing. It appears that the sterile neutrino is not real.

Failure to find sterile neutrinos is a disappointment to those who championed the idea, and also to those who thought that sterile neutrinos might also be dark matter. However, the history of science is full of ideas that didn’t pan out. 

The MicroBooNE experiment is not yet complete. Today’s announcement was based on only half of their data. In addition, they continue to develop new techniques and methods to comb through the data, looking for anything they might have overlooked. It remains possible that their full analysis might turn up something unexpected. It wouldn’t be the first time that neutrinos surprised scientists.

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Have astronomers found the first planet outside of our galaxy? It's complicated – CNET

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The Whirlpool Galaxy, some 28 million light-years from Earth, looks to our telescopes like a cosmic hurricane littered with sparkling gemstones. Huge, lean arms spiral out from the center of Whirlpool, also known as M51. Cradled within them are young stars flaring to life and old stars expanding, expiring and exploding. 

In 2012, NASA’s Chandra Observatory, which sees the sky in X-rays, spotted a curious flicker coming from the galaxy. An X-ray source in one of Whirlpool’s arms switched off for about two hours before suddenly flaring back to life. This isn’t particularly unusual for X-ray sources in the cosmos. Some flare, others periodically dim. 

This particular source emanated from an “X-ray binary,” known as M51-ULS-1, which is actually two objects: Cosmic dance partners that have been two-stepping around each other for potentially billions of years. One of these objects is either a black hole or a neutron star, and the other may be a large, very bright type of star known as a “blue supergiant.”

As astronomers looked a little more closely at the X-ray signal from the pair, they began to suspect the cause for the dimming may have been something we’ve never seen before: A world outside of the Milky Way, had briefly prevented X-rays from reaching our telescopes. The team dubbed it an “extroplanet.”

A research team led by astronomer Rosanne Di Stefano, of the Harvard-Smithsonian Center for Astrophysics, published details of their hypothesis in the journal Nature Astronomy on Oct. 25. Their study lays out evidence that the X-ray wink detected by Chandra was potentially caused by a planet, about the size of Saturn, passing in front of M51-ULS-1.

The extroplanet candidate goes by the name “M51-1” and is believed to orbit its host binary at about the same distance Uranus orbits our sun.

While many news sources have championed the detection as the “first planet discovered outside of the Milky Way,” there’s no way of confirming the find. At least, not for another few decades, when the proposed planet is supposed to make another transit of the binary. Di Stefano says the team modeled other objects that could potentially produce the dip in X-rays but came up short. Still, she stresses this is not a confirmed detection.

“We cannot claim that this is definitely a planet,” says Di Stefano, “but we do claim that the only model that fits all of the data … is the planet candidate model.”

While other astronomers are excited by the use of X-rays as a way of discovering distant worlds, they aren’t as convinced Di Stefano’s team has been able to rule out other objects such as large, failed stars known as brown dwarfs or smaller, cooler M stars. 

“Either this is a completely unexpected exoplanet discovered almost immediately in a small amount of data, or it’s something quite common or garden variety,” says Benjamin Pope, an astrophysicist studying exoplanets at the University of Queensland in Australia.

Hunting for hidden worlds

Astronomers have been probing the skies for decades, searching for planets outside of our solar system. The first confirmed detection of an exoplanet came in 1992 when two or more bodies were detected around the rapidly spinning neutron star PSR1257+12

Prior to these first detections, humans had mostly imagined planets very similar to those we become familiar with in preschool. Rocky planets like the Earth and Mars, gas giants like Jupiter and smaller worlds, like Pluto, far from the sun. Since 1992, our ideas have proven to be extremely unimaginative. 

Exoplanets are truly alien worlds with extremely strange features. There’s the planet where it rains iron, the mega Jupiter that orbits its home star in an egg-shaped orbit, a “naked” planet in the Neptune desert and a ton of super-Earths that seem to resemble home, just a little engorged. Dozens of strange, new worlds continue to be found by powerful planet-hunting telescopes each year. 

But all of these worlds have, so far, been located within the Milky Way. 

The Whirlpool Galaxy, M51, in X-ray and optical light.


NASA/CXC/SAO/R. DiStefano, et al.

It’s very likely (in fact, it’s practically certain) that planets exist outside of our galaxy — we just haven’t been able to detect them yet. Our closest galactic neighbor, Andromeda, is approximately 2.5 million light-years away. The farthest exoplanet we’ve found resides at just 28,000 light-years from Earth, according to the NASA Exoplanet Catalog

Finding planets outside the solar system is not easy because less and less light makes its way across the universe to our telescopes. Astronomers rarely “see” an exoplanet directly. This is because the bright light from a star in nearby planetary systems usually obscures any planets that might orbit around it. 

To “see” them, astronomers have to block out a star’s rays. Less than 2% of the exoplanets in NASA’s 4,538-strong catalog have been found by this method, known as “direct imaging.” 

But one highly successful method, accounting for over 3,000 exoplanet detections, is known as the “transit” method. Astronomers point their telescopes at stars and then wait for periodic dips in their brightness. If these dips come with a regular cadence, they can represent a planet, moving around the star and, from our view on Earth, periodically eclipsing its fiery host. It’s the same idea as a solar eclipse, when the moon passes directly in front of our sun and darkness descends over the Earth.

It’s this method that was critical to the discovery of M51-1. However, instead of detecting dips in visible light (a form of electromagnetic radiation), the team saw a dip in the X-rays (a different form of electromagnetic radiation). Because those X-rays were emanating from a relatively small region, Di Stefano says, a passing planet seems like it could block most or all of them.

M51-1

If M51-1 is a planet, Di Stefano’s team believe it may have had a tumultuous life. 

It’s gravitationally bound to the X-ray binary M51-ULS-1, which Di Stefano’s team posits consists of a black hole or neutron star orbiting a supergiant star. In the eons-old dance between the pair, the black hole or neutron star has been siphoning off mass from the supergiant. This mass, made of hot dust and gas, is constantly in motion around the black hole/neutron star in what’s known as an accretion disk. This hot disk gives off the X-rays detected by Chandra.

Regions of space around X-ray binaries are violent places, and this disk doesn’t give off X-rays in a stable manner. Sometimes, the X-rays seem to switch off for hours, but pinning down the reason is hard. “Within the very wide range of kinds of behaviors of these dynamic systems, it’s possible that some variation in the accretion rate or something like that could give rise to events like this,” says Duncan Galloway, an astrophysicist at Monash University studying neutron star binaries.

m51-lightcurve

The dip in X-ray brightness is apparent on this graph, just prior to 45 hours — but was it caused by a planet?


NASA/CXC/SAO/R. DiStefano, et al.

One belief is that the dimming could result from some of the hot gas and dust in the system obscuring the signal. Di Stefano says this is not the case, because gas and dust would provide a different signal. “As they pass in front of the X-ray source, some of the light from the source begins to interact with the outer regions of the cloud and this gives a distinctive spectral signature,” she notes.

Another possibility is that the X-ray dimming was caused by different types of stars obscuring our view. One type, known as a brown dwarf, arises when a star fails to properly ignite. Another, an M dwarf, is a common type of star sometimes dubbed a “red dwarf.” But due to the age of the M51-ULS-1 system, Di Stefano’s team believe these objects would be much larger than the object they’ve detected.

Di Stefano’s team ran a load of models exploring various scenarios for why the X-ray source switched off. In the end, she says, it was a Saturn-sized planet that seemed to fit what they were seeing best.

“The planet candidate model was the last one standing, so to speak,” says Di Stefano.

Pope is less convinced. “Personally, I wouldn’t bet that this is a planet,” he says. “In my view this is probably a stellar companion or something exotic happening in the disk.” 

Trust the process

This isn’t the first time NASA’s Chandra observatory has been swept up in a potential “extroplanet” find. Studying how radiation from distant stars is “bent” by gravity, a technique known as microlensing, astronomers at the University of Oklahoma believed they detected thousands of extragalactic planets back in 2018. Earlier studies have claimed to find evidence of extragalactic planets in the Andromeda galaxy.

Other astronomers were skeptical about these detections, too. The same skepticism has played out in the case of M51-1. And, importantly, that’s perfectly normal. 

This is the scientific process in action. Di Stefano’s team have argued their case: M51-1 is an extragalactic planet. Now there’s more work to do. Confirmation that M51-1 is planetary won’t be possible until it makes another transit of the X-ray binary in many decades’ time, but there are other ways for astronomers to vet their results. 

Pope notes that if we found analogous systems in the Milky Way, we’d be able to follow up with optical telescopes and get a better understanding of what might be happening at these types of systems. 

We know there must be planets outside of the Milky Way, and so, eventually, humans will discover them. For Galloway, the study is exciting not because of what caused the X-ray binary to dip in brightness, but what happens next. 

“The really exciting thing is there might be additional events in other data, so now we have a motivation where we can go and look for them,” he says.

Di Stefano feels the same way, hoping the publication will bring others into this type of research. She says the team is working hard, studying the skies for other X-ray binaries that may exhibit similar dimming.

“Ultimately,” she notes, “the best verification will be the discovery of more planets.”

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