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In the quest to build a quantum internet, scientists are putting their memories to the test. Quantum memories, that is.

Quantum memories are devices that store fragile information in the realm of the very small. Theyre an essential component for scientists vision of quantum networks that could allownew types of communication, from ultra-secure messaging to linking up far-flung quantum computers (SN: 6/28/23). Such memories would help scientistsestablish quantum connections, or entanglement, throughout a network (SN: 2/12/20).

Now, two teams of scientists have entangled quantum memories in networks nestled into cities, where the hustle and bustle of urban life can pose challenges to quantum communications.

These two impressive studies are pushing out of the lab and into real-world implementations, says physicist Benjamin Sussman of the University of Ottawa, who was not involved with the research. These are not just toy systems, but are really the first steps toward what future networks will look like.

In a network of two quantum memories connected by a telecommunications fiber link that traversed a 35-kilometer loop through Boston and Cambridge, Mass., scientistsmaintained entanglement for about a second, physicist Can Knaut and colleagues report in the May 16Nature. That doesnt sound like a lot for us, but in the domain of quantum, where everything is more fleeting, one second is actually a really long time, says Knaut, of Harvard University.

The researchers used quantum memories built from a tiny hunk of diamond in which two of the diamonds normal carbon atoms are replaced by one atom of silicon. That substitution creates a defect that serves as a quantum bit, or qubit. In fact, the defect serves as two qubits one thats short-lived, and another long-lived qubit that acts as the memory. Scientists prodded the short-lived qubit with a photon, or particle of light. The researchers used that qubit as a go-between in order to entangle the long-lived qubit with the photon. Then the scientists sent the photon through the fiber and repeated the process to entangle the long-lived qubits in each memory.

Meanwhile, in Hefei, China, entanglement was achieved in a network withthree quantum memoriesseparated by fiber links of about 20 kilometers, researchers report in the same issue ofNature.

This teams quantum memory was based on a large ensemble of rubidium atoms about 1 millimeter in diameter. When hit with a laser, the ensemble of atoms can emit a photon. Rather than shuttling the photon directly to another quantum memory, the photon was sent to a centrally located station, where it was measured along with a photon sent from another memory. That generated entanglement between the two distant parts of the network.

Meeting up in the middle meant the photons didnt have to travel all the way to the other side of the network, an added bonus. This scheme is rather efficient, but its experimental realization is rather challenging, says experimental physicist Xiao-Hui Bao of theUniversity of Science & Technology of China in Hefei. The technique required the team to find methods to correct for changes in the length of the fibers due to temperature shifts and other effects that could cause problems. This painstaking effort is called phase stabilization. This is the main technology advance we made in this paper, Bao says.

In contrast, the Boston network had no central station and didnt require phase stabilization. But both teams achieved whats called heralded entanglement. That means that a signal is sent to confirm that the entanglement was established, which demands that the entanglement persists long enough for information to make its way across the network. That confirmation is important for using such networks for practical applications, says physicist Wolfgang Tittel, who was not involved with either study.

If you compare how these two different groups have achieved [heralded entanglement], you see that there are more differences than similarities, and I find that great, says Tittel, of the University of Geneva. There are different approaches which are all still very, very promising.

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Two real-world tests of quantum memories bring a quantum internet closer to reality - Science News Magazine

It can be hard to wrap our minds round the very large and the very small. Ron Koeberer/Millennium Images, UK

Imagine setting off on a spacecraft that can travel at the speed of light. You wont get far. Even making it to the other side of the Milky Way would take 100,000 years. It is another 2.5 million years to Andromeda, our nearest galactic neighbour. And there are some 2 trillion galaxies beyond that.

The vastness of the cosmos defies comprehension. And yet, at the fundamental level, it is made of tiny particles.It is a bit of a foreign country both the small and the very big, says particle physicist Alan Barr at the University of Oxford. I dont think you ever really understand it, you just get used to it.

Still, you need to have some grasp of scale to have any chance of appreciating how reality works.

Lets start big, with the cosmic microwave background (CMB), the radiation released 380,000 years after the big bang. The biggest scales weve measured are features in the CMB, says astrophysicist Pedro Ferreira, also at the University of Oxford. These helped us put the diameter of the observable universe at 93 billion light years.

At the other end of the scale, the smallest entities are fundamental particles like quarks. Yet quantum physics paints these as dimensionless blips in a quantum field, with no size at all. So what is the shortest possible distance?

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Quantum to cosmos: Why scale is vital to our understanding of reality - New Scientist

Entanglement is a key part ofquantum computing

Bartlomiej K. Wroblewski/Alamy

While scientists generally try to find sensible explanations for weird phenomena, quantum entanglement has them tied in knots.

This link between subatomic particles, in which they appear to instantly influence one another no matter how far apart, defies our understanding of space and time. It famously confounded Albert Einstein, who dubbed it spooky action at a distance. And it continues to be a source of mystery today. These quantum correlations seem to appear somehow from outside space-time, in the sense that there is no story in space and time that explains them, says Nicolas Gisin at the University of Geneva, Switzerland.

But the truth is that, as physicists have come to accept the mysterious nature of entanglement and are using it to develop new technologies, they are doubtful that it has anything left to tell us about how the universe works.

You can create quantum entanglement between particles by bringing them close together so that they interact and their properties become intertwined. Alternatively, entangled particles can be created together in a process such as photon emission or the spontaneous breakup of a single particle such as a Higgs boson.

The spooky thing is that, in the right conditions, if you then send these particles to opposite sides of the universe, performing a measurement on one will instantaneously affect the outcome of a measurement on the other, despite the fact that there can be no information exchanged between them.

For Einstein,

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How quantum entanglement really works and why we accept its weirdness - New Scientist

Researchers have developed a new method that merges experimental data with advanced calculations to explore how gluons contribute to proton spin, revealing complex dynamics and setting the stage for future three-dimensional proton imaging. Credit: SciTechDaily.com

New research combining experimental and computational approaches provides deeper insights into proton spin contributions from gluons.

Nuclear physicists have been tirelessly exploring the origins of proton spin. A novel approach, merging experimental data with cutting-edge calculations, has now illuminated the spin contributions from gluonsthe particles that bind protons. This advancement also sets the stage for three-dimensional imaging of the proton structure.

Joseph Karpie, a postdoctoral associate at the Center for Theoretical and Computational Physics (Theory Center) at the U.S. Department of Energys Thomas Jefferson National Accelerator Facility, led this groundbreaking research.

He said that this decades-old mystery began with measurements of the sources of the protons spin in 1987. Physicists originally thought that the protons building blocks, its quarks, would be the main source of the protons spin. But thats not what they found. It turned out that the protons quarks only provide about 30% of the protons total measured spin. The rest comes from two other sources that have so far proven more difficult to measure.

One is the mysterious but powerful strong force. The strong force is one of the four fundamental forces in the universe. Its what glues quarks together to make up other subatomic particles, such as protons or neutrons. Manifestations of this strong force are called gluons, which are thought to contribute to the protons spin. The last bit of spin is thought to come from the movements of the protons quarks and gluons.

A global analysis of experimental data and lattice Quantum Chromodynamics calculations provides insight into the role of the gluons (purple squiggles) contributing to the spin of the nucleon. Credit: Jefferson Lab

This paper is sort of a bringing together of two groups in the Theory Center who have been working toward trying to understand the same bit of physics, which is how do the gluons that are inside of it contribute to how much the proton is spinning around, he said.

He said this study was inspired by a puzzling result that came from initial experimental measurements of the gluons spin. The measurements were made at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility based at Brookhaven National Laboratory in New York. The data at first seemed to indicate that the gluons may be contributing to the protons spin. They showed a positive result.

But as the data analysis was improved, a further possibility appeared.

When they improved their analysis, they started to get two sets of results that seemed quite different, one was positive and the other was negative, Karpie explained.

While the earlier positive result indicated that the gluons spins are aligned with that of the proton, the improved analysis allowed for the possibility that the gluons spins have an overall negative contribution. In that case, more of the proton spin would come from the movement of the quarks and gluons, or from the spin of the quarks themselves.

This puzzling result was published by the Jefferson Lab Angular Momentum (JAM) collaboration.

Meanwhile, the HadStruc collaboration had been addressing the same measurements in a different way. They were using supercomputers to calculate the underlying theory that describes the interactions among quarks and gluons in the proton, Quantum Chromodynamics (QCD).

To equip supercomputers to make this intense calculation, theorists somewhat simplify some aspects of the theory. This somewhat simplified version for computers is called lattice QCD.

Karpie led the work to bring together the data from both groups. He started with the combined data from experiments taken in facilities around the world. He then added the results from the lattice QCD calculation into his analysis.

This is putting everything together that we know about quark and gluon spin and how gluons contribute to the spin of the proton in one dimension, said David Richards, a Jefferson Lab senior staff scientist who worked on the study.

When we did, we saw that the negative things didnt go away, but they changed dramatically. That meant that theres something funny going on with those, Karpie said.

Karpie is lead author on the study that was recently published in Physical Review D. He said the main takeaway is that combining the data from both approaches provided a more informed result.

Were combining both of our datasets together and getting a better result out than either of us could get independently. Its really showing that we learn a lot more by combining lattice QCD and experiment together in one problem analysis, said Karpie. This is the first step, and we hope to keep doing this with more and more observables as well as we make more lattice data.

The next step is to further improve the datasets. As more powerful experiments provide more detailed information on the proton, these data begin painting a picture that goes beyond one dimension. And as theorists learn how to improve their calculations on ever-more powerful supercomputers, their solutions also become more precise and inclusive.

The goal is to eventually produce a three-dimensional understanding of the protons structure.

So, we learn our tools do work on the simpler one-dimension scenario. By testing our methods now, we hopefully will know what we need to do when we want to move up to do 3D structure, Richards said. This work will contribute to this 3D image of what a proton should look like. So its all about building our way up to the heart of the problem by doing this easier stuff now.

Reference: Gluon helicity from global analysis of experimental data and lattice QCD Ioffe time distributions by Jefferson Lab Angular Momentum and HadStruc Collaborations, J. Karpie, R. M. Whitehill, W. Melnitchouk, C. Monahan, K. Orginos, J.-W. Qiu, D. G. Richards, N. Sato and S. Zafeiropoulos, 27 February 2024,Physical Review D. DOI: 10.1103/PhysRevD.109.036031

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The Quantum Twist: Unveiling the Proton's Hidden Spin - SciTechDaily