modern_physics (7)

City-Sized, Secure Quantum Network...

Physicists Create City-Sized Ultrasecure Quantum Network

Quantum physics experiment has demonstrated an important step toward achieving quantum cryptography among many users, an essential requirement for a secure quantum Internet. Credit: ÖAW and Klaus Pichler Getty Images

Topics: Cryptography, Futurism, Internet of Things, Modern Physics, Quantum Computer, Quantum Mechanics

Quantum cryptography promises a future in which computers communicate with one another over ultrasecure links using the razzle-dazzle of quantum physics. But scaling up the breakthroughs in research labs to networks with a large number of nodes has proved difficult. Now an international team of researchers has built a scalable city-wide quantum network to share keys for encrypting messages.

The network can grow in size without incurring an unreasonable escalation in the costs of expensive quantum hardware. Also, this system does not require any node to be trustworthy, thus removing any security-sapping weak links.

“We have tested it both in the laboratory and in deployed fibers across the city of Bristol” in England, says Siddarth Koduru Joshi of the University of Bristol. He and his colleagues demonstrated their ideas using a quantum network with eight nodes in which the most distant nodes were 17 kilometers apart, as measured by the length of the optical fiber connecting them. The team’s findings appeared in Science Advances on September 2.

Physicists Create City-Sized Ultrasecure Quantum Network, Anil Ananthaswamy, Scientific American

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Strange Metals...


A diagram showing different states of matter as a function of temperature, T, and interaction strength, U (normalized to the amplitude, t, of electrons hopping between sites). Strange metals emerge in a regime separating a metallic spin glass and a Fermi liquid. P. Cha et al./Proceedings of the National Academy of Sciences 2020

Topics: Black Holes, Modern Physics, Quantum Mechanics, Superconductors, Theoretical Physics

Even by the standards of quantum physicists, strange metals are just plain odd. The materials are related to high-temperature superconductors and have surprising connections to the properties of black holes. Electrons in strange metals dissipate energy as fast as they’re allowed to under the laws of quantum mechanics, and the electrical resistivity of a strange metal, unlike that of ordinary metals, is proportional to the temperature.

Generating a theoretical understanding of strange metals is one of the biggest challenges in condensed matter physics. Now, using cutting-edge computational techniques, researchers from the Flatiron Institute in New York City and Cornell University have solved the first robust theoretical model of strange metals. The work reveals that strange metals are a new state of matter, the researchers report July 22 in the Proceedings of the National Academy of Sciences.

“The fact that we call them strange metals should tell you how well we understand them,” says study co-author Olivier Parcollet, a senior research scientist at the Flatiron Institute’s Center for Computational Quantum Physics (CCQ). “Strange metals share remarkable properties with black holes, opening exciting new directions for theoretical physics.”

Quantum Physicists Crack Mystery of ‘Strange Metals,’ a New State of Matter, Thomas Sumner, Simon Foundation

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Holographic Microscopy...


FIG. 1. (a) Schematic of an inline digital holographic microscope. In a typical setup, a collimated laser (light red) illuminates a sample, which scatters light (dark red wavefronts). The transmitted and scattered light passes through an objective and tube lens, which focuses the light onto a digital camera. (b) A hologram of a polystyrene particle obtained from an inline holographic microscope.

Topics: Holography, Optical Physics, Microscopy, Modern Physics

In the past few years, the venerable field of holographic microscopy has been revitalized by computational data analysis. It is now possible to fit a generative (forward) model of scattering directly to experimentally obtained holograms of complex microscopic objects. This approach enables precision measurements: it allows the motion of colloidal particles and biological organisms to be tracked with nanometer-scale precision and their optical properties to be inferred particle by particle. In this Perspective, we discuss how the model-based inference approach to holographic microscopy has opened up new applications. We also discuss how it must evolve to meet the needs of emerging applications that demand lower systematic uncertainties and higher precision. In this context, we present some new results on how modeling the optical train of the microscope can enable better measurements of the positions of spherical and nonspherical colloidal particles. Finally, we discuss how machine learning might play a role in future advances. Though we do not exhaustively catalog all the developments in this field, we show a few examples and some new results that spotlight open questions and opportunities.


Precise measurements in digital holographic microscopy by modeling the optical train, Ronald Alexander, Brian Leahy, Vinothan N. Manoharan, Journal of Applied Physics

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Hybrid Quantum Networking...

Credit: Getty Images


Topics: Computer Science, Modern Physics, Quantum Computer, Quantum Mechanics

In a world’s first, researchers in France and the U.S. have performed a pioneering experiment demonstrating “hybrid” quantum networking. The approach, which unites two distinct methods of encoding information in particles of light called photons, could eventually allow for more capable and robust communications and computing.

Similar to how classical electronics can represent information as digital or analog signals, quantum systems can encode information as either discrete variables (DVs) in particles or continuous variables (CVs) in waves. Researchers have historically used one approach or the other—but not both—in any given system.

“DV and CV encoding have distinct advantages and drawbacks,” says Hugues de Riedmatten of the Institute of Photonic Sciences in Barcelona, who was not a part of the research. CV systems encode information in the varying intensity, or phasing, of light waves. They tend to be more efficient than DV approaches but are also more delicate, exhibiting stronger sensitivity to signal losses. Systems using DVs, which transmit information by the counting of photons, are harder to pair with conventional information technologies than CV techniques. They are also less error-prone and more fault-tolerant, however. Combining the two, de Riedmatten says, could offer “the best of both worlds.”

‘Hybrid’ Quantum Networking Demonstrated for First Time, Dhananjay Khadilkar, Scientific American

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Majorana qubits...
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Topics: History, Modern Physics, Quantum Computer, Quantum Mechanics

Soon after Enrico Fermi became a professor of physics at Italy’s University of Rome in 1927, Ettore Majorana joined his research group. Majorana’s colleagues described him as humble because he considered some of his work unexceptional. For example, Majorana correctly predicted in 1932 the existence of the neutron, which he dubbed a neutral proton, based on an atomic-structure experiment by Irène Joliot-Curie and Frédéric Joliot-Curie. Despite Fermi’s urging, Majorana didn’t write a paper. Later that year James Chadwick experimentally confirmed the neutron’s existence and was awarded the 1935 Nobel Prize in Physics for the discovery.

Nevertheless, Fermi thought highly of Majorana, as is captured in the following quote: “There are various categories of scientists, people of a secondary or tertiary standing, who do their best but do not go very far. There are also those of high standing, who come to discoveries of great importance, fundamental for the development of science. But then there are geniuses like Galileo and Newton. Well, Ettore was one of them.” Majorana only wrote nine papers, and the last one, about the now-eponymous fermions, was published in 1937 at Fermi’s insistence. A few months later, Majorana took a night boat to Palermo and was never seen again.1

In that final article, Majorana presented an alternative representation of the relativistic Dirac equation in terms of real wavefunctions. The representation has profound consequences because a real wavefunction describes particles that are their own antiparticles, unlike electrons and positrons. Since particles and antiparticles have opposite charges, fermions in his new representation must have zero charge. Majorana postulated that the neutrino could be one of those exotic fermions.

Although physicists have observed neutrinos for more than 60 years, whether Majorana’s hypothesis is true remains unclear. For example, the discovery of neutrino oscillations, which earned Takaaki Kajita and Arthur McDonald the 2015 Nobel Prize in Physics, demonstrates that neutrinos have mass. But the standard model requires that neutrinos be massless, so various possibilities have been hypothesized to explain the discrepancy. One answer could come from massive neutrinos that do not interact through the weak nuclear force. Such sterile neutrinos could be the particles that Majorana predicted. Whereas conclusive evidence for the existence of Majorana neutrinos remains elusive, researchers are now using Majorana’s idea for other applications, including exotic excitations in superconductors.

Majorana qubits for topological quantum computing, Physics Today

Ramón Aguado is a senior researcher at the Spanish National Research Council (CSIC) in Madrid.

Leo Kouwenhoven is a researcher at the Microsoft Quantum Lab Delft and a professor of applied physics at Delft University of Technology in the Netherlands.

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Figure 1.
Spin–orbit coupling can open a bulk bandgap in materials with inverted valence and conduction bands. That gap is complete in a topological insulator, but in a Weyl semimetal, the bands still touch at certain points. Both phases also host surface states not shown here. (Adapted from ref. 4, B. Yan and C. Felser.)


Topics: Modern Physics, Particle Physics, Quantum Mechanics

When Paul Dirac introduced his famous equation for relativistic fermions in 1928, he aimed to describe one well-known particle: the electron. Shortly thereafter, Hermann Weyl observed that the equation has a special solution when the mass is set to zero. The so-called Weyl fermions embodied by that solution would be charged, like electrons, but being massless, they would travel faster and with less energy dissipation. The particles would also be chiral, like neutrinos, with each one’s handedness depending on whether its spin is aligned or antialigned with its momentum. Those features make Weyl fermions appealing candidates for use in electronic and spintronic devices.

No such elementary particle has yet been found. However, in 2015 three groups of researchers identified the first Weyl semimetal (WSM), tantalum arsenide, which hosts quasiparticles—collective excitations of electrons—with the properties of Weyl fermions.1 A WSM must have a broken symmetry, and in TaAs, it’s inversion symmetry. Researchers, however, have continued searching for materials, particularly ferromagnetic materials, that instead rely on broken time-reversal symmetry. Tying a WSM crystal’s properties to magnetism, which can be adjusted using temperature changes or external fields, makes them potentially tunable.

Three new papers provide experimental evidence for magnetic WSMs. Yulin Chen’s team at Oxford University and Haim Beidenkopf’s team at the Weizmann Institute of Science, together with collaborators,2 presented studies of Co3Sn2S2, and Zahid Hasan’s group at Princeton University3 looked at Co2MnGa. The works identify important features in the electronic structures of both materials’ bulk and surface states.


Magnetic semimetals host massless quasiparticles, Christine Middleton, Physics Today

#P4TC: Weyl Fermions...July 27, 2015

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Quantum Sound...

Credit: Getty Images


Topics: Modern Physics, Phonons, Quantum Mechanics, Theoretical Physics

Researchers have gained control of the elusive “particle” of sound, the phonon. Although phonons—the smallest units of the vibrational energy that makes up sound waves—are not matter, they can be considered particles the way photons are particles of light. Photons commonly store information in prototype quantum computers, which aim to harness quantum effects to achieve unprecedented processing power. Using sound instead may have advantages, although it would require manipulating phonons on very fine scales.

Until recently, scientists lacked this ability; just detecting an individual phonon destroyed it. Early methods involved converting phonons to electricity in quantum circuits called superconducting qubits. These circuits accept energy in specific amounts; if a phonon’s energy matches, the circuit can absorb it—destroying the phonon but giving an energy reading of its presence.

In a new study, scientists at JILA (a collaboration between the National Institute of Standards and Technology and the University of Colorado Boulder) tuned the energy units of their superconducting qubit so phonons would not be destroyed. Instead the phonons sped up the current in the circuit, thanks to a special material that created an electric field in response to vibrations. Experimenters could then detect how much change in current each phonon caused.

“There’s been a lot of recent and impressive successes using superconducting qubits to control the quantum states of light. And we were curious—what can you do with sound that you can’t with light?” says Lucas Sletten of U.C. Boulder, lead author of the study published in June in Physical Review X. One difference is speed: sound travels much slower than light. Sletten and his colleagues took advantage of this to coordinate circuit-phonon interactions that sped up the current. They trapped phonons of particular wavelengths (called modes) between two acoustic “mirrors,” which reflect sound, and the relatively long time sound takes to make a round trip allowed the precise coordination. The mirrors were a hair’s width apart—similar control of light would require mirrors separated by about 12 meters.


Trapping the Tiniest Sound, Leila Sloman, Scientific American

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