Category: quantum physics
Extending the Adiabatic Theorem
Jerk the support from which a swinging pendulum hangs, and you will change the pendulum’s motion. But move the support very gradually, and the system will adapt so that the pendulum’s motion relative to its support remains unchanged. A similar principle holds true for quantum systems. The quantum adiabatic theorem says that a system, when perturbed sufficiently slowly, remains in its instantaneous ground state. Sarah Damerow and Stefan Kehrein of the University of Göttingen in Germany now show that aspects of this principle remain true even for the opposite limit: The ground state remains the single most likely state even for a quantum system subjected to an instantaneous perturbation [1].
Formally, the quantum adiabatic theorem describes how a perturbed system’s Hamiltonian evolves in time. It shows that, for a slow perturbation, the system transitions from its initial ground state to the time-evolved Hamiltonian’s ground state with a probability greater than the combined probabilities of all the excited states.
Damerow and Kehrein used analytical and numerical tools to examine a quantum system undergoing rapid perturbation. They considered a quantum Ising model—a lattice of interacting magnetic spins—subjected to a rapidly changing external field. They found that the system was more likely to evolve from its initial ground state to the time-evolved Hamiltonian’s ground state than to any given excited state—provided that the lattice was in the same magnetic phase (paramagnetic or ferromagnetic) in both ground states.
Building desktop particle accelerators to unlock new realms of research
Using high-intensity lasers, researchers have taken an important step toward miniaturization of particle accelerators by demonstrating free-electron laser amplification at extreme ultraviolet wavelengths (27–50 nm), with an acceleration length of only a few millimeters. By generating high-quality, monoenergetic electron beams (i.e. beams where all the electrons have nearly the same energy), they have achieved a key milestone toward compact accelerator technologies.
The work is published in the journal Physical Review Research.
The research team led by The University of Osaka’s Institute of Scientific and Industrial Research (SANKEN) in collaboration with Kansai Institute for Photon Science (KPSI), National Institutes for Quantum Science and Technology (QST), RIKEN SPring-8 Center (RSC), High Energy Accelerator Research Organization (KEK), used a technique called laser wakefield acceleration to create plasma waves that generate extremely strong accelerating electric fields, thanks to waves within the plasma that travel at almost the speed of light.
Useful quantum computers could be built with as few as 10,000 qubits, team finds
Quantum computers of the future may be closer to reality thanks to new research from Caltech and Oratomic, a Caltech-linked start-up company. Theorists and experimentalists teamed up to develop a new approach for reducing the errors that riddle today’s rudimentary quantum computers. Whereas these machines were previously thought to require millions of qubits to work properly (qubits being the quantum equivalent to 1’s and 0’s in classical computers), the new results indicate that a fully realized quantum computer could be built with as few as 10,000 to 20,000 qubits. The need for fewer qubits means that quantum computers could, in theory, be operational by the end of the decade.
The team proposes a new quantum error-correction architecture that is significantly more efficient than previous approaches. Quantum error correction is a process by which extra, redundant qubits are introduced to correct errors, or faults, enabling the ultimate goal in the field: fault-tolerant quantum computing.
The results exploit special properties of quantum computing platforms built out of neutral atoms, which serve as the qubits. Alternative platforms in development include superconducting circuits and trapped ions (ions are charged whereas neutral atoms are not). In a neutral atom system, laser beams known as optical tweezers are used to arrange atoms into qubit arrays. Manuel Endres, a professor of physics at Caltech, and his colleagues recently created the largest qubit array ever assembled, containing 6,100 trapped neutral atoms.
Ultrafast quantum light pulses measured for the first time
Researchers at the Technion—Israel Institute of Technology have, for the first time, measured the temporal duration of individual pulses of an extraordinary form of quantum light known as bright squeezed vacuum (BSV). Their findings are published in Optica.
Bright squeezed vacuum is a unique quantum state of light. Although it is formally considered the “vacuum state” and the electric field of this light is zero on average, it exhibits enormous quantum fluctuations of its electric field due to the squeezing effect.
This is in stark contrast to typical light produced by intense lasers, known as coherent-state light, that exhibit only extremely weak quantum fluctuations. However, for BSV, the fluctuations can lead to extremely intense light pulses, containing up to one trillion (10¹²) photons in a single pulse, hence the term bright squeezed vacuum. Until now, no one had measured the temporal duration of single BSV pulses.
Superconductivity switched on in material once thought only magnetic
Superconductivity—the ability of a material to conduct electricity without any energy loss to heat—enables highly efficient, ultra-fast electronics essential for advanced technologies such as magnetic resonance imaging (MRI) machines, particle accelerators and, potentially, quantum computers. New research has now revealed that iron telluride (FeTe), a compound composed of the chemical elements iron and tellurium and long thought to be an ordinary magnetic metal, is in fact a superconductor. The researchers found that hidden excess iron atoms induce the material’s magnetism, and removing these atoms allows electricity to flow with zero resistance.
Two papers describing the research, both led by Penn State Professor of Physics Cui-Zu Chang, were published back-to-back today (April 1) in the journal Nature. The first paper focuses on how to “switch on” superconductivity in FeTe, while the second paper reveals a new kind of “quantum dance,” where superconductivity interacts with the material’s atomic structure when a different top layer is added, allowing researchers to tune its behavior.
“Unlike the well-known iron-based superconductor iron selenide (FeSe), FeTe has long been considered a magnetic metal without superconductivity, despite having an almost identical crystal structure,” Chang said. “It has remained a mystery why FeTe doesn’t share this important property.”
This New Quantum Theory Could Change Everything We Know About the Big Bang
A new quantum gravity theory suggests the Big Bang may have unfolded naturally—and could soon be testable.
Scientists at the University of Waterloo have introduced a new approach to understanding how the universe began, one that could reshape current ideas about the Big Bang and the earliest stages of cosmic history. Their research indicates that the universe’s rapid initial expansion may have developed naturally from a deeper and more complete theory known as quantum gravity.
Combining Gravity With Quantum Physics
Quantum magnetism: Spin-flip process in atomic nucleus does not account for all magnetic behavior
In the air people breathe, the water on Earth, the stars in the sky and more, atoms are the building blocks that make up the universe. Understanding the structure of the atomic nucleus is crucial for research with implications for astrophysics and in applications such as medical imaging and data storage.
A new study conducted by Department of Physics researchers using the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University examined titanium-50 nuclei and showed that a long-standing explanation for where magnetism in atomic nuclei comes from does not fully work for titanium-50. The research, which was published in Physical Review Letters, suggests that scientists may need to rethink how they explain nuclear magnetism.
“What current models propose is that magnetic strength is largely generated by spin-flip excitations, that means when flipping proton or neutron spins from up to down between so-called spin-orbit partner orbitals,” said Associate Professor Mark Spieker, a co-author on the multi-institution study. “For the first time, we showed that this type of spin-flip cannot be the only mechanism that generates nuclear magnetism.”
Racetrack-shaped lasers developed for bright, stable frequency combs
A new, miniature laser source developed by applied physicists in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Technical University of Vienna (TU Wien) could soon pack the power of a laboratory-based spectrometer—an important workhorse tool for precision environmental gas analysis—onto a single microchip.
The device, a ring-shaped, “racetrack” quantum cascade laser, generates a specific type of light source, called a frequency comb, in the difficult-to-access mid-infrared region of the electromagnetic spectrum. It was developed in the lab of Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, in collaboration with co-senior author Benedikt Schwarz and colleagues at TU Wien.
The research was co-led by first author Ted Letsou, a postdoctoral researcher in the Capasso group, and Johannes Fuchsberger, a graduate student at TU Wien, and is published in Optica.