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Category: quantum physics – Page 498

First Light for a Next-Generation Light Source

X-ray free-electron lasers (XFELs) first came into existence two decades ago. They have since enabled pioneering experiments that “see” both the ultrafast and the ultrasmall. Existing devices typically generate short and intense x-ray pulses at a rate of around 100 x-ray pulses per second. But one of these facilities, the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in California, is set to eclipse this pulse rate. The LCLS Collaboration has now announced “first light” for its upgraded machine, LCLS-II. When it is fully up and running, LCLS-II is expected to fire one million pulses per second, making it the world’s most powerful x-ray laser.

The LCLS-II upgrade signifies a quantum leap in the machine’s potential for discovery, says Robert Schoenlein, the LCLS’s deputy director for science. Now, rather than “demonstration” experiments on simple, model systems, scientists will be able to explore complex, real-world systems, he adds. For example, experimenters could peer into biological systems at ambient temperatures and physiological conditions, study photochemical systems and catalysts under the conditions in which they operate, and monitor nanoscale fluctuations of the electronic and magnetic correlations thought to govern the behavior of quantum materials.

The XFEL was first proposed in 1992 to tackle the challenge of building an x-ray laser. Conventional laser schemes excite large numbers of atoms into states from which they emit light. But excited states with energies corresponding to x-ray wavelengths are too short-lived to build up a sizeable excited-state population. XFELs instead rely on electrons traveling at relativistic speed through a periodic magnetic array called an undulator. Moving in a bunch, the electrons wiggle through the undulator, emitting x-ray radiation that interacts multiple times with the bunch and becomes amplified. The result is a bright x-ray beam with laser coherence.

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha’s Rule

Fluorescence exclusively occurs from the lowest excited state of a given multiplicity according to Kasha’s rule. However, this rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying mechanism is still understood merely on a phenomenological basis. This lack of understanding prevents the rational design and property-tuning of anti-Kasha fluorophores. Here, we propose a model explaining the photophysical properties of an archetypal anti-Kasha fluorophore, azulene, based on its ground-and excited-state (anti)aromaticity. We derived our model from a detailed analysis of the electronic structure of the ground singlet, first excited triplet, and quintet states and of the first and second excited singlet states using the perturbational molecular orbital theory and quantum-chemical aromaticity indices.

New quasi-particle bridges microwave and optical domains

In a paper published today (Sept. 18) in Nature Communications, researchers from the Paul-Drude-Institut in Berlin, Germany, and the Instituto Balseiro in Bariloche, Argentina, demonstrated that the mixing of confined quantum fluids of light and GHz sound leads to the emergence of an elusive phonoriton quasi-particle—in part a quantum of light (photon), a quantum of sound (phonon) and a semiconductor exciton. This discovery opens a novel way to coherently convert information between optical and microwave domains, bringing potential benefits to the fields of photonics, optomechanics and optical communication technologies.

The research team’s work draws inspiration from an everyday phenomenon: the transfer of energy between two coupled oscillators, such as, for instance, two pendulums connected by a spring. Under specific coupling conditions, known as the strong-coupling (SC) regime, energy continuously oscillates between the two pendulums, which are no longer independent, as their frequencies and decay rates are not those of the uncoupled ones. The oscillators can also be photonic or electronic quantum states: the SC regime, in this case, is fundamental for quantum state control and swapping.

In the above example, the two pendulums are assumed to have the same frequency, i.e., in resonance. However, hybrid quantum systems require coherent information transfer between oscillators with largely dissimilar frequencies. Here, one important example is in networks of quantum computers. While the most promising quantum computers operate with microwave qubits (i.e., at few GHz), quantum information is efficiently transferred using near infrared photons (100ds THz).

Dark Photons Could Explain One of The Universe’s Greatest Mysteries

A shadowy form of light within a universe of hypothetical particles is getting some serious consideration as a means of discovering the identity of dark matter.

According to a comprehensive new analysis under quantum chromodynamics, the dark photon is a much better fit for the observed results of particle collider experiments than the standard model of particle physics, by quite a wide margin.

In fact, a team of researchers led by physicist Nicholas Hunt-Smith of the ARC Centre of Excellence for Dark Matter Particle Physics and the University of Adelaide in Australia calculated a confidence level of 6.5 sigma, suggesting the odds that dark photons don’t explain the observations are in the ballpark of one in a billion.

Physicists Create New Magnetic Material to Unleash Quantum Computing

Quantum behavior is a strange, fragile thing that hovers on the edge of reality, between a world of possibility and a Universe of absolutes. In that mathematical haze lies the potential of quantum computing; the promise of devices that could quickly solve algorithms that would take classic computers too long to process.

For now, quantum computers are confined to cool rooms close to absolute zero (−273 degrees Celsius) where particles are less likely to tumble out of their critical quantum states.

Breaking through this temperature barrier to develop materials that still exhibit quantum properties at room temperatures has long been the goal of quantum computing. Though the low temperatures help keep the particle’s properties from collapsing out of their useful fog of possibility, the bulk and expense of the equipment limits their potential and ability to be scaled up for general use.

Quantum computing offers new insight into photochemical processes

Quantum computing has provided new insights into a fundamental aspect of photochemical reactions that has previously proven difficult to study. The findings could improve scientists’ understanding of light-driven processes such as photosynthesis, smog formation and ozone destruction.

Photochemical processes occur when atomic nuclei and their electrons take on different configurations after absorbing a photon. Some of these reactions are guided by a quantum phenomenon called a conical intersection, where the potential energy surfaces that describe a molecule in its ground state and in its excited state converge. In these situations, quantum mechanical interference can prevent certain molecular transformations from taking place – a constraint known as a geometric phase. This limits the path that the reaction can take and affects the reaction outcome. The geometric phase has been known about since the 1950s, but due to the femtosecond timescales involved, it has never been directly observed in a molecular system.

Quantum Gauge Networks: A New Kind of Tensor Network

Kevin Slagle, Quantum 7, 1113 (2023). Although tensor networks are powerful tools for simulating low-dimensional quantum physics, tensor network algorithms are very computationally costly in higher spatial dimensions. We introduce $\textit{quantum gauge networks}$: a different kind of tensor network ansatz for which the computation cost of simulations does not explicitly increase for larger spatial dimensions. We take inspiration from the gauge picture of quantum dynamics, which consists of a local wavefunction for each patch of space, with neighboring patches related by unitary connections. A quantum gauge network (QGN) has a similar structure, except the Hilbert space dimensions of the local wavefunctions and connections are truncated. We describe how a QGN can be obtained from a generic wavefunction or matrix product state (MPS). All $2k$-point correlation functions of any wavefunction for $M$ many operators can be encoded exactly by a QGN with bond dimension $O(M^k)$. In comparison, for just $k=1$, an exponentially larger bond dimension of $2^{M/6}$ is generically required for an MPS of qubits. We provide a simple QGN algorithm for approximate simulations of quantum dynamics in any spatial dimension. The approximate dynamics can achieve exact energy conservation for time-independent Hamiltonians, and spatial symmetries can also be maintained exactly. We benchmark the algorithm by simulating the quantum quench of fermionic Hamiltonians in up to three spatial dimensions.

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