3 minute read
INDUSTRY NEWS: Ultrafast Probing Reveals Intricate Dynamics of Quantum Coherence
Source: Sally Wood
Ultrafast, multidimensional spectroscopy unlocks macroscopic-scale effects of quantum electronic correlations.
Researchers have found that low-energy and high energy states are correlated in a layered, superconducting material LSCO (lanthanum, strontium, copper, oxygen). Exciting the material, with an ultrafast (<100fs) beam of near-infrared light, produces coherent excitations lasting a surprisingly ‘long’ time of around 500 femtoseconds, originating from a quantum superposition of excited states within the crystal. The strong correlation between the energy of this coherence, and the optical energy of the emitted signal, indicates a coherent interaction between the states at low and high energy. This kind of coherent interaction is the root of many intriguing and poorly-understood phenomena displayed by quantum materials. It is one of the first applications of multidimensional spectroscopy to study correlated electron systems, such as hightemperature superconductors. The intriguing magnetic and electronic properties of quantum materials hold significant promise for future technologies. However, controlling these properties requires an improved understanding of the ways in which macroscopic behaviour emerges in complex materials with strong electronic correlations. Potentially useful electric and magnetic properties of quantum materials with strong electronic correlations include: Mott transition, colossal magnetoresistance, topological insulators, and hightemperature superconductivity. Such macroscopic properties emerge out of microscopic complexity, rooted in the competing interactions between the degrees of freedom (charge, lattice, spin, orbital, and topology) of electronic states. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. In a new study, researchers applied multidimensional coherent spectroscopy to the challenge for the first time, utilising the technique’s unique capability to differentiate between competing signal pathways, selectively exciting and probing low-energy excitations. Researchers analysed the quantum coherence of excitations produced by hitting LSCO (lanthanum, strontium, copper and oxygen) crystals with a sequence of tailored, ultrafast beams of near-infrared light lasting less than 100 femtoseconds This coherence has unusual properties, lasts a surprisingly ‘long’ time of around 500 femtoseconds, and originates from a quantum superposition of excited states within the crystal. “We found a strong correlation between the energy of this coherence and the optical energy of the emitted signal, which indicates a special coherent interaction between the states, at low and high energy, in these complex systems,” said study author Jeff Davis from Swinburne University of Technology. Because the number of available excitations affects the band structure of a crystal, the effective energy structure changes transiently during measurement, which links low-energy excitations and optically excited electronic states. The study demonstrated that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways. Furthermore, the results demonstrate that multidimensional coherent spectroscopy is a powerful tool in the study of electronic correlations, and opens the door to addressing, directly, the quantum-coherent states of many-body matter. As well as representing a major advancement in ultrafast spectroscopy of correlated materials, the work has wider significance in optics and photonics, chemistry, nanoscience, and condensedmatter science. The research paper, ‘Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy’, was published in Science Advances. Within FLEET, Jeff Davis uses ultrafast spectroscopy to study and control the microscopic interactions in 2D materials and how they lead to macroscopic behaviour. In FLEET’s third research theme, lighttransformed materials, systems are temporarily driven out of thermal equilibrium in order to investigate the qualitatively different physics displayed, and new capabilities for dynamically controlling their behaviour. FLEET is an Australian Research Councilfunded research centre bringing together over a hundred Australian and international experts to develop a new generation of ultra-low energy electronics.
Three excitation pulses with wave vectors k1, k2, and k3 form three corners of a box with 4th pulse (local oscillation; LO) on the fourth corner.
2D spectrum showing energy difference between the states in the quantum superposition, shown before, during and after pulse overlap.
