Quantum spin dynamics

Abstract:

For the past three decades nanoscience has widely affected many areas in physics, chemistry and engineering, and has led to numerous fundamental discoveries, as well as applications and products. Concurrently, quantum science and technology has developed into a cross-disciplinary research endeavour connecting these same areas and holds burgeoning commercial promise. Although quantum physics dictates the behaviour of nanoscale objects, quantum coherence, which is central to quantum information, communication and sensing, has not played an explicit role in much of nanoscience. This Review describes fundamental principles and practical applications of quantum coherence in nanoscale systems, a research area we call quantum-coherent nanoscience. We structure this Review according to specific degrees of freedom that can be quantum-coherently controlled in a given nanoscale system, such as charge, spin, mechanical motion and photons. We review the current state of the art and focus on outstanding challenges and opportunities unlocked by the merging of nanoscience and coherent quantum operations.

Abstract:

Resonant optical excitation of certain molecular vibrations in κ−(BEDT−TTF)2Cu[N(CN)2]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium Tc. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ−(BEDT−TTF)2Cu[N(CN)2]Cl and the metallic compound κ−(BEDT−TTF)2Cu(NCS)2. We find nonequilibrium photoinduced superconductivity only in κ−(BEDT−TTF)2Cu[N(CN)2]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.

Abstract:

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics, because electric fields can be confined over shorter length scales than magnetic fields1,2,3,4,5. Thus, recent demonstrations of electric-field sensitivities in molecular spin materials6,7,8 are tantalizing, raising the viability of the quantum analogues of macroscopic magneto-electric devices9,10,11,12,13,14,15. However, the electric-field sensitivities reported so far are rather weak, prompting the question of how to design molecules with stronger spin–electric couplings. Here we show that one path is to identify an energy scale in the spin spectrum that is associated with a structural degree of freedom with a substantial electrical polarizability. We study an example of a molecular nanomagnet in which a small structural distortion establishes clock transitions (that is, transitions whose energy is to first order independent of the magnetic field) in the spin spectrum; the fact that this distortion is associated with an electric dipole allows us to control the clock-transition energy to an unprecedented degree. We demonstrate coherent electrical control of the quantum spin state and exploit it to independently manipulate the two magnetically identical but inversion-related molecules in the unit cell of the crystal. Our findings pave the way for the use of molecular spins in quantum technologies and spintronics.