Nonlinear quantum magnetophononics in SrCu2(BO3)2

Harnessing the most advanced capabilities of quantum technologies will require the ability to control macroscopic quantum states of matter. Quantum magnetic materials provide a valuable platform for realizing highly entangled many-body quantum systems, and have been used to investigate phenomena ranging from quantum phase transitions (QPTs) to fractionalization, topological order and the entanglement structure of the quantum wavefunction. Although multiple studies have controlled their properties by static applied pressures or magnetic fields, dynamical control at the fundamental timescales of their magnetic interactions remains completely unexplored. However, major progress in the technology of ultrafast laser pulses has enabled the dynamical modification of electronic properties, and now we demonstrate the ultrafast control of quantum magnetism. This we achieve by a magnetophononic mechanism, the driving of coherent lattice displacements to produce a resonant excitation of the quantum spin dynamics. Specifically, we apply intense terahertz laser pulses to excite a collective spin state of the quantum antiferromagnet SrCu2(BO3)2 by resonance with the nonlinear mixing frequency of the driven phonons that modulate the magnetic interactions. Our observations indicate a universal mechanism for controlling nonequilibrium quantum many-body physics on timescales many orders of magnitude faster than those achieved to date.

要充分发挥量子技术的前沿性能，必须具备调控物质宏观量子态的能力。量子磁材料 (Quantum magnetic materials) 是构建高纠缠多体量子系统的优质平台，已被用于研究从量子相变 (quantum phase transitions, QPTs)、分数化、拓扑序到量子波函数纠缠结构等各类物理现象。尽管已有多项研究通过静态外压或磁场调控其物性，但在磁相互作用的基本时间尺度上开展动态调控，迄今仍未被探索。不过，超快激光脉冲技术的长足进步已实现电子物性的动态调控，本研究首次展示了量子磁性的超快操控。我们通过磁声耦合机制达成这一目标：驱动相干晶格位移，以此对量子自旋动力学产生共振激发。具体而言，我们利用强太赫兹激光脉冲，通过与调控磁相互作用的受驱动声子的非线性混频频率共振，激发量子反铁磁体SrCu₂(BO₃)₂的集体自旋态。我们的观测结果表明，存在一种通用机制，可在比迄今已实现的调控快多个数量级的时间尺度上操控非平衡量子多体物理系统。