Electronic spins can form long-range entangled phases of condensed matter named quantum spin liquids. They are expected to form in frustrated magnets that do not exhibit symmetry-breaking order down to zero temperature. Quantum spin ice is a theoretically well-established example described by an emergent quantum electrodynamics, with quasiparticle excitations behaving like photons and fractionally charged matter. However, in frustrated magnets it remains difficult to establish convincing experimental evidence for quantum spin liquid ground states and their fractional excitations. Here we study the time-dependent magnetic response of the candidate quantum spin ice material Ce₂Sn₂O₇. We find a gapped spectrum that features a threshold and peaks that match theories for pair production and propagation of fractional matter excitations strongly coupled to a background quantum electrodynamic field. The multiple peaks in our neutron spectroscopy data are a specific signature of the so-called π-flux phase of quantum spin ice, providing spectroscopic evidence for fractionalization in a three-dimensional quantum spin liquid.

电子自旋可以形成凝聚态的长程纠缠相，称为量子自旋液体。预计它们会在受挫磁体中形成，这些磁体在零温度下不会表现出打破对称性的序。量子自旋冰是一个理论上公认的例子，由新兴量子电动力学描述，准粒子激发的行为类似于光子和带分数电荷的物质。然而，在受挫磁体中，仍然很难为量子自旋液体基态及其分数激发建立令人信服的实验证据。在这里，我们研究了候选量子自旋冰材料 Ce₂Sn₂O₇ 的瞬态磁响应。我们发现了一个间隙光谱，其阈值和峰值与与背景量子电动场强耦合的分数物质激发的成对产生和传播理论相匹配。我们的中子能谱数据中的多个峰是量子自旋冰的π通量相的特定特征，为三维量子自旋液体中的分馏提供了光谱证据。