‘Sudden’ quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter^1,2,3,4. Yet this remains challenging in electron matter^5,6,7,8,9, especially superconductors^{10,11,12,13,14}. The grand question of what is hidden underneath superconductivity (SC)¹⁵ appears universal, but poorly understood. Here we reveal a long-lived gapless quantum phase of prethermalized quasiparticles (QPs) after a single-cycle terahertz (THz) quench of a Nb₃Sn SC gap. Its conductivity spectra is characterized by a sharp coherent peak and a vanishing scattering rate that decreases almost linearly towards zero frequency, which is most pronounced around the full depletion of the condensate and absent for a high-frequency pump. Above a critical pump threshold, such a QP phase with coherent transport and memory persists as an unusual prethermalization plateau, without relaxation to normal and SC thermal states for an order of magnitude longer than the QP recombination and thermalization times. Switching to this metastable ‘quantum QP fluid’ signals non-thermal quench of coupled SC and charge-density-wave (CDW)-like orders and hints quantum control beneath the SC.

突然的量子猝灭和预热已经成为发现物质^1,2,3,4出现状态的一个横切主题。然而，这在电子物质^{5、6、7、8、9}，特别是超导体^{10、11、12、13、14中}仍然具有挑战性。在超导电性（SC）¹⁵下隐藏着什么的大问题似乎普遍存在，但人们对此知之甚少。在这里，我们揭示了Nb ₃的单周期太赫兹（THz）淬灭后，预热的准粒子（QP）的长寿命无间隙量子相Sn SC间隙。它的电导率光谱的特征是尖锐的相干峰和消失的散射率，该散射率几乎线性地向零频率减小，这在冷凝水完全耗尽的情况下最为明显，而对于高频泵则不存在。高于临界泵阈值，具有相干传输和记忆的QP相将以异常的预热平台状态持续存在，而不会松弛到正常和SC热态，其幅度要比QP重组和热化时间长一个数量级。切换到这种亚稳态的“量子QP流体”会发出耦合SC的非热猝灭信号和类似电荷密度波（CDW）的阶数，并暗示SC下的量子控制。